scsh-0.5/alt/bitwise.scm

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1995-10-13 23:34:21 -04:00
; Copyright (c) 1993, 1994 Richard Kelsey and Jonathan Rees. See file COPYING.
; Bitwise operators written in vanilla Scheme.
; Written for clarity and simplicity, not for speed.
; No need to use these in Scheme 48 since Scheme 48's virtual machine
; provides fast machine-level implementations.
(define (bitwise-not i)
(- -1 i))
(define (bitwise-and x y)
(cond ((= x 0) 0)
((= x -1) y)
(else
(+ (* (bitwise-and (arithmetic-shift x -1)
(arithmetic-shift y -1))
2)
(* (modulo x 2) (modulo y 2))))))
(define (bitwise-ior x y)
(bitwise-not (bitwise-and (bitwise-not x)
(bitwise-not y))))
(define (bitwise-xor x y)
(bitwise-and (bitwise-not (bitwise-and x y))
(bitwise-ior x y)))
(define (bitwise-eqv x y)
(bitwise-not (bitwise-xor x y)))
(define (arithmetic-shift n m)
(floor (* n (expt 2 m))))
(define (count-bits x) ; Count 1's in the positive 2's comp rep
(let ((x (if (< x 0) (bitwise-not x) x)))
(do ((x x (arithmetic-shift x 1))
(result 0 (+ result (modulo x 2))))
((= x 0) result))))
;(define (integer-length integer) ...) ;?