-- implementation of Luhn's checksum algorithm, computing and checking of checksum digit
local luhn = {}

-- main part of the algorithm, double each 2 digits, clamp them and return the sum.
function luhn:_sum_digits(num)
	assert(num >= 0, "number must be positive")
	local sum = 0
	local digits_n = 0

	repeat -- iterate over digits from right to left
		local tmp = num % 10 -- get current unit
		-- each 2 digits, double the current digit
		if (digits_n % 2) == 0 then tmp = tmp * 2 end
		if tmp > 9 then tmp = tmp - 9 end

		digits_n = digits_n + 1
		sum = sum + tmp
		-- remove unit from number
		num = math.floor(num / 10)
	until num == 0
	return sum
end

-- returns the computed check digit
function luhn:compute_check_digit(num)
	return (self:_sum_digits(num) * 9) % 10
end

-- returns true if its valid, false otherwise
function luhn:validate_check_digit(num)
	local check_digit = num % 10 -- get check digit
	return self:compute_check_digit(math.floor(num / 10)) == check_digit
end

-- returns the original number with the check digit concatenated at the end
function luhn:make_number(num)
	return ((num * 10) + self:compute_check_digit(num))
end

return luhn