init.lua

-- Copyright 2015-2024 Mitchell. See LICENSE.

--- Two-way file comparison for Textadept.
--
-- Install this module by copying it into your *~/.textadept/modules/* directory or Textadept's
-- *modules/* directory, and then putting the following in your *~/.textadept/init.lua*:
--
--	require('file_diff')
--
-- ### Compiling
--
-- Releases include binaries, so building this modules should not be necessary. If you want
-- to build manually, use CMake. For example:
--
--	cmake -S . -B build_dir
--	cmake --build build_dir --target diff
--	cmake --install build_dir
--
-- ### Usage
--
-- A sample workflow is this:
--
-- 1. Start comparing two files via the "Compare Files" submenu in the "Tools" menu.
-- 2. The caret is initially placed in the file on the left.
-- 3. Go to the next change via menu or key binding.
-- 4. Merge the change from the other buffer into the current one (right to left) via menu or
--	key binding.
-- 5. Go to the next change via menu or key binding.
-- 6. Merge the change from the current buffer into the other one (left to right) via menu or
--	key binding.
-- 7. Repeat as necessary.
--
-- Note: merging can be performed wherever the caret is placed when jumping between changes,
-- even if one buffer has a change and the other does not (additions or deletions).
--
-- ### Key Bindings
--
-- Windows and Linux | macOS | Terminal | Command
-- -|-|-|-
-- **Tools**| | |
-- F6 | F6 | None | Compare files...
-- Shift+F6 | ⇧F6 | None | Compare the buffers in two split views
-- Ctrl+F6 | ⌘F6 | None | Stop comparing
-- Ctrl+Alt+. | ^⌘. | None | Goto next difference
-- Ctrl+Alt+, | ^⌘, | None | Goto previous difference
-- Ctrl+Alt+< | ^⌘< | None | Merge left
-- Ctrl+Alt+> | ^⌘> | None | Merge right
-- @module file_diff
local M = {}

--- The marker for line additions.
M.MARK_ADDITION = view.new_marker_number()
--- The marker for line deletions.
M.MARK_DELETION = view.new_marker_number()
--- The marker for line modifications.
M.MARK_MODIFICATION = view.new_marker_number()
--- The indicator number for text added within lines.
M.INDIC_ADDITION = view.new_indic_number()
--- The indicator number for text deleted within lines.
M.INDIC_DELETION = view.new_indic_number()
local MARK_ADDITION = M.MARK_ADDITION
local MARK_DELETION = M.MARK_DELETION
local MARK_MODIFICATION = M.MARK_MODIFICATION
local INDIC_ADDITION = M.INDIC_ADDITION
local INDIC_DELETION = M.INDIC_DELETION
--- The name of the theme color used to mark additions.
-- The default value is 'green'. If your theme does not define that color, set this field to
-- your theme's equivalent.
M.addition_color_name = 'green'
--- The name of the theme color used to mark deletions.
-- The default value is 'red'. If your theme does not define that color, set this field to your
-- theme's equivalent.
M.deletion_color_name = 'red'
--- The name of the theme color used to mark modifications.
-- The default value is 'yellow'. If your theme does not define that color, set this field to
-- your theme's equivalent.
M.modification_color_name = 'yellow'

-- Localizations.
local _L = _L
if not rawget(_L, 'Compare Files') then
	-- Dialogs.
	_L['Select the first file to compare'] = 'Select the first file to compare'
	_L['Select the file to compare to'] = 'Select the file to compare to'
	-- Status.
	_L['No more differences'] = 'No more differences'
	-- Menu.
	_L['Compare Files'] = 'Compare _Files'
	_L['Compare Files...'] = '_Compare Files...'
	_L['Compare This File With...'] = 'Compare This File _With...'
	_L['Compare Buffers'] = 'Compare _Buffers'
	_L['Next Change'] = '_Next Change'
	_L['Previous Change'] = '_Previous Change'
	_L['Merge Left'] = 'Merge _Left'
	_L['Merge Right'] = 'Merge _Right'
	_L['Stop Comparing'] = '_Stop Comparing'
end

local lib = 'file_diff.diff'
if OSX then lib = lib .. 'osx' end
local diff = require(lib)
local DELETE, INSERT = 0, 1 -- C++: "enum Operation {DELETE, INSERT, EQUAL};"

local view1, view2

--- Clear markers, indicators, and placeholder lines.
-- Used when re-marking changes or finished comparing.
local function clear_marked_changes()
	local buffer1 = _VIEWS[view1] and view1.buffer
	local buffer2 = _VIEWS[view2] and view2.buffer
	for _, mark in ipairs{MARK_ADDITION, MARK_DELETION, MARK_MODIFICATION} do
		if buffer1 then buffer1:marker_delete_all(mark) end
		if buffer2 then buffer2:marker_delete_all(mark) end
	end
	for _, indic in ipairs{INDIC_ADDITION, INDIC_DELETION} do
		if buffer1 then
			buffer1.indicator_current = indic
			buffer1:indicator_clear_range(1, buffer1.length)
		end
		if buffer2 then
			buffer2.indicator_current = indic
			buffer2:indicator_clear_range(1, buffer2.length)
		end
	end
	if buffer1 then buffer1:annotation_clear_all() end
	if buffer2 then buffer2:annotation_clear_all() end
end

local synchronizing = false
--- Synchronize the scroll and line position of the other buffer.
local function synchronize()
	synchronizing = true
	local line = buffer:line_from_position(buffer.current_pos)
	local visible_line = view:visible_from_doc_line(line)
	local first_visible_line = view.first_visible_line
	local x_offset = view.x_offset
	ui.goto_view(view == view1 and view2 or view1)
	buffer:goto_line(view:doc_line_from_visible(visible_line))
	view.first_visible_line, view.x_offset = first_visible_line, x_offset
	ui.goto_view(view == view2 and view1 or view2)
	synchronizing = false
end

--- Returns the number of lines contained in the given string.
local function count_lines(text)
	local lines = 1
	for _ in text:gmatch('\n') do lines = lines + 1 end
	return lines
end

--- Mark the differences between the two buffers.
local function mark_changes()
	if not _VIEWS[view1] or not _VIEWS[view2] then return end
	clear_marked_changes() -- clear previous marks
	local buffer1, buffer2 = view1.buffer, view2.buffer
	-- Perform the diff.
	local diffs = diff(buffer1:get_text(), buffer2:get_text())
	-- Parse the diff, marking modified lines and changed text.
	local pos1, pos2 = 1, 1
	for i = 1, #diffs, 2 do
		local op, text = diffs[i], diffs[i + 1]
		local text_len = #text
		if op == DELETE then
			local next_op, next_text = diffs[i + 2], diffs[i + 3]
			-- Mark partial lines as modified and full lines as deleted.
			local start_line = buffer1:line_from_position(pos1)
			local end_line = buffer1:line_from_position(pos1 + text_len)
			local mark = MARK_MODIFICATION -- assume partial initially
			if next_op ~= INSERT then
				-- Deleting full line(s), either from line start to next line(s) start, or from line
				-- end to next line(s) end. Adjust `start_line` and `end_line` accordingly for accurate
				-- line markers.
				if pos1 == buffer1:position_from_line(start_line) and
					(pos1 + text_len == buffer1:position_from_line(end_line)) then
					mark = MARK_DELETION
					end_line = end_line - 1
				elseif pos1 == buffer1.line_end_position[start_line] and
					(pos1 + text_len == buffer1.line_end_position[end_line]) then
					mark = MARK_DELETION
					start_line = start_line + 1
				end
			end
			for j = start_line, end_line do buffer1:marker_add(j, mark) end
			-- Highlight deletion from partially changed line(s) and mark other line as modified.
			if mark == MARK_MODIFICATION then
				buffer1.indicator_current = INDIC_DELETION
				buffer1:indicator_fill_range(pos1, text_len)
				buffer2:marker_add(buffer2:line_from_position(pos2), mark)
			end
			pos1 = pos1 + text_len
			-- Calculate net change in lines and fill in empty space in either buffer with annotations
			-- if necessary.
			if next_op == INSERT then
				-- Deleting line(s) in favor of other lines. If those other lines are more numerous,
				-- then fill empty space in this buffer.
				local num_lines = count_lines(text)
				local next_num_lines = count_lines(next_text)
				if num_lines < next_num_lines then
					local annotation_lines = next_num_lines - num_lines - 1
					local annotation_text = ' ' ..
						string.rep('\n', annotation_lines + buffer1.annotation_lines[end_line])
					buffer1.annotation_text[end_line] = annotation_text
				end
			elseif mark == MARK_DELETION then
				-- Deleting full line(s) with no replacement, so fill empty space in other buffer.
				local offset = not text:find('^\n') and 1 or 0
				local line = math.max(buffer2:line_from_position(pos2) - offset, 1)
				local annotation_lines = end_line - start_line
				local annotation_text = ' ' .. string.rep('\n', annotation_lines)
				buffer2.annotation_text[line] = annotation_text
			else
				-- Deleting partial line(s) with no replacement, so fill empty space in other buffer.
				local extra_lines = count_lines(text) - 1
				if extra_lines > 0 then
					local line = buffer2:line_from_position(pos2)
					local annotation_lines = extra_lines - 1
					local annotation_text = ' ' .. string.rep('\n', annotation_lines)
					buffer2.annotation_text[line] = annotation_text
				end
			end
		elseif op == INSERT then
			local prev_op, prev_text = diffs[i - 2], diffs[i - 1]
			-- Mark partial lines as modified and full lines as deleted.
			local start_line = buffer2:line_from_position(pos2)
			local end_line = buffer2:line_from_position(pos2 + text_len)
			local mark = MARK_MODIFICATION -- assume partial initially
			if prev_op ~= DELETE then
				-- Adding full line(s), either from line start to next line(s) start, or from line end
				-- to next line(s) end. Adjust `start_line` and `end_line` accordingly for accurate
				-- line markers.
				if pos2 == buffer2:position_from_line(start_line) and
					(pos2 + text_len == buffer2:position_from_line(end_line)) then
					mark = MARK_ADDITION
					end_line = end_line - 1
				elseif pos2 == buffer2.line_end_position[start_line] and
					(pos2 + text_len == buffer2.line_end_position[end_line]) then
					mark = MARK_ADDITION
					start_line = start_line + 1
				end
			end
			for j = start_line, end_line do buffer2:marker_add(j, mark) end
			-- Highlight addition from partially changed line(s) and mark other line as modified.
			if mark == MARK_MODIFICATION then
				buffer2.indicator_current = INDIC_ADDITION
				buffer2:indicator_fill_range(pos2, text_len)
				buffer1:marker_add(buffer1:line_from_position(pos1), mark)
			end
			pos2 = pos2 + text_len
			-- Calculate net change in lines and fill in empty space in either buffer with annotations
			-- if necessary.
			if prev_op == DELETE then
				-- Adding line(s) in favor of other lines. If those other lines are more numerous,
				-- then fill empty space in this buffer.
				local num_lines = count_lines(text)
				local prev_num_lines = count_lines(prev_text)
				if num_lines < prev_num_lines then
					local annotation_lines = prev_num_lines - num_lines - 1
					local annotation_text = ' ' ..
						string.rep('\n', annotation_lines + buffer2.annotation_lines[end_line])
					buffer2.annotation_text[end_line] = annotation_text
				end
			elseif mark == MARK_ADDITION then
				-- Adding full line(s) with no replacement, so fill empty space in other buffer.
				local offset = not text:find('^\n') and 1 or 0
				local line = math.max(buffer1:line_from_position(pos1) - offset, 1)
				local annotation_lines = end_line - start_line
				local annotation_text = ' ' .. string.rep('\n', annotation_lines)
				buffer1.annotation_text[line] = annotation_text
			else
				-- Adding partial line(s) with no replacement, so fill empty space in other buffer.
				local extra_lines = count_lines(text) - 1
				if extra_lines > 0 then
					local line = buffer1:line_from_position(pos1)
					local annotation_lines = extra_lines - 1
					local annotation_text = ' ' .. string.rep('\n', annotation_lines)
					buffer1.annotation_text[line] = annotation_text
				end
			end
		else
			pos1, pos2 = pos1 + text_len, pos2 + text_len
		end
	end
	synchronize()
end

local starting_diff = false

--- Highlight differences between files *file1* and *file2*, or the user-selected files.
-- @param file1 Optional name of the older file. If `-`, uses the current buffer. If `nil`,
--	the user is prompted for a file.
-- @param file2 Optional name of the newer file. If `-`, uses the current buffer. If `nil`,
--	the user is prompted for a file.
-- @param horizontal Optional flag specifying whether or not to split the view horizontally. The
--	default value is `false`, comparing the two files side-by-side.
function M.start(file1, file2, horizontal)
	file1 = file1 or ui.dialogs.open{
		title = _L['Select the first file to compare'],
		dir = (buffer.filename or ''):match('^(.+)[/\\]') or lfs.currentdir()
	}
	if not file1 then return end
	file2 = file2 or ui.dialogs.open{
		title = string.format('%s %s', _L['Select the file to compare to'], file1:match('[^/\\]+$')),
		dir = file1:match('^(.+)[/\\]') or lfs.currentdir()
	}
	if not file2 then return end
	starting_diff = true
	if not _VIEWS[view1] or not _VIEWS[view2] and #_VIEWS > 1 then
		view1, view2 = _VIEWS[1], _VIEWS[2] -- preserve current split views
	end
	if _VIEWS[view1] and view ~= view1 then ui.goto_view(view1) end
	if file1 ~= '-' then io.open_file(file1) end
	view.annotation_visible = view.ANNOTATION_STANDARD -- view1
	if not _VIEWS[view1] or not _VIEWS[view2] then
		view1, view2 = view:split(not horizontal)
	else
		ui.goto_view(view2)
	end
	if file2 ~= '-' then io.open_file(file2) end
	view.annotation_visible = view.ANNOTATION_STANDARD -- view2
	ui.goto_view(view1)
	starting_diff = false
	if file1 == '-' or file2 == '-' then mark_changes() end
end

--- Stops comparing.
local function stop()
	if not _VIEWS[view1] or not _VIEWS[view2] then return end
	clear_marked_changes()
	view1, view2 = nil, nil
end

-- Stop comparing when one of the buffer's being compared is switched or closed.
events.connect(events.BUFFER_BEFORE_SWITCH, function() if not starting_diff then stop() end end)
events.connect(events.BUFFER_DELETED, stop)

--- Retrieves the equivalent of line number *line* in the other buffer.
-- @param line Line to get the synchronized equivalent of in the other buffer.
-- @return line
local function get_synchronized_line(line)
	local visible_line = view:visible_from_doc_line(line)
	local pos = buffer.current_pos
	ui.goto_view(view == view1 and view2 or view1)
	line = view:doc_line_from_visible(visible_line)
	ui.goto_view(view == view2 and view1 or view2)
	buffer:set_empty_selection(pos)
	return line
end

--- Jumps to the next or previous difference between the two files depending on boolean *next*.
-- `file_diff.start()` must have been called previously.
-- @param next Whether to go to the next or previous difference relative to the current line.
function M.goto_change(next)
	if not _VIEWS[view1] or not _VIEWS[view2] then return end
	-- Determine the line to start on, keeping in mind the synchronized line numbers may be different.
	local line1, line2
	local step = next and 1 or -1
	if view == view1 then
		line1 = buffer:line_from_position(buffer.current_pos) + step
		if line1 < 1 then line1 = 1 end
		line2 = get_synchronized_line(line1)
	else
		line2 = buffer:line_from_position(buffer.current_pos) + step
		if line2 < 1 then line2 = 1 end
		line1 = get_synchronized_line(line2)
	end
	-- Search for the next change or set of changes, wrapping as necessary.
	-- A block of additions, deletions, or modifications should be treated as a single change.
	local buffer1, buffer2 = view1.buffer, view2.buffer
	local diff_marker = 1 << MARK_ADDITION - 1 | 1 << MARK_DELETION - 1 | 1 << MARK_MODIFICATION - 1
	local f = next and buffer.marker_next or buffer.marker_previous
	line1 = f(buffer1, line1, diff_marker)
	while line1 >= 1 and
		(buffer1:marker_get(line1) & diff_marker == buffer1:marker_get(line1 - step) & diff_marker) do
		line1 = f(buffer1, line1 + step, diff_marker)
	end
	line2 = f(buffer2, line2, diff_marker)
	while line2 >= 1 and
		(buffer2:marker_get(line2) & diff_marker == buffer2:marker_get(line2 - step) & diff_marker) do
		line2 = f(buffer2, line2 + step, diff_marker)
	end
	if line1 < 1 and line2 < 1 then
		line1 = f(buffer1, next and 1 or buffer1.line_count, diff_marker)
		line2 = f(buffer2, next and 1 or buffer2.line_count, diff_marker)
	end
	if line1 < 1 and line2 < 1 then
		ui.statusbar_text = _L['No more differences']
		return
	end
	-- Determine which change is closer to the current line, keeping in mind the synchronized
	-- line numbers may be different. (For example, one buffer may have a block of modifications
	-- next while the other buffer has a block of additions next, and those additions logically
	-- come first.)
	if view == view1 then
		if line2 >= 1 then
			ui.goto_view(view2)
			local visible_line = view:visible_from_doc_line(line2)
			ui.goto_view(view1)
			local line2_1 = view:doc_line_from_visible(visible_line)
			buffer:goto_line(line1 >= 1 and (next and line1 < line2_1 or not next and line1 > line2_1) and
				line1 or line2_1)
		else
			buffer:goto_line(line1)
		end
	else
		if line1 >= 1 then
			ui.goto_view(view1)
			local visible_line = view:visible_from_doc_line(line1)
			ui.goto_view(view2)
			local line1_2 = view:doc_line_from_visible(visible_line)
			buffer:goto_line(line2 >= 1 and (next and line2 < line1_2 or not next and line2 > line1_2) and
				line2 or line1_2)
		else
			buffer:goto_line(line2)
		end
	end
	view:vertical_center_caret()
end

--- Merges a change from one buffer to another, depending on the change under the caret and the
-- merge direction.
-- @param left Whether to merge from right to left or left to right.
function M.merge(left)
	if not _VIEWS[view1] or not _VIEWS[view2] then return end
	local buffer1, buffer2 = view1.buffer, view2.buffer
	-- Determine whether or not there is a change to merge.
	local start_line = buffer:line_from_position(buffer.current_pos)
	local end_line = start_line + 1
	local diff_marker = 1 << MARK_ADDITION - 1 | 1 << MARK_DELETION - 1 | 1 << MARK_MODIFICATION - 1
	local marker = buffer:marker_get(start_line) & diff_marker
	if marker == 0 then
		-- Look for additions or deletions from the other buffer, which are offset one line down
		-- (side-effect of Scintilla's visible line -> doc line conversions).
		local line = get_synchronized_line(start_line) + 1
		if (view == view1 and buffer2 or buffer1):marker_get(line) & diff_marker > 0 then
			ui.goto_view(view == view1 and view2 or view1)
			buffer:set_empty_selection(buffer:position_from_line(line))
			M.merge(left)
			ui.goto_view(view == view2 and view1 or view2)
			buffer:set_empty_selection(buffer:position_from_line(start_line))
		end
		return
	end
	-- Determine the bounds of the change target it.
	while buffer:marker_get(start_line - 1) & diff_marker == marker do start_line = start_line - 1 end
	buffer.target_start = buffer:position_from_line(start_line)
	while buffer:marker_get(end_line) & diff_marker == marker do end_line = end_line + 1 end
	buffer.target_end = buffer:position_from_line(end_line)
	-- Perform the merge, depending on context.
	if marker == 1 << MARK_ADDITION - 1 then
		if left then
			-- Merge addition from right to left.
			local line = get_synchronized_line(end_line)
			buffer1:insert_text(buffer1:position_from_line(line), buffer2.target_text)
		else
			-- Merge "deletion" (empty text) from left to right.
			buffer2:replace_target('')
		end
	elseif marker == 1 << MARK_DELETION - 1 then
		if left then
			-- Merge "addition" (empty text) from right to left.
			buffer1:replace_target('')
		else
			-- Merge deletion from left to right.
			local line = get_synchronized_line(end_line)
			buffer2:insert_text(buffer2:position_from_line(line), buffer1.target_text)
		end
	elseif marker == 1 << MARK_MODIFICATION - 1 then
		local target_text = buffer.target_text
		start_line = get_synchronized_line(start_line)
		end_line = get_synchronized_line(end_line)
		ui.goto_view(view == view1 and view2 or view1)
		buffer.target_start = buffer:position_from_line(start_line)
		buffer.target_end = buffer:position_from_line(end_line)
		if view == view2 and left or view == view1 and not left then
			-- Merge change from opposite view.
			target_text = buffer.target_text
			ui.goto_view(view == view2 and view1 or view2)
			buffer:replace_target(target_text)
		else
			-- Merge change to opposite view.
			buffer:replace_target(target_text)
			ui.goto_view(view == view2 and view1 or view2)
		end
	end
	mark_changes() -- refresh
end

-- TODO: connect to these in `start()` and disconnect in `stop()`?

-- Ensure the diff buffers are scrolled in sync.
events.connect(events.UPDATE_UI, function(updated)
	if _VIEWS[view1] and _VIEWS[view2] and updated and not synchronizing then
		if updated & (view.UPDATE_H_SCROLL | view.UPDATE_V_SCROLL | buffer.UPDATE_SELECTION) > 0 then
			synchronize()
		end
	end
end)

-- Highlight differences as text is typed and deleted.
events.connect(events.MODIFIED, function(position, modification_type)
	if not _VIEWS[view1] or not _VIEWS[view2] then return end
	if modification_type & (0x01 | 0x02) > 0 then mark_changes() end -- insert text | delete text
end)

events.connect(events.VIEW_NEW, function()
	local markers = {
		[MARK_ADDITION] = M.addition_color_name, [MARK_DELETION] = M.deletion_color_name,
		[MARK_MODIFICATION] = M.modification_color_name
	}
	for mark, color in pairs(markers) do
		view:marker_define(mark, not CURSES and view.MARK_BACKGROUND or view.MARK_FULLRECT)
		if view.colors[color] then view.marker_back[mark] = view.colors[color] end
		if not CURSES then
			view.marker_layer[mark], view.marker_alpha[mark] = view.LAYER_UNDER_TEXT, 0x60
		end
	end
	local indicators = {
		[INDIC_ADDITION] = M.addition_color_name, [INDIC_DELETION] = M.deletion_color_name
	}
	for indic, color in pairs(indicators) do
		view.indic_style[indic] = not CURSES and view.INDIC_FULLBOX or view.INDIC_STRAIGHTBOX
		if view.colors[color] then view.indic_fore[indic] = view.colors[color] end
		if not CURSES then view.indic_alpha[indic], view.indic_under[indic] = 0x60, true end
	end
end)

args.register('-d', '--diff', 2, M.start, 'Compares two files')

-- Add a menu and configure key bindings.
-- (Insert 'Compare Files' menu in alphabetical order.)
local m_tools = textadept.menu.menubar['Tools']
local found_area
for i = 1, #m_tools - 1 do
	if not found_area and m_tools[i + 1].title == _L['Bookmarks'] then
		found_area = true
	elseif found_area then
		local label = m_tools[i].title or m_tools[i][1]
		if 'Compare Files' < label:gsub('^_', '') or m_tools[i][1] == '' then
			table.insert(m_tools, i, {
				title = _L['Compare Files'], --
				{_L['Compare Files...'], M.start}, {
					_L['Compare This File With...'],
					function() if buffer.filename then M.start(buffer.filename) end end
				}, {_L['Compare Buffers'], function() M.start('-', '-') end}, --
				{''}, --
				{_L['Next Change'], function() M.goto_change(true) end},
				{_L['Previous Change'], M.goto_change}, --
				{''}, --
				{_L['Merge Left'], function() M.merge(true) end}, --
				{_L['Merge Right'], M.merge}, --
				{''}, --
				{_L['Stop Comparing'], stop}
			})
			break
		end
	end
end
if not CURSES then
	keys.f6 = M.start
	keys['shift+f6'] = m_tools[_L['Compare Files']][_L['Compare Buffers']][2]
	keys[not OSX and 'ctrl+f6' or 'cmd+f6'] = m_tools[_L['Compare Files']][_L['Stop Comparing']][2]
	keys[not OSX and 'ctrl+alt+.' or 'ctrl+cmd+.'] =
		m_tools[_L['Compare Files']][_L['Next Change']][2]
	keys[not OSX and 'ctrl+alt+,' or 'ctrl+cmd+,'] = M.goto_change
	keys[not OSX and 'ctrl+alt+<' or 'ctrl+cmd+<'] = m_tools[_L['Compare Files']][_L['Merge Left']][2]
	keys[not OSX and 'ctrl+alt+>' or 'ctrl+cmd+>'] = M.merge
end

return M

-- The function below is a Lua C function.

--- Returns a list that represents the differences between strings *text1* and *text2*.
-- Each consecutive pair of elements in the returned list represents a "diff". The first element
-- is an integer: 0 for a deletion, 1 for an insertion, and 2 for equality. The second element
-- is the associated diff text.
-- @param text1 String to compare against.
-- @param text2 String to compare.
-- @return list of differences
-- @usage diffs = diff(text1, text2)
--	for i = 1, #diffs, 2 do print(diffs[i], diffs[i + 1]) end
-- @function _G.diff