Re: #<byte>?

[Richard Cobbe <cobbe@directlink.net>]

Lo, on Saturday, 26 August, 2000, John R. Hall did write:

<SNIP>

> Perhaps some operators for bit shifting and masking would also
> be helpful.

These, at least, exist; search through the MzScheme docs for

bitwise-{and,ior,xor,not}
arithmetic-shift

Given that, it would be pretty straightforward to implement a byte type;
something like this should work, if you want to treat bytes as a separate
type.

(define-struct byte (value))

(define integer->byte
  (lambda (n)
    (make-byte (bitwise-and n #xFF))))

(define signed-integer->byte
  (lambda (n)
    (let ([sign (if (negative? n) #x80 #x00)])
      (make-byte (bitwise-or (bitwise-and n #x7F) sign)))))

(define byte->integer
  (lambda (b)
    (byte-value b)))

(define signed-byte->integer
  (lambda (b)
    (let* ([value (byte-value b)]
           [sign (if (zero? (bitwise-and value #x80))
                     1
                     -1)])
      (* (bitwise-and value #x7f) sign))))

(Warning: the above code was written on the spot; I haven't tested it or
even checked it for syntactic correctness!)

The only thing missing is a means to do binary I/O on these objects; I
suspect you might have to go out to C for that.

Richard