Merge Fortress Forever lang key value pairs from one file to another

merge_lang.lua

-- NOTE: 
-- the input files can't be UTF-16 with BOM encoded like the language files are;
-- they need to be converted to UTF-8 before running this script

local function file_exists(name)
   local f=io.open(name,"r")
   if f~=nil then io.close(f) return true else return false end
end

local function read_whole_file(filepath)
	if not file_exists(filepath) then return nil end
	local f = io.open(filepath, "rb")
	local content = f:read("*all")
	content = content:gsub("\r\n", "\n")
	f:close()
	return content
end

local function write_to_file(filepath, contents)
	if contents == nil then return end
	local outputfile = io.open(filepath, "w+")
	io.output(outputfile)
	io.write(contents)
	io.close(outputfile)
end

local function make_pattern_plain(str)
	return str:gsub("[%(%)%.%%%+%-%*%?%[%]%^%$]", function(c) return "%" .. c end)
end

local function make_pattern_plain(str)
	return str:gsub("[%(%)%.%%%+%-%*%?%[%]%^%$]", function(c) return "%" .. c end)
end

local function make_replacement_safe(str)
	return str:gsub("%%", "%%%%")
end

local function plain_gsub(haystack, needle, repl, n)
	needle = make_pattern_plain(needle)
	repl = make_replacement_safe(repl)
	return string.gsub(haystack, needle, repl, n)
end

local function safe_gsub(haystack, needle, repl, n)
	repl = make_replacement_safe(repl)
	return string.gsub(haystack, needle, repl, n)
end

local function parse_lang_file(filepath)
	local contents = read_whole_file(filepath)
	local parsed = {}
	if contents then
		for indent, key, spacing, str in contents:gmatch("(%s*)\"([^\"]+)\"(%s+)\"([^\"]+)\"") do
			parsed[key] = str
		end
	end
	return parsed
end

local function merge_lang_file(from, to, output)
	local parsed_from = parse_lang_file(from)
	local output_lines = {}
	local key_merged_status = {}
	for key, str in pairs(parsed_from) do
		key_merged_status[key] = false
	end
	if file_exists(to) then
		for line in io.lines(to) do
			local key = line:match("%s*\"([^\"]+)\"%s+\"[^\"]+\"")
			if key then
				if parsed_from[key] ~= nil then
					line = line:gsub("(%s*\""..make_pattern_plain(key).."\"%s+)\"([^\"]+)\"", "%1\""..make_replacement_safe(parsed_from[key]).."\"")
					key_merged_status[key] = true
				else
					print("Added key: " .. key)
				end
			end
			table.insert(output_lines, line)
		end
		for key, merged in pairs(key_merged_status) do
			if not merged then
				print("Removed key: " .. key .. " ("..parsed_from[key]..")")
			end
		end
	end
	return write_to_file(output, table.concat(output_lines, "\n"))
end

local from = "from.txt"
local to = "to.txt"
local output = "merged.txt"
merge_lang_file(from, to, output)