55 lines
1.9 KiB
Scheme
55 lines
1.9 KiB
Scheme
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; Copyright (c) 1993, 1994 Richard Kelsey and Jonathan Rees. See file COPYING.
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; Random number generator, extracted from T sources. Original
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; probably by Richard Kelsey,
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(define half-log 14)
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(define full-log (* half-log 2))
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(define half-mask (- (arithmetic-shift 1 half-log) 1))
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(define full-mask (- (arithmetic-shift 1 full-log) 1))
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(define index-log 6)
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(define random-1 (bitwise-and 314159265 full-mask))
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(define random-2 (bitwise-and 271828189 full-mask))
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; (MAKE-RANDOM <seed>) takes an integer seed and returns a procedure of no
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; arguments that returns a new pseudo-random number each time it is called.
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(define (make-random seed)
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(make-random-vector seed
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(lambda (vec a b)
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(lambda ()
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(set! a (randomize a random-1 random-2))
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(set! b (randomize b random-2 random-1))
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(let* ((index (arithmetic-shift a (- index-log full-log)))
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(c (vector-ref vec index)))
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(vector-set! vec index b)
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c)))))
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(define (randomize x mult ad)
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(bitwise-and (+ (low-bits-of-product x mult) ad)
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full-mask))
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(define (make-random-vector seed return)
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(let* ((size (arithmetic-shift 1 index-log))
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(vec (make-vector size 0)))
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(do ((i 0 (+ i 1))
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(b seed (randomize b random-2 random-1)))
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((>= i size)
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(return vec seed b))
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(vector-set! vec i b))))
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; Compute low bits of product of two fixnums using only fixnum arithmetic.
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; [x1 x2] * [y1 y2] = [x1y1 (x1y2+x2y1) x2y2]
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(define (low-bits-of-product x y)
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(let ((x1 (arithmetic-shift x (- 0 half-log)))
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(y1 (arithmetic-shift y (- 0 half-log)))
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(x2 (bitwise-and x half-mask))
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(y2 (bitwise-and y half-mask)))
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(bitwise-and (+ (* x2 y2)
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(arithmetic-shift (bitwise-and (+ (* x1 y2) (* x2 y1))
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half-mask)
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half-log))
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full-mask)))
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