မဝ်ဂျူ:string/char

Reimplementation of mw.ustring.char in pure Lua, which is faster up to around 15 arguments, and much faster if given only one argument.

---

local char = string.char
local concat = table.concat
local tonumber = tonumber

local function err(cp)
	error("Codepoint " .. cp .. " is out of range: codepoints must be between 0x0 and 0x10FFFF.", 2)
end

local function utf8_char(cp)
	cp = tonumber(cp)
	if cp < 0 then
		err("-0x" .. ("%X"):format(-cp + 1))
	elseif cp < 0x80 then
		return char(cp)
	elseif cp < 0x800 then
		return char(
			0xC0 + cp / 0x40,
			0x80 + cp % 0x40
		)
	elseif cp < 0x10000 then
		if cp >= 0xD800 and cp < 0xE000 then
			return "?" -- mw.ustring.char returns "?" for surrogates.
		end
		return char(
			0xE0 + cp / 0x1000,
			0x80 + cp / 0x40 % 0x40,
			0x80 + cp % 0x40
		)
	elseif cp < 0x110000 then
		return char(
			0xF0 + cp / 0x40000,
			0x80 + cp / 0x1000 % 0x40,
			0x80 + cp / 0x40 % 0x40,
			0x80 + cp % 0x40
		)
	end
	err("0x" .. ("%X"):format(cp))
end

return function(cp, ...)
	if ... == nil then
		return utf8_char(cp)
	end
	local ret = {cp, ...}
	for i = 1, #ret do
		ret[i] = utf8_char(ret[i])
	end
	return concat(ret)
end