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.
Building the answers provided by Inaimathi and Shrein, I also added an encode function. Here is an implementation of both encode and decode, for both string and region arguments:
;; ASCII-HEX converion
(defun my/hex-decode-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)))))
(defun my/hex-encode-string (ascii-string)
(let ((res nil))
(dotimes (i (length ascii-string) (apply #'concat (reverse res)))
(let ((ascii-char (substring ascii-string i (+ i 1))))
(push (format "%x" (string-to-char ascii-char)) res)))))
(defun my/hex-decode-region (start end)
"Decode a hex string in the selected region."
(interactive "r")
(save-excursion
(let* ((decoded-text
(my/hex-decode-string
(buffer-substring start end))))
(delete-region start end)
(insert decoded-text))))
(defun my/hex-encode-region (start end)
"Encode a hex string in the selected region."
(interactive "r")
(save-excursion
(let* ((encoded-text
(my/hex-encode-string
(buffer-substring start end))))
(delete-region start end)
(insert encoded-text))))