How do I convert a string of hex into ASCII using elisp?

Today I received a reply to one of my emails in the form of a string of hex bytes:

"686170707920333974682068617665206120676f6f64206f6e6521"

And I was thinking of the most efficient clean way to convert the string into it's ASCII equivalent. I'll add my answer to the question but I didn't feel it was as elegant as it could have been.

Solution

Here's an iterative solution

(defun decode-hex-string (hex-string)
  (let ((res nil))
    (dotimes (i (/ (length hex-string) 2) (apply #'concat (reverse res)))
      (let ((hex-byte (substring hex-string (* 2 i) (* 2 (+ i 1)))))
        (push (format "%c" (string-to-number hex-byte 16)) res)))))

And one using loop, if you're looking to avoid side-effect operations (you may need to (require 'cl) in order to use this one):

(defun decode-hex-string (hex-string)
  (apply #'concat 
     (loop for i from 0 to (- (/ (length hex-string) 2) 1) 
           for hex-byte = (substring hex-string (* 2 i) (* 2 (+ i 1)))
           collect (format "%c" (string-to-number hex-byte 16)))))

In general, it's best to avoid recursion in Elisp and Common Lisp; your stack is going to keel over with a big enough input, and neither language guarantees tail recursion (which you aren't using, but still). In Scheme, it's a different story.

Incidentally, Happy 39th.