kahl

 1(define (from-twocomp numbits input)
 2  (let ((mask (expt 2 (- numbits 1))))
 3    (+ (* -1 (bitwise-and input mask))
 4       (bitwise-and input (bitwise-not mask)))))
 5
 6(define (bytevector->number size bv)
 7  (let ((shift-proc (lambda (idx) (* idx 8))))
 8    (apply bitwise-ior
 9           (map (lambda (index)
10                   (arithmetic-shift
11                     (bytevector-u8-ref bv index)
12                     (shift-proc index)))
13                (range size)))))
14
15(define (bytevector->uint bv)
16  (if (> (bytevector-length bv) 10)
17    (error "maximum length of encoded uint is 10 bytes")
18    (list->bits
19      (bytevector-fold-right
20        (lambda (x ys)
21          (let* ((lst    (bits->list x))
22                 (no-msb (if (msb-set? x)
23                           (init lst)
24                           lst)))
25            (append no-msb ys)))
26        '() bv))))
27
28(define (uint->number n)
29  (let ((x (arithmetic-shift n -1)))
30    (if (lsb-set? n)
31      (bitwise-eqv x)
32      x)))