; coin flips
(define randint #f)
(let ((two31 #x80000000) (a (make-vector 56 -1)) (fptr #f))
(define (mod-diff x y) (modulo (- x y) two31)) ; generic version
; (define (mod-diff x y) (logand (- x y) #x7FFFFFFF)) ; fast version
(define (flip-cycle)
(do ((ii 1 (+ ii 1)) (jj 32 (+ jj 1))) ((< 55 jj))
(vector-set! a ii (mod-diff (vector-ref a ii) (vector-ref a jj))))
(do ((ii 25 (+ ii 1)) (jj 1 (+ jj 1))) ((< 55 ii))
(vector-set! a ii (mod-diff (vector-ref a ii) (vector-ref a jj))))
(set! fptr 54) (vector-ref a 55))
(let* ((seed (mod-diff seed 0)) (prev seed) (next 1))
(vector-set! a 55 prev)
(do ((i 21 (modulo (+ i 21) 55))) ((zero? i))
(vector-set! a i next) (set! next (mod-diff prev next))
(set! seed (+ (quotient seed 2) (if (odd? seed) #x40000000 0)))
(set! next (mod-diff next seed)) (set! prev (vector-ref a i)))
(flip-cycle) (flip-cycle) (flip-cycle) (flip-cycle) (flip-cycle)))
(if (negative? (vector-ref a fptr)) (flip-cycle)
(let ((next (vector-ref a fptr))) (set! fptr (- fptr 1)) next)))
(let ((t (- two31 (modulo two31 m))))
(let loop
((r
(next
-rand))) (if (<= t r
) (loop
(next
-rand)) (modulo r m
))))) (init
-rand 19380110) ; happy birthday donald e knuth
(cond
((null? seed
) (/ (next
-rand) two31
)) ((eq? (car seed) 'get) (cons fptr (vector->list a)))
((eq? (car seed) 'set) (set! fptr (caadr seed))
(set! a (list->vector (cdadr seed))))
(else (/ (init
-rand (modulo
(numerator
(inexact->exact (car seed))) two31)) two31)))))
(set! randint (lambda args
(cond ((null? (cdr args))
(if (< (car args
) two31
) (unif
-rand (car args
)) ((< (car args) (cadr args))
(let ((span (- (cadr args) (car args))))
(+ (car args)
(if (< span two31
) (unif
-rand span
) (else (let ((span (- (car args) (cadr args))))
(- (car args)
(if (< span two31
) (unif
-rand span
)
(define
(biased
) (if (< (rand) 0.4) 1 0))
(do ((n 100000 (- n 1))
(s 0 (+ s (biased))))
((zero? n) (display s)))
(newline)
(define (unbiased)
(if (zero? (biased))
(if (zero? (biased)) (unbiased) 0)
(if (zero? (biased)) 1 (unbiased))))
(do ((n 100000 (- n 1))
(s 0 (+ s (unbiased))))
((zero? n) (display s)))
(newline)