[racket-dev] hex decoding?

There doesn't appear to be a library function for going the reverse
direction but here's one way to write it up:

#lang racket

(define ASCII-ZERO (char->integer #\0))

;; [0-9A-Fa-f] -> Number from 0 to 15
(define (hex-char->number c)
  (if (char-numeric? c)
      (- (char->integer c) ASCII-ZERO)
      (match c
        [(or #\a #\A) 10]
        [(or #\b #\B) 11]
        [(or #\c #\C) 12]
        [(or #\d #\D) 13]
        [(or #\e #\E) 14]
        [(or #\f #\F) 15]
        [_ (error 'hex-char->number "invalid hex char: ~a\n" c)])))

(define (hex-string->bytes str) (list->bytes (hex-string->bytelist str)))
(define (hex-string->bytelist str)
  (with-input-from-string
   str
   (thunk
    (let loop ()
      (define c1 (read-char))
      (define c2 (read-char))
      (cond [(eof-object? c1) null]
            [(eof-object? c2) (list (hex-char->number c1))]
            [else (cons (+ (* (hex-char->number c1) 16)
                           (hex-char->number c2))
                        (loop))])))))

(require file/sha1)
(hex-string->bytes (bytes->hex-string #"turtles"))


Welcome to DrRacket, version 5.3.4.6 [3m].
Language: racket [custom].
#"turtles"
>

On Sun, Jun 9, 2013 at 6:30 PM, David Vanderson
<david.vanderson at gmail.com> wrote:
> I'm doing some cryptography exercises that involve a lot of hex
> encoding/decoding.  I see there's a bytes->hex-string function in file/sha1
> and openssl/sha1, but I can't find a decode function.
>
> Is a hex decode function in the distribution?
>
> Thanks,
> Dave
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