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Replace arithmetic with bit operations

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quotient/remainder is replaced with word-index and bit-index, expt with arithmetic-shift.
This commit is contained in:
Michael Myers 2017-01-09 06:07:04 -05:00 committed by Vincent St-Amour
parent 63182523df
commit e2070b882d
1 changed files with 12 additions and 3 deletions
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15
racket/collects/data/bit-vector.rkt
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@ -11,10 +11,16 @@
"private/count-bits-in-fixnum.rkt") "private/count-bits-in-fixnum.rkt")
(define bits-in-a-word 8) (define bits-in-a-word 8)
(define bits-in-a-word-log2 3)
(define largest-word (define largest-word
(- (expt 2 bits-in-a-word) 1)) (- (expt 2 bits-in-a-word) 1))
(define (word-index n)
(arithmetic-shift n (- bits-in-a-word-log2)))
(define (bit-index n)
(bitwise-and n (sub1 bits-in-a-word)))
(define (make-bit-vector size [fill #f]) (define (make-bit-vector size [fill #f])
(define-values (q r) (quotient/remainder size bits-in-a-word)) (define-values (q r) (quotient/remainder size bits-in-a-word))
(define word-size (+ q (if (zero? r) 0 1))) (define word-size (+ q (if (zero? r) 0 1)))
@ -46,7 +52,8 @@
[else default])])) [else default])]))
(define (unsafe-bit-vector-ref bv n) (define (unsafe-bit-vector-ref bv n)
(define-values (wi bi) (quotient/remainder n bits-in-a-word)) (define wi (word-index n))
(define bi (bit-index n))
(match bv (match bv
[(struct bit-vector (words size)) [(struct bit-vector (words size))
(define word (bytes-ref words wi)) (define word (bytes-ref words wi))
@ -68,13 +75,15 @@
(bit-vector-ref bv key)) (bit-vector-ref bv key))
(define (bit-vector-set! bv n b) (define (bit-vector-set! bv n b)
(define-values (wi bi) (quotient/remainder n bits-in-a-word)) (define wi (word-index n))
(define bi (bit-index n))
(match bv (match bv
[(struct bit-vector (words size)) [(struct bit-vector (words size))
(define word (bytes-ref words wi)) (define word (bytes-ref words wi))
(define bit (bitwise-bit-set? word bi)) (define bit (bitwise-bit-set? word bi))
(unless (eq? bit b) (unless (eq? bit b)
(bytes-set! words wi (bitwise-xor word (expt 2 bi))))])) (define new-word (bitwise-xor word (arithmetic-shift 1 bi)))
(bytes-set! words wi new-word))]))
(define (bit-vector-length bv) (define (bit-vector-length bv)
(bit-vector-size bv)) (bit-vector-size bv))