Takes in a number or a string coercible to a number, and returns a string where every 3rd digit from the decimal point is separated by a comma

commify.lua

-- Takes in a number or a string coercible to a number, and returns a string where every 3rd digit from the decimal point is separated by sep (default = ",")
-- e.g. 1000 -> 1,000 and 1234567 -> 1,234,567
-- scales linearly, basically O(2n), and doesn't create any unnecessary intermediary strings
-- @author Validark
local function commify(num, sep)
	local t = type(num)

	if t == "number" then
		num = string.format("%f", num)
	elseif t ~= "string" then
		error("An invalid value was passed to commify (got " .. t .. ", expected a number or a string coercible to a number)")
	end

	local signPos, start, nextPos = string.match(num, "^[+-]?()0*()%d+()")

	if nextPos == nil
		-- Don't allow trailing 0's
		or signPos ~= start
		-- If a string was passed in, make sure it validates as coercible to a non-scientific number
		or (t == "string" and (string.match(num, "^%.%d+()", nextPos) or nextPos) <= #num)
	then
		error("An invalid number was passed to commify: " .. num)
	end

	local n = nextPos - start
	if n < 4 then return num end
	local k = (n - 1) % 3
	local s = k + start
	local chunks = { string.sub(num, 1, s) }
	local count = 1

	for i = s + 1, n - 3, 3 do
		count = count + 1
		chunks[count] = string.sub(num, i, i + 2)
	end

	count = count + 1
	chunks[count] = string.sub(num, nextPos - 3)

	return table.concat(chunks, sep or ",", 1, count)
end

return commify