Sorry it took so long, but I've submitted a pull request to make this function public in file/sha1:

https://github.com/plt/racket/pull/426

Let me know if I screwed up, it's my first pull request.

Thanks,
Dave

On 06/11/2013 05:11 PM, Robby Findler wrote:
Yes, I think file/sha1 is the right place.

Thanks!

Robby


On Tue, Jun 11, 2013 at 3:26 PM, David Vanderson <david.vander...@gmail.com <mailto:david.vander...@gmail.com>> wrote:

    Thank you Stephen and Tony for your examples.  I found the
    following private function in db/private/mysql/connection.rkt:

    (define (hex-string->bytes s)
      (define (hex-digit->int c)
        (let ([c (char->integer c)])
          (cond [(<= (char->integer #\0) c (char->integer #\9))
                 (- c (char->integer #\0))]
                [(<= (char->integer #\a) c (char->integer #\f))
                 (+ 10 (- c (char->integer #\a)))]
                [(<= (char->integer #\A) c (char->integer #\F))
                 (+ 10 (- c (char->integer #\A)))])))
      (unless (and (string? s) (even? (string-length s))
                   (regexp-match? #rx"[0-9a-zA-Z]*" s))
        (raise-type-error 'hex-string->bytes
                          "string containing an even number of
    hexadecimal digits" s))
      (let* ([c (quotient (string-length s) 2)]
             [b (make-bytes c)])
        (for ([i (in-range c)])
          (let ([high (hex-digit->int (string-ref s (+ i i)))]
                [low  (hex-digit->int (string-ref s (+ i i 1)))])
            (bytes-set! b i (+ (arithmetic-shift high 4) low))))
        b))


    Can this function be exported?  I'm willing to make a patch with
    docs and tests - is file/sha1 the right place?


    Thanks,
    Dave

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