Re: binary representation of numbers

[Mikael Djurfeldt]

Rohan Drape <address@hidden> writes:

> Hello All,
>
> I have searched the documentation but cannot find procedures to convert 
> between scheme numbers and common machine byte representations.  I need 
> something equivalent to the PLT procedures:
>
> (integer-byte-string->integer string signed? [big-endian?])
> (integer->integer-byte-string n size-n signed? [big-endian? to-string])
> (floating-point-byte-string->real string [big-endian?])
> (floating-point-byte-string->real string [big-endian?])
>
> which assume of course that strings are byte vectors.  These are required to 
> implement a simple byte protocol over UDP.  I am sure this is `under my nose' 
> but I cannot find it, any pointers would be appreciated.

My suspicion is that you can't do this in a reasonable way in Guile.
>From the C level it's easy enough to do it, but not on the Scheme
level.  Someone should implement these primitives for Guile.

There is a clumsy way to do it, though.  It's based on the use of
string-ports and uniform-vector-read/write.

This is a hint of how to do it.  It's an incomplete implementation of
integer->integer-byte-string:

(define (integer->integer-byte-string n size-n signed?)
  (with-output-to-string
    (lambda ()
      (uniform-vector-write
       (list->uniform-vector (select-proto size-n signed?) (list n))))))

(define (select-proto . spec)
  (cdr (assoc spec '(((2 #t) . s)
                     ((4 #f) . 1)
                     ((4 #t) . -1)
                     ((8 #t) . l)))))