Port of punycode encode function in lua from https://github.com/bestiejs/punycode.js

punycode.lua

local bit = require "bit"

local _M = {}

local maxInt = 2147483647
local base = 36
local tMin = 1
local tMax = 26
local skew = 38
local damp = 700
local initialBias = 72
local initialN = 128
local delimiter = 45
local baseMinusTMin = base - tMin

local function utf8_decode(str)
	local i = 1
	local length = string.len(str)

	local output = {}
	local codes_left = 0
	local prev_multioctet = 0

	while i <= length do
		local c = string.byte(str, i)
		i = i + 1

		if codes_left > 0 then
			prev_multioctet = prev_multioctet + (c - 0x80)
			codes_left = codes_left - 1

			if codes_left == 0 then
				table.insert(output, prev_multioctet)
			else
				prev_multioctet = prev_multioctet * 0x40
			end
		elseif c <= 0x7f then
			table.insert(output, c)
		elseif c <= 0xdf then
			codes_left = 1
			prev_multioctet = (c - 0xc0) * 0x40
		elseif c <= 0xef then
			codes_left = 2
			prev_multioctet = (c - 0xe0) * 0x40
		else
			codes_left = 3
			prev_multioctet = (c - 0xf0) * 0x40
		end
	end

	return output
end

local function ucs2decode(str)
	local output = {}
	local counter = 1
	local length = table.getn(str)
	while counter <= length do
		local value = str[counter]
		counter = counter + 1

		if (value >= 0xD800 and value <= 0xDBFF and counter <= length) then
			local extra = str[counter]
			counter = counter + 1

			if bit.band(extra, 0xfc00) == 0xdc00 then
				table.insert(output, bit.lshift(bit.band(value, 0x3ff), 10) + bit.band(extra, 0x3ff) + 0x10000)
			else
				table.insert(output, value)
				counter = counter - 1
			end
		else
			table.insert(output, value)
		end
	end

	return output
end

local function digit_to_basic(digit, flag)
	local result = digit + 22
	if flag ~= 0 then
		result = result - 32
	end
	if digit < 26 then
		result = result + 75
	end
	return result
end

local function adapt(delta, numPoints, firstTime)
	local k = 0
	if firstTime then
		delta = math.floor(delta / damp)
	else
		delta = bit.rshift(delta, 1)
	end
	delta = delta + math.floor(delta / numPoints)
	while delta > baseMinusTMin * bit.rshift(tMax, 1) do
		delta = math.floor(delta / baseMinusTMin)
		k = k + base
	end
	return math.floor(k + (baseMinusTMin + 1) * delta / (delta + skew))
end

local function from_char_codes(arr)
	local result = ""

	for _, code in ipairs(arr) do
		result = result .. string.char(code)
	end

	return result
end

local function raw_encode(input)
	local output = {}
	input = ucs2decode(input)
	local inputLength = table.getn(input)
	local n = initialN
	local delta = 0
	local bias = initialBias

	for _, val in ipairs(input) do
		if val < 0x80 then
			table.insert(output, val)
		end
	end

	local basicLength = table.getn(output)
	local handledCPCount = basicLength

	if basicLength > 0 then
		table.insert(output, delimiter)
	end

	while handledCPCount < inputLength do
		local m = maxInt
		for _, val in ipairs(input) do
			if val >= n and val < m then
				m = val
			end
		end

		local handledCPCountPlusOne = handledCPCount + 1
		if m - n > math.floor((maxInt - delta) / handledCPCountPlusOne) then
			error('overflow')
		end

		delta = delta + (m - n) * handledCPCountPlusOne
		n = m

		for _, val in ipairs(input) do
			if val < n then
				delta = delta + 1
				if delta > maxInt then
					error('overflow')
				end
			end

			if val == n then
				local q = delta
				local k = base
				while true do
					local t
					if k <= bias then
						t = tMin
					elseif k >= bias + tMax then
						t = tMax
					else
						t = k - bias
					end
					if q < t then
						break
					end
					local qMinusT = q - t
					local baseMinusT = base - t
					table.insert(output, digit_to_basic(t + qMinusT % baseMinusT, 0))
					q = math.floor(qMinusT / baseMinusT)

					k = k + base
				end

				table.insert(output, digit_to_basic(q, 0))
				bias = adapt(delta, handledCPCountPlusOne, handledCPCount == basicLength)
				delta = 0
				handledCPCount = handledCPCount + 1
			end
		end

		delta = delta + 1
		n = n + 1
	end

	return output
end

function _M.encode_domain(domain)
	local has_non_ascii = false
	local length = string.len(domain)
	for i = 1,length do
		if string.byte(domain, i) > 0x7e then
			has_non_ascii = true
			break
		end
	end

	if not has_non_ascii then
		return domain
	end

	-- TODO: implement split by \u3002\uFF0E\uFF61 too
	domain = utf8_decode(domain)

	local result = ""
	local part = {}
	local part_has_non_ascii = false
	for _, c in ipairs(domain) do
		if c ~= 0x2e then
			if c > 0x7e then
				part_has_non_ascii = true
			end

			table.insert(part, c)
		else
			if part_has_non_ascii then
				result = result .. "xn--" .. from_char_codes(raw_encode(part)) .. "."
			else
				result = result .. from_char_codes(part) .. "."
			end

			part = {}
			part_has_non_ascii = false
		end
	end

	if part_has_non_ascii then
		result = result .. "xn--" .. from_char_codes(raw_encode(part))
	else
		result = result .. from_char_codes(part)
	end

	return result
end

return _M