local math_module = "Module:math"

local char = string.char
local error = error
local format = string.format
local pcall = pcall
local select = select
local tonumber = tonumber
local type = type

local function to_hex(...)
	to_hex = require(math_module).to_hex
	return to_hex(...)
end

local function codepoint_err(cp, i)
	-- Throw error: to_hex can only return integers, so only show the bad value
	-- if it can be converted into something that looks like a codepoint.
	local success, result = pcall(to_hex, cp, true)
	error(format(
		"bad argument #%d to 'string/char' (codepoint between 0x0 and 0x10FFFF expected%s)",
		i, success and "; got " .. result or ""),
	i + 3)
end

local function utf8_char(n, i, v, ...)
	local cp = tonumber(v)
	if cp == nil then
		error(format("bad argument #%d to 'char' (number expected; got %s)", i, type(v)), i + 2)
	elseif cp < 0 then
		codepoint_err(cp, i)
	elseif cp < 0x80 then
		if i == n then
			return cp
		end
		return cp, utf8_char(n, i + 1, ...)
	elseif cp < 0x800 then
		if i == n then
			return 0xC0 + cp / 0x40,
				0x80 + cp % 0x40
		end
		return 0xC0 + cp / 0x40,
			0x80 + cp % 0x40,
			utf8_char(n, i + 1, ...)
	elseif cp < 0x10000 then
		-- Don't return "?" for surrogates, like mw.ustring.char does, as they
		-- have legitimate uses (e.g. in JSON).
		if i == n then
			return 0xE0 + cp / 0x1000,
				0x80 + cp / 0x40 % 0x40,
				0x80 + cp % 0x40
		end
		return 0xE0 + cp / 0x1000,
			0x80 + cp / 0x40 % 0x40,
			0x80 + cp % 0x40,
			utf8_char(n, i + 1, ...)
	elseif cp < 0x110000 then
		if i == n then
			return 0xF0 + cp / 0x40000,
				0x80 + cp / 0x1000 % 0x40,
				0x80 + cp / 0x40 % 0x40,
				0x80 + cp % 0x40
		end
		return 0xF0 + cp / 0x40000,
			0x80 + cp / 0x1000 % 0x40,
			0x80 + cp / 0x40 % 0x40,
			0x80 + cp % 0x40,
			utf8_char(n, i + 1, ...)
	end
	codepoint_err(cp, i)
end

return function(...)
	local n = select("#", ...)
	if n ~= 0 then
		return char(utf8_char(n, 1, ...))
	end
end