Tough_Crowd/src/main.py

# Diffuse: a graphical tool for merging and comparing text files.
#
# Copyright (C) 2019 Derrick Moser <derrick_moser@yahoo.com>
# Copyright (C) 2021 Romain Failliot <romain.failliot@foolstep.com>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License along
# with this program; if not, write to the Free Software Foundation, Inc.,
# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.

import os
import sys
import codecs
import difflib
import encodings
import glob
import re
import shlex
import stat
import unicodedata
import webbrowser

import gi

gi.require_version('GObject', '2.0')
from gi.repository import GObject

gi.require_version('Gtk', '3.0')
from gi.repository import Gtk

gi.require_version('Gdk', '3.0')
from gi.repository import Gdk

gi.require_version('GdkPixbuf', '2.0')
from gi.repository import GdkPixbuf

gi.require_version('Pango', '1.0')
from gi.repository import Pango

gi.require_version('PangoCairo', '1.0')
from gi.repository import PangoCairo

from urllib.parse import urlparse

from diffuse import utils
from diffuse import constants
from diffuse.vcs.vcs_registry import VcsRegistry

if not hasattr(__builtins__, 'WindowsError'):
    # define 'WindowsError' so 'except' statements will work on all platforms
    WindowsError = IOError

# avoid some dictionary lookups when string.whitespace is used in loops
# this is sorted based upon frequency to speed up code for stripping whitespace
whitespace = ' \t\n\r\x0b\x0c'

# colour resources
class Colour:
    def __init__(self, r, g, b, a=1.0):
        # the individual colour components as floats in the range [0, 1]
        self.red = r
        self.green = g
        self.blue = b
        self.alpha = a

    # multiply by scalar
    def __mul__(self, s):
        return Colour(s * self.red, s * self.green, s * self.blue, s * self.alpha)

    # add colours
    def __add__(self, other):
        return Colour(self.red + other.red, self.green + other.green, self.blue + other.blue, self.alpha + other.alpha)

    # over operator
    def over(self, other):
        return self + other * (1 - self.alpha)

# class to build and run a finite state machine for identifying syntax tokens
class SyntaxParser:
    # create a new state machine that begins in initial_state and classifies
    # all characters not matched by the patterns as default_token_type
    def __init__(self, initial_state, default_token_type):
        # initial state for the state machine when parsing a new file
        self.initial_state = initial_state
        # default classification of characters that are not explicitly matched
        # by any state transition patterns
        self.default_token_type = default_token_type
        # mappings from a state to a list of (pattern, token_type, next_state)
        # tuples indicating the new state for the state machine when 'pattern'
        # is matched and how to classify the matched characters
        self.transitions_lookup = { initial_state : [] }

    # Adds a new edge to the finite state machine from prev_state to
    # next_state.  Characters will be identified as token_type when pattern is
    # matched.  Any newly referenced state will be added.  Patterns for edges
    # leaving a state will be tested in the order they were added to the finite
    # state machine.
    def addPattern(self, prev_state, next_state, token_type, pattern):
        lookup = self.transitions_lookup
        for state in prev_state, next_state:
            if state not in lookup:
                lookup[state] = []
        lookup[prev_state].append([pattern, token_type, next_state])

    # given a string and an initial state, identify the final state and tokens
    def parse(self, state_name, s):
        lookup = self.transitions_lookup
        transitions, blocks, start = lookup[state_name], [], 0
        while start < len(s):
            for pattern, token_type, next_state in transitions:
                m = pattern.match(s, start)
                if m is not None:
                    end, state_name = m.span()[1], next_state
                    transitions = lookup[state_name]
                    break
            else:
                end, token_type = start + 1, self.default_token_type
            if len(blocks) > 0 and blocks[-1][2] == token_type:
                blocks[-1][1] = end
            else:
                blocks.append([start, end, token_type])
            start = end
        return state_name, blocks

# split string into lines based upon DOS, Mac, and Unix line endings
def splitlines(s):
    # split on new line characters
    temp, i, n = [], 0, len(s)
    while i < n:
        j = s.find('\n', i)
        if j < 0:
            temp.append(s[i:])
            break
        j += 1
        temp.append(s[i:j])
        i = j
    # split on carriage return characters
    ss = []
    for s in temp:
        i, n = 0, len(s)
        while i < n:
            j = s.find('\r', i)
            if j < 0:
                ss.append(s[i:])
                break
            j += 1
            if j < n and s[j] == '\n':
                j += 1
            ss.append(s[i:j])
            i = j
    return ss

# also recognise old Mac OS line endings
def readlines(fd):
    return strip_eols(splitlines(fd.read()))

def readconfiglines(fd):
    return fd.read().replace('\r', '').split('\n')

# This class to hold all customisable behaviour not exposed in the preferences
# dialogue: hotkey assignment, colours, syntax highlighting, etc.
# Syntax highlighting is implemented in supporting '*.syntax' files normally
# read from the system wide initialisation file '/etc/diffuserc'.
# The personal initialisation file '~/diffuse/diffuserc' can be used to change
# default behaviour.
class Resources:
    def __init__(self):
        # default keybindings
        self.keybindings = {}
        self.keybindings_lookup = {}
        set_binding = self.setKeyBinding
        set_binding('menu', 'open_file', 'Ctrl+o')
        set_binding('menu', 'open_file_in_new_tab', 'Ctrl+t')
        set_binding('menu', 'open_modified_files', 'Shift+Ctrl+O')
        set_binding('menu', 'open_commit', 'Shift+Ctrl+T')
        set_binding('menu', 'reload_file', 'Shift+Ctrl+R')
        set_binding('menu', 'save_file', 'Ctrl+s')
        set_binding('menu', 'save_file_as', 'Shift+Ctrl+A')
        set_binding('menu', 'save_all', 'Shift+Ctrl+S')
        set_binding('menu', 'new_2_way_file_merge', 'Ctrl+2')
        set_binding('menu', 'new_3_way_file_merge', 'Ctrl+3')
        set_binding('menu', 'new_n_way_file_merge', 'Ctrl+4')
        set_binding('menu', 'close_tab', 'Ctrl+w')
        set_binding('menu', 'undo_close_tab', 'Shift+Ctrl+W')
        set_binding('menu', 'quit', 'Ctrl+q')
        set_binding('menu', 'undo', 'Ctrl+z')
        set_binding('menu', 'redo', 'Shift+Ctrl+Z')
        set_binding('menu', 'cut', 'Ctrl+x')
        set_binding('menu', 'copy', 'Ctrl+c')
        set_binding('menu', 'paste', 'Ctrl+v')
        set_binding('menu', 'select_all', 'Ctrl+a')
        set_binding('menu', 'clear_edits', 'Ctrl+r')
        set_binding('menu', 'dismiss_all_edits', 'Ctrl+d')
        set_binding('menu', 'find', 'Ctrl+f')
        set_binding('menu', 'find_next', 'Ctrl+g')
        set_binding('menu', 'find_previous', 'Shift+Ctrl+G')
        set_binding('menu', 'go_to_line', 'Shift+Ctrl+L')
        set_binding('menu', 'realign_all', 'Ctrl+l')
        set_binding('menu', 'isolate', 'Ctrl+i')
        set_binding('menu', 'first_difference', 'Shift+Ctrl+Up')
        set_binding('menu', 'previous_difference', 'Ctrl+Up')
        set_binding('menu', 'next_difference', 'Ctrl+Down')
        set_binding('menu', 'last_difference', 'Shift+Ctrl+Down')
        set_binding('menu', 'first_tab', 'Shift+Ctrl+Page_Up')
        set_binding('menu', 'previous_tab', 'Ctrl+Page_Up')
        set_binding('menu', 'next_tab', 'Ctrl+Page_Down')
        set_binding('menu', 'last_tab', 'Shift+Ctrl+Page_Down')
        set_binding('menu', 'shift_pane_right', 'Shift+Ctrl+parenright')
        set_binding('menu', 'shift_pane_left', 'Shift+Ctrl+parenleft')
        set_binding('menu', 'convert_to_upper_case', 'Ctrl+u')
        set_binding('menu', 'convert_to_lower_case', 'Shift+Ctrl+U')
        set_binding('menu', 'sort_lines_in_ascending_order', 'Ctrl+y')
        set_binding('menu', 'sort_lines_in_descending_order', 'Shift+Ctrl+Y')
        set_binding('menu', 'remove_trailing_white_space', 'Ctrl+k')
        set_binding('menu', 'convert_tabs_to_spaces', 'Ctrl+b')
        set_binding('menu', 'convert_leading_spaces_to_tabs', 'Shift+Ctrl+B')
        set_binding('menu', 'increase_indenting', 'Shift+Ctrl+greater')
        set_binding('menu', 'decrease_indenting', 'Shift+Ctrl+less')
        set_binding('menu', 'convert_to_dos', 'Shift+Ctrl+E')
        set_binding('menu', 'convert_to_mac', 'Shift+Ctrl+C')
        set_binding('menu', 'convert_to_unix', 'Ctrl+e')
        set_binding('menu', 'copy_selection_right', 'Shift+Ctrl+Right')
        set_binding('menu', 'copy_selection_left', 'Shift+Ctrl+Left')
        set_binding('menu', 'copy_left_into_selection', 'Ctrl+Right')
        set_binding('menu', 'copy_right_into_selection', 'Ctrl+Left')
        set_binding('menu', 'merge_from_left_then_right', 'Ctrl+m')
        set_binding('menu', 'merge_from_right_then_left', 'Shift+Ctrl+M')
        set_binding('menu', 'help_contents', 'F1')
        set_binding('line_mode', 'enter_align_mode', 'space')
        set_binding('line_mode', 'enter_character_mode', 'Return')
        set_binding('line_mode', 'enter_character_mode', 'KP_Enter')
        set_binding('line_mode', 'first_line', 'Home')
        set_binding('line_mode', 'first_line', 'g')
        set_binding('line_mode', 'extend_first_line', 'Shift+Home')
        set_binding('line_mode', 'last_line', 'End')
        set_binding('line_mode', 'last_line', 'Shift+G')
        set_binding('line_mode', 'extend_last_line', 'Shift+End')
        set_binding('line_mode', 'up', 'Up')
        set_binding('line_mode', 'up', 'k')
        set_binding('line_mode', 'extend_up', 'Shift+Up')
        set_binding('line_mode', 'extend_up', 'Shift+K')
        set_binding('line_mode', 'down', 'Down')
        set_binding('line_mode', 'down', 'j')
        set_binding('line_mode', 'extend_down', 'Shift+Down')
        set_binding('line_mode', 'extend_down', 'Shift+J')
        set_binding('line_mode', 'left', 'Left')
        set_binding('line_mode', 'left', 'h')
        set_binding('line_mode', 'extend_left', 'Shift+Left')
        set_binding('line_mode', 'right', 'Right')
        set_binding('line_mode', 'right', 'l')
        set_binding('line_mode', 'extend_right', 'Shift+Right')
        set_binding('line_mode', 'page_up', 'Page_Up')
        set_binding('line_mode', 'page_up', 'Ctrl+u')
        set_binding('line_mode', 'extend_page_up', 'Shift+Page_Up')
        set_binding('line_mode', 'extend_page_up', 'Shift+Ctrl+U')
        set_binding('line_mode', 'page_down', 'Page_Down')
        set_binding('line_mode', 'page_down', 'Ctrl+d')
        set_binding('line_mode', 'extend_page_down', 'Shift+Page_Down')
        set_binding('line_mode', 'extend_page_down', 'Shift+Ctrl+D')
        set_binding('line_mode', 'delete_text', 'BackSpace')
        set_binding('line_mode', 'delete_text', 'Delete')
        set_binding('line_mode', 'delete_text', 'x')
        set_binding('line_mode', 'clear_edits', 'r')
        set_binding('line_mode', 'isolate', 'i')
        set_binding('line_mode', 'first_difference', 'Ctrl+Home')
        set_binding('line_mode', 'first_difference', 'Shift+P')
        set_binding('line_mode', 'previous_difference', 'p')
        set_binding('line_mode', 'next_difference', 'n')
        set_binding('line_mode', 'last_difference', 'Ctrl+End')
        set_binding('line_mode', 'last_difference', 'Shift+N')
        #set_binding('line_mode', 'copy_selection_right', 'Shift+L')
        #set_binding('line_mode', 'copy_selection_left', 'Shift+H')
        set_binding('line_mode', 'copy_left_into_selection', 'Shift+L')
        set_binding('line_mode', 'copy_right_into_selection', 'Shift+H')
        set_binding('line_mode', 'merge_from_left_then_right', 'm')
        set_binding('line_mode', 'merge_from_right_then_left', 'Shift+M')
        set_binding('align_mode', 'enter_line_mode', 'Escape')
        set_binding('align_mode', 'align', 'space')
        set_binding('align_mode', 'enter_character_mode', 'Return')
        set_binding('align_mode', 'enter_character_mode', 'KP_Enter')
        set_binding('align_mode', 'first_line', 'g')
        set_binding('align_mode', 'last_line', 'Shift+G')
        set_binding('align_mode', 'up', 'Up')
        set_binding('align_mode', 'up', 'k')
        set_binding('align_mode', 'down', 'Down')
        set_binding('align_mode', 'down', 'j')
        set_binding('align_mode', 'left', 'Left')
        set_binding('align_mode', 'left', 'h')
        set_binding('align_mode', 'right', 'Right')
        set_binding('align_mode', 'right', 'l')
        set_binding('align_mode', 'page_up', 'Page_Up')
        set_binding('align_mode', 'page_up', 'Ctrl+u')
        set_binding('align_mode', 'page_down', 'Page_Down')
        set_binding('align_mode', 'page_down', 'Ctrl+d')
        set_binding('character_mode', 'enter_line_mode', 'Escape')

        # default colours
        self.colours = {
            'alignment' : Colour(1.0, 1.0, 0.0),
            'character_selection' : Colour(0.7, 0.7, 1.0),
            'cursor' : Colour(0.0, 0.0, 0.0),
            'difference_1' : Colour(1.0, 0.625, 0.625),
            'difference_2' : Colour(0.85, 0.625, 0.775),
            'difference_3' : Colour(0.85, 0.775, 0.625),
            'hatch' : Colour(0.8, 0.8, 0.8),
            'line_number' : Colour(0.0, 0.0, 0.0),
            'line_number_background' : Colour(0.75, 0.75, 0.75),
            'line_selection' : Colour(0.7, 0.7, 1.0),
            'map_background' : Colour(0.6, 0.6, 0.6),
            'margin' : Colour(0.8, 0.8, 0.8),
            'edited' : Colour(0.5, 1.0, 0.5),
            'preedit' : Colour(0.0, 0.0, 0.0),
            'text' : Colour(0.0, 0.0, 0.0),
            'text_background' : Colour(1.0, 1.0, 1.0) }

        # default floats
        self.floats = {
            'alignment_opacity' : 1.0,
            'character_difference_opacity' : 0.4,
            'character_selection_opacity' : 0.4,
            'edited_opacity' : 0.4,
            'line_difference_opacity' : 0.3,
            'line_selection_opacity' : 0.4 }

        # default strings
        self.strings = {}

        # syntax highlighting support
        self.syntaxes = {}
        self.syntax_file_patterns = {}
        self.syntax_magic_patterns = {}
        self.current_syntax = None

        # list of imported resources files (we only import each file once)
        self.resource_files = set()

        # special string resources
        self.setDifferenceColours('difference_1 difference_2 difference_3')

    # keyboard action processing
    def setKeyBinding(self, ctx, s, v):
        action_tuple = (ctx, s)
        modifiers = Gdk.ModifierType(0)
        key = None
        for token in v.split('+'):
            if token == 'Shift':
                modifiers |= Gdk.ModifierType.SHIFT_MASK
            elif token == 'Ctrl':
                modifiers |= Gdk.ModifierType.CONTROL_MASK
            elif token == 'Alt':
                modifiers |= Gdk.ModifierType.MOD1_MASK
            elif len(token) == 0 or token[0] == '_':
                raise ValueError()
            else:
                token = 'KEY_' + token
                if not hasattr(Gdk, token):
                    raise ValueError()
                key = getattr(Gdk, token)
        if key is None:
            raise ValueError()
        key_tuple = (ctx, (key, modifiers))

        # remove any existing binding
        if key_tuple in self.keybindings_lookup:
            self._removeKeyBinding(key_tuple)

        # ensure we have a set to hold this action
        if action_tuple not in self.keybindings:
            self.keybindings[action_tuple] = {}
        bindings = self.keybindings[action_tuple]

        # menu items can only have one binding
        if ctx == 'menu':
            for k in bindings.keys():
                self._removeKeyBinding(k)

        # add the binding
        bindings[key_tuple] = None
        self.keybindings_lookup[key_tuple] = action_tuple

    def _removeKeyBinding(self, key_tuple):
        action_tuple = self.keybindings_lookup[key_tuple]
        del self.keybindings_lookup[key_tuple]
        del self.keybindings[action_tuple][key_tuple]

    def getActionForKey(self, ctx, key, modifiers):
        try:
            return self.keybindings_lookup[(ctx, (key, modifiers))][1]
        except KeyError:
            pass

    def getKeyBindings(self, ctx, s):
        try:
            return [ t for c, t in self.keybindings[(ctx, s)].keys() ]
        except KeyError:
            return []

    # colours used for indicating differences
    def setDifferenceColours(self, s):
        colours = s.split()
        if len(colours) > 0:
            self.difference_colours = colours

    def getDifferenceColour(self, i):
        n = len(self.difference_colours)
        return self.getColour(self.difference_colours[(i + n - 1) % n])

    # colour resources
    def getColour(self, symbol):
        try:
            return self.colours[symbol]
        except KeyError:
            utils.logDebug(f'Warning: unknown colour "{symbol}"')
            self.colours[symbol] = v = Colour(0.0, 0.0, 0.0)
            return v

    # float resources
    def getFloat(self, symbol):
        try:
            return self.floats[symbol]
        except KeyError:
            utils.logDebug(f'Warning: unknown float "{symbol}"')
            self.floats[symbol] = v = 0.5
            return v

    # string resources
    def getString(self, symbol):
        try:
            return self.strings[symbol]
        except KeyError:
            utils.logDebug(f'Warning: unknown string "{symbol}"')
            self.strings[symbol] = v = ''
            return v

    # syntax highlighting
    def getSyntaxNames(self):
        return list(self.syntaxes.keys())

    def getSyntax(self, name):
        return self.syntaxes.get(name, None)

    def guessSyntaxForFile(self, name, ss):
        name = os.path.basename(name)
        for key, pattern in self.syntax_file_patterns.items():
            if pattern.search(name):
                return key
        # fallback to analysing the first line of the file
        if len(ss) > 0:
            s = ss[0]
            for key, pattern in self.syntax_magic_patterns.items():
                if pattern.search(s):
                    return key

    # parse resource files
    def parse(self, file_name):
        # only process files once
        if file_name in self.resource_files:
            return

        self.resource_files.add(file_name)
        f = open(file_name, 'r')
        ss = readconfiglines(f)
        f.close()

        # FIXME: improve validation
        for i, s in enumerate(ss):
            args = shlex.split(s, True)
            if len(args) == 0:
                continue

            try:
                # eg. add Python syntax highlighting:
                #    import /usr/share/diffuse/syntax/python.syntax
                if args[0] == 'import' and len(args) == 2:
                    path = os.path.expanduser(args[1])
                    # relative paths are relative to the parsed file
                    path = os.path.join(utils.globEscape(os.path.dirname(file_name)), path)
                    paths = glob.glob(path)
                    if len(paths) == 0:
                        paths = [ path ]
                    for path in paths:
                        # convert to absolute path so the location of
                        # any processing errors are reported with
                        # normalised file names
                        self.parse(os.path.abspath(path))
                # eg. make Ctrl+o trigger the open_file menu item
                #    keybinding menu open_file Ctrl+o
                elif args[0] == 'keybinding' and len(args) == 4:
                    self.setKeyBinding(args[1], args[2], args[3])
                # eg. set the regular background colour to white
                #    colour text_background 1.0 1.0 1.0
                elif args[0] in [ 'colour', 'color' ] and len(args) == 5:
                    self.colours[args[1]] = Colour(float(args[2]), float(args[3]), float(args[4]))
                # eg. set opacity of the line_selection colour
                #    float line_selection_opacity 0.4
                elif args[0] == 'float' and len(args) == 3:
                    self.floats[args[1]] = float(args[2])
                # eg. set the help browser
                #    string help_browser gnome-help
                elif args[0] == 'string' and len(args) == 3:
                    self.strings[args[1]] = args[2]
                    if args[1] == 'difference_colours':
                        self.setDifferenceColours(args[2])
                # eg. start a syntax specification for Python
                #    syntax Python normal text
                # where 'normal' is the name of the default state and
                # 'text' is the classification of all characters not
                # explicitly matched by a syntax highlighting rule
                elif args[0] == 'syntax' and (len(args) == 2 or len(args) == 4):
                    key = args[1]
                    if len(args) == 2:
                        # remove file pattern for a syntax specification
                        try:
                            del self.syntax_file_patterns[key]
                        except KeyError:
                            pass
                        # remove magic pattern for a syntax specification
                        try:
                            del self.syntax_magic_patterns[key]
                        except KeyError:
                            pass
                        # remove a syntax specification
                        self.current_syntax = None
                        try:
                            del self.syntaxes[key]
                        except KeyError:
                            pass
                    else:
                        self.current_syntax = SyntaxParser(args[2], args[3])
                        self.syntaxes[key] = self.current_syntax
                # eg. transition from state 'normal' to 'comment' when
                # the pattern '#' is matched and classify the matched
                # characters as 'python_comment'
                #    syntax_pattern normal comment python_comment '#'
                elif args[0] == 'syntax_pattern' and self.current_syntax is not None and len(args) >= 5:
                    flags = 0
                    for arg in args[5:]:
                        if arg == 'ignorecase':
                            flags |= re.IGNORECASE
                        else:
                            raise ValueError()
                    self.current_syntax.addPattern(args[1], args[2], args[3], re.compile(args[4], flags))
                # eg. default to the Python syntax rules when viewing
                # a file ending with '.py' or '.pyw'
                #    syntax_files Python '\.pyw?$'
                elif args[0] == 'syntax_files' and (len(args) == 2 or len(args) == 3):
                    key = args[1]
                    if len(args) == 2:
                        # remove file pattern for a syntax specification
                        try:
                            del self.syntax_file_patterns[key]
                        except KeyError:
                            pass
                    else:
                        flags = 0
                        if utils.isWindows():
                            flags |= re.IGNORECASE
                        self.syntax_file_patterns[key] = re.compile(args[2], flags)
                # eg. default to the Python syntax rules when viewing
                # a files starting with patterns like #!/usr/bin/python
                #    syntax_magic Python '^#!/usr/bin/python$'
                elif args[0] == 'syntax_magic' and len(args) > 1:
                    key = args[1]
                    if len(args) == 2:
                        # remove magic pattern for a syntax specification
                        try:
                            del self.syntax_magic_patterns[key]
                        except KeyError:
                            pass
                    else:
                        flags = 0
                        for arg in args[3:]:
                            if arg == 'ignorecase':
                                flags |= re.IGNORECASE
                            else:
                                raise ValueError()
                        self.syntax_magic_patterns[key] = re.compile(args[2], flags)
                else:
                    raise ValueError()
            except: # Grr... the 're' module throws weird errors
            #except ValueError:
                utils.logError(_('Error processing line %(line)d of %(file)s.') % { 'line': i + 1, 'file': file_name })

theResources = Resources()

# map an encoding name to its standard form
def norm_encoding(e):
    if e is not None:
        return e.replace('-', '_').lower()

# widget to help pick an encoding
class EncodingMenu(Gtk.Box):
    def __init__(self, prefs, autodetect=False):
        Gtk.Box.__init__(self, orientation=Gtk.Orientation.HORIZONTAL)
        self.combobox = combobox = Gtk.ComboBoxText.new()
        self.encodings = prefs.getEncodings()[:]
        for e in self.encodings:
            combobox.append_text(e)
        if autodetect:
            self.encodings.insert(0, None)
            combobox.prepend_text(_('Auto Detect'))
        self.pack_start(combobox, False, False, 0)
        combobox.show()

    def set_text(self, encoding):
        encoding = norm_encoding(encoding)
        if encoding in self.encodings:
            self.combobox.set_active(self.encodings.index(encoding))

    def get_text(self):
        i = self.combobox.get_active()
        if i >= 0:
            return self.encodings[i]

# text entry widget with a button to help pick file names
class FileEntry(Gtk.Box):
    def __init__(self, parent, title):
        Gtk.Box.__init__(self, orientation=Gtk.Orientation.HORIZONTAL)
        self.toplevel = parent
        self.title = title
        self.entry = entry = Gtk.Entry.new()
        self.pack_start(entry, True, True, 0)
        entry.show()
        button = Gtk.Button.new()
        image = Gtk.Image.new()
        image.set_from_stock(Gtk.STOCK_OPEN, Gtk.IconSize.MENU)
        button.add(image)
        image.show()
        button.connect('clicked', self.chooseFile)
        self.pack_start(button, False, False, 0)
        button.show()

    # action performed when the pick file button is pressed
    def chooseFile(self, widget):
        dialog = Gtk.FileChooserDialog(self.title, self.toplevel, Gtk.FileChooserAction.OPEN, (Gtk.STOCK_CANCEL, Gtk.ResponseType.CANCEL, Gtk.STOCK_OPEN, Gtk.ResponseType.OK))
        dialog.set_current_folder(os.path.realpath(os.curdir))
        if dialog.run() == Gtk.ResponseType.OK:
            self.entry.set_text(dialog.get_filename())
        dialog.destroy()

    def set_text(self, s):
        self.entry.set_text(s)

    def get_text(self):
        return self.entry.get_text()

# adaptor class to allow a Gtk.FontButton to be read like a Gtk.Entry
class FontButton(Gtk.FontButton):
    def __init__(self):
        Gtk.FontButton.__init__(self)

    def get_text(self):
        return self.get_font_name()

# class to store preferences and construct a dialogue for manipulating them
class Preferences:
    def __init__(self, path):
        self.bool_prefs = {}
        self.int_prefs = {}
        self.string_prefs = {}
        self.int_prefs_min = {}
        self.int_prefs_max = {}
        self.string_prefs_enums = {}

        # find available encodings
        self.encodings = sorted(set(encodings.aliases.aliases.values()))

        if utils.isWindows():
            svk_bin = 'svk.bat'
        else:
            svk_bin = 'svk'

        auto_detect_codecs = [ 'utf_8', 'utf_16', 'latin_1' ]
        e = norm_encoding(sys.getfilesystemencoding())
        if e not in auto_detect_codecs:
            # insert after UTF-8 as the default encoding may prevent UTF-8 from
            # being tried
            auto_detect_codecs.insert(2, e)

        # self.template describes how preference dialogue layout
        #
        # this will be traversed later to build the preferences dialogue and
        # discover which preferences exist
        #
        # folders are described using:
        #    [ 'FolderSet', label1, template1, label2, template2, ... ]
        # lists are described using:
        #    [ 'List', template1, template2, template3, ... ]
        # individual preferences are described using one of the following
        # depending upon its type and the desired widget:
        #    [ 'Boolean', name, default, label ]
        #    [ 'Integer', name, default, label ]
        #    [ 'String', name, default, label ]
        #    [ 'File', name, default, label ]
        #    [ 'Font', name, default, label ]
        self.template = [ 'FolderSet',
            _('Display'),
            [ 'List',
              [ 'Font', 'display_font', 'Monospace 10', _('Font') ],
              [ 'Integer', 'display_tab_width', 8, _('Tab width'), 1, 1024 ],
              [ 'Boolean', 'display_show_right_margin', True, _('Show right margin') ],
              [ 'Integer', 'display_right_margin', 80, _('Right margin'), 1, 8192 ],
              [ 'Boolean', 'display_show_line_numbers', True, _('Show line numbers') ],
              [ 'Boolean', 'display_show_whitespace', False, _('Show white space characters') ],
              [ 'Boolean', 'display_ignore_case', False, _('Ignore case differences') ],
              [ 'Boolean', 'display_ignore_whitespace', False, _('Ignore white space differences') ],
              [ 'Boolean', 'display_ignore_whitespace_changes', False, _('Ignore changes to white space') ],
              [ 'Boolean', 'display_ignore_blanklines', False, _('Ignore blank line differences') ],
              [ 'Boolean', 'display_ignore_endofline', False, _('Ignore end of line differences') ]
            ],
            _('Alignment'),
            [ 'List',
              [ 'Boolean', 'align_ignore_case', False, _('Ignore case') ],
              [ 'Boolean', 'align_ignore_whitespace', True, _('Ignore white space') ],
              [ 'Boolean', 'align_ignore_whitespace_changes', False, _('Ignore changes to white space') ],
              [ 'Boolean', 'align_ignore_blanklines', False, _('Ignore blank lines') ],
              [ 'Boolean', 'align_ignore_endofline', True, _('Ignore end of line characters') ]
            ],
            _('Editor'),
            [ 'List',
              [ 'Boolean', 'editor_auto_indent', True, _('Auto indent') ],
              [ 'Boolean', 'editor_expand_tabs', False, _('Expand tabs to spaces') ],
              [ 'Integer', 'editor_soft_tab_width', 8, _('Soft tab width'), 1, 1024 ]
            ],
            _('Tabs'),
            [ 'List',
              [ 'Integer', 'tabs_default_panes', 2, _('Default panes'), 2, 16 ],
              [ 'Boolean', 'tabs_always_show', False, _('Always show the tab bar') ],
              [ 'Boolean', 'tabs_warn_before_quit', True, _('Warn me when closing a tab will quit %s') % constants.APP_NAME ]
            ],
            _('Regional Settings'),
            [ 'List',
              [ 'Encoding', 'encoding_default_codec', sys.getfilesystemencoding(), _('Default codec') ],
              [ 'String', 'encoding_auto_detect_codecs', ' '.join(auto_detect_codecs), _('Order of codecs used to identify encoding') ]
            ],
        ]
        # conditions used to determine if a preference should be greyed out
        self.disable_when = {
            'display_right_margin': ('display_show_right_margin', False),
            'display_ignore_whitespace_changes': ('display_ignore_whitespace', True),
            'display_ignore_blanklines': ('display_ignore_whitespace', True),
            'display_ignore_endofline': ('display_ignore_whitespace', True),
            'align_ignore_whitespace_changes': ('align_ignore_whitespace', True),
            'align_ignore_blanklines': ('align_ignore_whitespace', True),
            'align_ignore_endofline': ('align_ignore_whitespace', True)
        }
        if utils.isWindows():
            root = os.environ.get('SYSTEMDRIVE', None)
            if root is None:
                root = 'C:\\'
            elif not root.endswith('\\'):
                root += '\\'
            self.template.extend([
                    _('Cygwin'),
                    [ 'List',
                      [ 'File', 'cygwin_root', os.path.join(root, 'cygwin'), _('Root directory') ],
                      [ 'String', 'cygwin_cygdrive_prefix', '/cygdrive', _('Cygdrive prefix') ]
                    ]
                ])

        # create template for Version Control options
        vcs = [ ('bzr', 'Bazaar', 'bzr'),
                ('cvs', 'CVS', 'cvs'),
                ('darcs', 'Darcs', 'darcs'),
                ('git', 'Git', 'git'),
                ('hg', 'Mercurial', 'hg'),
                ('mtn', 'Monotone', 'mtn'),
                ('rcs', 'RCS', None),
                ('svn', 'Subversion', 'svn'),
                ('svk', 'SVK', svk_bin) ]

        vcs_template = [ 'List',
              [ 'String', 'vcs_search_order', 'bzr cvs darcs git hg mtn rcs svn svk', _('Version control system search order') ] ]
        vcs_folders_template = [ 'FolderSet' ]
        for key, name, cmd in vcs:
            temp = [ 'List' ]
            if key == 'rcs':
                # RCS uses multiple commands
                temp.extend([ [ 'File', key + '_bin_co', 'co', _('"co" command') ],
                              [ 'File', key + '_bin_rlog', 'rlog', _('"rlog" command') ] ])
            else:
                temp.extend([ [ 'File', key + '_bin', cmd, _('Command') ] ])
            if utils.isWindows():
                temp.append([ 'Boolean', key + '_bash', False, _('Launch from a Bash login shell') ])
                if key != 'git':
                    temp.append([ 'Boolean', key + '_cygwin', False, _('Update paths for Cygwin') ])
            vcs_folders_template.extend([ name, temp ])
        vcs_template.append(vcs_folders_template)

        self.template.extend([ _('Version Control'), vcs_template ])
        self._initFromTemplate(self.template)
        self.default_bool_prefs = self.bool_prefs.copy()
        self.default_int_prefs = self.int_prefs.copy()
        self.default_string_prefs = self.string_prefs.copy()
        # load the user's preferences
        self.path = path
        if os.path.isfile(self.path):
            try:
                f = open(self.path, 'r')
                ss = readconfiglines(f)
                f.close()
                for j, s in enumerate(ss):
                    try:
                        a = shlex.split(s, True)
                        if len(a) > 0:
                            p = a[0]
                            if len(a) == 2 and p in self.bool_prefs:
                                self.bool_prefs[p] = (a[1] == 'True')
                            elif len(a) == 2 and p in self.int_prefs:
                                self.int_prefs[p] = max(self.int_prefs_min[p], min(int(a[1]), self.int_prefs_max[p]))
                            elif len(a) == 2 and p in self.string_prefs:
                                self.string_prefs[p] = a[1]
                            else:
                                raise ValueError()
                    except ValueError:
                        # this may happen if the prefs were written by a
                        # different version -- don't bother the user
                        utils.logDebug(f'Error processing line {j + 1} of {self.path}.')
            except IOError:
                # bad $HOME value? -- don't bother the user
                utils.logDebug(f'Error reading {self.path}.')

    # recursively traverses 'template' to discover the preferences and
    # initialise their default values in self.bool_prefs, self.int_prefs, and
    # self.string_prefs
    def _initFromTemplate(self, template):
        if template[0] == 'FolderSet' or template[0] == 'List':
            i = 1
            while i < len(template):
                if template[0] == 'FolderSet':
                    i += 1
                self._initFromTemplate(template[i])
                i += 1
        elif template[0] == 'Boolean':
            self.bool_prefs[template[1]] = template[2]
        elif template[0] == 'Integer':
            self.int_prefs[template[1]] = template[2]
            self.int_prefs_min[template[1]] = template[4]
            self.int_prefs_max[template[1]] = template[5]
        elif template[0] in [ 'String', 'File', 'Font', 'Encoding' ]:
            self.string_prefs[template[1]] = template[2]

    # callback used when a preference is toggled
    def _toggled_cb(self, widget, widgets, name):
        # disable any preferences than are no longer relevant
        for k, v in self.disable_when.items():
            p, t = v
            if p == name:
                widgets[k].set_sensitive(widgets[p].get_active() != t)

    # display the dialogue and update the preference values if the accept
    # button was pressed
    def runDialog(self, parent):
        dialog = Gtk.Dialog(_('Preferences'), parent=parent, destroy_with_parent=True)
        dialog.add_button(Gtk.STOCK_CANCEL, Gtk.ResponseType.REJECT)
        dialog.add_button(Gtk.STOCK_OK, Gtk.ResponseType.OK)

        widgets = {}
        w = self._buildPrefsDialog(parent, widgets, self.template)
        # disable any preferences than are not relevant
        for k, v in self.disable_when.items():
            p, t = v
            if widgets[p].get_active() == t:
                widgets[k].set_sensitive(False)
        dialog.vbox.add(w) # pylint: disable=no-member
        w.show()

        accept = (dialog.run() == Gtk.ResponseType.OK)
        if accept:
            for k in self.bool_prefs.keys():
                self.bool_prefs[k] = widgets[k].get_active()
            for k in self.int_prefs.keys():
                self.int_prefs[k] = widgets[k].get_value_as_int()
            for k in self.string_prefs.keys():
                self.string_prefs[k] = nullToEmpty(widgets[k].get_text())
            try:
                ss = []
                for k, v in self.bool_prefs.items():
                    if v != self.default_bool_prefs[k]:
                        ss.append(f'{k} {v}\n')
                for k, v in self.int_prefs.items():
                    if v != self.default_int_prefs[k]:
                        ss.append(f'{k} {v}\n')
                for k, v in self.string_prefs.items():
                    if v != self.default_string_prefs[k]:
                        v_escaped = v.replace('\\', '\\\\').replace('"', '\\"')
                        ss.append(f'{k} "{v_escaped}"\n')
                ss.sort()
                f = open(self.path, 'w')
                f.write(f'# This prefs file was generated by {constants.APP_NAME} {constants.VERSION}.\n\n')
                for s in ss:
                    f.write(s)
                f.close()
            except IOError:
                utils.logErrorAndDialog(_('Error writing %s.') % (self.path, ), parent)
        dialog.destroy()
        return accept

    # recursively traverses 'template' to build the preferences dialogue
    # and the individual preference widgets into 'widgets' so their value
    # can be easily queried by the caller
    def _buildPrefsDialog(self, parent, widgets, template):
        type = template[0]
        if type == 'FolderSet':
            notebook = Gtk.Notebook.new()
            notebook.set_border_width(10)
            i = 1
            while i < len(template):
                label = Gtk.Label.new(template[i])
                i += 1
                w = self._buildPrefsDialog(parent, widgets, template[i])
                i += 1
                notebook.append_page(w, label)
                w.show()
                label.show()
            return notebook
        else:
            table = Gtk.Grid.new()
            table.set_border_width(10)
            for i, tpl in enumerate(template[1:]):
                type = tpl[0]
                if type == 'FolderSet':
                    w = self._buildPrefsDialog(parent, widgets, tpl)
                    table.attach(w, 0, i, 2, 1)
                    w.show()
                elif type == 'Boolean':
                    button = Gtk.CheckButton.new_with_mnemonic(tpl[3])
                    button.set_active(self.bool_prefs[tpl[1]])
                    widgets[tpl[1]] = button
                    table.attach(button, 1, i, 1, 1)
                    button.connect('toggled', self._toggled_cb, widgets, tpl[1])
                    button.show()
                else:
                    label = Gtk.Label.new(tpl[3] + ': ')
                    label.set_xalign(1.0)
                    label.set_yalign(0.5)
                    table.attach(label, 0, i, 1, 1)
                    label.show()
                    if tpl[0] in [ 'Font', 'Integer' ]:
                        entry = Gtk.Box.new(Gtk.Orientation.HORIZONTAL, 0)
                        if tpl[0] == 'Font':
                            button = FontButton()
                            button.set_font(self.string_prefs[tpl[1]])
                        else:
                            button = Gtk.SpinButton.new(Gtk.Adjustment.new(self.int_prefs[tpl[1]], tpl[4], tpl[5], 1, 0, 0), 1.0, 0)
                        widgets[tpl[1]] = button
                        entry.pack_start(button, False, False, 0)
                        button.show()
                    else:
                        if tpl[0] == 'Encoding':
                            entry = EncodingMenu(self)
                            entry.set_text(tpl[3])
                        elif tpl[0] == 'File':
                            entry = FileEntry(parent, tpl[3])
                        else:
                            entry = Gtk.Entry.new()
                        widgets[tpl[1]] = entry
                        entry.set_text(self.string_prefs[tpl[1]])
                    table.attach(entry, 1, i, 1, 1)
                    entry.show()
                table.show()
            return table

    # get/set methods to manipulate the preference values
    def getBool(self, name):
        return self.bool_prefs[name]

    def setBool(self, name, value):
        self.bool_prefs[name] = value

    def getInt(self, name):
        return self.int_prefs[name]

    def getString(self, name):
        return self.string_prefs[name]

    def setString(self, name, value):
        self.string_prefs[name] = value

    def getEncodings(self):
        return self.encodings

    def _getDefaultEncodings(self):
        return self.string_prefs['encoding_auto_detect_codecs'].split()

    def getDefaultEncoding(self):
        return self.string_prefs['encoding_default_codec']

    # attempt to convert a string to unicode from an unknown encoding
    def convertToUnicode(self, s):
        # a BOM is required for autodetecting UTF16 and UTF32
        magic = { 'utf16': [ codecs.BOM_UTF16_BE, codecs.BOM_UTF16_LE ],
                  'utf32': [ codecs.BOM_UTF32_BE, codecs.BOM_UTF32_LE ] }
        for encoding in self._getDefaultEncodings():
            try:
                encoding = encoding.lower().replace('-', '').replace('_', '')
                for m in magic.get(encoding, [ b'' ]):
                    if s.startswith(m):
                        break
                else:
                    continue
                return str(s, encoding=encoding), encoding
            except (UnicodeDecodeError, LookupError):
                pass
        return ''.join([ chr(ord(c)) for c in s ]), None

    # cygwin and native applications can be used on windows, use this method
    # to convert a path to the usual form expected on sys.platform
    def convertToNativePath(self, s):
        if utils.isWindows() and s.find('/') >= 0:
            # treat as a cygwin path
            s = s.replace(os.sep, '/')
            # convert to a Windows native style path
            p = [ a for a in s.split('/') if a != '' ]
            if s.startswith('//'):
                p[:0] = [ '', '' ]
            elif s.startswith('/'):
                pr = [ a for a in self.getString('cygwin_cygdrive_prefix').split('/') if a != '' ]
                n = len(pr)
                if len(p) > n and len(p[n]) == 1 and p[:n] == pr:
                    # path starts with cygdrive prefix
                    p[:n + 1] = [ p[n] + ':' ]
                else:
                    # full path
                    p[:0] = [ a for a in self.getString('cygwin_root').split(os.sep) if a != '' ]
            # add trailing slash
            if p[-1] != '' and s.endswith('/'):
                p.append('')
            s = os.sep.join(p)
        return s

# convenience method for creating a menu according to a template
def createMenu(specs, radio=None, accel_group=None):
    menu = Gtk.Menu.new()
    for spec in specs:
        if len(spec) > 0:
            if len(spec) > 7 and spec[7] is not None:
                g, k = spec[7]
                if g not in radio:
                    item = Gtk.RadioMenuItem.new_with_mnemonic_from_widget(None, spec[0])
                    radio[g] = (item, {})
                else:
                    item = Gtk.RadioMenuItem.new_with_mnemonic_from_widget(radio[g][0], spec[0])
                radio[g][1][k] = item
            else:
                item = Gtk.ImageMenuItem.new_with_mnemonic(spec[0])
            cb = spec[1]
            if cb is not None:
                data = spec[2]
                item.connect('activate', cb, data)
            if len(spec) > 3 and spec[3] is not None:
                image = Gtk.Image.new()
                image.set_from_stock(spec[3], Gtk.IconSize.MENU)
                item.set_image(image)
            if accel_group is not None and len(spec) > 4:
                a = theResources.getKeyBindings('menu', spec[4])
                if len(a) > 0:
                    key, modifier = a[0]
                    item.add_accelerator('activate', accel_group, key, modifier, Gtk.AccelFlags.VISIBLE)
            if len(spec) > 5:
                item.set_sensitive(spec[5])
            if len(spec) > 6 and spec[6] is not None:
                item.set_submenu(createMenu(spec[6], radio, accel_group))
            item.set_use_underline(True)
        else:
            item = Gtk.SeparatorMenuItem.new()
        item.show()
        menu.append(item)
    return menu

# convenience method for creating a menu bar according to a template
def createMenuBar(specs, radio, accel_group):
    menu_bar = Gtk.MenuBar.new()
    for label, spec in specs:
        menu = Gtk.MenuItem.new_with_mnemonic(label)
        menu.set_submenu(createMenu(spec, radio, accel_group))
        menu.set_use_underline(True)
        menu.show()
        menu_bar.append(menu)
    return menu_bar

# convenience method for packing buttons into a container according to a
# template
def appendButtons(box, size, specs):
    for spec in specs:
        if len(spec) > 0:
            button = Gtk.Button.new()
            button.set_relief(Gtk.ReliefStyle.NONE)
            button.set_can_focus(False)
            image = Gtk.Image.new()
            image.set_from_stock(spec[0], size)
            button.add(image)
            image.show()
            if len(spec) > 2:
                button.connect('clicked', spec[1], spec[2])
                if len(spec) > 3:
                    button.set_tooltip_text(spec[3])
            box.pack_start(button, False, False, 0)
            button.show()
        else:
            separator = Gtk.Separator.new(Gtk.Orientation.VERTICAL)
            box.pack_start(separator, False, False, 5)
            separator.show()

# masks used to indicate the presence of particular line endings
DOS_FORMAT = 1
MAC_FORMAT = 2
UNIX_FORMAT = 4

# True if the string ends with '\r\n'
def has_dos_line_ending(s):
    return s.endswith('\r\n')

# True if the string ends with '\r'
def has_mac_line_ending(s):
    return s.endswith('\r')

# True if the string ends with '\n' but not '\r\n'
def has_unix_line_ending(s):
    return s.endswith('\n') and not s.endswith('\r\n')

# returns the format mask for a list of strings
def getFormat(ss):
    flags = 0
    for s in ss:
        if s is not None:
            if has_dos_line_ending(s):
                flags |= DOS_FORMAT
            elif has_mac_line_ending(s):
                flags |= MAC_FORMAT
            elif has_unix_line_ending(s):
                flags |= UNIX_FORMAT
    return flags

# returns the number of characters in the string excluding any line ending
# characters
def len_minus_line_ending(s):
    if s is None:
        return 0
    n = len(s)
    if s.endswith('\r\n'):
        n -= 2
    elif s.endswith('\r') or s.endswith('\n'):
        n -= 1
    return n

# returns the string without the line ending characters
def strip_eol(s):
    if s is not None:
        return s[:len_minus_line_ending(s)]

# returns the list of strings without line ending characters
def strip_eols(ss):
    return [ strip_eol(s) for s in ss ]

# convenience method to change the line ending of a string
def convert_to_format(s, format):
    if s is not None and format != 0:
        old_format = getFormat([ s ])
        if old_format != 0 and (old_format & format) == 0:
            s = strip_eol(s)
            # prefer the host line ending style
            if (format & DOS_FORMAT) and os.linesep == '\r\n':
                s += os.linesep
            elif (format & MAC_FORMAT) and os.linesep == '\r':
                s += os.linesep
            elif (format & UNIX_FORMAT) and os.linesep == '\n':
                s += os.linesep
            elif format & UNIX_FORMAT:
                s += '\n'
            elif format & DOS_FORMAT:
                s += '\r\n'
            elif format & MAC_FORMAT:
                s += '\r'
    return s

theVCSs = VcsRegistry()

# utility method to step advance an adjustment
def step_adjustment(adj, delta):
    v = adj.get_value() + delta
    # clamp to the allowed range
    v = max(v, int(adj.get_lower()))
    v = min(v, int(adj.get_upper() - adj.get_page_size()))
    adj.set_value(v)

# This is a replacement for Gtk.ScrolledWindow as it forced expose events to be
# handled immediately after changing the viewport position.  This could cause
# the application to become unresponsive for a while as it processed a large
# queue of keypress and expose event pairs.
class ScrolledWindow(Gtk.Grid):
    scroll_directions = set((Gdk.ScrollDirection.UP,
                             Gdk.ScrollDirection.DOWN,
                             Gdk.ScrollDirection.LEFT,
                             Gdk.ScrollDirection.RIGHT))

    def __init__(self, hadj, vadj):
        Gtk.Grid.__init__(self)
        self.position = (0, 0)
        self.scroll_count = 0
        self.partial_redraw = False

        self.hadj, self.vadj = hadj, vadj
        vport = Gtk.Viewport.new()
        darea = Gtk.DrawingArea.new()
        darea.add_events(Gdk.EventMask.SCROLL_MASK)
        self.darea = darea
        # replace darea's queue_draw_area with our own so we can tell when
        # to disable/enable our scrolling optimisation
        self.darea_queue_draw_area = darea.queue_draw_area
        darea.queue_draw_area = self.redraw_region
        vport.add(darea)
        darea.show()
        self.attach(vport, 0, 0, 1, 1)
        vport.set_vexpand(True)
        vport.set_hexpand(True)
        vport.show()

        self.vbar = bar = Gtk.Scrollbar.new(Gtk.Orientation.VERTICAL, vadj)
        self.attach(bar, 1, 0, 1, 1)
        bar.show()

        self.hbar = bar = Gtk.Scrollbar.new(Gtk.Orientation.HORIZONTAL, hadj)
        self.attach(bar, 0, 1, 1, 1)
        bar.show()

        # listen to our signals
        hadj.connect('value-changed', self.value_changed_cb)
        vadj.connect('value-changed', self.value_changed_cb)
        darea.connect('configure-event', self.configure_cb)
        darea.connect('scroll-event', self.scroll_cb)
        darea.connect('draw', self.draw_cb)

    # updates the adjustments to match the new widget size
    def configure_cb(self, widget, event):
        w, h = event.width, event.height
        for adj, d in (self.hadj, w), (self.vadj, h):
            v = adj.get_value()
            if v + d > adj.get_upper():
                adj.set_value(max(0, adj.get_upper() - d))
            adj.set_page_size(d)
            adj.set_page_increment(d)

    # update the vertical adjustment when the mouse's scroll wheel is used
    def scroll_cb(self, widget, event):
        d = event.direction
        if d in self.scroll_directions:
            delta = 100
            if d in (Gdk.ScrollDirection.UP, Gdk.ScrollDirection.LEFT):
                delta = -delta
            vertical = (d in (Gdk.ScrollDirection.UP, Gdk.ScrollDirection.DOWN))
            if event.state & Gdk.ModifierType.SHIFT_MASK:
                vertical = not vertical
            if vertical:
                adj = self.vadj
            else:
                adj = self.hadj
            step_adjustment(adj, delta)

    def value_changed_cb(self, widget):
        old_x, old_y = self.position
        pos_x = int(self.hadj.get_value())
        pos_y = int(self.vadj.get_value())
        self.position = (pos_x, pos_y)
        if self.darea.get_window() is not None:
            # window.scroll() although visually nice, is slow, revert to
            # queue_draw() if scroll a lot without seeing an expose event
            if self.scroll_count < 2 and not self.partial_redraw:
                self.scroll_count += 1
                self.darea.get_window().scroll(old_x - pos_x, old_y - pos_y)
            else:
                self.partial_redraw = False
                self.darea.queue_draw()

    def draw_cb(self, widget, cr):
        self.scroll_count = 0

    # replacement for darea.queue_draw_area that notifies us when a partial
    # redraw happened
    def redraw_region(self, x, y, w, h):
        self.partial_redraw = True
        self.darea_queue_draw_area(x, y, w, h)

# Enforcing manual alignment is accomplished by dividing the lines of text into
# sections that are matched independently.  'blocks' is an array of integers
# describing how many lines (including null lines for spacing) that are in each
# section starting from the beginning of the files.  When no manual alignment
# has been specified, all lines are in the same section so 'blocks' should
# contain a single entry equal to the number of lines.  Zeros are not allowed
# in this array so 'blocks' will be an empty array when there are no lines.  A
# 'cut' at location 'i' means a line 'i-1' and line 'i' belong to different
# sections

def createBlock(n):
    if n > 0:
        return [ n ]
    return []

# returns the two sets of blocks after cutting at 'i'
def cutBlocks(i, blocks):
    pre, post, nlines = [], [], 0
    for b in blocks:
        if nlines >= i:
            post.append(b)
        elif nlines + b <= i:
            pre.append(b)
        else:
            n = i - nlines
            pre.append(n)
            post.append(b - n)
        nlines += b
    return pre, post

# returns a set of blocks containing all of the cuts in the inputs
def mergeBlocks(leftblocks, rightblocks):
    leftblocks, rightblocks, b = leftblocks[:], rightblocks[:], []
    while len(leftblocks) > 0:
        nleft, nright = leftblocks[0], rightblocks[0]
        n = min(nleft, nright)
        if n < nleft:
            leftblocks[0] -= n
        else:
            del leftblocks[0]
        if n < nright:
            rightblocks[0] -= n
        else:
            del rightblocks[0]
        b.append(n)
    return b

# utility method to simplify working with structures used to describe character
# differences of a line
#
# ranges of character differences are indicated by (start, end, flags) tuples
# where 'flags' is a mask used to indicate if the characters are different from
# the line to the left, right, or both
#
# this method will return the union of two sorted lists of ranges
def mergeRanges(r1, r2):
    r1, r2, result, start = r1[:], r2[:], [], 0
    rs = [ r1, r2 ]
    while len(r1) > 0 and len(r2) > 0:
        flags, start = 0, min(r1[0][0], r2[0][0])
        if start == r1[0][0]:
            r1end = r1[0][1]
            flags |= r1[0][2]
        else:
            r1end = r1[0][0]
        if start == r2[0][0]:
            r2end = r2[0][1]
            flags |= r2[0][2]
        else:
            r2end = r2[0][0]
        end = min(r1end, r2end)
        result.append((start, end, flags))
        for r in rs:
            if start == r[0][0]:
                if end == r[0][1]:
                    del r[0]
                else:
                    r[0] = (end, r[0][1], r[0][2])
    result.extend(r1)
    result.extend(r2)
    return result

# eliminates lines that are spacing lines in all panes
def removeNullLines(blocks, lines_set):
    bi, bn, i = 0, 0, 0
    while bi < len(blocks):
        while i < bn + blocks[bi]:
            for lines in lines_set:
                if lines[i] is not None:
                    i += 1
                    break
            else:
                for lines in lines_set:
                    del lines[i]
                blocks[bi] -= 1
        if blocks[bi] == 0:
            del blocks[bi]
        else:
            bn += blocks[bi]
            bi += 1

def nullToEmpty(s):
    if s is None:
        s = ''
    return s

# returns true if the string only contains whitespace characters
def isBlank(s):
    for c in whitespace:
        s = s.replace(c, '')
    return len(s) == 0

# use Pango.SCALE instead of Pango.PIXELS to avoid overflow exception
def pixels(size):
    return int(size / Pango.SCALE + 0.5)

# constructs a full URL for the named file
def path2url(path, proto='file'):
    r = [ proto, ':///' ]
    s = os.path.abspath(path)
    i = 0
    while i < len(s) and s[i] == os.sep:
        i += 1
    for c in s[i:]:
        if c == os.sep:
            c = '/'
        elif c == ':' and utils.isWindows():
            c = '|'
        else:
            v = ord(c)
            if v <= 0x20 or v >= 0x7b or c in '$&+,/:;=?@"<>#%\\^[]`':
                c = '%%%02X' % (v, )
        r.append(c)
    return ''.join(r)

# the file diff viewer is always in one of these modes defining the cursor,
# and hotkey behaviour
LINE_MODE = 0
CHAR_MODE = 1
ALIGN_MODE = 2

# mapping to column width of a character (tab will never be in this map)
char_width_cache = {}

ALPHANUMERIC_CLASS = 0
WHITESPACE_CLASS = 1
OTHER_CLASS = 2

# maps similar types of characters to a group
def getCharacterClass(c):
    if c.isalnum() or c == '_':
        return ALPHANUMERIC_CLASS
    elif c.isspace():
        return WHITESPACE_CLASS
    return OTHER_CLASS

# longest common subsequence of unique elements common to 'a' and 'b'
def __patience_subsequence(a, b):
    # value unique lines by their order in each list
    value_a, value_b = {}, {}
    # find unique values in 'a'
    for i, s in enumerate(a):
        if s in value_a:
            value_a[s] = -1
        else:
            value_a[s] = i
    # find unique values in 'b'
    for i, s in enumerate(b):
        if s in value_b:
            value_b[s] = -1
        else:
            value_b[s] = i
    # lay down items in 'b' as if playing patience if the item is unique in
    # 'a' and 'b'
    pile, pointers, atob = [], {}, {}
    get, append = value_a.get, pile.append
    for s in b:
        v = get(s, -1)
        if v != -1:
            vb = value_b[s]
            if vb != -1:
                atob[v] = vb
                # find appropriate pile for v
                start, end = 0, len(pile)
                # optimisation as values usually increase
                if end and v > pile[-1]:
                    start = end
                else:
                    while start < end:
                        mid = (start + end) // 2
                        if v < pile[mid]:
                            end = mid
                        else:
                            start = mid + 1
                if start < len(pile):
                    pile[start] = v
                else:
                    append(v)
                if start:
                    pointers[v] = pile[start-1]
    # examine our piles to determine the longest common subsequence
    result = []
    if pile:
        v, append = pile[-1], result.append
        append((v, atob[v]))
        while v in pointers:
            v = pointers[v]
            append((v, atob[v]))
        result.reverse()
    return result

# difflib-style approximation of the longest common subsequence
def __lcs_approx(a, b):
    count1, lookup = {}, {}
    # count occurrences of each element in 'a'
    for s in a:
        count1[s] = count1.get(s, 0) + 1
    # construct a mapping from a element to where it can be found in 'b'
    for i, s in enumerate(b):
        if s in lookup:
            lookup[s].append(i)
        else:
            lookup[s] = [ i ]
    if set(lookup).intersection(count1):
        # we have some common elements
        # identify popular entries
        popular = {}
        n = len(a)
        if n > 200:
            for k, v in count1.items():
                if 100 * v > n:
                    popular[k] = 1
        n = len(b)
        if n > 200:
            for k, v in lookup.items():
                if 100 * len(v) > n:
                    popular[k] = 1
        # while walk through entries in 'a', incrementally update the list of
        # matching subsequences in 'b' and keep track of the longest match
        # found
        prev_matches, matches, max_length, max_indices = {}, {}, 0, []
        for ai, s in enumerate(a):
            if s in lookup:
                if s in popular:
                    # we only extend existing previously found matches to avoid
                    # performance issues
                    for bi in prev_matches:
                        if bi + 1 < n and b[bi + 1] == s:
                            matches[bi] = v = prev_matches[bi] + 1
                            # check if this is now the longest match
                            if v >= max_length:
                                if v == max_length:
                                    max_indices.append((ai, bi))
                                else:
                                    max_length = v
                                    max_indices = [ (ai, bi) ]
                else:
                    prev_get = prev_matches.get
                    for bi in lookup[s]:
                        matches[bi] = v = prev_get(bi - 1, 0) + 1
                        # check if this is now the longest match
                        if v >= max_length:
                            if v == max_length:
                                max_indices.append((ai, bi))
                            else:
                                max_length = v
                                max_indices = [ (ai, bi) ]
            prev_matches, matches = matches, {}
        if max_indices:
            # include any popular entries at the beginning
            aidx, bidx, nidx = 0, 0, 0
            for ai, bi in max_indices:
                n = max_length
                ai += 1 - n
                bi += 1 - n
                while ai and bi and a[ai - 1] == b[bi - 1]:
                    ai -= 1
                    bi -= 1
                    n += 1
                if n > nidx:
                    aidx, bidx, nidx = ai, bi, n
            return aidx, bidx, nidx

# patinence diff with difflib-style fallback
def patience_diff(a, b):
    matches, len_a, len_b = [], len(a), len(b)
    if len_a and len_b:
        blocks = [ (0, len_a, 0, len_b, 0) ]
        while blocks:
            start_a, end_a, start_b, end_b, match_idx = blocks.pop()
            aa, bb = a[start_a:end_a], b[start_b:end_b]
            # try patience
            pivots = __patience_subsequence(aa, bb)
            if pivots:
                offset_a, offset_b = start_a, start_b
                for pivot_a, pivot_b in pivots:
                    pivot_a += offset_a
                    pivot_b += offset_b
                    if start_a <= pivot_a:
                        # extend before
                        idx_a, idx_b = pivot_a, pivot_b
                        while start_a < idx_a and start_b < idx_b and a[idx_a - 1] == b[idx_b - 1]:
                            idx_a -= 1
                            idx_b -= 1
                        # if anything is before recurse on the section
                        if start_a < idx_a and start_b < idx_b:
                            blocks.append((start_a, idx_a, start_b, idx_b, match_idx))
                        # extend after
                        start_a, start_b = pivot_a + 1, pivot_b + 1
                        while start_a < end_a and start_b < end_b and a[start_a] == b[start_b]:
                            start_a += 1
                            start_b += 1
                        # record match
                        matches.insert(match_idx, (idx_a, idx_b, start_a - idx_a))
                        match_idx += 1
                # if anything is after recurse on the section
                if start_a < end_a and start_b < end_b:
                    blocks.append((start_a, end_a, start_b, end_b, match_idx))
            else:
                # fallback if patience fails
                pivots = __lcs_approx(aa, bb)
                if pivots:
                    idx_a, idx_b, n = pivots
                    idx_a += start_a
                    idx_b += start_b
                    # if anything is before recurse on the section
                    if start_a < idx_a and start_b < idx_b:
                        blocks.append((start_a, idx_a, start_b, idx_b, match_idx))
                    # record match
                    matches.insert(match_idx, (idx_a, idx_b, n))
                    match_idx += 1
                    idx_a += n
                    idx_b += n
                    # if anything is after recurse on the section
                    if idx_a < end_a and idx_b < end_b:
                        blocks.append((idx_a, end_a, idx_b, end_b, match_idx))
    # try matching from beginning to first match block
    if matches:
        end_a, end_b = matches[0][:2]
    else:
        end_a, end_b = len_a, len_b
    i = 0
    while i < end_a and i < end_b and a[i] == b[i]:
        i += 1
    if i:
        matches.insert(0, (0, 0, i))
    # try matching from last match block to end
    if matches:
        start_a, start_b, n = matches[-1]
        start_a += n
        start_b += n
    else:
        start_a, start_b = 0, 0
    end_a, end_b = len_a, len_b
    while start_a < end_a and start_b < end_b and a[end_a - 1] == b[end_b - 1]:
        end_a -= 1
        end_b -= 1
    if end_a < len_a:
        matches.append((end_a, end_b, len_a - end_a))
    # add a zero length block to the end
    matches.append((len_a, len_b, 0))
    return matches

# widget used to compare and merge text files
class FileDiffViewer(Gtk.Grid):
    # class describing a text pane
    class Pane:
        def __init__(self):
            # list of lines displayed in this pane (including spacing lines)
            self.lines = []
            # high water mark for line length in Pango units (used to determine
            # the required horizontal scroll range)
            self.line_lengths = 0
            # highest line number
            self.max_line_number = 0
            # cache of syntax highlighting information for each line
            # self.syntax_cache[i] corresponds to self.lines[i]
            # the list is truncated when a change to a line invalidates a
            # portion of the cache
            self.syntax_cache = []
            # cache of character differences for each line
            # self.diff_cache[i] corresponds to self.lines[i]
            # portion of the cache are cleared by setting entries to None
            self.diff_cache = []
            # mask indicating the type of line endings present
            self.format = 0
            # number of lines with edits
            self.num_edits = 0

    # class describing a single line of a pane
    class Line:
        def __init__(self, line_number = None, text = None):
            # line number
            self.line_number = line_number
            # original text for the line
            self.text = text
            # flag indicating modifications are present
            self.is_modified = False
            # actual modified text
            self.modified_text = None
            # cache used to speed up comparison of strings
            # this should be cleared whenever the comparison preferences change
            self.compare_string = None

        # returns the current text for this line
        def getText(self):
            if self.is_modified:
                return self.modified_text
            return self.text

    def __init__(self, n, prefs):
        # verify we have a valid number of panes
        if n < 2:
            raise ValueError('Invalid number of panes')

        Gtk.Grid.__init__(self)
        self.set_can_focus(True)
        self.prefs = prefs
        self.string_width_cache = {}
        self.options = {}

        # diff blocks
        self.blocks = []

        # undos
        self.undos = []
        self.redos = []
        self.undoblock = None

        # cached data
        self.syntax = None
        self.diffmap_cache = None

        # editing mode
        self.mode = LINE_MODE
        self.current_pane = 1
        self.current_line = 0
        self.current_char = 0
        self.selection_line = 0
        self.selection_char = 0
        self.align_pane = 0
        self.align_line = 0
        self.cursor_column = -1

        # keybindings
        self._line_mode_actions = {
            'enter_align_mode': self._line_mode_enter_align_mode,
            'enter_character_mode': self.setCharMode,
            'first_line': self._first_line,
            'extend_first_line': self._extend_first_line,
            'last_line': self._last_line,
            'extend_last_line': self._extend_last_line,
            'up': self._line_mode_up,
            'extend_up': self._line_mode_extend_up,
            'down': self._line_mode_down,
            'extend_down': self._line_mode_extend_down,
            'left': self._line_mode_left,
            'extend_left': self._line_mode_extend_left,
            'right': self._line_mode_right,
            'extend_right': self._line_mode_extend_right,
            'page_up': self._line_mode_page_up,
            'extend_page_up': self._line_mode_extend_page_up,
            'page_down': self._line_mode_page_down,
            'extend_page_down': self._line_mode_extend_page_down,
            'delete_text': self._delete_text,
            'clear_edits': self.clear_edits,
            'isolate': self.isolate,
            'first_difference': self.first_difference,
            'previous_difference': self.previous_difference,
            'next_difference': self.next_difference,
            'last_difference': self.last_difference,
            'copy_selection_right': self.copy_selection_right,
            'copy_selection_left': self.copy_selection_left,
            'copy_left_into_selection': self.copy_left_into_selection,
            'copy_right_into_selection': self.copy_right_into_selection,
            'merge_from_left_then_right': self.merge_from_left_then_right,
            'merge_from_right_then_left': self.merge_from_right_then_left }
        self._align_mode_actions = {
            'enter_line_mode': self._align_mode_enter_line_mode,
            'enter_character_mode': self.setCharMode,
            'first_line': self._first_line,
            'last_line': self._last_line,
            'up': self._line_mode_up,
            'down': self._line_mode_down,
            'left': self._line_mode_left,
            'right': self._line_mode_right,
            'page_up': self._line_mode_page_up,
            'page_down': self._line_mode_page_down,
            'align': self._align_text }
        self._character_mode_actions = {
            'enter_line_mode': self.setLineMode }
        self._button_actions = {
            'undo': self.undo,
            'redo': self.redo,
            'cut': self.cut,
            'copy': self.copy,
            'paste': self.paste,
            'select_all': self.select_all,
            'clear_edits': self.clear_edits,
            'dismiss_all_edits': self.dismiss_all_edits,
            'realign_all': self.realign_all,
            'isolate': self.isolate,
            'first_difference': self.first_difference,
            'previous_difference': self.previous_difference,
            'next_difference': self.next_difference,
            'last_difference': self.last_difference,
            'shift_pane_right': self.shift_pane_right,
            'shift_pane_left': self.shift_pane_left,
            'convert_to_upper_case': self.convert_to_upper_case,
            'convert_to_lower_case': self.convert_to_lower_case,
            'sort_lines_in_ascending_order': self.sort_lines_in_ascending_order,
            'sort_lines_in_descending_order': self.sort_lines_in_descending_order,
            'remove_trailing_white_space': self.remove_trailing_white_space,
            'convert_tabs_to_spaces': self.convert_tabs_to_spaces,
            'convert_leading_spaces_to_tabs': self.convert_leading_spaces_to_tabs,
            'increase_indenting': self.increase_indenting,
            'decrease_indenting': self.decrease_indenting,
            'convert_to_dos': self.convert_to_dos,
            'convert_to_mac': self.convert_to_mac,
            'convert_to_unix': self.convert_to_unix,
            'copy_selection_right': self.copy_selection_right,
            'copy_selection_left': self.copy_selection_left,
            'copy_left_into_selection': self.copy_left_into_selection,
            'copy_right_into_selection': self.copy_right_into_selection,
            'merge_from_left_then_right': self.merge_from_left_then_right,
            'merge_from_right_then_left': self.merge_from_right_then_left }

        # create panes
        self.dareas = []
        self.panes = []
        self.hadj = Gtk.Adjustment.new(0, 0, 0, 0, 0, 0)
        self.vadj = Gtk.Adjustment.new(0, 0, 0, 0, 0, 0)
        for i in range(n):
            pane = FileDiffViewer.Pane()
            self.panes.append(pane)

            # pane contents
            sw = ScrolledWindow(self.hadj, self.vadj)
            sw.set_vexpand(True)
            sw.set_hexpand(True)
            darea = sw.darea
            darea.add_events(Gdk.EventMask.BUTTON_PRESS_MASK |
                             Gdk.EventMask.BUTTON1_MOTION_MASK)
            darea.connect('button-press-event', self.darea_button_press_cb, i)
            darea.connect('motion-notify-event', self.darea_motion_notify_cb, i)
            darea.connect('draw', self.darea_draw_cb, i)
            self.dareas.append(darea)
            self.attach(sw, i, 1, 1, 1)
            sw.show()

        self.hadj.connect('value-changed', self.hadj_changed_cb)
        self.vadj.connect('value-changed', self.vadj_changed_cb)

        # add diff map
        self.diffmap = diffmap = Gtk.DrawingArea.new()
        diffmap.add_events(Gdk.EventMask.BUTTON_PRESS_MASK |
                       Gdk.EventMask.BUTTON1_MOTION_MASK |
                       Gdk.EventMask.SCROLL_MASK)
        diffmap.connect('button-press-event', self.diffmap_button_press_cb)
        diffmap.connect('motion-notify-event', self.diffmap_button_press_cb)
        diffmap.connect('scroll-event', self.diffmap_scroll_cb)
        diffmap.connect('draw', self.diffmap_draw_cb)
        self.attach(diffmap, n, 1, 1, 1)
        diffmap.show()
        diffmap.set_size_request(16 * n, 0)
        self.add_events(Gdk.EventMask.KEY_PRESS_MASK |
                        Gdk.EventMask.FOCUS_CHANGE_MASK)
        self.connect('focus-in-event', self.focus_in_cb)
        self.connect('focus-out-event', self.focus_out_cb)
        self.connect('key-press-event', self.key_press_cb)

        # input method
        self.im_context = im = Gtk.IMMulticontext.new()
        im.connect('commit', self.im_commit_cb)
        im.connect('preedit-changed', self.im_preedit_changed_cb)
        im.connect('retrieve-surrounding', self.im_retrieve_surrounding_cb)
        self.im_preedit = None
        # Cache our keyboard focus state so we know when to change the
        # input method's focus.  We ensure the input method is given focus
        # if and only if (self.has_focus and self.mode == CHAR_MODE).
        self.has_focus = False

        # font
        self.setFont(Pango.FontDescription.from_string(prefs.getString('display_font')))
        self.cursor_pos = (0, 0)

        # scroll to first difference when realised
        darea.connect_after('realize', self._realise_cb)

    # callback used when the viewer is first displayed
    # this must be connected with 'connect_after()' so the final widget sizes
    # are known and the scroll bar can be moved to the first difference
    def _realise_cb(self, widget):
        self.im_context.set_client_window(self.get_window())
        try:
            self.go_to_line(self.options['line'])
        except KeyError:
            self.first_difference()

    # callback for most menu items and buttons
    def button_cb(self, widget, data):
        self.openUndoBlock()
        self._button_actions[data]()
        self.closeUndoBlock()

    # set startup options
    def setOptions(self, options):
        self.options = options

    # updates the display font and resizes viewports as necessary
    def setFont(self, font):
        self.font = font
        metrics = self.get_pango_context().get_metrics(self.font)
        self.font_height = max(pixels(metrics.get_ascent() + metrics.get_descent()), 1)
        self.digit_width = metrics.get_approximate_digit_width()
        self.updateSize(True)
        self.diffmap.queue_draw()

    # returns the 'column width' for a string -- used to help position
    # characters when tabs and other special characters are present
    # This is an inline loop over self.characterWidth() for performance reasons.
    def stringWidth(self, s):
        if not self.prefs.getBool('display_show_whitespace'):
            s = strip_eol(s)
        col = 0
        for c in s:
            try:
                w = char_width_cache[c]
            except KeyError:
                v = ord(c)
                if v < 32:
                    if c == '\t':
                        tab_width = self.prefs.getInt('display_tab_width')
                        w = tab_width - col % tab_width
                    elif c == '\n':
                        w = 1
                        char_width_cache[c] = w
                    else:
                        w = 2
                        char_width_cache[c] = w
                else:
                    if unicodedata.east_asian_width(c) in 'WF':
                        w = 2
                    else:
                        w = 1
                    char_width_cache[c] = w
            col += w
        return col

    # returns the 'column width' for a single character created at column 'i'
    def characterWidth(self, i, c):
        try:
            return char_width_cache[c]
        except KeyError:
            v = ord(c)
            if v < 32:
                if c == '\t':
                    tab_width = self.prefs.getInt('display_tab_width')
                    return tab_width - i % tab_width
                elif c == '\n':
                    w = 1
                else:
                    w = 2
            elif unicodedata.east_asian_width(c) in 'WF':
                w = 2
            else:
                w = 1
            char_width_cache[c] = w
            return w

    # translates a string into an array of the printable representation for
    # each character
    def expand(self, s):
        visible = self.prefs.getBool('display_show_whitespace')
        if not visible:
            s = strip_eol(s)
        tab_width = self.prefs.getInt('display_tab_width')
        col = 0
        result = []
        for c in s:
            v = ord(c)
            if v <= 32:
                if c == ' ':
                    if visible:
                        # show spaces using a centre-dot
                        result.append('\u00b7')
                    else:
                        result.append(c)
                elif c == '\t':
                    width = tab_width - col % tab_width
                    if visible:
                        # show tabs using a right double angle quote
                        result.append('\u00bb' + (width - 1) * ' ')
                    else:
                        result.append(width * ' ')
                elif c == '\n' and visible:
                    # show newlines using a pilcrow
                    result.append('\u00b6')
                else:
                    # prefix with a ^
                    result.append('^' + chr(v + 64))
            else:
                result.append(c)
            col += self.characterWidth(col, c)
        return result

    # changes the viewer's mode to LINE_MODE
    def setLineMode(self):
        if self.mode != LINE_MODE:
            if self.mode == CHAR_MODE:
                self._im_focus_out()
                self.im_context.reset()
                self._im_set_preedit(None)
                self.current_char = 0
                self.selection_char = 0
                self.dareas[self.current_pane].queue_draw()
            elif self.mode == ALIGN_MODE:
                self.dareas[self.align_pane].queue_draw()
                self.dareas[self.current_pane].queue_draw()
                self.align_pane = 0
                self.align_line = 0
            self.mode = LINE_MODE
            self.emit('cursor_changed')
            self.emit('mode_changed')

    # changes the viewer's mode to CHAR_MODE
    def setCharMode(self):
        if self.mode != CHAR_MODE:
            if self.mode == LINE_MODE:
                self.cursor_column = -1
                self.setCurrentChar(self.current_line, 0)
            elif self.mode == ALIGN_MODE:
                self.dareas[self.align_pane].queue_draw()
                self.cursor_column = -1
                self.align_pane = 0
                self.align_line = 0
                self.setCurrentChar(self.current_line, 0)
            self._im_focus_in()
            self.im_context.reset()
            self.mode = CHAR_MODE
            self.emit('cursor_changed')
            self.emit('mode_changed')

    # sets the syntax highlighting rules
    def setSyntax(self, s):
        if self.syntax is not s:
            self.syntax = s
            # invalidate the syntax caches
            for pane in self.panes:
                pane.syntax_cache = []
            self.emit('syntax_changed', s)
            # force all panes to redraw
            for darea in self.dareas:
                darea.queue_draw()

    # gets the syntax
    def getSyntax(self):
        return self.syntax

    # returns True if any pane contains edits
    def hasEdits(self):
        for pane in self.panes:
            if pane.num_edits > 0:
                return True
        return False

    # Changes to the diff viewer's state is recorded so they can be later
    # undone.  The recorded changes are organised into blocks that correspond
    # to high level action initiated by the user.  For example, pasting some
    # text may modify some lines and cause insertion of spacing lines to keep
    # proper alignment with the rest of the panes.  An undo operation initiated
    # by the user should undo all of these changes in a single step.
    # openUndoBlock() should be called when the action from a user, like a
    # mouse button press, menu item, etc. may cause change to the diff viewer's
    # state
    def openUndoBlock(self):
        self.undoblock = []

    # all changes to the diff viewer's state should create an Undo object and
    # add it to the current undo block using this method
    # this method does not need to be called when the state change is a result
    # of an undo/redo operation (self.undoblock is None in these cases)
    def addUndo(self, u):
        self.undoblock.append(u)

    # all openUndoBlock() calls should also have a matching closeUndoBlock()
    # this method collects all Undos created since the openUndoBlock() call
    # and pushes them onto the undo stack as a single unit
    def closeUndoBlock(self):
        if len(self.undoblock) > 0:
            self.redos = []
            self.undos.append(self.undoblock)
        self.undoblock = None

    # 'undo' action
    def undo(self):
        self.undoblock, old_block = None, self.undoblock
        if self.mode == CHAR_MODE:
            # avoid implicit preedit commit when an undo changes the mode
            self.im_context.reset()
        if self.mode == LINE_MODE or self.mode == CHAR_MODE:
            if len(self.undos) > 0:
                # move the block to the redo stack
                block = self.undos.pop()
                self.redos.append(block)
                # undo all changes in the block in reverse order
                for u in block[::-1]:
                    u.undo(self)
        self.undoblock = old_block

    # 'redo' action
    def redo(self):
        self.undoblock, old_block = None, self.undoblock
        if self.mode == LINE_MODE or self.mode == CHAR_MODE:
            if self.mode == CHAR_MODE:
                # avoid implicit preedit commit when an redo changes the mode
                self.im_context.reset()
            if len(self.redos) > 0:
                # move the block to the undo stack
                block = self.redos.pop()
                self.undos.append(block)
                # re-apply all changes in the block
                for u in block:
                    u.redo(self)
        self.undoblock = old_block

    # returns the width of the viewport's line number column in Pango units
    def getLineNumberWidth(self):
        # find the maximum number of digits for a line number from all panes
        n = 0
        if self.prefs.getBool('display_show_line_numbers'):
            for pane in self.panes:
                n = max(n, len(str(pane.max_line_number)))
            # pad the total width by the width of a digit on either side
            n = (n + 2) * self.digit_width
        return n

    # returns the width of a string in Pango units
    def getTextWidth(self, text):
        if len(text) == 0:
            return 0
        layout = self.create_pango_layout(text)
        layout.set_font_description(self.font)
        return layout.get_size()[0]

    # updates the size of the viewport
    # set 'compute_width' to False if the high water mark for line length can
    # be used to determine the required width for the viewport, use True for
    # this value otherwise
    def updateSize(self, compute_width, f=None):
        digit_width, stringWidth = self.digit_width, self.stringWidth
        string_width_cache = self.string_width_cache
        if compute_width:
            if f is None:
                panes = self.panes
            else:
                panes = [ self.panes[f] ]
            for f, pane in enumerate(panes):
                del pane.syntax_cache[:]
                del pane.diff_cache[:]
                # re-compute the high water mark
                pane.line_lengths = 0
                for line in pane.lines:
                    if line is not None:
                        line.compare_string = None
                        text = [ line.text ]
                        if line.is_modified:
                            text.append(line.modified_text)
                        for s in text:
                            if s is not None:
                                w = string_width_cache.get(s, None)
                                if w is None:
                                    string_width_cache[s] = w = stringWidth(s)
                                pane.line_lengths = max(pane.line_lengths, digit_width * w)
        # compute the maximum extents
        num_lines, line_lengths = 0, 0
        for pane in self.panes:
            num_lines = max(num_lines, len(pane.lines))
            line_lengths = max(line_lengths, pane.line_lengths)
        # account for any preedit text
        if self.im_preedit is not None:
            w = self._preedit_layout().get_size()[0]
            s = self.getLineText(self.current_pane, self.current_line)
            if s is not None:
                w += digit_width * stringWidth(s)
            line_lengths = max(line_lengths, w)
        # the cursor can move one line past the last line of text, add it so we
        # can scroll to see this line
        num_lines += 1
        width = self.getLineNumberWidth() + digit_width + line_lengths
        width = pixels(width)
        height = self.font_height * num_lines
        # update the adjustments
        self.hadj.set_upper(width)
        self.hadj.step_increment = self.font_height
        self.vadj.set_upper(height)
        self.vadj.step_increment = self.font_height

    # returns a line from the specified pane and offset
    def getLine(self, f, i):
        lines = self.panes[f].lines
        if i < len(lines):
            return lines[i]

    # returns the text for the specified line
    def getLineText(self, f, i):
        line = self.getLine(f, i)
        if line is not None:
            return line.getText()

    # Undo for changes to the cached line ending style
    class SetFormatUndo:
        def __init__(self, f, format, old_format):
            self.data = (f, format, old_format)

        def undo(self, viewer):
            f, _, old_format = self.data
            viewer.setFormat(f, old_format)

        def redo(self, viewer):
            f, format, _ = self.data
            viewer.setFormat(f, format)

    # sets the cached line ending style
    def setFormat(self, f, format):
        pane = self.panes[f]
        if self.undoblock is not None:
            # create an Undo object for the action
            self.addUndo(FileDiffViewer.SetFormatUndo(f, format, pane.format))
        pane.format = format
        self.emit('format_changed', f, format)

    # Undo for the creation of Line objects
    class InstanceLineUndo:
        def __init__(self, f, i, reverse):
            self.data = (f, i, reverse)

        def undo(self, viewer):
            f, i, reverse = self.data
            viewer.instanceLine(f, i, not reverse)

        def redo(self, viewer):
            f, i, reverse = self.data
            viewer.instanceLine(f, i, reverse)

    # creates an instance of a Line object for the specified pane and offset
    # deletes an instance when 'reverse' is set to True
    def instanceLine(self, f, i, reverse=False):
        if self.undoblock is not None:
            # create an Undo object for the action
            self.addUndo(FileDiffViewer.InstanceLineUndo(f, i, reverse))
        pane = self.panes[f]
        if reverse:
            pane.lines[i] = None
        else:
            line = FileDiffViewer.Line()
            pane.lines[i] = line

    # Undo for changing the text for a Line object
    class UpdateLineTextUndo:
        def __init__(self, f, i, old_is_modified, old_text, is_modified, text):
            self.data = (f, i, old_is_modified, old_text, is_modified, text)

        def undo(self, viewer):
            f, i, old_is_modified, old_text, _, _ = self.data
            viewer.updateLineText(f, i, old_is_modified, old_text)

        def redo(self, viewer):
            f, i, _, _, is_modified, text = self.data
            viewer.updateLineText(f, i, is_modified, text)

    def getMapFlags(self, f, i):
        flags = 0
        compare_text = self.getCompareString(f, i)
        if f > 0 and self.getCompareString(f - 1, i) != compare_text:
            flags |= 1
        if f + 1 < len(self.panes) and self.getCompareString(f + 1, i) != compare_text:
            flags |= 2
        line = self.getLine(f, i)
        if line is not None and line.is_modified:
            flags |= 4
        return flags

    # update line 'i' in pane 'f' to contain 'text'
    def updateLineText(self, f, i, is_modified, text):
        pane = self.panes[f]
        line = pane.lines[i]
        flags = self.getMapFlags(f, i)
        if self.undoblock is not None:
            # create an Undo object for the action
            self.addUndo(FileDiffViewer.UpdateLineTextUndo(f, i, line.is_modified, line.modified_text, is_modified, text))
        old_num_edits = pane.num_edits
        if is_modified and not line.is_modified:
            pane.num_edits += 1
        elif not is_modified and line.is_modified:
            pane.num_edits -= 1
        if pane.num_edits != old_num_edits:
            self.emit('num_edits_changed', f)
        line.is_modified = is_modified
        line.modified_text = text
        line.compare_string = None

        # update/invalidate all relevant caches and queue widgets for redraw
        if text is not None:
            pane.line_lengths = max(pane.line_lengths, self.digit_width * self.stringWidth(text))
        self.updateSize(False)

        fs = []
        if f > 0:
            fs.append(f - 1)
        if f + 1 < len(self.panes):
            fs.append(f + 1)
        for fn in fs:
            otherpane = self.panes[fn]
            if i < len(otherpane.diff_cache):
                otherpane.diff_cache[i] = None
            self._queue_draw_lines(fn, i)
        if i < len(pane.syntax_cache):
            del pane.syntax_cache[i:]
        if i < len(pane.diff_cache):
            pane.diff_cache[i] = None
        self.dareas[f].queue_draw()
        if self.getMapFlags(f, i) != flags:
            self.diffmap_cache = None
            self.diffmap.queue_draw()

    # Undo for inserting a spacing line in a single pane
    class InsertNullUndo:
        def __init__(self, f, i, reverse):
            self.data = (f, i, reverse)

        def undo(self, viewer):
            f, i, reverse = self.data
            viewer.insertNull(f, i, not reverse)

        def redo(self, viewer):
            f, i, reverse = self.data
            viewer.insertNull(f, i, reverse)

    # insert a spacing line at line 'i' in pane 'f'
    # this caller must ensure the blocks and number of lines in each pane
    # are valid again
    def insertNull(self, f, i, reverse):
        if self.undoblock is not None:
            # create an Undo object for the action
            self.addUndo(FileDiffViewer.InsertNullUndo(f, i, reverse))
        pane = self.panes[f]
        lines = pane.lines
        # update/invalidate all relevant caches
        if reverse:
            del lines[i]
            if i < len(pane.syntax_cache):
                del pane.syntax_cache[i]
        else:
            lines.insert(i, None)
            if i < len(pane.syntax_cache):
                state = pane.syntax_cache[i][0]
                pane.syntax_cache.insert(i, [state, state, None, None])

    # Undo for manipulating a section of the line matching data
    class InvalidateLineMatchingUndo:
        def __init__(self, i, n, new_n):
            self.data = (i, n, new_n)

        def undo(self, viewer):
            i, n, new_n = self.data
            viewer.invalidateLineMatching(i, new_n, n)

        def redo(self, viewer):
            i, n, new_n = self.data
            viewer.invalidateLineMatching(i, n, new_n)

    # manipulate a section of the line matching data
    def invalidateLineMatching(self, i, n, new_n):
        if self.undoblock is not None:
            # create an Undo object for the action
            self.addUndo(FileDiffViewer.InvalidateLineMatchingUndo(i, n, new_n))
        # update/invalidate all relevant caches and queue widgets for redraw
        i2 = i + n
        for f, pane in enumerate(self.panes):
            if i < len(pane.diff_cache):
                if i2 + 1 < len(pane.diff_cache):
                    pane.diff_cache[i:i2] = new_n * [ None ]
                else:
                    del pane.diff_cache[i:]
            self.dareas[f].queue_draw()
        self.diffmap_cache = None
        self.diffmap.queue_draw()

    # Undo for alignment changes
    class AlignmentChangeUndo:
        def __init__(self, finished):
            self.data = finished

        def undo(self, viewer):
            finished = self.data
            viewer.alignmentChange(not finished)

        def redo(self, viewer):
            finished = self.data
            viewer.alignmentChange(finished)

    # update viewer in response to alignment changes
    def alignmentChange(self, finished):
        if self.undoblock is not None:
            # create an Undo object for the action
            self.addUndo(FileDiffViewer.AlignmentChangeUndo(finished))
        if finished:
            self.updateSize(False)

    # updates the alignment of 'n' lines starting from 'i'
    def updateAlignment(self, i, n, lines):
        self.alignmentChange(False)
        new_n = len(lines[0])
        i2 = i + n
        # insert spacing lines
        for f in range(len(self.panes)):
            for j in range(i2-1, i-1, -1):
                if self.getLine(f, j) is None:
                    self.insertNull(f, j, True)
            temp = lines[f]
            for j in range(new_n):
                if temp[j] is None:
                    self.insertNull(f, i + j, False)
        # update line matching for this block

        # FIXME: we should be able to do something more intelligent here...
        # the syntax cache will become invalidated.... we don't really need to
        # do that...
        self.invalidateLineMatching(i, n, new_n)
        self.alignmentChange(True)

    # Undo for changing how lines are cut into blocks for alignment
    class UpdateBlocksUndo:
        def __init__(self, old_blocks, blocks):
            self.data = (old_blocks, blocks)

        def undo(self, viewer):
            old_blocks, _ = self.data
            viewer.updateBlocks(old_blocks)

        def redo(self, viewer):
            _, blocks = self.data
            viewer.updateBlocks(blocks)

    # change how lines are cut into blocks for alignment
    def updateBlocks(self, blocks):
        if self.undoblock is not None:
            # create an Undo object for the action
            self.addUndo(FileDiffViewer.UpdateBlocksUndo(self.blocks, blocks))
        self.blocks = blocks

    # insert 'n' blank lines in all panes
    def insertLines(self, i, n):
        # insert lines
        self.updateAlignment(i, 0, [ n * [ None ] for pane in self.panes ])
        pre, post = cutBlocks(i, self.blocks)
        pre.append(n)
        pre.extend(post)
        self.updateBlocks(pre)

        # update selection
        if self.current_line >= i:
            self.current_line += n
        if self.selection_line >= i:
            self.selection_line += n
        # queue redraws
        self.updateSize(False)
        self.diffmap_cache = None
        self.diffmap.queue_draw()

    # remove a line
    def removeSpacerLines(self, i, n, skip=-1):
        npanes, removed = len(self.panes), []
        for j in range(i, i + n):
            for f in range(npanes):
                line = self.getLine(f, j)
                if line is not None:
                    if line.line_number is not None or (f != skip and line.getText() is not None):
                        break
            else:
                # remove line
                for f in range(npanes):
                    line = self.getLine(f, j)
                    if line is not None:
                        # create undo to record any edits to the line before
                        # removing it
                        self.updateLineText(f, j, False, None)
                        # remove the line so it doesn't persist as a spacer
                        self.instanceLine(f, j, True)
                removed.append(j)

        nremoved = len(removed)
        if nremoved > 0:
            # update blocks
            bi, bii, blocks = 0, 0, self.blocks[:]
            for j in removed:
                while bii + blocks[bi] < j:
                    bii += blocks[bi]
                    bi += 1
                if blocks[bi] == 1:
                    del blocks[bi]
                else:
                    blocks[bi] -= 1
                bii += 1
            self.updateBlocks(blocks)

            self.alignmentChange(False)
            removed.reverse()
            for j in removed:
                for f in range(npanes):
                    self.insertNull(f, j, True)
            # FIXME: we should be able to do something more intelligent here...
            # the syntax cache will become invalidated.... we don't really need
            # to do that...
            self.invalidateLineMatching(i, n, n - nremoved)
            self.alignmentChange(True)

            # queue redraws
            self.updateSize(False)
            self.diffmap_cache = None
            self.diffmap.queue_draw()
        return nremoved

    # Undo for replacing the lines for a single pane with a new set
    class ReplaceLinesUndo:
        def __init__(self, f, lines, new_lines, max_num, new_max_num):
            self.data = (f, lines, new_lines, max_num, new_max_num)

        def undo(self, viewer):
            f, lines, new_lines, max_num, new_max_num = self.data
            viewer.replaceLines(f, new_lines, lines, new_max_num, max_num)

        def redo(self, viewer):
            f, lines, new_lines, max_num, new_max_num = self.data
            viewer.replaceLines(f, lines, new_lines, max_num, new_max_num)

    # replace the lines for a single pane with a new set
    def replaceLines(self, f, lines, new_lines, max_num, new_max_num):
        if self.undoblock is not None:
            # create an Undo object for the action
            self.addUndo(FileDiffViewer.ReplaceLinesUndo(f, lines, new_lines, max_num, new_max_num))
        pane = self.panes[f]
        pane.lines = new_lines
        # update/invalidate all relevant caches and queue widgets for redraw
        old_num_edits = pane.num_edits
        pane.num_edits = 0
        for line in new_lines:
            if line is not None and line.is_modified:
                pane.num_edits += 1
        if pane.num_edits != old_num_edits:
            self.emit('num_edits_changed', f)
        del pane.syntax_cache[:]
        pane.max_line_number = new_max_num
        self.dareas[f].queue_draw()
        self.updateSize(True, f)
        self.diffmap_cache = None
        self.diffmap.queue_draw()

    # create a hash for a line to use for line matching
    def _alignmentHash(self, line):
        text = line.getText()
        if text is None:
            return None
        pref = self.prefs.getBool
        if pref('align_ignore_whitespace'):
            # strip all white space from the string
            for c in whitespace:
                text = text.replace(c, '')
        else:
            # hashes for non-null lines should start with '+' to distinguish
            # them from blank lines
            if pref('align_ignore_endofline'):
                text = strip_eol(text)
            if pref('align_ignore_blanklines') and isBlank(text):
                # consider all lines containing only white space as the same
                return ''

            if pref('align_ignore_whitespace_changes'):
                # replace all blocks of white space with a single space
                pc = True
                r = []
                append = r.append
                for c in text:
                    if c in whitespace:
                        if pc:
                            append(' ')
                            pc = False
                    else:
                        append(c)
                        pc = True
                text = ''.join(r)
        if pref('align_ignore_case'):
            # convert everything to upper case
            text = text.upper()
        return text

    # align sets of lines by inserting null spacers and updating the size
    # of blocks to which they belong
    #
    # Leftlines and rightlines are list of list of lines.  Only the inner list
    # of lines are aligned (leftlines[-1] and rightlines[0]). Any spacers
    # needed for alignment are inserted in all lists of lines for a particular
    # side to keep them all in sync.
    def alignBlocks(self, leftblocks, leftlines, rightblocks, rightlines):
        blocks = ( leftblocks, rightblocks )
        lines = ( leftlines, rightlines )
        # get the inner lines we are to match
        middle = ( leftlines[-1], rightlines[0] )
        # eliminate any existing spacer lines
        mlines = ( [ line for line in middle[0] if line is not None ],
                   [ line for line in middle[1] if line is not None ] )
        s1, s2 = mlines
        n1, n2 = 0, 0
        # hash lines according to the alignment preferences
        a = self._alignmentHash
        t1 = [ a(s) for s in s1 ]
        t2 = [ a(s) for s in s2 ]
        # align s1 and s2 by inserting spacer lines
        # this will be used to determine which lines from the inner lists of
        # lines should be neighbours
        for block in patience_diff(t1, t2):
            delta = (n1 + block[0]) - (n2 + block[1])
            if delta < 0:
                # insert spacer lines in s1
                i = n1 + block[0]
                s1[i:i] = -delta * [ None ]
                n1 -= delta
            elif delta > 0:
                # insert spacer lines in s2
                i = n2 + block[1]
                s2[i:i] = delta * [ None ]
                n2 += delta
        nmatch = len(s1)

        # insert spacer lines in leftlines and rightlines and increase the
        # size of blocks in leftblocks and rightblocks as spacer lines are
        # inserted
        #
        # advance one row at a time inserting spacer lines as we go
        # 'i' indicates which row we are processing
        # 'k' indicates which pair of neighbours we are processing
        i, k = 0, 0
        bi = [ 0, 0 ]
        bn = [ 0, 0 ]
        while True:
            # if we have reached the end of the list for any side, it needs
            # spacer lines to align with the other side
            insert = [ i >= len(m) for m in middle ]
            if insert == [ True, True ]:
                # we have reached the end of both inner lists of lines
                # we are done
                break
            if insert == [ False, False ] and k < nmatch:
                # determine if either side needs spacer lines to make the
                # inner list of lines match up
                accept = True
                for j in range(2):
                    m = mlines[j][k]
                    if middle[j][i] is not m:
                        # this line does not correspond to the pair of
                        # neighbours we expected
                        if m is None:
                            # we expected to find a null here so insert one
                            insert[j] = True
                        else:
                            # we have a null but didn't expect one we will not
                            # obtain the pairing we expected by inserting nulls
                            accept = False
                if accept:
                    # our lines will be correctly paired up
                    # move on to the next pair
                    k += 1
                else:
                    # insert spacer lines as needed
                    insert = [ m[i] is not None for m in middle ]
            for j in range(2):
                if insert[j]:
                    # insert spacers lines for side 'j'
                    for temp in lines[j]:
                        temp.insert(i, None)
                    blocksj = blocks[j]
                    bij = bi[j]
                    bnj = bn[j]
                    # append a new block if needed
                    if len(blocksj) == 0:
                        blocksj.append(0)
                    # advance to the current block
                    while bnj + blocksj[bij] < i:
                        bnj += blocksj[bij]
                        bij += 1
                    # increase the current block size
                    blocksj[bij] += 1
            # advance to the next row
            i += 1

    # replace the contents of pane 'f' with the strings list of strings 'ss'
    def replaceContents(self, f, ss):
        self.alignmentChange(False)
        # determine the format for the text
        self.setFormat(f, getFormat(ss))

        # create an initial set of blocks for the lines
        blocks = []
        n = len(ss)
        if n > 0:
            blocks.append(n)
        # create line objects for the text
        Line = FileDiffViewer.Line
        mid = [ [ Line(j + 1, ss[j]) for j in range(n) ] ]

        if f > 0:
            # align with panes to the left
            # use copies so the originals can be used by the Undo object
            leftblocks = self.blocks[:]
            leftlines = [ pane.lines[:] for pane in self.panes[:f] ]
            removeNullLines(leftblocks, leftlines)
            self.alignBlocks(leftblocks, leftlines, blocks, mid)
            mid[:0] = leftlines
            blocks = mergeBlocks(leftblocks, blocks)
        if f + 1 < len(self.panes):
            # align with panes to the right
            # use copies so the originals can be used by the Undo object
            rightblocks = self.blocks[:]
            rightlines = [ pane.lines[:] for pane in self.panes[f + 1:] ]
            removeNullLines(rightblocks, rightlines)
            self.alignBlocks(blocks, mid, rightblocks, rightlines)
            mid.extend(rightlines)
            blocks = mergeBlocks(blocks, rightblocks)

        # update the lines for this pane
        pane = self.panes[f]
        old_n = len(pane.lines)
        new_n = len(mid[f])
        self.replaceLines(f, pane.lines, mid[f], pane.max_line_number, n)

        # insert or remove spacer lines from the other panes
        insertNull, getLine = self.insertNull, self.getLine
        for f_idx in range(len(self.panes)):
            if f_idx != f:
                for j in range(old_n-1, -1, -1):
                    if getLine(f_idx, j) is None:
                        insertNull(f_idx, j, True)
                temp = mid[f_idx]
                for j in range(new_n):
                    if temp[j] is None:
                        insertNull(f_idx, j, False)

        # update the blocks
        self.invalidateLineMatching(0, old_n, new_n)
        self.updateBlocks(blocks)
        self.alignmentChange(True)
        # update cursor
        self.setLineMode()
        self.setCurrentLine(self.current_pane, min(self.current_line, len(pane.lines) + 1))

    # refresh the lines to contain new objects with updated line numbers and
    # no local edits
    def bakeEdits(self, f):
        pane, lines, line_num = self.panes[f], [], 0
        for i in range(len(pane.lines)):
            s = self.getLineText(f, i)
            if s is None:
                lines.append(None)
            else:
                line_num += 1
                lines.append(FileDiffViewer.Line(line_num, s))

        # update loaded pane
        self.replaceLines(f, pane.lines, lines, pane.max_line_number, line_num)

    # update the contents for a line, creating the line if necessary
    def updateText(self, f, i, text, is_modified=True):
        if self.panes[f].lines[i] is None:
            self.instanceLine(f, i)
        self.updateLineText(f, i, is_modified, text)

    # replace the current selection with 'text'
    def replaceText(self, text):
        # record the edit mode as we will be updating the selection too
        self.recordEditMode()

        # find the extents of the current selection
        f = self.current_pane
        pane = self.panes[f]
        nlines = len(pane.lines)
        line0, line1 = self.selection_line, self.current_line
        if self.mode == LINE_MODE:
            col0, col1 = 0, 0
            if line1 < line0:
                line0, line1 = line1, line0
            if line1 < nlines:
                line1 += 1
        else:
            col0, col1 = self.selection_char, self.current_char
            if line1 < line0 or (line1 == line0 and col1 < col0):
                line0, col0, line1, col1 = line1, col1, line0, col0

        # update text
        if text is None:
            text = ''
        # split the replacement text into lines
        ss = splitlines(text)
        if len(ss) == 0 or len(ss[-1]) != len_minus_line_ending(ss[-1]):
            ss.append('')
        # change the format to that of the target pane
        if pane.format == 0:
            self.setFormat(f, getFormat(ss))
        ss = [ convert_to_format(s, pane.format) for s in ss ]
        # prepend original text that was before the selection
        if col0 > 0:
            pre = self.getLineText(f, line0)[:col0]
            ss[0] = pre + ss[0]
        # remove the last line as it needs special casing
        lastcol = 0
        if len(ss) > 0:
            last = ss[-1]
            if len(last) == len_minus_line_ending(last):
                del ss[-1]
                lastcol = len(last)
        cur_line = line0 + len(ss)
        if lastcol > 0:
            # the replacement text does not end with a new line character
            # we need more text to finish the line, search forward for some
            # more text
            while line1 < nlines:
                s = self.getLineText(f, line1)
                line1 += 1
                if s is not None:
                    last = last + s[col1:]
                    break
                col1 = 0
            ss.append(last)
        elif col1 > 0:
            # append original text that was after the selection
            s = self.getLineText(f, line1)
            ss.append(s[col1:])
            line1 += 1

        # remove blank rows if possible
        n_need = len(ss)
        n_have = line1 - line0
        n_have -= self.removeSpacerLines(line0, n_have, f)
        delta = n_have - n_need
        if delta < 0:
            self.insertLines(line0 + n_have, -delta)
            delta = 0
        # update the text
        for i, s in enumerate(ss):
            self.updateText(f, line0 + i, s)
        # clear all unused lines
        for i in range(delta):
            self.updateText(f, line0 + n_need + i, None)
        # update selection
        if self.mode == LINE_MODE:
            self.setCurrentLine(f, line0 + max(n_need, 1) - 1, line0)
        else:
            self.setCurrentChar(cur_line, lastcol)
        self.recordEditMode()

    # manually adjust line matching so 'line1' of pane 'f' is a neighbour of
    # 'line2' from pane 'f+1'
    def align(self, f, line1, line2):
        # record the edit mode as we will be updating the selection too
        self.recordEditMode()

        # find the smallest span of blocks that includes line1 and line2
        start = line1
        end = line2
        if end < start:
            start, end = end, start
        pre_blocks = []
        mid = []
        post_blocks = []
        n = 0
        for b in self.blocks:
            if n + b <= start:
                dst = pre_blocks
            elif n <= end:
                dst = mid
            else:
                dst = post_blocks
            dst.append(b)
            n += b
        start = sum(pre_blocks)
        end = start + sum(mid)

        # cut the span of blocks into three sections:
        # 1. lines before the matched pair
        # 2. the matched pair
        # 3. lines after the matched pair
        # each section has lines and blocks for left and right sides
        lines_s = [ [], [], [] ]
        cutblocks = [ [], [], [] ]
        lines = [ pane.lines for pane in self.panes ]
        nlines = len(lines[0])
        for temp, m in zip([ lines[:f + 1], lines[f + 1:] ], [ line1, line2 ]):
            # cut the blocks just before the line being matched
            pre, post = cutBlocks(m - start, mid)
            if len(temp) == 1:
                # if we only have one pane on this side, we don't need to
                # preserve other cuts
                pre = createBlock(sum(pre))
            # the first section of lines to match
            lines_s[0].append([ s[start:m] for s in temp ])
            cutblocks[0].append(pre)
            # the line to match may be after the actual lines
            if m < nlines:
                m1 = [ [ s[m] ] for s in temp ]
                m2 = [ s[m + 1:end] for s in temp ]
                # cut the blocks just after the line being matched
                b1, b2 = cutBlocks(1, post)
                if len(temp) == 1:
                    # if we only have one pane on this side, we don't need to
                    # preserve other cuts
                    b2 = createBlock(sum(b2))
            else:
                m1 = [ [] for s in temp ]
                m2 = [ [] for s in temp ]
                b1, b2 = [], []
            # the second section of lines to match
            lines_s[1].append(m1)
            cutblocks[1].append(b1)
            # the third section of lines to match
            lines_s[2].append(m2)
            cutblocks[2].append(b2)

        # align each section and concatenate the results
        finallines = [ [] for s in lines ]
        for b, lines_t in zip(cutblocks, lines_s):
            removeNullLines(b[0], lines_t[0])
            removeNullLines(b[1], lines_t[1])
            self.alignBlocks(b[0], lines_t[0], b[1], lines_t[1])
            temp = lines_t[0]
            temp.extend(lines_t[1])
            for dst, s in zip(finallines, temp):
                dst.extend(s)
            pre_blocks.extend(mergeBlocks(b[0], b[1]))
        pre_blocks.extend(post_blocks)

        # update the actual lines and blocks
        self.updateAlignment(start, end - start, finallines)
        self.updateBlocks(pre_blocks)

        i = len(lines_s[0][0][0])
        self.removeSpacerLines(start + i, len(finallines[0]) - i)
        i -= min(self.removeSpacerLines(start, i), i - 1)

        # update selection
        self.setCurrentLine(self.current_pane, start + i)
        self.recordEditMode()

    # Undo for changing the selection mode and range
    class EditModeUndo:
        def __init__(self, mode, current_pane, current_line, current_char, selection_line, selection_char, cursor_column):
            self.data = (mode, current_pane, current_line, current_char, selection_line, selection_char, cursor_column)

        def undo(self, viewer):
            mode, current_pane, current_line, current_char, selection_line, selection_char, cursor_column = self.data
            viewer.setEditMode(mode, current_pane, current_line, current_char, selection_line, selection_char, cursor_column)

        def redo(self, viewer):
            self.undo(viewer)

    # appends an undo to reset to the specified selection mode and range
    # this should be called before and after actions that also change the
    # selection
    def recordEditMode(self):
        if self.undoblock is not None:
            self.addUndo(FileDiffViewer.EditModeUndo(self.mode, self.current_pane, self.current_line, self.current_char, self.selection_line, self.selection_char, self.cursor_column))

    # change the selection mode
    def setEditMode(self, mode, f, current_line, current_char, selection_line, selection_char, cursor_column):
        old_f = self.current_pane
        self.mode = mode
        self.current_pane = f
        self.current_line = current_line
        self.current_char = current_char
        self.selection_line = selection_line
        self.selection_char = selection_char
        self.cursor_column = cursor_column
        if mode == CHAR_MODE:
            self.setCurrentChar(self.current_line, self.current_char, True)
        else:
            self.setCurrentLine(self.current_pane, self.current_line, self.selection_line)
        self.emit('cursor_changed')
        self.emit('mode_changed')
        # queue a redraw to show the updated selection
        self.dareas[old_f].queue_draw()

    # queue a range of lines for redrawing
    def _queue_draw_lines(self, f, line0, line1=None):
        if line1 is None:
            line1 = line0
        elif line0 > line1:
            line0, line1 = line1, line0
        darea = self.dareas[f]
        w, h = darea.get_allocation().width, self.font_height
        darea.queue_draw_area(0, line0 * h - int(self.vadj.get_value()), w, (line1 - line0 + 1) * h)

    # scroll vertically to ensure the current line is visible
    def _ensure_line_is_visible(self, i):
        h = self.font_height
        lower = i * h
        upper = lower + h
        vadj = self.vadj
        v = vadj.get_value()
        ps = vadj.get_page_size()
        if lower < v:
            vadj.set_value(lower)
        elif upper > v + ps:
            vadj.set_value(upper - ps)

    # change the current selection in LINE_MODE
    # use extend=True to extend the selection
    def setCurrentLine(self, f, i, selection=None):
        # remember old cursor position so we can just redraw what is necessary
        old_f = self.current_pane
        line0, line1 = self.current_line, self.selection_line

        # clamp input values
        f = max(min(f, len(self.panes) - 1), 0)
        i = max(min(i, len(self.panes[f].lines)), 0)

        # update cursor
        self.current_pane = f
        self.current_line = i
        if selection is None:
            self.selection_line = i
        else:
            self.selection_line = selection

        self.emit('cursor_changed')

        # invalidate old selection area
        self._queue_draw_lines(old_f, line0, line1)
        # invalidate new selection area
        self._queue_draw_lines(f, i, self.selection_line)

        # ensure the new cursor position is visible
        self._ensure_line_is_visible(i)

    # returns True if the line has preedit text
    def hasPreedit(self, f, i):
        return self.mode == CHAR_MODE and self.current_pane == f and self.current_line == i and self.im_preedit is not None

    # create a layout for the existing preedit text
    def _preedit_layout(self, partial=False):
        s, a, c = self.im_preedit
        if partial:
            s = s[:c]
        layout = self.create_pango_layout(s)
        layout.set_font_description(self.font)
        layout.set_attributes(a)
        return layout

    # inform input method about cursor motion
    def _cursor_position_changed(self, recompute):
        if self.mode == CHAR_MODE:
            # update input method
            h = self.font_height
            if recompute:
                self.cursor_pos = (pixels(self._get_cursor_x_offset()), self.current_line * h)
            x, y = self.cursor_pos
            x -= int(self.hadj.get_value())
            y -= int(self.vadj.get_value())
            # translate to a position relative to the window
            x, y = self.dareas[self.current_pane].translate_coordinates(self.get_toplevel(), x, y)
            # input methods support widgets are centred horizontally about the
            # cursor, a width of 50 seems to give a better widget positions
            rect = Gdk.Rectangle()
            rect.x = x
            rect.y = y
            rect.width = 50
            rect.height = h

            self.im_context.set_cursor_location(rect)

    # get the position of the cursor in Pango units
    def _get_cursor_x_offset(self):
        j = self.current_char
        if j > 0:
            text = self.getLineText(self.current_pane, self.current_line)[:j]
            return self.getTextWidth(''.join(self.expand(text)))
        return 0

    # scroll to ensure the current cursor position is visible
    def _ensure_cursor_is_visible(self):
        current_line = self.current_line

        # find the cursor's horizontal range
        lower = self._get_cursor_x_offset()
        if self.im_preedit is not None:
            lower += self._preedit_layout(True).get_size()[0]
        upper = lower + self.getLineNumberWidth() + self.digit_width
        lower, upper = pixels(lower), pixels(upper)

        # scroll horizontally
        hadj = self.hadj
        v = hadj.get_value()
        ps = hadj.get_page_size()
        if lower < v:
            hadj.set_value(lower)
        elif upper > v + ps:
            hadj.set_value(upper - ps)

        # scroll vertically to current line
        self._ensure_line_is_visible(current_line)

    def __set_clipboard_text(self, clipboard, s):
        # remove embedded nulls as the clipboard cannot handle them
        Gtk.Clipboard.get(clipboard).set_text(s.replace('\0', ''), -1)

    # change the current selection in CHAR_MODE
    # use extend=True to extend the selection
    def setCurrentChar(self, i, j, si=None, sj=None):
        f = self.current_pane

        # remember old cursor position so we can just redraw what is necessary
        line0, line1 = self.current_line, self.selection_line

        # clear remembered cursor column
        self.cursor_column = -1
        # update cursor and selection
        extend = (si is not None and sj is not None)
        if not extend:
            si, sj = i, j
        self.current_line = i
        self.current_char = j
        self.selection_line = si
        self.selection_char = sj

        if extend:
            self.__set_clipboard_text(Gdk.SELECTION_PRIMARY, self.getSelectedText())

        self._cursor_position_changed(True)
        self.emit('cursor_changed')

        # invalidate old selection area
        self._queue_draw_lines(f, line0, line1)
        # invalidate new selection area
        self._queue_draw_lines(f, i, self.selection_line)

        # ensure the new cursor position is visible
        self._ensure_cursor_is_visible()

    # returns the currently selected text
    def getSelectedText(self):
        f = self.current_pane
        start, end = self.selection_line, self.current_line
        # find extents of selection
        if self.mode == LINE_MODE:
            if end < start:
                start, end = end, start
            end += 1
            col0, col1 = 0, 0
        else:
            col0, col1 = self.selection_char, self.current_char
            if end < start or (end == start and col1 < col0):
                start, col0, end, col1 = end, col1, start, col0
            if col1 > 0:
               end += 1
        # get the text for the selected lines
        end = min(end, len(self.panes[f].lines))
        ss = [ self.getLineText(f, i) for i in range(start, end) ]
        # trim out the unselected parts of the lines
        # check for col > 0 as some lines may be null
        if col1 > 0:
            ss[-1] = ss[-1][:col1]
        if col0 > 0:
            ss[0] = ss[0][col0:]
        return ''.join([ s for s in ss if s is not None ])

    # expands the selection to include everything
    def select_all(self):
        if self.mode == LINE_MODE or self.mode == CHAR_MODE:
            f = self.current_pane
            self.selection_line = 0
            self.current_line = len(self.panes[f].lines)
            if self.mode == CHAR_MODE:
                self.selection_char = 0
                self.current_char = 0
            self.dareas[f].queue_draw()

    # returns the index of the last character in text that should be left of
    # 'x' pixels from the edge of the darea widget
    # if partial=True, include characters only partially to the left of 'x'
    def _getPickedCharacter(self, text, x, partial):
        if text is None:
            return 0
        n = len(text)
        w = self.getLineNumberWidth()
        for i, s in enumerate(self.expand(text)):
            width = self.getTextWidth(s)
            tmp = w
            if partial:
                tmp += width // 2
            else:
                tmp += width
            if x < pixels(tmp):
                return i
            w += width
        return n

    # update the selection in response to a mouse button press
    def button_press(self, f, x, y, extend):
        if y < 0:
            x, y = -1, 0
        i = min(y // self.font_height, len(self.panes[f].lines))
        if self.mode == CHAR_MODE and f == self.current_pane:
            text = strip_eol(self.getLineText(f, i))
            j = self._getPickedCharacter(text, x, True)
            if extend:
                si, sj = self.selection_line, self.selection_char
            else:
                si, sj = None, None
            self.setCurrentChar(i, j, si, sj)
        else:
            if self.mode == ALIGN_MODE:
                extend = False
            elif self.mode == CHAR_MODE:
                self.setLineMode()
            if extend and f == self.current_pane:
                selection = self.selection_line
            else:
                selection = None
            self.setCurrentLine(f, i, selection)

    # callback for mouse button presses in the text window
    def darea_button_press_cb(self, widget, event, f):
        self.get_toplevel().set_focus(self)
        x = int(event.x + self.hadj.get_value())
        y = int(event.y + self.vadj.get_value())
        nlines = len(self.panes[f].lines)
        i = min(y // self.font_height, nlines)
        if event.button == 1:
            # left mouse button
            if event.type == Gdk.EventType._2BUTTON_PRESS:
                # double click
                if self.mode == ALIGN_MODE:
                    self.setLineMode()
                if self.mode == LINE_MODE:
                    # change to CHAR_MODE
                    self.setCurrentLine(f, i)
                    # silently switch mode so the viewer does not scroll yet.
                    self.mode = CHAR_MODE
                    self._im_focus_in()
                    self.button_press(f, x, y, False)
                    self.emit('mode_changed')
                elif self.mode == CHAR_MODE and self.current_pane == f:
                    # select word
                    text = strip_eol(self.getLineText(f, i))
                    if text is not None:
                        n = len(text)
                        j = self._getPickedCharacter(text, x, False)
                        if j < n:
                            c = getCharacterClass(text[j])
                            k = j
                            while k > 0 and getCharacterClass(text[k - 1]) == c:
                                k -= 1
                            while j < n and getCharacterClass(text[j]) == c:
                                j += 1
                            self.setCurrentChar(i, j, i, k)
            elif event.type == Gdk.EventType._3BUTTON_PRESS:
                # triple click, select a whole line
                if self.mode == CHAR_MODE and self.current_pane == f:
                    i2 = min(i + 1, nlines)
                    self.setCurrentChar(i2, 0, i, 0)
            else:
                # update the selection
                is_shifted = event.state & Gdk.ModifierType.SHIFT_MASK
                extend = (is_shifted and f == self.current_pane)
                self.button_press(f, x, y, extend)
        elif event.button == 2:
            # middle mouse button, paste primary selection
            if self.mode == CHAR_MODE and f == self.current_pane:
                self.button_press(f, x, y, False)
                self.openUndoBlock()
                Gtk.Clipboard.get(Gdk.SELECTION_PRIMARY).request_text(self.receive_clipboard_text_cb, None)
                self.closeUndoBlock()
        elif event.button == 3:
            # right mouse button, raise context sensitive menu
            can_align = (self.mode == LINE_MODE and (f == self.current_pane + 1 or f == self.current_pane - 1))
            can_isolate = (self.mode == LINE_MODE and f == self.current_pane)
            can_merge = (self.mode == LINE_MODE and f != self.current_pane)
            can_select = ((self.mode == LINE_MODE or self.mode == CHAR_MODE) and f == self.current_pane)
            can_swap = (f != self.current_pane)

            menu = createMenu(
                      [ [_('Align with Selection'), self.align_with_selection_cb, [f, i], Gtk.STOCK_EXECUTE, None, can_align],
                      [_('Isolate'), self.button_cb, 'isolate', None, None, can_isolate ],
                      [_('Merge Selection'), self.merge_lines_cb, f, None, None, can_merge],
                      [],
                      [_('Cut'), self.button_cb, 'cut', Gtk.STOCK_CUT, None, can_select],
                      [_('Copy'), self.button_cb, 'copy', Gtk.STOCK_COPY, None, can_select],
                      [_('Paste'), self.button_cb, 'paste', Gtk.STOCK_PASTE, None, can_select],
                      [],
                      [_('Select All'), self.button_cb, 'select_all', None, None, can_select],
                      [_('Clear Edits'), self.button_cb, 'clear_edits', Gtk.STOCK_CLEAR, None, can_isolate],
                      [],
                      [_('Swap with Selected Pane'), self.swap_panes_cb, f, None, None, can_swap] ])
            menu.attach_to_widget(self)
            menu.popup(None, None, None, None, event.button, event.time)

    # callback used to notify us about click and drag motion
    def darea_motion_notify_cb(self, widget, event, f):
        if event.state & Gdk.ModifierType.BUTTON1_MASK:
            # left mouse button
            extend = (f == self.current_pane)
            x = int(event.x + self.hadj.get_value())
            y = int(event.y + self.vadj.get_value())
            self.button_press(f, x, y, extend)

    # return a list of (begin, end, flag) tuples marking characters that differ
    # from the text in line 'i' from panes 'f' and 'f+1'
    # return the results for pane 'f' if idx=0 and 'f+1' if idx=1
    def getDiffRanges(self, f, i, idx, flag):
        result = []
        s1 = nullToEmpty(self.getLineText(f, i))
        s2 = nullToEmpty(self.getLineText(f + 1, i))

        # ignore blank lines if specified
        if self.prefs.getBool('display_ignore_blanklines') and isBlank(s1) and isBlank(s2):
            return result

        # ignore white space preferences
        ignore_whitespace = self.prefs.getBool('display_ignore_whitespace')
        if ignore_whitespace or self.prefs.getBool('display_ignore_whitespace_changes'):
            if idx == 0:
                s = s1
            else:
                s = s2
            if self.prefs.getBool('display_ignore_endofline'):
                s = strip_eol(s)

            s1 = nullToEmpty(self.getCompareString(f, i))
            s2 = nullToEmpty(self.getCompareString(f + 1, i))

            # build a mapping from characters in compare string to those in the
            # original string
            v = 0
            lookup = []
            # we only need to consider white space here as those are the only
            # ones that can change the number of characters in the compare
            # string
            if ignore_whitespace:
                # all white space characters were removed
                for c in s:
                    if c not in whitespace:
                        lookup.append(v)
                    v += 1
            else:
                # all white space characters were replaced with a single space
                first = True
                for c in s:
                    if c in whitespace:
                        # only include the first white space character of a span
                        if first:
                            lookup.append(v)
                            first = False
                    else:
                        lookup.append(v)
                        first = True
                    v += 1
            lookup.append(v)
        else:
            lookup = None

        start = 0
        for block in difflib.SequenceMatcher(None, s1, s2).get_matching_blocks():
            end = block[idx]
            # skip zero length blocks
            if start < end:
                if lookup is None:
                    result.append((start, end, flag))
                else:
                    # map to indices for the original string
                    lookup_start = lookup[start]
                    lookup_end = lookup[end]
                    # scan for whitespace and skip those sections if specified
                    for j in range(lookup_start, lookup_end):
                        if ignore_whitespace and s[j] in whitespace:
                            if lookup_start != j:
                                result.append((lookup_start, j, flag))
                            lookup_start = j + 1
                    if lookup_start != lookup_end:
                        result.append((lookup_start, lookup_end, flag))
            start = end + block[2]
        return result

    # returns a hash of a string that can be used to quickly compare strings
    # according to the display preferences
    def getCompareString(self, f, i):
        line = self.getLine(f, i)
        if line is None:
            return None
        # if a cached value exists, use it
        s = line.compare_string
        if s is not None:
            return s
        # compute a new hash and cache it
        s = line.getText()
        if s is not None:
            if self.prefs.getBool('display_ignore_endofline'):
                s = strip_eol(s)
            if self.prefs.getBool('display_ignore_blanklines') and isBlank(s):
                return None
            if self.prefs.getBool('display_ignore_whitespace'):
                # strip all white space characters
                for c in whitespace:
                    s = s.replace(c, '')
            elif self.prefs.getBool('display_ignore_whitespace_changes'):
                # map all spans of white space characters to a single space
                first = True
                temp = []
                for c in s:
                    if c in whitespace:
                        if first:
                            temp.append(' ')
                            first = False
                    else:
                        temp.append(c)
                        first = True
                s = ''.join(temp)
            if self.prefs.getBool('display_ignore_case'):
                # force everything to be upper case
                s = s.upper()
            # cache the hash
            line.compare_string = s
        return s

    # draw the text viewport
    def darea_draw_cb(self, widget, cr, f):
        pane = self.panes[f]
        syntax = theResources.getSyntax(self.syntax)

        rect = widget.get_allocation()
        x = rect.x + int(self.hadj.get_value())
        y = rect.y + int(self.vadj.get_value())

        cr.translate(-x, -y)

        maxx = x + rect.width
        maxy = y + rect.height
        line_number_width = pixels(self.getLineNumberWidth())
        h = self.font_height

        diffcolours = [ theResources.getDifferenceColour(f), theResources.getDifferenceColour(f + 1) ]
        diffcolours.append((diffcolours[0] + diffcolours[1]) * 0.5)

        # iterate over each exposed line
        i = y // h
        y_start = i * h
        while y_start < maxy:
            line = self.getLine(f, i)

            # line numbers
            if line_number_width > 0 and 0 < maxx and line_number_width > x:
                cr.save()
                cr.rectangle(0, y_start, line_number_width, h)
                cr.clip()
                colour = theResources.getColour('line_number_background')
                cr.set_source_rgb(colour.red, colour.green, colour.blue)
                cr.paint()

                # draw the line number
                if line is not None and line.line_number is not None:
                    colour = theResources.getColour('line_number')
                    cr.set_source_rgb(colour.red, colour.green, colour.blue)
                    layout = self.create_pango_layout(str(line.line_number))
                    layout.set_font_description(self.font)
                    w = pixels(layout.get_size()[0] + self.digit_width)
                    cr.move_to(line_number_width - w, y_start)
                    PangoCairo.show_layout(cr, layout)
                cr.restore()

            x_start = line_number_width
            if x_start < maxx:
                cr.save()
                cr.rectangle(x_start, y_start, maxx - x_start, h)
                cr.clip()

                text = self.getLineText(f, i)
                ss = None

                # enlarge cache to fit pan.diff_cache[i]
                if i >= len(pane.diff_cache):
                    pane.diff_cache.extend((i - len(pane.diff_cache) + 1) * [ None ])
                # construct a list of ranges for this lines character
                # differences if not already cached
                if pane.diff_cache[i] is None:
                    flags = 0
                    temp_diff = []
                    comptext = self.getCompareString(f, i)
                    if f > 0:
                        # compare with neighbour to the left
                        if self.getCompareString(f - 1, i) != comptext:
                            flags |= 1
                            if text is not None:
                                temp_diff = mergeRanges(temp_diff, self.getDiffRanges(f - 1, i, 1, 1))
                    if f + 1 < len(self.panes):
                        # compare with neighbour to the right
                        if self.getCompareString(f + 1, i) != comptext:
                            flags |= 2
                            if text is not None:
                                temp_diff = mergeRanges(temp_diff, self.getDiffRanges(f, i, 0, 2))

                    chardiff = []
                    if text is not None:
                        # expand text into a list of visual representations
                        ss = self.expand(text)

                        # find the size of each region in Pango units
                        old_end = 0
                        x_temp = 0
                        for start, end, tflags in temp_diff:
                            layout = self.create_pango_layout(''.join(ss[old_end:start]))
                            layout.set_font_description(self.font)
                            x_temp += layout.get_size()[0]
                            layout = self.create_pango_layout(''.join(ss[start:end]))
                            layout.set_font_description(self.font)
                            w = layout.get_size()[0]
                            chardiff.append((start, end, x_temp, w, diffcolours[tflags - 1]))
                            old_end = end
                            x_temp += w
                    # cache flags and character diff ranges
                    pane.diff_cache[i] = (flags, chardiff)
                else:
                    flags, chardiff = pane.diff_cache[i]

                # account for preedit changes
                if f > 0 and self.hasPreedit(f - 1, i):
                    flags |= 1
                if f + 1 < len(self.panes) and self.hasPreedit(f + 1, i):
                    flags |= 2
                has_preedit = self.hasPreedit(f, i)
                if has_preedit:
                    # we have preedit text
                    preeditlayout = self._preedit_layout()
                    preeditwidth = preeditlayout.get_size()[0]
                    if f > 0:
                        flags |= 1
                    if f + 1 < len(self.panes):
                        flags |= 2
                else:
                    preeditwidth = 0
                # draw background
                colour = theResources.getColour('text_background')
                alpha = theResources.getFloat('character_difference_opacity')
                if flags != 0:
                    colour = (diffcolours[flags - 1] * theResources.getFloat('line_difference_opacity')).over(colour)
                cr.set_source_rgb(colour.red, colour.green, colour.blue)
                cr.paint()

                # make preedit text appear as a modified line that differs from
                # both neighbours
                preedit_bg_colour = (diffcolours[flags - 1] * alpha).over(colour)

                if text is not None:
                    # draw char diffs
                    for starti, endi, start, w, colour in chardiff:
                        if has_preedit:
                            # make space for preedit text
                            if self.current_char <= starti:
                                start += preeditwidth
                            elif self.current_char < endi:
                                w += preeditwidth
                        cr.set_source_rgba(colour.red, colour.green, colour.blue, alpha)
                        cr.rectangle(x_start + pixels(start), y_start, pixels(w), h)
                        cr.fill()

                if has_preedit or (line is not None and line.is_modified):
                    # draw modified
                    colour = theResources.getColour('edited')
                    alpha = theResources.getFloat('edited_opacity')
                    preedit_bg_colour = (colour * alpha).over(preedit_bg_colour)
                    cr.set_source_rgba(colour.red, colour.green, colour.blue, alpha)
                    cr.paint()
                if self.mode == ALIGN_MODE:
                    # draw align
                    if self.align_pane == f and self.align_line == i:
                        colour = theResources.getColour('alignment')
                        alpha = theResources.getFloat('alignment_opacity')
                        cr.set_source_rgba(colour.red, colour.green, colour.blue, alpha)
                        cr.paint()
                elif self.mode == LINE_MODE:
                    # draw line selection
                    if self.current_pane == f:
                        start, end = self.selection_line, self.current_line
                        if end < start:
                            start, end = end, start
                        if i >= start and i <= end:
                            colour = theResources.getColour('line_selection')
                            alpha = theResources.getFloat('line_selection_opacity')
                            cr.set_source_rgba(colour.red, colour.green, colour.blue, alpha)
                            cr.paint()
                elif self.mode == CHAR_MODE:
                    # draw char selection
                    if self.current_pane == f and text is not None:
                        start, end = self.selection_line, self.current_line
                        start_char, end_char = self.selection_char, self.current_char
                        if end < start or (end == start and end_char < start_char):
                            start, start_char, end, end_char = end, end_char, start, start_char
                        if start <= i and end >= i:
                            if start < i:
                                start_char = 0
                            if end > i:
                                end_char = len(text)
                            if start_char < end_char:
                                if ss is None:
                                    ss = self.expand(text)
                                layout = self.create_pango_layout(''.join(ss[:start_char]))
                                layout.set_font_description(self.font)
                                x_temp = layout.get_size()[0]
                                layout = self.create_pango_layout(''.join(ss[start_char:end_char]))
                                layout.set_font_description(self.font)
                                w = layout.get_size()[0]
                                colour = theResources.getColour('character_selection')
                                alpha = theResources.getFloat('character_selection_opacity')
                                cr.set_source_rgba(colour.red, colour.green, colour.blue, alpha)
                                cr.rectangle(x_start + pixels(x_temp), y_start, pixels(w), h)
                                cr.fill()

                if self.prefs.getBool('display_show_right_margin'):
                    # draw margin
                    x_temp = line_number_width + pixels(self.prefs.getInt('display_right_margin') * self.digit_width)
                    if x_temp >= x and x_temp < maxx:
                        colour = theResources.getColour('margin')
                        cr.set_source_rgb(colour.red, colour.green, colour.blue)
                        cr.set_line_width(1)
                        cr.move_to(x_temp, y_start)
                        cr.rel_line_to(0, h)
                        cr.stroke()

                if text is None:
                    # draw hatching
                    colour = theResources.getColour('hatch')
                    cr.set_source_rgb(colour.red, colour.green, colour.blue)
                    cr.set_line_width(1)
                    h2 = 2 * h
                    temp = line_number_width
                    if temp < x:
                        temp += ((x - temp) // h) * h
                    h_half = 0.5 * h
                    phase = [ h_half, h_half, -h_half, -h_half ]
                    for j in range(4):
                        x_temp = temp
                        y_temp = y_start
                        for k in range(j):
                            y_temp += phase[k]
                        cr.move_to(x_temp, y_temp)
                        for k in range(j, 4):
                            cr.rel_line_to(h_half, phase[k])
                            x_temp += h_half
                        while x_temp < maxx:
                            cr.rel_line_to(h, h)
                            cr.rel_line_to(h, -h)
                            x_temp += h2
                        cr.stroke()
                else:
                    # continue populating the syntax highlighting cache until
                    # line 'i' is included
                    n = len(pane.syntax_cache)
                    while i >= n:
                        temp = self.getLineText(f, n)
                        if syntax is None:
                            initial_state, end_state = None, None
                            if temp is None:
                                blocks = None
                            else:
                                blocks = [ (0, len(temp), 'text') ]
                        else:
                            # apply the syntax highlighting rules to identify
                            # ranges of similarly coloured characters
                            if n == 0:
                                initial_state = syntax.initial_state
                            else:
                                initial_state = pane.syntax_cache[-1][1]
                            if temp is None:
                                end_state, blocks = initial_state, None
                            else:
                                end_state, blocks = syntax.parse(initial_state, temp)
                        pane.syntax_cache.append([initial_state, end_state, blocks, None])
                        n += 1

                    # use the cache the position, layout, and colour of each
                    # span of characters
                    blocks = pane.syntax_cache[i][3]
                    if blocks is None:
                        # populate the cache item if it didn't exist
                        if ss is None:
                            ss = self.expand(text)
                        x_temp = 0
                        blocks = []
                        for start, end, tag in pane.syntax_cache[i][2]:
                            layout = self.create_pango_layout(''.join(ss[start:end]))
                            layout.set_font_description(self.font)
                            colour = theResources.getColour(tag)
                            blocks.append((start, end, x_temp, layout, colour))
                            x_temp += layout.get_size()[0]
                        pane.syntax_cache[i][3] = blocks

                    # draw text
                    for starti, endi, start, layout, colour in blocks:
                        cr.set_source_rgb(colour.red, colour.green, colour.blue)
                        if has_preedit:
                            # make space for preedit text
                            if self.current_char <= starti:
                                start += preeditwidth
                            elif self.current_char < endi:
                                # divide text into 2 segments
                                ss = self.expand(text)
                                layout = self.create_pango_layout(''.join(ss[starti:self.current_char]))
                                layout.set_font_description(self.font)
                                cr.move_to(x_start + pixels(start), y_start)
                                PangoCairo.show_layout(cr, layout)
                                start += layout.get_size()[0] + preeditwidth
                                layout = self.create_pango_layout(''.join(ss[self.current_char:endi]))
                                layout.set_font_description(self.font)
                        cr.move_to(x_start + pixels(start), y_start)
                        PangoCairo.show_layout(cr, layout)

                if self.current_pane == f and self.current_line == i:
                    # draw the cursor and preedit text
                    if self.mode == CHAR_MODE:
                        x_pos = x_start + pixels(self._get_cursor_x_offset())
                        if has_preedit:
                            # we have preedit text
                            layout = self._preedit_layout()
                            w = pixels(layout.get_size()[0])

                            # clear the background
                            colour = preedit_bg_colour
                            cr.set_source_rgb(colour.red, colour.green, colour.blue)
                            cr.rectangle(x_pos, y_start, w, h)
                            cr.fill()
                            # draw the preedit text
                            colour = theResources.getColour('preedit')
                            cr.set_source_rgb(colour.red, colour.green, colour.blue)
                            cr.move_to(x_pos, y_start)
                            PangoCairo.show_layout(cr, layout)
                            # advance to the preedit's cursor position
                            x_pos += pixels(self._preedit_layout(True).get_size()[0])
                        # draw the character editing cursor
                        colour = theResources.getColour('cursor')
                        cr.set_source_rgb(colour.red, colour.green, colour.blue)
                        cr.set_line_width(1)
                        cr.move_to(x_pos + 0.5, y_start)
                        cr.rel_line_to(0, h)
                        cr.stroke()
                    elif self.mode == LINE_MODE or self.mode == ALIGN_MODE:
                        # draw the line editing cursor
                        colour = theResources.getColour('cursor')
                        cr.set_source_rgb(colour.red, colour.green, colour.blue)
                        cr.set_line_width(1)
                        cr.move_to(maxx, y_start + 0.5)
                        cr.line_to(x_start + 0.5, y_start + 0.5)
                        cr.line_to(x_start + 0.5, y_start + h - 0.5)
                        cr.line_to(maxx, y_start + h - 0.5)
                        cr.stroke()
                cr.restore()
            # advance to the next exposed line
            i += 1
            y_start += h

    # callback used when panes are scrolled horizontally
    def hadj_changed_cb(self, adj):
        self._cursor_position_changed(False)

    # callback used when panes are scrolled vertically
    def vadj_changed_cb(self, adj):
        self._cursor_position_changed(False)
        self.diffmap.queue_draw()

    # callback to handle button presses on the overview map
    def diffmap_button_press_cb(self, widget, event):
        vadj = self.vadj

        h = widget.get_allocation().height
        hmax = max(int(vadj.get_upper()), h)

        # centre view about picked location
        y = event.y * hmax // h
        v = y - int(vadj.get_page_size() / 2)
        v = max(v, int(vadj.get_lower()))
        v = min(v, int(vadj.get_upper() - vadj.get_page_size()))
        vadj.set_value(v)

    # callback to handle mouse scrollwheel events
    def diffmap_scroll_cb(self, widget, event):
        delta = 100
        if event.direction == Gdk.ScrollDirection.UP:
            step_adjustment(self.vadj, -delta)
        elif event.direction == Gdk.ScrollDirection.DOWN:
            step_adjustment(self.vadj, delta)

    # redraws the overview map when a portion is exposed
    def diffmap_draw_cb(self, widget, cr):
        n = len(self.panes)

        # compute map if it hasn't already been cached
        # the map is a list of (start, end, flags) tuples for each pane
        # flags & 1 indicates differences with the pane to the left
        # flags & 2 indicates differences with the pane to the right
        # flags & 4 indicates modified lines
        # flags & 8 indicates regular lines with text
        if self.diffmap_cache is None:
            nlines = len(self.panes[0].lines)
            start = n * [ 0 ]
            flags = n * [ 0 ]
            self.diffmap_cache = [ [] for f in range(n) ]
            # iterate over each row of lines
            for i in range(nlines):
                nextflag = 0
                # iterate over each pane
                for f in range(n):
                    flag = nextflag
                    nextflag = 0
                    s0 = self.getCompareString(f, i)
                    # compare with neighbour to the right
                    if f + 1 < n:
                        if s0 != self.getCompareString(f + 1, i):
                            flag |= 2
                            nextflag |= 1
                    line = self.getLine(f, i)
                    if line is not None and line.is_modified:
                        # modified line
                        flag = 4
                    elif line is None or line.getText() is None:
                        # empty line
                        flag = 0
                    elif flag == 0:
                        # regular line
                        flag = 8
                    if flags[f] != flag:
                        if flags[f] != 0:
                            self.diffmap_cache[f].append([start[f], i, flags[f]])
                        start[f] = i
                        flags[f] = flag
            # finish any incomplete ranges
            for f in range(n):
                if flags[f] != 0:
                    self.diffmap_cache[f].append([start[f], nlines, flags[f]])

        # clear
        colour = theResources.getColour('map_background')
        cr.set_source_rgb(colour.red, colour.green, colour.blue)
        cr.paint()
        bg_colour = theResources.getColour('text_background')
        edited_colour = theResources.getColour('edited')

        rect = widget.get_allocation()

        # get scroll position and total size
        vadj = self.vadj
        hmax = max(vadj.get_upper(), rect.height)

        # draw diff blocks
        wn = rect.width / n
        pad = 1
        for f in range(n):
            diffcolours = [ theResources.getDifferenceColour(f), theResources.getDifferenceColour(f + 1) ]
            diffcolours.append((diffcolours[0] + diffcolours[1]) * 0.5)
            wx = f * wn
            # draw in two passes, more important stuff in the second pass
            # this ensures less important stuff does not obscure more important
            # data
            for p in range(2):
                for start, end, flag in self.diffmap_cache[f]:
                    if p == 0 and flag == 8:
                        colour = bg_colour
                    elif p == 1 and flag & 7:
                        if flag & 4:
                            colour = edited_colour
                        else:
                            colour = diffcolours[(flag & 3) - 1]
                    else:
                        continue

                    # ensure the line is visible in the map
                    ymin = rect.height * self.font_height * start // hmax
                    if ymin >= rect.y + rect.height:
                        break
                    yh = max(rect.height * self.font_height * end // hmax - ymin, 1)

                    #if ymin + yh <= rect.y:
                    #    continue

                    cr.set_source_rgb(colour.red, colour.green, colour.blue)
                    cr.rectangle(wx + pad, ymin, wn - 2 * pad, yh)
                    cr.fill()

        # draw cursor
        vmin = int(vadj.get_value())
        vmax = vmin + vadj.get_page_size()
        ymin = rect.height * vmin // hmax
        if ymin < rect.y + rect.height:
            yh = rect.height * vmax // hmax - ymin
            if yh > 1:
                yh -= 1
            #if ymin + yh > rect.y:
            colour = theResources.getColour('line_selection')
            alpha = theResources.getFloat('line_selection_opacity')
            cr.set_source_rgba(colour.red, colour.green, colour.blue, alpha)
            cr.rectangle(0.5, ymin + 0.5, rect.width - 1, yh - 1)
            cr.fill()

            colour = theResources.getColour('cursor')
            cr.set_source_rgb(colour.red, colour.green, colour.blue)
            cr.set_line_width(1)
            cr.rectangle(0.5, ymin + 0.5, rect.width - 1, yh - 1)
            cr.stroke()

    # returns the maximum valid offset for a cursor position
    # cursors cannot be moved to the right of line ending characters
    def getMaxCharPosition(self, i):
        return len_minus_line_ending(self.getLineText(self.current_pane, i))

    # 'enter_align_mode' keybinding action
    def _line_mode_enter_align_mode(self):
        if self.mode == CHAR_MODE:
            self._im_focus_out()
            self.im_context.reset()
            self._im_set_preedit(None)
        self.mode = ALIGN_MODE
        self.selection_line = self.current_line
        self.align_pane = self.current_pane
        self.align_line = self.current_line
        self.emit('mode_changed')
        self.dareas[self.align_pane].queue_draw()

    # 'first_line' keybinding action
    def _first_line(self):
        self.setCurrentLine(self.current_pane, 0)

    # 'extend_first_line' keybinding action
    def _extend_first_line(self):
        self.setCurrentLine(self.current_pane, 0, self.selection_line)

    # 'last_line' keybinding action
    def _last_line(self):
        f = self.current_pane
        self.setCurrentLine(f, len(self.panes[f].lines))

    # 'extend_last_line' keybinding action
    def _extend_last_line(self):
        f = self.current_pane
        self.setCurrentLine(f, len(self.panes[f].lines), self.selection_line)

    # 'up' keybinding action
    def _line_mode_up(self, selection=None):
        self.setCurrentLine(self.current_pane, self.current_line - 1, selection)

    # 'extend_up' keybinding action
    def _line_mode_extend_up(self):
        self._line_mode_up(self.selection_line)

    # 'down' keybinding action
    def _line_mode_down(self, selection=None):
        self.setCurrentLine(self.current_pane, self.current_line + 1, selection)

    # 'extend_down' keybinding action
    def _line_mode_extend_down(self):
        self._line_mode_down(self.selection_line)

    # 'left' keybinding action
    def _line_mode_left(self, selection=None):
        self.setCurrentLine(self.current_pane - 1, self.current_line, selection)

    # 'extend_left' keybinding action
    def _line_mode_extend_left(self):
        self._line_mode_left(self.selection_line)

    # 'right' keybinding action
    def _line_mode_right(self, selection=None):
        self.setCurrentLine(self.current_pane + 1, self.current_line, selection)

    # 'extend_right' keybinding action
    def _line_mode_extend_right(self):
        self._line_mode_right(self.selection_line)

    # 'page_up' keybinding action
    def _line_mode_page_up(self, selection=None):
        delta = int(self.vadj.get_page_size() // self.font_height)
        self.setCurrentLine(self.current_pane, self.current_line - delta, selection)

    # 'extend_page_up' keybinding action
    def _line_mode_extend_page_up(self):
        self._line_mode_page_up(self.selection_line)

    # 'page_down' keybinding action
    def _line_mode_page_down(self, selection=None):
        delta = int(self.vadj.get_page_size() // self.font_height)
        self.setCurrentLine(self.current_pane, self.current_line + delta, selection)

    # 'extend_page_down' keybinding action
    def _line_mode_extend_page_down(self):
        self._line_mode_page_down(self.selection_line)

    # 'delete_text' keybinding action
    def _delete_text(self):
        self.replaceText('')

    # 'enter_line_mode' keybinding action
    def _align_mode_enter_line_mode(self):
        self.selection_line = self.current_line
        self.setLineMode()

    # 'align' keybinding action
    def _align_text(self):
        f1 = self.align_pane
        line1 = self.align_line
        line2 = self.current_line
        self.selection_line = line2
        self.setLineMode()
        if self.current_pane == f1 + 1:
            self.align(f1, line1, line2)
        elif self.current_pane + 1 == f1:
            self.align(self.current_pane, line2, line1)

    # give the input method focus
    def _im_focus_in(self):
        if self.has_focus:
            self.im_context.focus_in()

    # remove input method focus
    def _im_focus_out(self):
        if self.has_focus:
            self.im_context.focus_out()

    # input method callback for committed text
    def im_commit_cb(self, im, s):
        if self.mode == CHAR_MODE:
            self.openUndoBlock()
            self.replaceText(s)
            self.closeUndoBlock()

    # update the cached preedit text
    def _im_set_preedit(self, p):
        self.im_preedit = p
        if self.mode == CHAR_MODE:
            f, i = self.current_pane, self.current_line
            self._queue_draw_lines(f, i)
            if f > 0:
                self._queue_draw_lines(f - 1, i)
            if f + 1 < len(self.panes):
                self._queue_draw_lines(f + 1, i)
        self.updateSize(False)

    # queue a redraw for location of preedit text
    def im_preedit_changed_cb(self, im):
        if self.mode == CHAR_MODE:
            s, a, c = self.im_context.get_preedit_string()
            if len(s) > 0:
                # we have preedit text, draw that instead
                p = (s, a, c)
            else:
                p = None
            self._im_set_preedit(p)
            self._ensure_cursor_is_visible()

    # callback to respond to retrieve_surrounding signals from input methods
    def im_retrieve_surrounding_cb(self, im):
        if self.mode == CHAR_MODE:
            # notify input method of text surrounding the cursor
            s = nullToEmpty(self.getLineText(self.current_pane, self.current_line))
            im.set_surrounding(s, len(s), self.current_char)

    # callback for 'focus_in_event'
    def focus_in_cb(self, widget, event):
        self.has_focus = True
        if self.mode == CHAR_MODE:
            # notify the input method of the focus change
            self._im_focus_in()

    # callback for 'focus_out_event'
    def focus_out_cb(self, widget, event):
        if self.mode == CHAR_MODE:
            # notify the input method of the focus change
            self._im_focus_out()
        self.has_focus = False

    # callback for keyboard events
    # only keypresses that are not handled by menu item accelerators reach here
    def key_press_cb(self, widget, event):
        if self.mode == CHAR_MODE:
            # update input method
            if self.im_context.filter_keypress(event):
                return True
        retval = False
        # determine the modified keys used
        mask = event.state & (Gdk.ModifierType.SHIFT_MASK | Gdk.ModifierType.CONTROL_MASK)
        if event.state & Gdk.ModifierType.LOCK_MASK:
            mask ^= Gdk.ModifierType.SHIFT_MASK
        self.openUndoBlock()
        if self.mode == LINE_MODE:
            # check if the keyval matches a line mode action
            action = theResources.getActionForKey('line_mode', event.keyval, mask)
            if action in self._line_mode_actions:
                self._line_mode_actions[action]()
                retval = True
        elif self.mode == CHAR_MODE:
            f = self.current_pane
            if event.state & Gdk.ModifierType.SHIFT_MASK:
                si, sj = self.selection_line, self.selection_char
            else:
                si, sj = None, None
            is_ctrl = event.state & Gdk.ModifierType.CONTROL_MASK
            retval = True
            # check if the keyval matches a character mode action
            action = theResources.getActionForKey('character_mode', event.keyval, mask)
            if action in self._character_mode_actions:
                self._character_mode_actions[action]()
            # allow CTRL-Tab for widget navigation
            elif event.keyval == Gdk.KEY_Tab and event.state & Gdk.ModifierType.CONTROL_MASK:
                retval = False
            # up/down cursor navigation
            elif event.keyval in [ Gdk.KEY_Up, Gdk.KEY_Down, Gdk.KEY_Page_Up, Gdk.KEY_Page_Down ]:
                i = self.current_line
                # move back to the remembered cursor column if possible
                col = self.cursor_column
                if col < 0:
                    # find the current cursor column
                    s = nullToEmpty(self.getLineText(f, i))[:self.current_char]
                    col = self.stringWidth(s)
                if event.keyval in [ Gdk.KEY_Up, Gdk.KEY_Down ]:
                    delta = 1
                else:
                    delta = int(self.vadj.get_page_size() // self.font_height)
                if event.keyval in [ Gdk.KEY_Up, Gdk.KEY_Page_Up ]:
                    delta = -delta
                i += delta
                j = 0
                nlines = len(self.panes[f].lines)
                if i < 0:
                    i = 0
                elif i > nlines:
                    i = nlines
                else:
                    # move the cursor to column 'col' if possible
                    s = self.getLineText(f, i)
                    if s is not None:
                        s = strip_eol(s)
                        idx = 0
                        for c in s:
                            w = self.characterWidth(idx, c)
                            if idx + w > col:
                                break
                            idx += w
                            j += 1
                self.setCurrentChar(i, j, si, sj)
                self.cursor_column = col
            # home key
            elif event.keyval == Gdk.KEY_Home:
                if is_ctrl:
                    i = 0
                else:
                    i = self.current_line
                self.setCurrentChar(i, 0, si, sj)
            # end key
            elif event.keyval == Gdk.KEY_End:
                if is_ctrl:
                    i = len(self.panes[f].lines)
                    j = 0
                else:
                    i = self.current_line
                    j = self.getMaxCharPosition(i)
                self.setCurrentChar(i, j, si, sj)
            # cursor left and cursor right navigation
            elif event.keyval == Gdk.KEY_Left or event.keyval == Gdk.KEY_Right:
                i = self.current_line
                j = self.current_char
                while True:
                    if event.keyval == Gdk.KEY_Left:
                        if j > 0:
                            j -= 1
                        elif i > 0:
                            i -= 1
                            j = self.getMaxCharPosition(i)
                        else:
                            break
                    else:
                        if j < self.getMaxCharPosition(i):
                            j += 1
                        elif i < len(self.panes[f].lines):
                            i += 1
                            j = 0
                        else:
                            break
                    if event.state & Gdk.ModifierType.CONTROL_MASK == 0:
                        break
                    # break if we are at the beginning of a word
                    text = self.getLineText(f, i)
                    if text is not None and j < len(text):
                        c = getCharacterClass(text[j])
                        if c != WHITESPACE_CLASS and (j < 1 or j - 1 >= len(text) or getCharacterClass(text[j - 1]) != c):
                            break
                self.setCurrentChar(i, j, si, sj)
            # backspace
            elif event.keyval == Gdk.KEY_BackSpace:
                s = ''
                i = self.current_line
                j = self.current_char
                if self.selection_line == i and self.selection_char == j:
                    if j > 0:
                        # delete back to the last soft-tab location if there
                        # are only spaces and tabs from the beginning of the
                        # line to the current cursor position
                        text = self.getLineText(f, i)[:j]
                        for c in text:
                            if c not in ' \t':
                                j -= 1
                                break
                        else:
                            w = self.stringWidth(text)
                            width = self.prefs.getInt('editor_soft_tab_width')
                            w = (w - 1) // width * width
                            if self.prefs.getBool('editor_expand_tabs'):
                                s = ' ' * w
                            else:
                                width = self.prefs.getInt('display_tab_width')
                                s = '\t' * (w // width) + ' ' * (w % width)
                            j = 0
                    else:
                        # delete back to an end of line character from the
                        # previous line
                        while i > 0:
                            i -= 1
                            text = self.getLineText(f, i)
                            if text is not None:
                                j = self.getMaxCharPosition(i)
                                break
                    self.current_line = i
                    self.current_char = j
                self.replaceText(s)
            # delete key
            elif event.keyval == Gdk.KEY_Delete:
                i = self.current_line
                j = self.current_char
                if self.selection_line == i and self.selection_char == j:
                    # advance the selection to the next character so we can
                    # delete it
                    text = self.getLineText(f, i)
                    while text is None and i < len(self.panes[f].lines):
                        i += 1
                        j = 0
                        text = self.getLineText(f, i)
                    if text is not None:
                        if j < self.getMaxCharPosition(i):
                            j += 1
                        else:
                            i += 1
                            j = 0
                    self.current_line = i
                    self.current_char = j
                self.replaceText('')
            # return key, add the platform specific end of line characters
            elif event.keyval in [ Gdk.KEY_Return, Gdk.KEY_KP_Enter ]:
                s = os.linesep
                if self.prefs.getBool('editor_auto_indent'):
                    start_i, start_j = self.selection_line, self.selection_char
                    end_i, end_j = self.current_line, self.current_char
                    if end_i < start_i or (end_i == start_i and end_j < start_j):
                        start_i, start_j = end_i, end_j
                    if start_j > 0:
                        j, text = 0, self.getLineText(f, start_i)
                        while j < start_j and text[j] in ' \t':
                            j += 1
                        w = self.stringWidth(text[:j])
                        if self.prefs.getBool('editor_expand_tabs'):
                            # convert to spaces
                            s += ' ' * w
                        else:
                            tab_width = self.prefs.getInt('display_tab_width')
                            # replace with tab characters where possible
                            s += '\t' * (w // tab_width)
                            s += ' ' * (w % tab_width)
                self.replaceText(s)
            # insert key
            elif event.keyval in [ Gdk.KEY_Tab, Gdk.KEY_ISO_Left_Tab ]:
                start_i, start_j = self.selection_line, self.selection_char
                end_i, end_j = self.current_line, self.current_char
                if start_i != end_i or start_j != end_j or event.keyval == Gdk.KEY_ISO_Left_Tab:
                    # find range of lines to operate upon
                    start, end, offset = start_i, end_i, 1
                    if end < start:
                        start, end = end, start
                    if event.keyval == Gdk.KEY_ISO_Left_Tab:
                        offset = -1
                    self.recordEditMode()
                    for i in range(start, end + 1):
                        text = self.getLineText(f, i)
                        if text is not None and len_minus_line_ending(text) > 0:
                            # count spacing before the first non-whitespace character
                            j, w = 0, 0
                            while j < len(text) and text[j] in ' \t':
                                w += self.characterWidth(w, text[j])
                                j += 1
                            # adjust by a multiple of the soft tab width
                            ws = max(0, w + offset * self.prefs.getInt('editor_soft_tab_width'))
                            if ws != w:
                                if self.prefs.getBool('editor_expand_tabs'):
                                    s = ' ' * ws
                                else:
                                    tab_width = self.prefs.getInt('display_tab_width')
                                    s = '\t' * (ws // tab_width) + ' ' * (ws % tab_width)
                                if i == start_i:
                                    start_j = len(s) + max(0, start_j - j)
                                if i == end_i:
                                    end_j = len(s) + max(0, end_j - j)
                                self.updateText(f, i, s + text[j:])
                    self.setCurrentChar(end_i, end_j, start_i, start_j)
                    self.recordEditMode()
                else:
                    # insert soft-tabs if there are only spaces and tabs from the
                    # beginning of the line to the cursor location
                    if end_i < start_i or (end_i == start_i and end_j < start_j):
                        start_i, start_j, end_i, end_j = end_i, end_j, start_i, start_j
                    temp = start_j
                    if temp > 0:
                        text = self.getLineText(f, start_i)[:start_j]
                        w = self.stringWidth(text)
                        while temp > 0 and text[temp - 1] in ' \t':
                            temp -= 1
                    else:
                        w = 0
                    tab_width = self.prefs.getInt('display_tab_width')
                    if temp > 0:
                        # insert a regular tab
                        ws = tab_width - w % tab_width
                    else:
                        # insert a soft tab
                        self.selection_line = start_i
                        self.selection_char = 0
                        self.current_line = end_i
                        self.current_char = end_j
                        width = self.prefs.getInt('editor_soft_tab_width')
                        ws = w + width - w % width
                        w = 0
                    if self.prefs.getBool('editor_expand_tabs'):
                        # convert to spaces
                        s = ' ' * ws
                    else:
                        # replace with tab characters where possible
                        s = '\t' * ((w + ws) // tab_width - w // tab_width)
                        s += ' ' * ((w + ws) % tab_width)
                    self.replaceText(s)
            # handle all other printable characters
            elif len(event.string) > 0:
                self.replaceText(event.string)
        elif self.mode == ALIGN_MODE:
            # check if the keyval matches an align mode action
            action = theResources.getActionForKey('align_mode', event.keyval, mask)
            if action in self._align_mode_actions:
                self._align_mode_actions[action]()
                retval = True
        self.closeUndoBlock()
        return retval

    # 'copy' action
    def copy(self):
        if self.mode == LINE_MODE or self.mode == CHAR_MODE:
            self.__set_clipboard_text(Gdk.SELECTION_CLIPBOARD, self.getSelectedText())

    # 'cut' action
    def cut(self):
        if self.mode == LINE_MODE or self.mode == CHAR_MODE:
            self.copy()
            self.replaceText('')

    # callback used when receiving clipboard text
    def receive_clipboard_text_cb(self, clipboard, text, data):
        if self.mode == LINE_MODE or self.mode == CHAR_MODE:
            # there is no guarantee this will be called before finishing
            # Gtk.Clipboard.get so we may need to create our own undo block
            needs_block = (self.undoblock is None)
            if needs_block:
                self.openUndoBlock()
            self.replaceText(nullToEmpty(text))
            if needs_block:
                self.closeUndoBlock()

    # 'paste' action
    def paste(self):
         Gtk.Clipboard.get(Gdk.SELECTION_CLIPBOARD).request_text(self.receive_clipboard_text_cb, None)

    # 'clear_edits' action
    def clear_edits(self):
        self.setLineMode()
        self.recordEditMode()
        f = self.current_pane
        start, end = self.selection_line, self.current_line
        if end < start:
            start, end = end, start
        end = min(end + 1, len(self.panes[0].lines))
        for i in range(start, end):
            line = self.getLine(f, i)
            if line is not None and line.is_modified:
                # remove the edits to the line
                self.updateText(f, i, None, False)
                if line.text is None:
                    # remove the line so it doesn't persist as a spacer
                    self.instanceLine(f, i, True)
        self.recordEditMode()

    # 'dismiss_all_edits' action
    def dismiss_all_edits(self):
        if self.mode == LINE_MODE or self.mode == CHAR_MODE:
            self.bakeEdits(self.current_pane)

    # callback for find menu item
    def find(self, pattern, match_case, backwards, from_start):
        self.setCharMode()
        # determine where to start searching from
        f = self.current_pane
        nlines = len(self.panes[f].lines)
        i, j = self.current_line, self.current_char
        si, sj = self.selection_line, self.selection_char
        if backwards:
            if from_start:
                i, j = nlines, 0
            elif si < i or (i == si and sj < j):
                i, j = si, sj
        else:
            if from_start:
                i, j = 0, 0
            elif i < si or (i == si and j < sj):
                i, j = si, sj

        if not match_case:
            pattern = pattern.upper()

        # iterate over all valid lines
        while i < nlines + 1:
            text = self.getLineText(f, i)
            if text is not None:
                if not match_case:
                    text = text.upper()
                # search for pattern
                if backwards:
                    idx = text.rfind(pattern, 0, j)
                else:
                    idx = text.find(pattern, j)
                if idx >= 0:
                    # we found a match
                    end = idx + len(pattern)
                    if backwards:
                        idx, end = end, idx
                    self.setCurrentChar(i, end, i, idx)
                    return True
            # advance
            if backwards:
                if i == 0:
                    break
                i -= 1
                text = self.getLineText(f, i)
                if text is None:
                    j = 0
                else:
                    j = len(text)
            else:
                i += 1
                j = 0
        # we have reached the end without finding a match
        return False

    # move cursor to a given line
    def go_to_line(self, i):
        f, idx = self.current_pane, 0
        if i > 0:
            # search for a line matching that number
            # we want to leave the cursor at the end of the file
            # if 'i' is greater than the last numbered line
            lines = self.panes[f].lines
            while idx < len(lines):
                line = lines[idx]
                if line is not None and line.line_number == i:
                    break
                idx += 1
        # select the line and make sure it is visible
        self.setLineMode()
        self.centre_view_about_y((idx + 0.5) * self.font_height)
        self.setCurrentLine(f, idx)

    # recompute viewport size and redraw as the display preferences may have
    # changed
    def prefsUpdated(self):
        # clear cache as tab width may have changed
        self.string_width_cache = {}
        self.setFont(Pango.FontDescription.from_string(self.prefs.getString('display_font')))
        # update preedit text
        self._cursor_position_changed(True)

        for pane in self.panes:
            del pane.diff_cache[:]
        # tab width may have changed
        self.emit('cursor_changed')
        for darea in self.dareas:
            darea.queue_draw()
        self.diffmap_cache = None
        self.diffmap.queue_draw()

    # 'realign_all' action
    def realign_all(self):
        self.setLineMode()
        f = self.current_pane
        self.recordEditMode()
        lines = []
        blocks = []
        for pane in self.panes:
            # create a new list of lines with no spacers
            newlines = [ [ line for line in pane.lines if line is not None ] ]
            newblocks = createBlock(len(newlines[0]))
            if len(lines) > 0:
                # match with neighbour to the left
                self.alignBlocks(blocks, lines, newblocks, newlines)
                blocks = mergeBlocks(blocks, newblocks)
            else:
                blocks = newblocks
            lines.extend(newlines)
        self.updateAlignment(0, len(self.panes[f].lines), lines)
        self.updateBlocks(blocks)
        self.setCurrentLine(f, min(self.current_line, len(self.panes[f].lines)))
        self.recordEditMode()

    # callback for the align with selection menu item
    def align_with_selection_cb(self, widget, data):
        self.setLineMode()
        self.openUndoBlock()
        self.recordEditMode()
        # get the line and pane where the user right clicked
        f, line1 = data
        f2 = self.current_pane
        line2 = self.current_line
        if f2 < f:
            f = f2
            line1, line2 = line2, line1
        self.align(f, line1, line2)
        self.recordEditMode()
        self.closeUndoBlock()

    # 'isolate' action
    def isolate(self):
        self.setLineMode()
        self.recordEditMode()
        f = self.current_pane
        start, end = self.selection_line, self.current_line
        if end < start:
            start, end = end, start
        end += 1
        nlines = len(self.panes[f].lines)
        end = min(end, nlines)
        n = end - start
        if n > 0:
            lines = [ pane.lines[start:end] for pane in self.panes ]
            space = [ n * [ None ] for pane in self.panes ]
            lines[f], space[f] = space[f], lines[f]

            pre, post = cutBlocks(end, self.blocks)
            pre, middle = cutBlocks(start, pre)

            # remove nulls
            b = createBlock(n)
            removeNullLines(b, space)
            end = start + sum(b)
            if end > start:
                end -= 1
            removeNullLines(middle, lines)

            for s, line in zip(space, lines):
                s.extend(line)

            # update lines and blocks
            self.updateAlignment(start, n, space)
            pre.extend(b)
            pre.extend(middle)
            pre.extend(post)
            self.updateBlocks(pre)
            self.removeSpacerLines(end, sum(middle))
            end -= self.removeSpacerLines(start, sum(b))
            self.setCurrentLine(f, end, start)
        self.recordEditMode()

    # returns True if line 'i' in pane 'f' has an edit or is different from its
    # neighbour
    def hasEditsOrDifference(self, f, i):
        line = self.getLine(f, i)
        if line is not None and line.is_modified:
            return True
        text = self.getCompareString(f, i)
        return (f > 0 and self.getCompareString(f - 1, i) != text) or (f + 1 < len(self.panes) and text != self.getCompareString(f + 1, i))

    # returns True if there are any differences
    def hasDifferences(self):
        n = len(self.panes)
        nlines = len(self.panes[0].lines)
        for i in range(nlines):
            text = self.getCompareString(0, i)
            for f in range(1, n):
                if self.getCompareString(f, i) != text:
                    return True
        return False

    # scroll the viewport so pixels at position 'y' are centred
    def centre_view_about_y(self, y):
        vadj = self.vadj
        y = min(max(0, y - vadj.get_page_size() / 2), vadj.get_upper() - vadj.get_page_size())
        vadj.set_value(y)

    # move the cursor from line 'i' to the next difference in direction 'delta'
    def go_to_difference(self, i, delta):
        f = self.current_pane
        nlines = len(self.panes[f].lines)
        # back up to beginning of difference
        if i >= 0 and i <= nlines:
            while self.hasEditsOrDifference(f, i):
                i2 = i - delta
                if i2 < 0 or i2 > nlines:
                    break
                i = i2
        # step over non-difference
        while i >= 0 and i <= nlines and not self.hasEditsOrDifference(f, i):
            i += delta
        # find extent of difference
        if i >= 0 and i <= nlines:
            start = i
            while i >= 0 and i <= nlines and self.hasEditsOrDifference(f, i):
                i += delta
            i -= delta
            if i < start:
                start, i = i, start
            # centre the view on the selection
            self.centre_view_about_y((start + i) * self.font_height / 2)
            self.setCurrentLine(f, start, i)

    # 'first_difference' action
    def first_difference(self):
        self.setLineMode()
        self.go_to_difference(0, 1)

    # 'previous_difference' action
    def previous_difference(self):
        self.setLineMode()
        i = min(self.current_line, self.selection_line) - 1
        self.go_to_difference(i, -1)

    # 'next_difference' action
    def next_difference(self):
        self.setLineMode()
        i = max(self.current_line, self.selection_line) + 1
        self.go_to_difference(i, 1)

    # 'last_difference' action
    def last_difference(self):
        self.setLineMode()
        i = len(self.panes[self.current_pane].lines)
        self.go_to_difference(i, -1)

    # Undo for changes to the pane ordering
    class SwapPanesUndo:
        def __init__(self, f_dst, f_src):
            self.data = (f_dst, f_src)

        def undo(self, viewer):
            f_dst, f_src = self.data
            viewer.swapPanes(f_src, f_dst)

        def redo(self, viewer):
            f_dst, f_src = self.data
            viewer.swapPanes(f_dst, f_src)

    # swap the contents of two panes
    def swapPanes(self, f_dst, f_src):
        if self.undoblock is not None:
            self.addUndo(FileDiffViewer.SwapPanesUndo(f_dst, f_src))
        self.current_pane = f_dst
        f0 = self.panes[f_dst]
        f1 = self.panes[f_src]
        self.panes[f_dst], self.panes[f_src] = f1, f0
        npanes = len(self.panes)
        for f_idx in f_dst, f_src:
            for f in range(f_idx - 1, f_idx + 2):
                if f >= 0 and f < npanes:
                    # clear the diff cache and redraw as the pane has a new
                    # neighbour
                    del self.panes[f].diff_cache[:]
                    self.dareas[f].queue_draw()
        # queue redraw
        self.diffmap_cache = None
        self.diffmap.queue_draw()
        self.emit('swapped_panes', f_dst, f_src)

    # swap the contents of two panes
    def swap_panes(self, f_dst, f_src):
        if f_dst >= 0 and f_dst < len(self.panes):
            if self.mode == ALIGN_MODE:
                self.setLineMode()
            self.recordEditMode()
            self.swapPanes(f_dst, f_src)
            self.recordEditMode()

    # callback for swap panes menu item
    def swap_panes_cb(self, widget, data):
        self.openUndoBlock()
        self.swap_panes(data, self.current_pane)
        self.closeUndoBlock()

    # 'shift_pane_left' action
    def shift_pane_left(self):
        f = self.current_pane
        self.swap_panes(f - 1, f)

    # 'shift_pane_right' action
    def shift_pane_right(self):
        f = self.current_pane
        self.swap_panes(f + 1, f)

    # 'convert_to_upper_case' action
    def _convert_case(self, to_upper):
        # find range of characters to operate upon
        if self.mode == CHAR_MODE:
            start, end = self.current_line, self.selection_line
            j0, j1 = self.current_char, self.selection_char
            if end < start or (start == end and j1 < j0):
                start, j0, end, j1 = end, j1, start, j0
        else:
            self.setLineMode()
            start, end = self.current_line, self.selection_line
            if end < start:
                start, end = end, start
            end += 1
            j0, j1 = 0, 0
        self.recordEditMode()
        f = self.current_pane
        for i in range(start, end + 1):
            text = self.getLineText(f, i)
            if text is not None:
                s = text
                # skip characters after the selection
                if i == end:
                    s, post = s[:j1], s[j1:]
                else:
                    post = ''
                # skip characters before the selection
                if i == start:
                    pre, s = s[:j0], s[j0:]
                else:
                    pre = ''
                # change the case
                if to_upper:
                    s = s.upper()
                else:
                    s = s.lower()
                s = ''.join([pre, s, post])
                # only update the line if it changed
                if s != text:
                    self.updateText(f, i, s)

    # 'convert_to_upper_case' action
    def convert_to_upper_case(self):
        self._convert_case(True)

    # 'convert_to_lower_case' action
    def convert_to_lower_case(self):
        self._convert_case(False)

    # sort lines
    def _sort_lines(self, descending):
        if self.mode != CHAR_MODE:
            self.setLineMode()
        self.recordEditMode()
        f = self.current_pane
        # find cursor range
        start, end = self.selection_line, self.current_line
        if end < start:
            start, end = end, start
        # get set of lines
        ss = [ self.getLineText(f, i) for i in range(start, end + 1) ]
        # create sorted list, removing any nulls
        temp = [ s for s in ss if s is not None ]
        temp.sort()
        if descending:
            temp.reverse()
        # add back in the nulls
        temp.extend((len(ss) - len(temp)) * [ None ])
        for i, s in enumerate(temp):
            # update line if it changed
            if ss[i] != s:
                self.updateText(f, start + i, s)
        if self.mode == CHAR_MODE:
            # ensure the cursor position is valid
            self.setCurrentChar(self.current_line, 0, self.selection_line, 0)
        self.recordEditMode()

    # 'sort_lines_in_ascending_order' action
    def sort_lines_in_ascending_order(self):
        self._sort_lines(False)

    # 'sort_lines_in_descending_order' action
    def sort_lines_in_descending_order(self):
        self._sort_lines(True)

    # 'remove_trailing_white_space' action
    def remove_trailing_white_space(self):
        if self.mode != CHAR_MODE:
            self.setLineMode()
        self.recordEditMode()
        f = self.current_pane
        # find cursor range
        start, end = self.selection_line, self.current_line
        if end < start:
            start, end = end, start
        # get set of lines
        for i in range(start, end + 1):
            text = self.getLineText(f, i)
            if text is not None:
                # locate trailing whitespace
                old_n = n = len_minus_line_ending(text)
                while n > 0 and text[n - 1] in whitespace:
                    n -= 1
                # update line if it changed
                if n < old_n:
                    self.updateText(f, i, text[:n] + text[old_n:])
        if self.mode == CHAR_MODE:
            # ensure the cursor position is valid
            self.setCurrentChar(self.current_line, 0, self.selection_line, 0)
        self.recordEditMode()

    # 'convert_tabs_to_spaces' action
    def convert_tabs_to_spaces(self):
        # find range of characters to operate upon
        if self.mode == CHAR_MODE:
            start, end = self.current_line, self.selection_line
            j0, j1 = self.current_char, self.selection_char
            if end < start or (start == end and j1 < j0):
                start, j0, end, j1 = end, j1, start, j0
        else:
            self.setLineMode()
            start, end = self.current_line, self.selection_line
            if end < start:
                start, end = end, start
            end += 1
            j0, j1 = 0, 0
        self.recordEditMode()
        f = self.current_pane
        for i in range(start, end + 1):
            text = self.getLineText(f, i)
            if text is not None:
                # expand tabs
                ss, col = [], 0
                for c in text:
                    w = self.characterWidth(col, c)
                    # replace tab with spaces
                    if c == '\t':
                        c = w * ' '
                    ss.append(c)
                    col += w
                # determine the range of interest
                if i == start:
                    k0 = j0
                else:
                    k0 = 0
                if i == end:
                    k1 = j1
                else:
                    k1 = len(ss)
                # compute leading and converted text
                s = text[:k0] + ''.join(ss[k0:k1])
                if i == end:
                    # update the end cursor location
                    j1 = len(s)
                # append the trailing text
                s += text[k1:]
                # update line only if it changed
                if text != s:
                    self.updateText(f, i, s)
        if self.mode == CHAR_MODE:
            # ensure the cursor position is valid
            self.setCurrentChar(end, j1, start, j0)
        self.recordEditMode()

    # 'convert_leading_spaces_to_tabs' action
    def convert_leading_spaces_to_tabs(self):
        if self.mode != CHAR_MODE:
            self.setLineMode()
        self.recordEditMode()
        f = self.current_pane
        tab_width = self.prefs.getInt('display_tab_width')
        # find cursor range
        start, end = self.selection_line, self.current_line
        if end < start:
            start, end = end, start
        for i in range(start, end + 1):
            text = self.getLineText(f, i)
            if text is not None:
                # find leading white space
                j, col = 0, 0
                while j < len(text) and text[j] in ' \t':
                    col += self.characterWidth(col, text[j])
                    j += 1
                if col >= tab_width:
                    # convert to tabs
                    s = ''.join([ '\t' * (col // tab_width), ' ' * (col % tab_width), text[j:] ])
                    # update line only if it changed
                    if text != s:
                        self.updateText(f, i, s)
        if self.mode == CHAR_MODE:
            # ensure the cursor position is valid
            self.setCurrentChar(self.current_line, 0, self.selection_line, 0)
        self.recordEditMode()

    # adjust indenting of the selected lines by 'offset' soft tabs
    def _adjust_indenting(self, offset):
        if self.mode != CHAR_MODE:
            self.setLineMode()
        # find range of lines to operate upon
        f = self.current_pane
        start, end = self.selection_line, self.current_line
        if end < start:
            start, end = end, start

        self.recordEditMode()
        for i in range(start, end + 1):
            text = self.getLineText(f, i)
            if text is not None and len_minus_line_ending(text) > 0:
                # count spacing before the first non-whitespace character
                j, w = 0, 0
                while j < len(text) and text[j] in ' \t':
                    w += self.characterWidth(w, text[j])
                    j += 1
                # adjust by a multiple of the soft tab width
                ws = max(0, w + offset * self.prefs.getInt('editor_soft_tab_width'))
                if ws != w:
                    if self.prefs.getBool('editor_expand_tabs'):
                        s = ' ' * ws
                    else:
                        tab_width = self.prefs.getInt('display_tab_width')
                        s = '\t' * (ws // tab_width) + ' ' * (ws % tab_width)
                    self.updateText(f, i, s + text[j:])
        if self.mode == CHAR_MODE:
            # ensure the cursor position is valid
            self.setCurrentChar(self.current_line, 0, self.selection_line, 0)
        self.recordEditMode()

    # 'increase_indenting' action
    def increase_indenting(self):
        self._adjust_indenting(1)

    # 'decrease_indenting' action
    def decrease_indenting(self):
        self._adjust_indenting(-1)

    def convert_format(self, format):
        self.setLineMode()
        self.recordEditMode()
        f = self.current_pane
        for i in range(len(self.panes[f].lines)):
            text = self.getLineText(f, i)
            s = convert_to_format(text, format)
            # only modify lines that actually change
            if s != text:
                self.updateText(f, i, s)
        self.setFormat(f, format)

    # 'convert_to_dos' action
    def convert_to_dos(self):
        self.convert_format(DOS_FORMAT)

    # 'convert_to_mac' action
    def convert_to_mac(self):
        self.convert_format(MAC_FORMAT)

    # 'convert_to_unix' action
    def convert_to_unix(self):
        self.convert_format(UNIX_FORMAT)

    # copies the selected range of lines from pane 'f_src' to 'f_dst'
    def merge_lines(self, f_dst, f_src):
        self.recordEditMode()
        self.setLineMode()
        pane = self.panes[f_dst]
        start, end = self.selection_line, self.current_line
        if end < start:
            start, end = end, start
        end = min(end + 1, len(pane.lines))
        ss = [ self.getLineText(f_src, i) for i in range(start, end) ]
        if pane.format == 0:
            # copy the format of the source pane if the format for the
            # destination pane as not yet been determined
            self.setFormat(f_dst, getFormat(ss))
        for i, s in enumerate(ss):
            self.updateText(f_dst, start + i, convert_to_format(s, pane.format))
        n = len(ss)
        delta = min(self.removeSpacerLines(start, n), n - 1)
        if self.selection_line > start:
            self.selection_line -= delta
        if self.current_line > start:
            self.current_line -= delta
        self.recordEditMode()

    # callback for merge lines menu item
    def merge_lines_cb(self, widget, data):
        self.openUndoBlock()
        self.merge_lines(data, self.current_pane)
        self.closeUndoBlock()

    # 'copy_selection_right' action
    def copy_selection_right(self):
        f = self.current_pane + 1
        if f > 0 and f < len(self.panes):
            self.merge_lines(f, f - 1)

    # 'copy_selection_left' action
    def copy_selection_left(self):
        f = self.current_pane - 1
        if f >= 0 and f + 1 < len(self.panes):
            self.merge_lines(f, f + 1)

    # 'copy_left_into_selection' action
    def copy_left_into_selection(self):
        f = self.current_pane
        if f > 0 and f < len(self.panes):
            self.merge_lines(f, f - 1)

    # 'copy_right_into_selection' action
    def copy_right_into_selection(self):
        f = self.current_pane
        if f >= 0 and f + 1 < len(self.panes):
            self.merge_lines(f, f + 1)

    # merge from both left and right into the current pane
    def _mergeBoth(self, right_first):
        self.recordEditMode()
        self.setLineMode()
        f = self.current_pane
        start, end = self.selection_line, self.current_line
        if end < start:
            start, end = end, start
        end += 1
        npanes = len(self.panes)
        nlines = len(self.panes[f].lines)
        end = min(end, nlines)
        n = end - start
        if n > 0:
            lines = [ pane.lines[start:end] for pane in self.panes ]
            spaces = [ n * [ None ] for pane in self.panes ]
            old_content = [ line for line in lines[f] if line is not None ]
            for i in range(f + 1, npanes):
                lines[i], spaces[i] = spaces[i], lines[i]
            # replace f's lines with merge content
            if f > 0:
                lines[f] = lines[f - 1][:]
            else:
                lines[f] = n * [ None ]
            if f + 1 < npanes:
                spaces[f] = spaces[f + 1][:]
            else:
                spaces[f] = n * [ None ]
            if right_first:
                lines, spaces = spaces, lines

            pre, post = cutBlocks(end, self.blocks)
            pre, b = cutBlocks(start, pre)

            #  join and remove null lines
            b.extend(b)
            for l, s in zip(lines, spaces):
                l.extend(s)
            removeNullLines(b, lines)

            # replace f's lines with original, growing if necessary
            new_content = lines[f]
            new_n = len(new_content)
            lines[f] = old_content
            min_n = len(old_content)

            delta = new_n - min_n
            if delta < 0:
                delta = -delta
                for i in range(npanes):
                    if i != f:
                        lines[i].extend(delta * [ None ])
                # grow last block
                if len(b) > 0:
                    b[-1] += delta
                else:
                    b = createBlock(delta)
            elif delta > 0:
                old_content.extend(delta * [ None ])
                new_n = len(old_content)

            # update lines and blocks
            self.updateAlignment(start, n, lines)
            pre.extend(b)
            pre.extend(post)
            self.updateBlocks(pre)

            for i in range(new_n):
                s = None
                if i < len(new_content):
                    line = new_content[i]
                    if line is not None:
                        s = line.getText()
                self.updateText(f, start + i, s)

            # update selection
            end = start + new_n
            if end > start:
                end -= 1
            self.setCurrentLine(f, end, start)
        self.recordEditMode()

    # 'merge_from_left_then_right' keybinding action
    def merge_from_left_then_right(self):
        self._mergeBoth(False)

    # 'merge_from_right_then_left' keybinding action
    def merge_from_right_then_left(self):
        self._mergeBoth(True)

# create 'title_changed' signal for FileDiffViewer
GObject.signal_new('swapped-panes', FileDiffViewer, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, (int, int))
GObject.signal_new('num-edits-changed', FileDiffViewer, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, (int, ))
GObject.signal_new('mode-changed', FileDiffViewer, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, ())
GObject.signal_new('cursor-changed', FileDiffViewer, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, ())
GObject.signal_new('syntax-changed', FileDiffViewer, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, (str, ))
GObject.signal_new('format-changed', FileDiffViewer, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, (int, int))

# dialogue used to search for text
class SearchDialog(Gtk.Dialog):
    def __init__(self, parent, pattern=None, history=None):
        Gtk.Dialog.__init__(self, title=_('Find...'), parent=parent, destroy_with_parent=True)
        self.add_button(Gtk.STOCK_CANCEL, Gtk.ResponseType.REJECT)
        self.add_button(Gtk.STOCK_OK, Gtk.ResponseType.ACCEPT)

        vbox = Gtk.Box.new(Gtk.Orientation.VERTICAL, 0)
        vbox.set_border_width(10)

        hbox = Gtk.Box.new(Gtk.Orientation.HORIZONTAL, 0)
        label = Gtk.Label.new(_('Search For: '))
        hbox.pack_start(label, False, False, 0)
        label.show()
        combo = Gtk.ComboBoxText.new_with_entry()
        self.entry = combo.get_child()
        self.entry.connect('activate', self.entry_cb)

        if pattern is not None:
            self.entry.set_text(pattern)

        if history is not None:
            completion = Gtk.EntryCompletion.new()
            liststore = Gtk.ListStore(GObject.TYPE_STRING)
            completion.set_model(liststore)
            completion.set_text_column(0)
            for h in history:
                liststore.append([h])
                combo.append_text(h)
            self.entry.set_completion(completion)

        hbox.pack_start(combo, True, True, 0)
        combo.show()
        vbox.pack_start(hbox, False, False, 0)
        hbox.show()

        button = Gtk.CheckButton.new_with_mnemonic(_('Match Case'))
        self.match_case_button = button
        vbox.pack_start(button, False, False, 0)
        button.show()

        button = Gtk.CheckButton.new_with_mnemonic(_('Search Backwards'))
        self.backwards_button = button
        vbox.pack_start(button, False, False, 0)
        button.show()

        self.vbox.pack_start(vbox, False, False, 0) # pylint: disable=no-member
        vbox.show()

    # callback used when the Enter key is pressed
    def entry_cb(self, widget):
        self.response(Gtk.ResponseType.ACCEPT)

# convenience method to request confirmation when closing the last tab
def confirmTabClose(parent):
    dialog = utils.MessageDialog(parent, Gtk.MessageType.WARNING, _('Closing this tab will quit %s.') % (constants.APP_NAME, ))
    end = (dialog.run() == Gtk.ResponseType.OK)
    dialog.destroy()
    return end

# custom dialogue for picking files with widgets for specifying the encoding
# and revision
class FileChooserDialog(Gtk.FileChooserDialog):
    # record last chosen folder so the file chooser can start at a more useful
    # location for empty panes
    last_chosen_folder = os.path.realpath(os.curdir)

    def __current_folder_changed_cb(self, widget):
        FileChooserDialog.last_chosen_folder = widget.get_current_folder()

    def __init__(self, title, parent, prefs, action, accept, rev=False):
        Gtk.FileChooserDialog.__init__(self, title=title, parent=parent, action=action)
        self.add_button(Gtk.STOCK_CANCEL, Gtk.ResponseType.CANCEL)
        self.add_button(accept, Gtk.ResponseType.OK)
        self.prefs = prefs
        hbox = Gtk.Box.new(Gtk.Orientation.HORIZONTAL, 0)
        hbox.set_border_width(5)
        label = Gtk.Label.new(_('Encoding: '))
        hbox.pack_start(label, False, False, 0)
        label.show()
        self.encoding = entry = EncodingMenu(prefs, action in [ Gtk.FileChooserAction.OPEN, Gtk.FileChooserAction.SELECT_FOLDER ])
        hbox.pack_start(entry, False, False, 5)
        entry.show()
        if rev:
            self.revision = entry = Gtk.Entry.new()
            hbox.pack_end(entry, False, False, 0)
            entry.show()
            label = Gtk.Label.new(_('Revision: '))
            hbox.pack_end(label, False, False, 0)
            label.show()

        self.vbox.pack_start(hbox, False, False, 0) # pylint: disable=no-member
        hbox.show()
        self.set_current_folder(self.last_chosen_folder)
        self.connect('current-folder-changed', self.__current_folder_changed_cb)

    def set_encoding(self, encoding):
        self.encoding.set_text(encoding)

    def get_encoding(self):
        return self.encoding.get_text()

    def get_revision(self):
        return self.revision.get_text()

    def get_filename(self):
        # convert from UTF-8 string to unicode
        return Gtk.FileChooserDialog.get_filename(self)

# dialogue used to search for text
class NumericDialog(Gtk.Dialog):
    def __init__(self, parent, title, text, val, lower, upper, step=1, page=0):
        Gtk.Dialog.__init__(self, title=title, parent=parent, destroy_with_parent=True)
        self.add_button(Gtk.STOCK_CANCEL, Gtk.ResponseType.REJECT)
        self.add_button(Gtk.STOCK_OK, Gtk.ResponseType.ACCEPT)

        vbox = Gtk.Box.new(Gtk.Orientation.VERTICAL, 0)
        vbox.set_border_width(10)

        hbox = Gtk.Box.new(Gtk.Orientation.HORIZONTAL, 0)
        label = Gtk.Label.new(text)
        hbox.pack_start(label, False, False, 0)
        label.show()
        adj = Gtk.Adjustment.new(val, lower, upper, step, page, 0)
        self.button = button = Gtk.SpinButton.new(adj, 1.0, 0)
        button.connect('activate', self.button_cb)
        hbox.pack_start(button, True, True, 0)
        button.show()

        vbox.pack_start(hbox, True, True, 0)
        hbox.show()

        self.vbox.pack_start(vbox, False, False, 0) # pylint: disable=no-member
        vbox.show()

    def button_cb(self, widget):
        self.response(Gtk.ResponseType.ACCEPT)

# establish callback for the about dialog's link to Diffuse's web site
def url_hook(dialog, link, userdata):
    webbrowser.open(link)



# the about dialog
class AboutDialog(Gtk.AboutDialog):
    def __init__(self):
        Gtk.AboutDialog.__init__(self)
        self.set_logo_icon_name('io.github.mightycreak.Diffuse')
        self.set_program_name(constants.APP_NAME)
        self.set_version(constants.VERSION)
        self.set_comments(_('Diffuse is a graphical tool for merging and comparing text files.'))
        self.set_copyright(constants.COPYRIGHT)
        self.set_website(constants.WEBSITE)
        self.set_authors([ 'Derrick Moser <derrick_moser@yahoo.com>',
                           'Romain Failliot <romain.failliot@foolstep.com>' ])
        self.set_translator_credits(_('translator-credits'))
        license_text = [
            constants.APP_NAME + ' ' + constants.VERSION + '\n\n',
            constants.COPYRIGHT + '\n\n',
            _('''This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License along
with this program; if not, write to the Free Software Foundation, Inc.,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.''') ]
        self.set_license(''.join(license_text))

# widget classed to create notebook tabs with labels and a close button
# use notebooktab.button.connect() to be notified when the button is pressed
# make this a Gtk.EventBox so signals can be connected for MMB and RMB button
# presses.
class NotebookTab(Gtk.EventBox):
    def __init__(self, name, stock):
        Gtk.EventBox.__init__(self)
        self.set_visible_window(False)
        hbox = Gtk.Box.new(Gtk.Orientation.HORIZONTAL, 0)
        if stock is not None:
            image = Gtk.Image.new()
            image.set_from_stock(stock, Gtk.IconSize.MENU)
            hbox.pack_start(image, False, False, 5)
            image.show()
        self.label = label = Gtk.Label.new(name)
        # left justify the widget
        label.set_xalign(0.0)
        label.set_yalign(0.5)
        hbox.pack_start(label, True, True, 0)
        label.show()
        self.button = button = Gtk.Button.new()
        button.set_relief(Gtk.ReliefStyle.NONE)
        image = Gtk.Image.new()
        image.set_from_stock(Gtk.STOCK_CLOSE, Gtk.IconSize.MENU)
        button.add(image)
        image.show()
        button.set_tooltip_text(_('Close Tab'))
        hbox.pack_start(button, False, False, 0)
        button.show()
        self.add(hbox)
        hbox.show()

    def get_text(self):
        return self.label.get_text()

    def set_text(self, s):
        self.label.set_text(s)

# contains information about a file
class FileInfo:
    def __init__(self, name=None, encoding=None, vcs=None, revision=None, label=None):
        # file name
        self.name = name
        # name of codec used to translate the file contents to unicode text
        self.encoding = encoding
        # the VCS object
        self.vcs = vcs
        # revision used to retrieve file from the VCS
        self.revision = revision
        # alternate text to display instead of the actual file name
        self.label = label
        # 'stat' for files read from disk -- used to warn about changes to the
        # file on disk before saving
        self.stat = None
        # most recent 'stat' for files read from disk -- used on focus change
        # to warn about changes to file on disk
        self.last_stat = None

# assign user specified labels to the corresponding files
def assign_file_labels(items, labels):
    new_items = []
    ss = labels[::-1]
    for name, data in items:
        if ss:
            s = ss.pop()
        else:
            s = None
        new_items.append((name, data, s))
    return new_items

# the main application class containing a set of file viewers
# this class displays tab for switching between viewers and dispatches menu
# commands to the current viewer
class Diffuse(Gtk.Window):
    # specialisation of FileDiffViewer for Diffuse
    class FileDiffViewer(FileDiffViewer):
        # pane header
        class PaneHeader(Gtk.Box):
            def __init__(self):
                Gtk.Box.__init__(self, orientation = Gtk.Orientation.HORIZONTAL, spacing = 0)
                appendButtons(self, Gtk.IconSize.MENU, [
                   [ Gtk.STOCK_OPEN, self.button_cb, 'open', _('Open File...') ],
                   [ Gtk.STOCK_REFRESH, self.button_cb, 'reload', _('Reload File') ],
                   [ Gtk.STOCK_SAVE, self.button_cb, 'save', _('Save File') ],
                   [ Gtk.STOCK_SAVE_AS, self.button_cb, 'save_as', _('Save File As...') ] ])

                self.label = label = Gtk.Label.new()
                label.set_selectable(True)
                label.set_ellipsize(Pango.EllipsizeMode.START)
                label.set_max_width_chars(1)

                self.pack_start(label, True, True, 0)

                # file's name and information about how to retrieve it from a
                # VCS
                self.info = FileInfo()
                self.has_edits = False
                self.updateTitle()
                self.show_all()

            # callback for buttons
            def button_cb(self, widget, s):
                self.emit(s)

            # creates an appropriate title for the pane header
            def updateTitle(self):
                ss = []
                info = self.info
                if info.label is not None:
                    # show the provided label instead of the file name
                    ss.append(info.label)
                else:
                    if info.name is not None:
                        ss.append(info.name)
                    if info.revision is not None:
                        ss.append('(' + info.revision + ')')
                if self.has_edits:
                    ss.append('*')
                s = ' '.join(ss)
                self.label.set_text(s)
                self.label.set_tooltip_text(s)
                self.emit('title_changed')

            # set num edits
            def setEdits(self, has_edits):
                if self.has_edits != has_edits:
                    self.has_edits = has_edits
                    self.updateTitle()

        # pane footer
        class PaneFooter(Gtk.Box):
            def __init__(self):
                Gtk.Box.__init__(self, orientation = Gtk.Orientation.HORIZONTAL, spacing = 0)
                self.cursor = label = Gtk.Label.new()
                self.cursor.set_size_request(-1, -1)
                self.pack_start(label, False, False, 0)

                separator = Gtk.Separator.new(Gtk.Orientation.VERTICAL)
                self.pack_end(separator, False, False, 10)

                self.encoding = label = Gtk.Label.new()
                self.pack_end(label, False, False, 0)

                separator = Gtk.Separator.new(Gtk.Orientation.VERTICAL)
                self.pack_end(separator, False, False, 10)

                self.format = label = Gtk.Label.new()
                self.pack_end(label, False, False, 0)

                separator = Gtk.Separator.new(Gtk.Orientation.VERTICAL)
                self.pack_end(separator, False, False, 10)

                self.set_size_request(0, self.get_size_request()[1])
                self.show_all()

            # set the cursor label
            def updateCursor(self, viewer, f):
                if viewer.mode == CHAR_MODE and viewer.current_pane == f:
                    ## TODO: Find a fix for the column bug (resizing issue when editing a line)
                    #j = viewer.current_char
                    #if j > 0:
                    #    text = viewer.getLineText(viewer.current_pane, viewer.current_line)[:j]
                    #    j = viewer.stringWidth(text)
                    #s = _('Column %d') % (j, )
                    s = ''
                else:
                    s = ''
                self.cursor.set_text(s)

            # set the format label
            def setFormat(self, s):
                v = []
                if s & DOS_FORMAT:
                    v.append('DOS')
                if s & MAC_FORMAT:
                    v.append('Mac')
                if s & UNIX_FORMAT:
                    v.append('Unix')
                self.format.set_text('/'.join(v))

            # set the format label
            def setEncoding(self, s):
                if s is None:
                    s = ''
                self.encoding.set_text(s)

        def __init__(self, n, prefs, title):
            FileDiffViewer.__init__(self, n, prefs)

            self.title = title
            self.status = ''

            self.headers = []
            self.footers = []
            for i in range(n):
                # pane header
                w = Diffuse.FileDiffViewer.PaneHeader()
                self.headers.append(w)
                self.attach(w, i, 0, 1, 1)
                w.connect('title-changed', self.title_changed_cb)
                w.connect('open', self.open_file_button_cb, i)
                w.connect('reload', self.reload_file_button_cb, i)
                w.connect('save', self.save_file_button_cb, i)
                w.connect('save-as', self.save_file_as_button_cb, i)
                w.show()

                # pane footer
                w = Diffuse.FileDiffViewer.PaneFooter()
                self.footers.append(w)
                self.attach(w, i, 2, 1, 1)
                w.show()

            self.connect('swapped-panes', self.swapped_panes_cb)
            self.connect('num-edits-changed', self.num_edits_changed_cb)
            self.connect('mode-changed', self.mode_changed_cb)
            self.connect('cursor-changed', self.cursor_changed_cb)
            self.connect('format-changed', self.format_changed_cb)

            for i, darea in enumerate(self.dareas):
                darea.drag_dest_set(Gtk.DestDefaults.ALL, [ Gtk.TargetEntry.new('text/uri-list', 0, 0) ], Gdk.DragAction.COPY)
                darea.connect('drag-data-received', self.drag_data_received_cb, i)
            # initialise status
            self.updateStatus()

        # convenience method to request confirmation before loading a file if
        # it will cause existing edits to be lost
        def loadFromInfo(self, f, info):
            if self.headers[f].has_edits:
                # warn users of any unsaved changes they might lose
                dialog = Gtk.MessageDialog(self.get_toplevel(), Gtk.DialogFlags.DESTROY_WITH_PARENT, Gtk.MessageType.WARNING, Gtk.ButtonsType.NONE, _('Save changes before loading the new file?'))
                dialog.set_title(constants.APP_NAME)
                dialog.add_button(Gtk.STOCK_CANCEL, Gtk.ResponseType.CANCEL)
                dialog.add_button(Gtk.STOCK_NO, Gtk.ResponseType.REJECT)
                dialog.add_button(Gtk.STOCK_YES, Gtk.ResponseType.OK)
                dialog.set_default_response(Gtk.ResponseType.CANCEL)
                response = dialog.run()
                dialog.destroy()
                if response == Gtk.ResponseType.OK:
                    # save the current pane contents
                    if not self.save_file(f):
                        # cancel if the save failed
                        return
                elif response != Gtk.ResponseType.REJECT:
                    # cancel if the user did not choose 'yes' or 'no'
                    return
            self.openUndoBlock()
            self.recordEditMode()
            self.load(f, info)
            self.recordEditMode()
            self.closeUndoBlock()

        # callback used when receiving drag-n-drop data
        def drag_data_received_cb(self, widget, context, x, y, selection, targettype, eventtime, f):
            # get uri list
            uris = selection.get_uris()
            # load the first valid file
            for uri in uris:
                path = urlparse(uri).path
                if os.path.isfile(path):
                    self.loadFromInfo(f, FileInfo(path))
                    break

        # change the file info for pane 'f' to 'info'
        def setFileInfo(self, f, info):
            h, footer = self.headers[f], self.footers[f]
            h.info = info
            h.updateTitle()
            footer.setFormat(self.panes[f].format)
            footer.setEncoding(info.encoding)
            footer.updateCursor(self, f)

        # callback used when a pane header's title changes
        def title_changed_cb(self, widget):
            # choose a short but descriptive title for the viewer
            has_edits = False
            names = []
            unique_names = set()
            for header in self.headers:
                has_edits |= header.has_edits
                s = header.info.label
                if s is None:
                    # no label provided, show the file name instead
                    s = header.info.name
                    if s is not None:
                        s = os.path.basename(s)
                if s is not None:
                    names.append(s)
                    unique_names.add(s)

            if len(unique_names) > 0:
                if len(unique_names) == 1:
                    self.title = names[0]
                else:
                    self.title = ' : '.join(names)
            s = self.title
            if has_edits:
                s += ' *'
            self.emit('title_changed', s)

        def setEncoding(self, f, encoding):
            h = self.headers[f]
            h.info.encoding = encoding
            self.footers[f].setEncoding(encoding)

        # load a new file into pane 'f'
        # 'info' indicates the name of the file and how to retrieve it from the
        # version control system if applicable
        def load(self, f, info):
            name = info.name
            encoding = info.encoding
            stat = None
            if name is None:
                # reset to an empty pane
                ss = []
            else:
                rev = info.revision
                try:
                    if rev is None:
                        # load the contents of a plain file
                        fd = open(name, 'rb')
                        s = fd.read()
                        fd.close()
                        # get the file's modification times so we can detect changes
                        stat = os.stat(name)
                    else:
                        if info.vcs is None:
                            raise IOError('Not under version control.')
                        fullname = os.path.abspath(name)
                        # retrieve the revision from the version control system
                        s = info.vcs.getRevision(self.prefs, fullname, rev)
                    # convert file contents to unicode
                    if encoding is None:
                        s, encoding = self.prefs.convertToUnicode(s)
                    else:
                        s = str(s, encoding=encoding)
                    ss = splitlines(s)
                except (IOError, OSError, UnicodeDecodeError, WindowsError, LookupError):
                    # FIXME: this can occur before the toplevel window is drawn
                    if rev is not None:
                        msg = _('Error reading revision %(rev)s of %(file)s.') % { 'rev': rev, 'file': name }
                    else:
                        msg = _('Error reading %s.') % (name, )
                    utils.logErrorAndDialog(msg, self.get_toplevel())
                    return
            # update the panes contents, last modified time, and title
            self.replaceContents(f, ss)
            info.encoding = encoding
            info.last_stat = info.stat = stat
            self.setFileInfo(f, info)
            # use the file name to choose appropriate syntax highlighting rules
            if name is not None:
                syntax = theResources.guessSyntaxForFile(name, ss)
                if syntax is not None:
                    self.setSyntax(syntax)

        # load a new file into pane 'f'
        def open_file(self, f, reload=False):
            h = self.headers[f]
            info = h.info
            if not reload:
                # we need to ask for a file name if we are not reloading the
                # existing file
                dialog = FileChooserDialog(_('Open File'), self.get_toplevel(), self.prefs, Gtk.FileChooserAction.OPEN, Gtk.STOCK_OPEN, True)
                if info.name is not None:
                    dialog.set_filename(os.path.realpath(info.name))
                dialog.set_encoding(info.encoding)
                dialog.set_default_response(Gtk.ResponseType.OK)
                end = (dialog.run() != Gtk.ResponseType.OK)
                name = dialog.get_filename()
                rev = None
                vcs = None
                revision = dialog.get_revision().strip()
                if revision != '':
                    rev = revision
                    vcs = theVCSs.findByFilename(name, self.prefs)
                info = FileInfo(name, dialog.get_encoding(), vcs, rev)
                dialog.destroy()
                if end:
                    return
            self.loadFromInfo(f, info)

        # callback for open file button
        def open_file_button_cb(self, widget, f):
            self.open_file(f)

        # callback for open file menu item
        def open_file_cb(self, widget, data):
            self.open_file(self.current_pane)

        # callback for reload file button
        def reload_file_button_cb(self, widget, f):
            self.open_file(f, True)

        # callback for reload file menu item
        def reload_file_cb(self, widget, data):
            self.open_file(self.current_pane, True)

        # check changes to files on disk when receiving keyboard focus
        def focus_in(self, widget, event):
            for f, h in enumerate(self.headers):
                info = h.info
                try:
                    if info.last_stat is not None:
                        info = h.info
                        new_stat = os.stat(info.name)
                        if info.last_stat[stat.ST_MTIME] < new_stat[stat.ST_MTIME]:
                            # update our notion of the most recent modification
                            info.last_stat = new_stat
                            if info.label is not None:
                                s = info.label
                            else:
                                s = info.name
                            msg = _('The file %s changed on disk.  Do you want to reload the file?') % (s, )
                            dialog = utils.MessageDialog(self.get_toplevel(), Gtk.MessageType.QUESTION, msg)
                            ok = (dialog.run() == Gtk.ResponseType.OK)
                            dialog.destroy()
                            if ok:
                                self.open_file(f, True)
                except OSError:
                    pass

        # save contents of pane 'f' to file
        def save_file(self, f, save_as=False):
            h = self.headers[f]
            info = h.info
            name, encoding, rev, label = info.name, info.encoding, info.revision, info.label
            if name is None or rev is not None:
                # we need to prompt for a file name the current contents were
                # not loaded from a regular file
                save_as = True
            if save_as:
                # prompt for a file name
                dialog = FileChooserDialog(_('Save %(title)s Pane %(pane)d') % { 'title': self.title, 'pane': f + 1 }, self.get_toplevel(), self.prefs, Gtk.FileChooserAction.SAVE, Gtk.STOCK_SAVE)
                if name is not None:
                    dialog.set_filename(os.path.abspath(name))
                if encoding is None:
                    encoding = self.prefs.getDefaultEncoding()
                dialog.set_encoding(encoding)
                name, label = None, None
                dialog.set_default_response(Gtk.ResponseType.OK)
                if dialog.run() == Gtk.ResponseType.OK:
                    name = dialog.get_filename()
                    encoding = dialog.get_encoding()
                    if encoding is None:
                        if info.encoding is not None:
                            # this case can occur if the user provided the
                            # encoding and it is not an encoding we know about
                            encoding = info.encoding
                        else:
                            encoding = self.prefs.getDefaultEncoding()
                dialog.destroy()
            if name is None:
                return False
            try:
                msg = None
                # warn if we are about to overwrite an existing file
                if save_as:
                    if os.path.exists(name):
                        msg = _('A file named %s already exists.  Do you want to overwrite it?') % (name, )
                # warn if we are about to overwrite a file that has changed
                # since we last read it
                elif info.stat is not None:
                    if info.stat[stat.ST_MTIME] < os.stat(name)[stat.ST_MTIME]:
                        msg = _('The file %s has been modified by another process since reading it.  If you save, all the external changes could be lost.  Save anyways?') % (name, )
                if msg is not None:
                    dialog = utils.MessageDialog(self.get_toplevel(), Gtk.MessageType.QUESTION, msg)
                    end = (dialog.run() != Gtk.ResponseType.OK)
                    dialog.destroy()
                    if end:
                        return False
            except OSError:
                pass
            try:
                # convert the text to the output encoding
                # refresh the lines to contain new objects with updated line
                # numbers and no local edits
                ss = []
                for line in self.panes[f].lines:
                    if line is not None:
                        s = line.getText()
                        if s is not None:
                            ss.append(s)
                encoded = codecs.encode(''.join(ss), encoding)

                # write file
                fd = open(name, 'wb')
                fd.write(encoded)
                fd.close()

                # make the edits look permanent
                self.openUndoBlock()
                self.bakeEdits(f)
                self.closeUndoBlock()
                # update the pane file info
                info.name, info.encoding, info.revision, info.label = name, encoding, None, label
                info.last_stat = info.stat = os.stat(name)
                self.setFileInfo(f, info)
                # update the syntax highlighting incase we changed the file
                # extension
                syntax = theResources.guessSyntaxForFile(name, ss)
                if syntax is not None:
                    self.setSyntax(syntax)
                return True
            except (UnicodeEncodeError, LookupError):
                utils.logErrorAndDialog(_('Error encoding to %s.') % (encoding, ), self.get_toplevel())
            except IOError:
                utils.logErrorAndDialog(_('Error writing %s.') % (name, ), self.get_toplevel())
            return False

        # callback for save file menu item
        def save_file_cb(self, widget, data):
            self.save_file(self.current_pane)

        # callback for save file as menu item
        def save_file_as_cb(self, widget, data):
            self.save_file(self.current_pane, True)

        # callback for save all menu item
        def save_all_cb(self, widget, data):
            for f, h in enumerate(self.headers):
                if h.has_edits:
                    self.save_file(f)

        # callback for save file button
        def save_file_button_cb(self, widget, f):
            self.save_file(f)

        # callback for save file as button
        def save_file_as_button_cb(self, widget, f):
            self.save_file(f, True)

        # callback for go to line menu item
        def go_to_line_cb(self, widget, data):
            parent = self.get_toplevel()
            dialog = NumericDialog(parent, _('Go To Line...'), _('Line Number: '), 1, 1, self.panes[self.current_pane].max_line_number + 1)
            okay = (dialog.run() == Gtk.ResponseType.ACCEPT)
            i = dialog.button.get_value_as_int()
            dialog.destroy()
            if okay:
                self.go_to_line(i)

        # callback to receive notification when the name of a file changes
        def swapped_panes_cb(self, widget, f_dst, f_src):
            f0, f1 = self.headers[f_dst], self.headers[f_src]
            f0.has_edits, f1.has_edits = f1.has_edits, f0.has_edits
            info0, info1 = f1.info, f0.info
            self.setFileInfo(f_dst, info0)
            self.setFileInfo(f_src, info1)

        # callback to receive notification when the name of a file changes
        def num_edits_changed_cb(self, widget, f):
            self.headers[f].setEdits(self.panes[f].num_edits > 0)

        # callback to record changes to the viewer's mode
        def mode_changed_cb(self, widget):
            self.updateStatus()

        # update the viewer's current status message
        def updateStatus(self):
            if self.mode == LINE_MODE:
                s = _('Press the enter key or double click to edit.  Press the space bar or use the RMB menu to manually align.')
            elif self.mode == CHAR_MODE:
                s = _('Press the escape key to finish editing.')
            elif self.mode == ALIGN_MODE:
                s = _('Select target line and press the space bar to align.  Press the escape key to cancel.')
            else:
                s = None
            self.status = s
            self.emit('status_changed', s)

        # gets the status bar text
        def getStatus(self):
            return self.status

        # callback to display the cursor in a pane
        def cursor_changed_cb(self, widget):
            for f, footer in enumerate(self.footers):
                footer.updateCursor(self, f)

        # callback to display the format of a pane
        def format_changed_cb(self, widget, f, format):
            self.footers[f].setFormat(format)

    def __init__(self, rc_dir):
        Gtk.Window.__init__(self, type = Gtk.WindowType.TOPLEVEL)

        self.prefs = Preferences(os.path.join(rc_dir, 'prefs'))
        # number of created viewers (used to label some tabs)
        self.viewer_count = 0

        # get monitor resolution
        monitor_geometry = Gdk.Display.get_default().get_monitor(0).get_geometry()

        # state information that should persist across sessions
        self.bool_state = { 'window_maximized': False, 'search_matchcase': False, 'search_backwards': False }
        self.int_state = { 'window_width': 1024, 'window_height': 768 }
        self.int_state['window_x'] = max(0, (monitor_geometry.width - self.int_state['window_width']) / 2)
        self.int_state['window_y'] = max(0, (monitor_geometry.height - self.int_state['window_height']) / 2)
        self.connect('configure-event', self.configure_cb)
        self.connect('window-state-event', self.window_state_cb)

        # search history is application wide
        self.search_pattern = None
        self.search_history = []

        self.connect('delete-event', self.delete_cb)
        accel_group = Gtk.AccelGroup()

        # create a Box for our contents
        vbox = Gtk.Box(orientation=Gtk.Orientation.VERTICAL)

        # create some custom icons for merging
        DIFFUSE_STOCK_NEW_2WAY_MERGE = 'diffuse-new-2way-merge'
        DIFFUSE_STOCK_NEW_3WAY_MERGE = 'diffuse-new-3way-merge'
        DIFFUSE_STOCK_LEFT_RIGHT = 'diffuse-left-right'
        DIFFUSE_STOCK_RIGHT_LEFT = 'diffuse-right-left'

        # get default theme and window scale factor
        default_theme = Gtk.IconTheme.get_default()
        scale_factor = self.get_scale_factor()

        icon_size = Gtk.IconSize.lookup(Gtk.IconSize.LARGE_TOOLBAR).height
        factory = Gtk.IconFactory()

        # render the base item used to indicate a new document
        p0 = default_theme.load_icon_for_scale("document-new", icon_size, scale_factor, 0)
        w, h = p0.get_width(), p0.get_height()

        # render new 2-way merge icon
        s = 0.8
        sw, sh = int(s * w), int(s * h)
        w1, h1 = w - sw, h - sh
        p = GdkPixbuf.Pixbuf.new(GdkPixbuf.Colorspace.RGB, True, 8, w, h)
        p.fill(0)
        p0.composite(p, 0, 0, sw, sh, 0, 0, s, s, GdkPixbuf.InterpType.BILINEAR, 255)
        p0.composite(p, w1, h1, sw, sh, w1, h1, s, s, GdkPixbuf.InterpType.BILINEAR, 255)
        factory.add(DIFFUSE_STOCK_NEW_2WAY_MERGE, Gtk.IconSet.new_from_pixbuf(p))

        # render new 3-way merge icon
        s = 0.7
        sw, sh = int(s * w), int(s * h)
        w1, h1 = (w - sw) / 2, (h - sh) / 2
        w2, h2 = w - sw, h - sh
        p = GdkPixbuf.Pixbuf.new(GdkPixbuf.Colorspace.RGB, True, 8, w, h)
        p.fill(0)
        p0.composite(p, 0, 0, sw, sh, 0, 0, s, s, GdkPixbuf.InterpType.BILINEAR, 255)
        p0.composite(p, w1, h1, sw, sh, w1, h1, s, s, GdkPixbuf.InterpType.BILINEAR, 255)
        p0.composite(p, w2, h2, sw, sh, w2, h2, s, s, GdkPixbuf.InterpType.BILINEAR, 255)
        factory.add(DIFFUSE_STOCK_NEW_3WAY_MERGE, Gtk.IconSet.new_from_pixbuf(p))

        # render the left and right arrow we will use in our custom icons
        p0 = default_theme.load_icon_for_scale("go-next", icon_size, scale_factor, 0)
        p1 = default_theme.load_icon_for_scale("go-previous", icon_size, scale_factor, 0)
        w, h, s = p0.get_width(), p0.get_height(), 0.65
        sw, sh = int(s * w), int(s * h)
        w1, h1 = w - sw, h - sh

        # create merge from left then right icon
        p = GdkPixbuf.Pixbuf.new(GdkPixbuf.Colorspace.RGB, True, 8, w, h)
        p.fill(0)
        p1.composite(p, w1, h1, sw, sh, w1, h1, s, s, GdkPixbuf.InterpType.BILINEAR, 255)
        p0.composite(p, 0, 0, sw, sh, 0, 0, s, s, GdkPixbuf.InterpType.BILINEAR, 255)
        factory.add(DIFFUSE_STOCK_LEFT_RIGHT, Gtk.IconSet.new_from_pixbuf(p))

        # create merge from right then left icon
        p = GdkPixbuf.Pixbuf.new(GdkPixbuf.Colorspace.RGB, True, 8, w, h)
        p.fill(0)
        p0.composite(p, 0, h1, sw, sh, 0, h1, s, s, GdkPixbuf.InterpType.BILINEAR, 255)
        p1.composite(p, w1, 0, sw, sh, w1, 0, s, s, GdkPixbuf.InterpType.BILINEAR, 255)
        factory.add(DIFFUSE_STOCK_RIGHT_LEFT, Gtk.IconSet.new_from_pixbuf(p))

        # make the icons available for use
        factory.add_default()

        menuspecs = []
        menuspecs.append([ _('_File'), [
                     [_('_Open File...'), self.open_file_cb, None, Gtk.STOCK_OPEN, 'open_file'],
                     [_('Open File In New _Tab...'), self.open_file_in_new_tab_cb, None, None, 'open_file_in_new_tab'],
                     [_('Open _Modified Files...'), self.open_modified_files_cb, None, None, 'open_modified_files'],
                     [_('Open Commi_t...'), self.open_commit_cb, None, None, 'open_commit'],
                     [_('_Reload File'), self.reload_file_cb, None, Gtk.STOCK_REFRESH, 'reload_file'],
                     [],
                     [_('_Save File'), self.save_file_cb, None, Gtk.STOCK_SAVE, 'save_file'],
                     [_('Save File _As...'), self.save_file_as_cb, None, Gtk.STOCK_SAVE_AS, 'save_file_as'],
                     [_('Save A_ll'), self.save_all_cb, None, None, 'save_all'],
                     [],
                     [_('New _2-Way File Merge'), self.new_2_way_file_merge_cb, None, DIFFUSE_STOCK_NEW_2WAY_MERGE, 'new_2_way_file_merge'],
                     [_('New _3-Way File Merge'), self.new_3_way_file_merge_cb, None, DIFFUSE_STOCK_NEW_3WAY_MERGE, 'new_3_way_file_merge'],
                     [_('New _N-Way File Merge...'), self.new_n_way_file_merge_cb, None, None, 'new_n_way_file_merge'],
                     [],
                     [_('_Close Tab'), self.close_tab_cb, None, Gtk.STOCK_CLOSE, 'close_tab'],
                     [_('_Undo Close Tab'), self.undo_close_tab_cb, None, None, 'undo_close_tab'],
                     [_('_Quit'), self.quit_cb, None, Gtk.STOCK_QUIT, 'quit'] ] ])

        menuspecs.append([ _('_Edit'), [
                     [_('_Undo'), self.button_cb, 'undo', Gtk.STOCK_UNDO, 'undo'],
                     [_('_Redo'), self.button_cb, 'redo', Gtk.STOCK_REDO, 'redo'],
                     [],
                     [_('Cu_t'), self.button_cb, 'cut', Gtk.STOCK_CUT, 'cut'],
                     [_('_Copy'), self.button_cb, 'copy', Gtk.STOCK_COPY, 'copy'],
                     [_('_Paste'), self.button_cb, 'paste', Gtk.STOCK_PASTE, 'paste'],
                     [],
                     [_('Select _All'), self.button_cb, 'select_all', None, 'select_all'],
                     [_('C_lear Edits'), self.button_cb, 'clear_edits', Gtk.STOCK_CLEAR, 'clear_edits'],
                     [_('_Dismiss All Edits'), self.button_cb, 'dismiss_all_edits', None, 'dismiss_all_edits'],
                     [],
                     [_('_Find...'), self.find_cb, None, Gtk.STOCK_FIND, 'find'],
                     [_('Find _Next'), self.find_next_cb, None, None, 'find_next'],
                     [_('Find Pre_vious'), self.find_previous_cb, None, None, 'find_previous'],
                     [_('_Go To Line...'), self.go_to_line_cb, None, Gtk.STOCK_JUMP_TO, 'go_to_line'],
                     [],
                     [_('Pr_eferences...'), self.preferences_cb, None, Gtk.STOCK_PREFERENCES, 'preferences'] ] ])

        submenudef = [ [_('None'), self.syntax_cb, None, None, 'no_syntax_highlighting', True, None, ('syntax', None) ] ]
        names = theResources.getSyntaxNames()
        if len(names) > 0:
            submenudef.append([])
            names.sort(key=str.lower)
            for name in names:
                submenudef.append([name, self.syntax_cb, name, None, 'syntax_highlighting_' + name, True, None, ('syntax', name) ])

        menuspecs.append([ _('_View'), [
                     [_('_Syntax Highlighting'), None, None, None, None, True, submenudef],
                     [],
                     [_('Re_align All'), self.button_cb, 'realign_all', Gtk.STOCK_EXECUTE, 'realign_all'],
                     [_('_Isolate'), self.button_cb, 'isolate', None, 'isolate'],
                     [],
                     [_('_First Difference'), self.button_cb, 'first_difference', Gtk.STOCK_GOTO_TOP, 'first_difference'],
                     [_('_Previous Difference'), self.button_cb, 'previous_difference', Gtk.STOCK_GO_UP, 'previous_difference'],
                     [_('_Next Difference'), self.button_cb, 'next_difference', Gtk.STOCK_GO_DOWN, 'next_difference'],
                     [_('_Last Difference'), self.button_cb, 'last_difference', Gtk.STOCK_GOTO_BOTTOM, 'last_difference'],
                     [],
                     [_('Fir_st Tab'), self.first_tab_cb, None, None, 'first_tab'],
                     [_('Pre_vious Tab'), self.previous_tab_cb, None, None, 'previous_tab'],
                     [_('Next _Tab'), self.next_tab_cb, None, None, 'next_tab'],
                     [_('Las_t Tab'), self.last_tab_cb, None, None, 'last_tab'],
                     [],
                     [_('Shift Pane _Right'), self.button_cb, 'shift_pane_right', None, 'shift_pane_right'],
                     [_('Shift Pane _Left'), self.button_cb, 'shift_pane_left', None, 'shift_pane_left'] ] ])

        menuspecs.append([ _('F_ormat'), [
                     [_('Convert To _Upper Case'), self.button_cb, 'convert_to_upper_case', None, 'convert_to_upper_case'],
                     [_('Convert To _Lower Case'), self.button_cb, 'convert_to_lower_case', None, 'convert_to_lower_case'],
                     [],
                     [_('Sort Lines In _Ascending Order'), self.button_cb, 'sort_lines_in_ascending_order', Gtk.STOCK_SORT_ASCENDING, 'sort_lines_in_ascending_order'],
                     [_('Sort Lines In D_escending Order'), self.button_cb, 'sort_lines_in_descending_order', Gtk.STOCK_SORT_DESCENDING, 'sort_lines_in_descending_order'],
                     [],
                     [_('Remove Trailing _White Space'), self.button_cb, 'remove_trailing_white_space', None, 'remove_trailing_white_space'],
                     [_('Convert Tabs To _Spaces'), self.button_cb, 'convert_tabs_to_spaces', None, 'convert_tabs_to_spaces'],
                     [_('Convert Leading Spaces To _Tabs'), self.button_cb, 'convert_leading_spaces_to_tabs', None, 'convert_leading_spaces_to_tabs'],
                     [],
                     [_('_Increase Indenting'), self.button_cb, 'increase_indenting', Gtk.STOCK_INDENT, 'increase_indenting'],
                     [_('De_crease Indenting'), self.button_cb, 'decrease_indenting', Gtk.STOCK_UNINDENT, 'decrease_indenting'],
                     [],
                     [_('Convert To _DOS Format'), self.button_cb, 'convert_to_dos', None, 'convert_to_dos'],
                     [_('Convert To _Mac Format'), self.button_cb, 'convert_to_mac', None, 'convert_to_mac'],
                     [_('Convert To Uni_x Format'), self.button_cb, 'convert_to_unix', None, 'convert_to_unix'] ] ])

        menuspecs.append([ _('_Merge'), [
                     [_('Copy Selection _Right'), self.button_cb, 'copy_selection_right', Gtk.STOCK_GOTO_LAST, 'copy_selection_right'],
                     [_('Copy Selection _Left'), self.button_cb, 'copy_selection_left', Gtk.STOCK_GOTO_FIRST, 'copy_selection_left'],
                     [],
                     [_('Copy Left _Into Selection'), self.button_cb, 'copy_left_into_selection', Gtk.STOCK_GO_FORWARD, 'copy_left_into_selection'],
                     [_('Copy Right I_nto Selection'), self.button_cb, 'copy_right_into_selection', Gtk.STOCK_GO_BACK, 'copy_right_into_selection'],
                     [_('_Merge From Left Then Right'), self.button_cb, 'merge_from_left_then_right', DIFFUSE_STOCK_LEFT_RIGHT, 'merge_from_left_then_right'],
                     [_('M_erge From Right Then Left'), self.button_cb, 'merge_from_right_then_left', DIFFUSE_STOCK_RIGHT_LEFT, 'merge_from_right_then_left'] ] ])

        menuspecs.append([ _('_Help'), [
                     [_('_Help Contents...'), self.help_contents_cb, None, Gtk.STOCK_HELP, 'help_contents'],
                     [],
                     [_('_About %s...') % (constants.APP_NAME, ), self.about_cb, None, Gtk.STOCK_ABOUT, 'about'] ] ])

        # used to disable menu events when switching tabs
        self.menu_update_depth = 0
        # build list of radio menu items so we can update them to match the
        # currently viewed pane
        self.radio_menus = radio_menus = {}
        menu_bar = createMenuBar(menuspecs, radio_menus, accel_group)
        vbox.pack_start(menu_bar, False, False, 0)
        menu_bar.show()

        # create button bar
        hbox = Gtk.Box.new(Gtk.Orientation.HORIZONTAL, 0)
        appendButtons(hbox, Gtk.IconSize.LARGE_TOOLBAR, [
           [ DIFFUSE_STOCK_NEW_2WAY_MERGE, self.new_2_way_file_merge_cb, None, _('New 2-Way File Merge') ],
           [ DIFFUSE_STOCK_NEW_3WAY_MERGE, self.new_3_way_file_merge_cb, None, _('New 3-Way File Merge') ],
           [],
           [ Gtk.STOCK_EXECUTE, self.button_cb, 'realign_all', _('Realign All') ],
           [ Gtk.STOCK_GOTO_TOP, self.button_cb, 'first_difference', _('First Difference') ],
           [ Gtk.STOCK_GO_UP, self.button_cb, 'previous_difference', _('Previous Difference') ],
           [ Gtk.STOCK_GO_DOWN, self.button_cb, 'next_difference', _('Next Difference') ],
           [ Gtk.STOCK_GOTO_BOTTOM, self.button_cb, 'last_difference', _('Last Difference') ],
           [],
           [ Gtk.STOCK_GOTO_LAST, self.button_cb, 'copy_selection_right', _('Copy Selection Right') ],
           [ Gtk.STOCK_GOTO_FIRST, self.button_cb, 'copy_selection_left', _('Copy Selection Left') ],
           [ Gtk.STOCK_GO_FORWARD, self.button_cb, 'copy_left_into_selection', _('Copy Left Into Selection') ],
           [ Gtk.STOCK_GO_BACK, self.button_cb, 'copy_right_into_selection', _('Copy Right Into Selection') ],
           [ DIFFUSE_STOCK_LEFT_RIGHT, self.button_cb, 'merge_from_left_then_right', _('Merge From Left Then Right') ],
           [ DIFFUSE_STOCK_RIGHT_LEFT, self.button_cb, 'merge_from_right_then_left', _('Merge From Right Then Left') ],
           [],
           [ Gtk.STOCK_UNDO, self.button_cb, 'undo', _('Undo') ],
           [ Gtk.STOCK_REDO, self.button_cb, 'redo', _('Redo') ],
           [ Gtk.STOCK_CUT, self.button_cb, 'cut', _('Cut') ],
           [ Gtk.STOCK_COPY, self.button_cb, 'copy', _('Copy') ],
           [ Gtk.STOCK_PASTE, self.button_cb, 'paste', _('Paste') ],
           [ Gtk.STOCK_CLEAR, self.button_cb, 'clear_edits', _('Clear Edits') ] ])
        # avoid the button bar from dictating the minimum window size
        hbox.set_size_request(0, hbox.get_size_request()[1])
        vbox.pack_start(hbox, False, False, 0)
        hbox.show()

        self.closed_tabs = []
        self.notebook = notebook = Gtk.Notebook.new()
        notebook.set_scrollable(True)
        notebook.connect('switch-page', self.switch_page_cb)
        vbox.pack_start(notebook, True, True, 0)
        notebook.show()

        # Add a status bar to the bottom
        self.statusbar = statusbar = Gtk.Statusbar.new()
        vbox.pack_start(statusbar, False, False, 0)
        statusbar.show()

        self.add_accel_group(accel_group)
        self.add(vbox)
        vbox.show()
        self.connect('focus-in-event', self.focus_in_cb)

    # notifies all viewers on focus changes so they may check for external
    # changes to files
    def focus_in_cb(self, widget, event):
        for i in range(self.notebook.get_n_pages()):
            self.notebook.get_nth_page(i).focus_in(widget, event)

    # record the window's position and size
    def configure_cb(self, widget, event):
        # read the state directly instead of using window_maximized as the order
        # of configure/window_state events is undefined
        if (widget.get_window().get_state() & Gdk.WindowState.MAXIMIZED) == 0:
            self.int_state['window_x'], self.int_state['window_y'] = widget.get_window().get_root_origin()
            self.int_state['window_width'] = event.width
            self.int_state['window_height'] = event.height

    # record the window's maximised state
    def window_state_cb(self, window, event):
        self.bool_state['window_maximized'] = ((event.new_window_state & Gdk.WindowState.MAXIMIZED) != 0)

    # load state information that should persist across sessions
    def loadState(self, statepath):
        if os.path.isfile(statepath):
            try:
                f = open(statepath, 'r')
                ss = readlines(f)
                f.close()
                for j, s in enumerate(ss):
                    try:
                        a = shlex.split(s, True)
                        if len(a) > 0:
                            if len(a) == 2 and a[0] in self.bool_state:
                                self.bool_state[a[0]] = (a[1] == 'True')
                            elif len(a) == 2 and a[0] in self.int_state:
                                self.int_state[a[0]] = int(a[1])
                            else:
                                raise ValueError()
                    except ValueError:
                        # this may happen if the state was written by a
                        # different version -- don't bother the user
                        utils.logDebug(f'Error processing line {j + 1} of {statepath}.')
            except IOError:
                # bad $HOME value? -- don't bother the user
                utils.logDebug(f'Error reading {statepath}.')

        self.move(self.int_state['window_x'], self.int_state['window_y'])
        self.resize(self.int_state['window_width'], self.int_state['window_height'])
        if self.bool_state['window_maximized']:
            self.maximize()

    # save state information that should persist across sessions
    def saveState(self, statepath):
        try:
            ss = []
            for k, v in self.bool_state.items():
                ss.append(f'{k} {v}\n')
            for k, v in self.int_state.items():
                ss.append(f'{k} {v}\n')
            ss.sort()
            f = open(statepath, 'w')
            f.write(f"# This state file was generated by {constants.APP_NAME} {constants.VERSION}.\n\n")
            for s in ss:
                f.write(s)
            f.close()
        except IOError:
            # bad $HOME value? -- don't bother the user
            utils.logDebug(f'Error writing {statepath}.')

    # select viewer for a newly selected file in the confirm close dialogue
    def __confirmClose_row_activated_cb(self, tree, path, col, model):
        self.notebook.set_current_page(self.notebook.page_num(model[path][3]))

    # toggle save state for a file listed in the confirm close dialogue
    def __confirmClose_toggle_cb(self, cell, path, model):
        model[path][0] = not model[path][0]

    # returns True if the list of viewers can be closed.  The user will be
    # given a chance to save any modified files before this method completes.
    def confirmCloseViewers(self, viewers):
        # make a list of modified files
        model = Gtk.ListStore.new([
            GObject.TYPE_BOOLEAN,
            GObject.TYPE_STRING,
            GObject.TYPE_INT,
            GObject.TYPE_OBJECT])
        for v in viewers:
            for f, h in enumerate(v.headers):
                if h.has_edits:
                    model.append((True, v.title, f + 1, v))
        if len(model) == 0:
            # there are no modified files, the viewers can be closed
            return True

        # ask the user which files should be saved
        dialog = Gtk.MessageDialog(parent=self.get_toplevel(),
                                   destroy_with_parent=True,
                                   message_type=Gtk.MessageType.WARNING,
                                   buttons=Gtk.ButtonsType.NONE,
                                   text=_('Some files have unsaved changes.  Select the files to save before closing.'))
        dialog.set_resizable(True)
        dialog.set_title(constants.APP_NAME)
        # add list of files with unsaved changes
        sw = Gtk.ScrolledWindow.new()
        sw.set_policy(Gtk.PolicyType.AUTOMATIC, Gtk.PolicyType.AUTOMATIC)
        treeview = Gtk.TreeView.new_with_model(model)
        r = Gtk.CellRendererToggle.new()
        r.connect('toggled', self.__confirmClose_toggle_cb, model)
        column = Gtk.TreeViewColumn(None, r)
        column.add_attribute(r, 'active', 0)
        treeview.append_column(column)
        r = Gtk.CellRendererText.new()
        column = Gtk.TreeViewColumn(_('Tab'), r, text=1)
        column.set_resizable(True)
        column.set_expand(True)
        column.set_sort_column_id(1)
        treeview.append_column(column)
        column = Gtk.TreeViewColumn(_('Pane'), r, text=2)
        column.set_resizable(True)
        column.set_sort_column_id(2)
        treeview.append_column(column)
        treeview.connect('row-activated', self.__confirmClose_row_activated_cb, model)
        sw.add(treeview)
        treeview.show()
        dialog.vbox.pack_start(sw, True, True, 0) # pylint: disable=no-member
        sw.show()
        # add custom set of action buttons
        dialog.add_button(Gtk.STOCK_CANCEL, Gtk.ResponseType.CANCEL)
        button = Gtk.Button.new_with_mnemonic(_('Close _Without Saving'))
        dialog.add_action_widget(button, Gtk.ResponseType.REJECT)
        button.show()
        dialog.add_button(Gtk.STOCK_SAVE, Gtk.ResponseType.OK)
        dialog.set_default_response(Gtk.ResponseType.CANCEL)
        response = dialog.run()
        dialog.destroy()
        if response == Gtk.ResponseType.OK:
            # save all checked files
            it = model.get_iter_first()
            while it:
                if model.get_value(it, 0):
                    f = model.get_value(it, 2) - 1
                    v = model.get_value(it, 3)
                    if not v.save_file(f):
                        # cancel if we failed to save a file
                        return False
                it = model.iter_next(it)
            return True
        # cancel if the user did not choose 'Close Without Saving' or 'Save'
        return response == Gtk.ResponseType.REJECT

    # callback for the close button on each tab
    def remove_tab_cb(self, widget, data):
        nb = self.notebook
        if nb.get_n_pages() > 1:
            # warn about losing unsaved changes before removing a tab
            if self.confirmCloseViewers([ data ]):
                self.closed_tabs.append((nb.page_num(data), data, nb.get_tab_label(data)))
                nb.remove(data)
                nb.set_show_tabs(self.prefs.getBool('tabs_always_show') or nb.get_n_pages() > 1)
        elif not self.prefs.getBool('tabs_warn_before_quit') or confirmTabClose(self.get_toplevel()):
            self.quit_cb(widget, data)

    # callback for RMB menu on notebook tabs
    def notebooktab_pick_cb(self, widget, data):
        self.notebook.set_current_page(data)

    # callback used when a mouse button is pressed on a notebook tab
    def notebooktab_button_press_cb(self, widget, event, data):
        if event.button == 2:
            # remove the tab on MMB
            self.remove_tab_cb(widget, data)
        elif event.button == 3:
            # create a popup to pick a tab for focus on RMB
            menu = Gtk.Menu.new()
            nb = self.notebook
            for i in range(nb.get_n_pages()):
                viewer = nb.get_nth_page(i)
                item = Gtk.MenuItem.new_with_label(nb.get_tab_label(viewer).get_text())
                item.connect('activate', self.notebooktab_pick_cb, i)
                menu.append(item)
                item.show()
                if viewer is data:
                    menu.select_item(item)
            menu.popup(None, None, None, event.button, event.time)

    # update window's title
    def updateTitle(self, viewer):
        title = self.notebook.get_tab_label(viewer).get_text()
        self.set_title(f'{title} - {constants.APP_NAME}')

    # update the message in the status bar
    def setStatus(self, s):
        sb = self.statusbar
        context = sb.get_context_id('Message')
        sb.pop(context)
        if s is None:
            s = ''
        sb.push(context, s)

    # update the label in the status bar
    def setSyntax(self, s):
        # update menu
        t = self.radio_menus.get('syntax', None)
        if t is not None:
            t = t[1]
            if s in t:
                self.menu_update_depth += 1
                t[s].set_active(True)
                self.menu_update_depth -= 1

    # callback used when switching notebook pages
    def switch_page_cb(self, widget, ptr, page_num):
        viewer = widget.get_nth_page(page_num)
        self.updateTitle(viewer)
        self.setStatus(viewer.getStatus())
        self.setSyntax(viewer.getSyntax())

    # callback used when a viewer's title changes
    def title_changed_cb(self, widget, title):
        # update the label in the notebook's tab
        self.notebook.get_tab_label(widget).set_text(title)
        if widget is self.getCurrentViewer():
            self.updateTitle(widget)

    # callback used when a viewer's status changes
    def status_changed_cb(self, widget, s):
        # update the label in the notebook's tab
        if widget is self.getCurrentViewer():
            self.setStatus(s)

    # callback used when a viewer's syntax changes
    def syntax_changed_cb(self, widget, s):
        # update the label
        if widget is self.getCurrentViewer():
            self.setSyntax(s)

    # create an empty viewer with 'n' panes
    def newFileDiffViewer(self, n):
        self.viewer_count += 1
        tabname = _('File Merge %d') % (self.viewer_count, )
        tab = NotebookTab(tabname, Gtk.STOCK_FILE)
        viewer = Diffuse.FileDiffViewer(n, self.prefs, tabname)
        tab.button.connect('clicked', self.remove_tab_cb, viewer)
        tab.connect('button-press-event', self.notebooktab_button_press_cb, viewer)
        self.notebook.append_page(viewer, tab)
        if hasattr(self.notebook, 'set_tab_reorderable'):
            # some PyGTK packages incorrectly omit this method
            self.notebook.set_tab_reorderable(viewer, True)
        tab.show()
        viewer.show()
        self.notebook.set_show_tabs(self.prefs.getBool('tabs_always_show') or self.notebook.get_n_pages() > 1)
        viewer.connect('title-changed', self.title_changed_cb)
        viewer.connect('status-changed', self.status_changed_cb)
        viewer.connect('syntax-changed', self.syntax_changed_cb)
        return viewer

    # create a new viewer to display 'items'
    def newLoadedFileDiffViewer(self, items):
        specs = []
        if len(items) == 0:
            for i in range(self.prefs.getInt('tabs_default_panes')):
                specs.append(FileInfo())
        elif len(items) == 1 and len(items[0][1]) == 1:
            # one file specified
            # determine which other files to compare it with
            name, data, label = items[0]
            rev, encoding = data[0]
            vcs = theVCSs.findByFilename(name, self.prefs)
            if vcs is None:
                # shift the existing file so it will be in the second pane
                specs.append(FileInfo())
                specs.append(FileInfo(name, encoding, None, None, label))
            else:
                if rev is None:
                    # no revision specified assume defaults
                    for name, rev in vcs.getFileTemplate(self.prefs, name):
                        if rev is None:
                            s = label
                        else:
                            s = None
                        specs.append(FileInfo(name, encoding, vcs, rev, s))
                else:
                    # single revision specified
                    specs.append(FileInfo(name, encoding, vcs, rev))
                    specs.append(FileInfo(name, encoding, None, None, label))
        else:
            # multiple files specified, use one pane for each file
            for name, data, label in items:
                for rev, encoding in data:
                    if rev is None:
                        vcs, s = None, label
                    else:
                        vcs, s = theVCSs.findByFilename(name, self.prefs), None
                    specs.append(FileInfo(name, encoding, vcs, rev, s))

        # open a new viewer
        viewer = self.newFileDiffViewer(max(2, len(specs)))

        # load the files
        for i, spec in enumerate(specs):
            viewer.load(i, spec)
        return viewer

    # create a new viewer for 'items'
    def createSingleTab(self, items, labels, options):
        if len(items) > 0:
            self.newLoadedFileDiffViewer(assign_file_labels(items, labels)).setOptions(options)

    # create a new viewer for each item in 'items'
    def createSeparateTabs(self, items, labels, options):
        # all tabs inherit the first tab's revision and encoding specifications
        items = [ (name, items[0][1]) for name, data in items ]
        for item in assign_file_labels(items, labels):
            self.newLoadedFileDiffViewer([ item ]).setOptions(options)

    # create a new viewer for each modified file found in 'items'
    def createCommitFileTabs(self, items, labels, options):
        new_items = []
        for item in items:
            name, data = item
            # get full path to an existing ancessor directory
            dn = os.path.abspath(name)
            while not os.path.isdir(dn):
                dn, old_dn = os.path.dirname(dn), dn
                if dn == old_dn:
                    break
            if len(new_items) == 0 or dn != new_items[-1][0]:
                new_items.append([ dn, None, [] ])
            dst = new_items[-1]
            dst[1] = data[-1][1]
            dst[2].append(name)
        for dn, encoding, names in new_items:
            vcs = theVCSs.findByFolder(dn, self.prefs)
            if vcs is not None:
                try:
                    for specs in vcs.getCommitTemplate(self.prefs, options['commit'], names):
                        viewer = self.newFileDiffViewer(len(specs))
                        for i, spec in enumerate(specs):
                            name, rev = spec
                            viewer.load(i, FileInfo(name, encoding, vcs, rev))
                        viewer.setOptions(options)
                except (IOError, OSError, WindowsError):
                    utils.logErrorAndDialog(_('Error retrieving commits for %s.') % (dn, ), self.get_toplevel())

    # create a new viewer for each modified file found in 'items'
    def createModifiedFileTabs(self, items, labels, options):
        new_items = []
        for item in items:
            name, data = item
            # get full path to an existing ancessor directory
            dn = os.path.abspath(name)
            while not os.path.isdir(dn):
                dn, old_dn = os.path.dirname(dn), dn
                if dn == old_dn:
                    break
            if len(new_items) == 0 or dn != new_items[-1][0]:
                new_items.append([ dn, None, [] ])
            dst = new_items[-1]
            dst[1] = data[-1][1]
            dst[2].append(name)
        for dn, encoding, names in new_items:
            vcs = theVCSs.findByFolder(dn, self.prefs)
            if vcs is not None:
                try:
                    for specs in vcs.getFolderTemplate(self.prefs, names):
                        viewer = self.newFileDiffViewer(len(specs))
                        for i, spec in enumerate(specs):
                            name, rev = spec
                            viewer.load(i, FileInfo(name, encoding, vcs, rev))
                        viewer.setOptions(options)
                except (IOError, OSError, WindowsError):
                    utils.logErrorAndDialog(_('Error retrieving modifications for %s.') % (dn, ), self.get_toplevel())

    # close all tabs without differences
    def closeOnSame(self):
        for i in range(self.notebook.get_n_pages() - 1, -1, -1):
            if not self.notebook.get_nth_page(i).hasDifferences():
                self.notebook.remove_page(i)

    # returns True if the application can safely quit
    def confirmQuit(self):
        nb = self.notebook
        return self.confirmCloseViewers([ nb.get_nth_page(i) for i in range(nb.get_n_pages()) ])

    # respond to close window request from the window manager
    def delete_cb(self, widget, event):
        if self.confirmQuit():
            Gtk.main_quit()
            return False
        return True

    # returns the currently focused viewer
    def getCurrentViewer(self):
        return self.notebook.get_nth_page(self.notebook.get_current_page())

    # callback for the open file menu item
    def open_file_cb(self, widget, data):
        self.getCurrentViewer().open_file_cb(widget, data)

    # callback for the open file menu item
    def open_file_in_new_tab_cb(self, widget, data):
        dialog = FileChooserDialog(_('Open File In New Tab'), self.get_toplevel(), self.prefs, Gtk.FileChooserAction.OPEN, Gtk.STOCK_OPEN, True)
        dialog.set_default_response(Gtk.ResponseType.OK)
        accept = (dialog.run() == Gtk.ResponseType.OK)
        name, encoding = dialog.get_filename(), dialog.get_encoding()
        rev = dialog.get_revision().strip()
        if rev == '':
            rev = None
        dialog.destroy()
        if accept:
            viewer = self.newLoadedFileDiffViewer([ (name, [ (rev, encoding) ], None) ])
            self.notebook.set_current_page(self.notebook.get_n_pages() - 1)
            viewer.grab_focus()

    # callback for the open modified files menu item
    def open_modified_files_cb(self, widget, data):
        parent = self.get_toplevel()
        dialog = FileChooserDialog(_('Choose Folder With Modified Files'), parent, self.prefs, Gtk.FileChooserAction.SELECT_FOLDER, Gtk.STOCK_OPEN)
        dialog.set_default_response(Gtk.ResponseType.OK)
        accept = (dialog.run() == Gtk.ResponseType.OK)
        name, encoding = dialog.get_filename(), dialog.get_encoding()
        dialog.destroy()
        if accept:
            n = self.notebook.get_n_pages()
            self.createModifiedFileTabs([ (name, [ (None, encoding) ]) ], [], {})
            if self.notebook.get_n_pages() > n:
                # we added some new tabs, focus on the first one
                self.notebook.set_current_page(n)
                self.getCurrentViewer().grab_focus()
            else:
                utils.logErrorAndDialog(_('No modified files found.'), parent)

    # callback for the open commit menu item
    def open_commit_cb(self, widget, data):
        parent = self.get_toplevel()
        dialog = FileChooserDialog(_('Choose Folder With Commit'), parent, self.prefs, Gtk.FileChooserAction.SELECT_FOLDER, Gtk.STOCK_OPEN, True)
        dialog.set_default_response(Gtk.ResponseType.OK)
        accept = (dialog.run() == Gtk.ResponseType.OK)
        name, rev, encoding = dialog.get_filename(), dialog.get_revision(), dialog.get_encoding()
        dialog.destroy()
        if accept:
            n = self.notebook.get_n_pages()
            self.createCommitFileTabs([ (name, [ (None, encoding) ]) ], [], { 'commit': rev })
            if self.notebook.get_n_pages() > n:
                # we added some new tabs, focus on the first one
                self.notebook.set_current_page(n)
                self.getCurrentViewer().grab_focus()
            else:
                utils.logErrorAndDialog(_('No committed files found.'), parent)

    # callback for the reload file menu item
    def reload_file_cb(self, widget, data):
        self.getCurrentViewer().reload_file_cb(widget, data)

    # callback for the save file menu item
    def save_file_cb(self, widget, data):
        self.getCurrentViewer().save_file_cb(widget, data)

    # callback for the save file as menu item
    def save_file_as_cb(self, widget, data):
        self.getCurrentViewer().save_file_as_cb(widget, data)

    # callback for the save all menu item
    def save_all_cb(self, widget, data):
        for i in range(self.notebook.get_n_pages()):
            self.notebook.get_nth_page(i).save_all_cb(widget, data)

    # callback for the new 2-way file merge menu item
    def new_2_way_file_merge_cb(self, widget, data):
        viewer = self.newFileDiffViewer(2)
        self.notebook.set_current_page(self.notebook.get_n_pages() - 1)
        viewer.grab_focus()

    # callback for the new 3-way file merge menu item
    def new_3_way_file_merge_cb(self, widget, data):
        viewer = self.newFileDiffViewer(3)
        self.notebook.set_current_page(self.notebook.get_n_pages() - 1)
        viewer.grab_focus()

    # callback for the new n-way file merge menu item
    def new_n_way_file_merge_cb(self, widget, data):
        parent = self.get_toplevel()
        dialog = NumericDialog(parent, _('New N-Way File Merge...'), _('Number of panes: '), 4, 2, 16)
        okay = (dialog.run() == Gtk.ResponseType.ACCEPT)
        npanes = dialog.button.get_value_as_int()
        dialog.destroy()
        if okay:
            viewer = self.newFileDiffViewer(npanes)
            self.notebook.set_current_page(self.notebook.get_n_pages() - 1)
            viewer.grab_focus()

    # callback for the close tab menu item
    def close_tab_cb(self, widget, data):
        self.remove_tab_cb(widget, self.notebook.get_nth_page(self.notebook.get_current_page()))

    # callback for the undo close tab menu item
    def undo_close_tab_cb(self, widget, data):
        if len(self.closed_tabs) > 0:
            i, tab, tab_label = self.closed_tabs.pop()
            self.notebook.insert_page(tab, tab_label, i)
            self.notebook.set_current_page(i)
            self.notebook.set_show_tabs(True)

    # callback for the quit menu item
    def quit_cb(self, widget, data):
        if self.confirmQuit():
            Gtk.main_quit()

    # request search parameters if force=True and then perform a search in the
    # current viewer pane
    def find(self, force, reverse):
        viewer = self.getCurrentViewer()
        if force or self.search_pattern is None:
            # construct search dialog
            history = self.search_history
            pattern = viewer.getSelectedText()
            for c in '\r\n':
                i = pattern.find(c)
                if i >= 0:
                    pattern = pattern[:i]
            dialog = SearchDialog(self.get_toplevel(), pattern, history)
            dialog.match_case_button.set_active(self.bool_state['search_matchcase'])
            dialog.backwards_button.set_active(self.bool_state['search_backwards'])
            keep = (dialog.run() == Gtk.ResponseType.ACCEPT)
            # persist the search options
            pattern = dialog.entry.get_text()
            match_case = dialog.match_case_button.get_active()
            backwards = dialog.backwards_button.get_active()
            dialog.destroy()
            if not keep or pattern == '':
                return
            # perform the search
            self.search_pattern = pattern
            if pattern in history:
                del history[history.index(pattern)]
            history.insert(0, pattern)
            self.bool_state['search_matchcase'] = match_case
            self.bool_state['search_backwards'] = backwards

        # determine where to start searching from
        reverse ^= self.bool_state['search_backwards']
        from_start, more = False, True
        while more:
            if viewer.find(self.search_pattern, self.bool_state['search_matchcase'], reverse, from_start):
                break

            if reverse:
                msg = _('Phrase not found.  Continue from the end of the file?')
            else:
                msg = _('Phrase not found.  Continue from the start of the file?')
            dialog = utils.MessageDialog(self.get_toplevel(), Gtk.MessageType.QUESTION, msg)
            dialog.set_default_response(Gtk.ResponseType.OK)
            more = (dialog.run() == Gtk.ResponseType.OK)
            dialog.destroy()
            from_start = True

    # callback for the find menu item
    def find_cb(self, widget, data):
        self.find(True, False)

    # callback for the find next menu item
    def find_next_cb(self, widget, data):
        self.find(False, False)

    # callback for the find previous menu item
    def find_previous_cb(self, widget, data):
        self.find(False, True)

    # callback for the go to line menu item
    def go_to_line_cb(self, widget, data):
        self.getCurrentViewer().go_to_line_cb(widget, data)

    # notify all viewers of changes to the preferences
    def preferences_updated(self):
        n = self.notebook.get_n_pages()
        self.notebook.set_show_tabs(self.prefs.getBool('tabs_always_show') or n > 1)
        for i in range(n):
            self.notebook.get_nth_page(i).prefsUpdated()

    # callback for the preferences menu item
    def preferences_cb(self, widget, data):
        if self.prefs.runDialog(self.get_toplevel()):
            self.preferences_updated()

    # callback for all of the syntax highlighting menu items
    def syntax_cb(self, widget, data):
        # ignore events while we update the menu when switching tabs
        # also ignore notification of the newly disabled item
        if self.menu_update_depth == 0 and widget.get_active():
            self.getCurrentViewer().setSyntax(data)

    # callback for the first tab menu item
    def first_tab_cb(self, widget, data):
        self.notebook.set_current_page(0)

    # callback for the previous tab menu item
    def previous_tab_cb(self, widget, data):
        i, n = self.notebook.get_current_page(), self.notebook.get_n_pages()
        self.notebook.set_current_page((n + i - 1) % n)

    # callback for the next tab menu item
    def next_tab_cb(self, widget, data):
        i, n = self.notebook.get_current_page(), self.notebook.get_n_pages()
        self.notebook.set_current_page((i + 1) % n)

    # callback for the last tab menu item
    def last_tab_cb(self, widget, data):
        self.notebook.set_current_page(self.notebook.get_n_pages() - 1)

    # callback for most menu items and buttons
    def button_cb(self, widget, data):
        self.getCurrentViewer().button_cb(widget, data)

    # display help documentation
    def help_contents_cb(self, widget, data):
        help_url = None
        if utils.isWindows():
            # help documentation is distributed as local HTML files
            # search for localised manual first
            parts = [ 'manual' ]
            if utils.lang is not None:
                parts = [ 'manual' ]
                parts.extend(utils.lang.split('_'))
            while len(parts) > 0:
                help_file = os.path.join(utils.bin_dir, '_'.join(parts) + '.html')
                if os.path.isfile(help_file):
                    # we found a help file
                    help_url = path2url(help_file)
                    break
                del parts[-1]
        else:
            # verify gnome-help is available
            browser = None
            p = os.environ.get('PATH', None)
            if p is not None:
                for s in p.split(os.pathsep):
                    fp = os.path.join(s, 'gnome-help')
                    if os.path.isfile(fp):
                        browser = fp
                        break
            if browser is not None:
                # find localised help file
                if utils.lang is None:
                    parts = []
                else:
                    parts = utils.lang.split('_')
                s = os.path.abspath(os.path.join(utils.bin_dir, '../share/gnome/help/diffuse'))
                while True:
                    if len(parts) > 0:
                        d = '_'.join(parts)
                    else:
                        # fall back to using 'C'
                        d = 'C'
                    help_file = os.path.join(os.path.join(s, d), 'diffuse.xml')
                    if os.path.isfile(help_file):
                        args = [ browser, path2url(help_file, 'ghelp') ]
                        # spawnvp is not available on some systems, use spawnv instead
                        os.spawnv(os.P_NOWAIT, args[0], args)
                        return
                    if len(parts) == 0:
                        break
                    del parts[-1]
        if help_url is None:
            # no local help file is available, show on-line help
            help_url = constants.WEBSITE + 'manual.html'
            # ask for localised manual
            if utils.lang is not None:
                help_url += '?lang=' + utils.lang
        # use a web browser to display the help documentation
        webbrowser.open(help_url)

    # callback for the about menu item
    def about_cb(self, widget, data):
        dialog = AboutDialog()
        dialog.run()
        dialog.destroy()

GObject.signal_new('title-changed', Diffuse.FileDiffViewer, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, (str, ))
GObject.signal_new('status-changed', Diffuse.FileDiffViewer, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, (str, ))
GObject.signal_new('title-changed', Diffuse.FileDiffViewer.PaneHeader, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, ())
GObject.signal_new('open', Diffuse.FileDiffViewer.PaneHeader, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, ())
GObject.signal_new('reload', Diffuse.FileDiffViewer.PaneHeader, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, ())
GObject.signal_new('save', Diffuse.FileDiffViewer.PaneHeader, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, ())
GObject.signal_new('save-as', Diffuse.FileDiffViewer.PaneHeader, GObject.SignalFlags.RUN_LAST, GObject.TYPE_NONE, ())

def main():
    # app = Application()
    # return app.run(sys.argv)

    args = sys.argv
    argc = len(args)

    if argc == 2 and args[1] in [ '-v', '--version' ]:
        print('%s %s\n%s' % (constants.APP_NAME, constants.VERSION, constants.COPYRIGHT))
        return 0

    if argc == 2 and args[1] in [ '-h', '-?', '--help' ]:
        print(_('''Usage:
    diffuse [ [OPTION...] [FILE...] ]...
    diffuse ( -h | -? | --help | -v | --version )

Diffuse is a graphical tool for merging and comparing text files.  Diffuse is
able to compare an arbitrary number of files side-by-side and gives users the
ability to manually adjust line matching and directly edit files.  Diffuse can
also retrieve revisions of files from Bazaar, CVS, Darcs, Git, Mercurial,
Monotone, RCS, Subversion, and SVK repositories for comparison and merging.

Help Options:
  ( -h | -? | --help )             Display this usage information
  ( -v | --version )               Display version and copyright information

Configuration Options:
  --no-rcfile                      Do not read any resource files
  --rcfile <file>                  Specify explicit resource file

General Options:
  ( -c | --commit ) <rev>          File revisions <rev-1> and <rev>
  ( -D | --close-if-same )         Close all tabs with no differences
  ( -e | --encoding ) <codec>      Use <codec> to read and write files
  ( -L | --label ) <label>         Display <label> instead of the file name
  ( -m | --modified )              Create a new tab for each modified file
  ( -r | --revision ) <rev>        File revision <rev>
  ( -s | --separate )              Create a new tab for each file
  ( -t | --tab )                   Start a new tab
  ( -V | --vcs ) <vcs-list>        Version control system search order
  --line <line>                    Start with line <line> selected
  --null-file                      Create a blank file comparison pane

Display Options:
  ( -b | --ignore-space-change )   Ignore changes to white space
  ( -B | --ignore-blank-lines )    Ignore changes in blank lines
  ( -E | --ignore-end-of-line )    Ignore end of line differences
  ( -i | --ignore-case )           Ignore case differences
  ( -w | --ignore-all-space )      Ignore white space differences'''))
        return 0

    # find the config directory and create it if it didn't exist
    rc_dir = os.environ.get('XDG_CONFIG_HOME', None)
    subdirs = ['diffuse']
    if rc_dir is None:
        rc_dir = os.path.expanduser('~')
        subdirs.insert(0, '.config')
    rc_dir = utils.make_subdirs(rc_dir, subdirs)

    # find the local data directory and create it if it didn't exist
    data_dir = os.environ.get('XDG_DATA_HOME', None)
    subdirs = ['diffuse']
    if data_dir is None:
        data_dir = os.path.expanduser('~')
        subdirs[:0] = [ '.local', 'share' ]
    data_dir = utils.make_subdirs(data_dir, subdirs)

    # load resource files
    i = 1
    rc_files = []
    if i < argc  and args[i] == '--no-rcfile':
        i += 1
    elif i + 1 < argc and args[i] == '--rcfile':
        i += 1
        rc_files.append(args[i])
        i += 1
    else:
        # parse system wide then personal initialisation files
        if utils.isWindows():
            rc_file = os.path.join(utils.bin_dir, 'diffuserc')
        else:
            rc_file = os.path.join(utils.bin_dir, f'{constants.sysconfigdir}/diffuserc')
        for rc_file in rc_file, os.path.join(rc_dir, 'diffuserc'):
            if os.path.isfile(rc_file):
                rc_files.append(rc_file)
    for rc_file in rc_files:
        # convert to absolute path so the location of any processing errors are
        # reported with normalised file names
        rc_file = os.path.abspath(rc_file)
        try:
            # diffuse.theResources.parse(rc_file) # Modularization
            theResources.parse(rc_file)
        except IOError:
            utils.logError(_('Error reading %s.') % (rc_file, ))

    # diff = diffuse.Diffuse(rc_dir) # Modularization
    diff = Diffuse(rc_dir)

    # load state
    statepath = os.path.join(data_dir, 'state')
    diff.loadState(statepath)

    # process remaining command line arguments
    encoding = None
    revs = []
    close_on_same = False
    specs = []
    had_specs = False
    labels = []
    funcs = {
        'modified': diff.createModifiedFileTabs,
        'commit': diff.createCommitFileTabs,
        'separate': diff.createSeparateTabs,
        'single': diff.createSingleTab
    }
    mode = 'single'
    options = {}
    while i < argc:
        arg = args[i]
        if len(arg) > 0 and arg[0] == '-':
            if i + 1 < argc and arg in [ '-c', '--commit' ]:
                # specified revision
                funcs[mode](specs, labels, options)
                i += 1
                rev = args[i]
                specs, labels, options = [], [], { 'commit': args[i] }
                mode = 'commit'
            elif arg in [ '-D', '--close-if-same' ]:
                close_on_same = True
            elif i + 1 < argc and arg in [ '-e', '--encoding' ]:
                i += 1
                encoding = args[i]
                encoding = encodings.aliases.aliases.get(encoding, encoding)
            elif arg in [ '-m', '--modified' ]:
                funcs[mode](specs, labels, options)
                specs, labels, options = [], [], {}
                mode = 'modified'
            elif i + 1 < argc and arg in [ '-r', '--revision' ]:
                # specified revision
                i += 1
                revs.append((args[i], encoding))
            elif arg in [ '-s', '--separate' ]:
                funcs[mode](specs, labels, options)
                specs, labels, options = [], [], {}
                # open items in separate tabs
                mode = 'separate'
            elif arg in [ '-t', '--tab' ]:
                funcs[mode](specs, labels, options)
                specs, labels, options = [], [], {}
                # start a new tab
                mode = 'single'
            elif i + 1 < argc and arg in [ '-V', '--vcs' ]:
                i += 1
                diff.prefs.setString('vcs_search_order', args[i])
                diff.preferences_updated()
            elif arg in [ '-b', '--ignore-space-change' ]:
                diff.prefs.setBool('display_ignore_whitespace_changes', True)
                diff.prefs.setBool('align_ignore_whitespace_changes', True)
                diff.preferences_updated()
            elif arg in [ '-B', '--ignore-blank-lines' ]:
                diff.prefs.setBool('display_ignore_blanklines', True)
                diff.prefs.setBool('align_ignore_blanklines', True)
                diff.preferences_updated()
            elif arg in [ '-E', '--ignore-end-of-line' ]:
                diff.prefs.setBool('display_ignore_endofline', True)
                diff.prefs.setBool('align_ignore_endofline', True)
                diff.preferences_updated()
            elif arg in [ '-i', '--ignore-case' ]:
                diff.prefs.setBool('display_ignore_case', True)
                diff.prefs.setBool('align_ignore_case', True)
                diff.preferences_updated()
            elif arg in [ '-w', '--ignore-all-space' ]:
                diff.prefs.setBool('display_ignore_whitespace', True)
                diff.prefs.setBool('align_ignore_whitespace', True)
                diff.preferences_updated()
            elif i + 1 < argc and arg == '-L':
                i += 1
                labels.append(args[i])
            elif i + 1 < argc and arg == '--line':
                i += 1
                try:
                    options['line'] = int(args[i])
                except ValueError:
                    utils.logError(_('Error parsing line number.'))
            elif arg == '--null-file':
                # add a blank file pane
                if mode == 'single' or mode == 'separate':
                    if len(revs) == 0:
                        revs.append((None, encoding))
                    specs.append((None, revs))
                    revs = []
                had_specs = True
            else:
                utils.logError(_('Skipping unknown argument "%s".') % (args[i], ))
        else:
            filename = diff.prefs.convertToNativePath(args[i])
            if (mode == 'single' or mode == 'separate') and os.path.isdir(filename):
                if len(specs) > 0:
                    filename = os.path.join(filename, os.path.basename(specs[-1][0]))
                else:
                    utils.logError(_('Error processing argument "%s".  Directory not expected.') % (args[i], ))
                    filename = None
            if filename is not None:
                if len(revs) == 0:
                    revs.append((None, encoding))
                specs.append((filename, revs))
                revs = []
            had_specs = True
        i += 1
    if mode in [ 'modified', 'commit' ] and len(specs) == 0:
        specs.append((os.curdir, [ (None, encoding) ]))
        had_specs = True
    funcs[mode](specs, labels, options)

    # create a file diff viewer if the command line arguments haven't
    # implicitly created any
    if not had_specs:
        diff.newLoadedFileDiffViewer([])
    elif close_on_same:
        diff.closeOnSame()
    nb = diff.notebook
    n = nb.get_n_pages()
    if n > 0:
        nb.set_show_tabs(diff.prefs.getBool('tabs_always_show') or n > 1)
        nb.get_nth_page(0).grab_focus()
        diff.show()
        Gtk.main()
        # save state
        diff.saveState(statepath)

    return 0