Luhn algorithm in elisp

I was looking for the checksum validation of credit card numbers and came across the Luhn Algorithm. I had emacs open and on a whim decided I'll try and implement it using elisp. I'm no lisp programmer but, have in the past managed to write some basic elisp in .emacs so, I guessed it would take me about .5hr at most.

I guessed wrong. It took me a lot longer to wrap my head around even some of the simple elisp constructs like let and lambda took quite a while and it took a lot longer than I anticipated. Here, I present to you the fruit of my labors :)

	(defun luhn-sum (list n)
	(if (null list)
	0
	(+ (let ((x (car list)))
	(if (= 1 (mod n 2))
	(let ((y (* 2 x)))
	(if (> y 9)
	(+ 1 (mod y 10))
	y))
	x))
	(luhn-sum (cdr list) (+ 1 n)))
	)
	)
	(defun card-no-str-to-num (card-no)
	(mapcar (lambda (x) (string-to-number x 10)) (cdr (reverse (cdr (split-string card-no "")))))
	)
	(defun luhn-check (card-no)
	(eq 0 (mod (luhn-sum (card-no-str-to-num card-no) 0) 10))
	)
	(luhn-check "49927398716")

view raw luhn-check.el hosted with ❤ by GitHub