local function _parse(str, init)
	local pos = init
	if string.find(str, "^%(", pos) then
		pos = pos + 1
		local list = {};
		while true do
			if string.find(str, "^%)", pos) then
				pos = pos + 1
				return list, pos
			end
			local child, new_pos = _parse(str, pos)
			if not child then
				error(string.format("expected ')', '(', or atom at index %d", pos))
			end
			table.insert(list, child)
			pos = new_pos
		end
	else
		local pos = init
		local len_str = string.match(str, "^%d+", pos)
		if not len_str then
			error(string.format("expected '(' or atom at index %d", pos))
		end
		local len = tonumber(len_str)
		pos = pos+#len_str
	
		if not string.find(str, "^:", pos) then error(string.format("expected ':' at index %d", pos)) end
		pos = pos + 1
		local value = string.sub(str, pos, pos + len - 1)
		if #value < len then error(string.format("unexpected end of string at index %d", pos)) end
		pos = pos + len
	
		return value, pos
	end
end

local function parse(str)
	local value, pos = _parse(str, 1)
	if pos <= #str then
		error(string.format("extra characters at index %d", pos))
	end
	return value
end

local function _serialize(tokens, obj)
	if type(obj) == "table" then
		table.insert(tokens, "(")
		for _, item in ipairs(obj) do
			_serialize(tokens, item)
		end
		table.insert(tokens, ")")
	else
		local s = tostring(obj)
		table.insert(tokens, tostring(#s))
		table.insert(tokens, ":")
		table.insert(tokens, s)
	end
end

local function serialize(obj)
	local tokens = {}
	_serialize(tokens, obj)
	return table.concat(tokens)
end

return {parse = parse, serialize = serialize}