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  1. ; an impossible interview question
  2.  
  3. (define rand #f)
  4. (define randint #f)
  5. (let ((two31 #x80000000) (a (make-vector 56 -1)) (fptr #f))
  6.   (define (mod-diff x y) (modulo (- x y) two31)) ; generic version
  7.   ; (define (mod-diff x y) (logand (- x y) #x7FFFFFFF)) ; fast version
  8.   (define (flip-cycle)
  9.     (do ((ii 1 (+ ii 1)) (jj 32 (+ jj 1))) ((< 55 jj))
  10.       (vector-set! a ii (mod-diff (vector-ref a ii) (vector-ref a jj))))
  11.     (do ((ii 25 (+ ii 1)) (jj 1 (+ jj 1))) ((< 55 ii))
  12.       (vector-set! a ii (mod-diff (vector-ref a ii) (vector-ref a jj))))
  13.     (set! fptr 54) (vector-ref a 55))
  14.   (define (init-rand seed)
  15.     (let* ((seed (mod-diff seed 0)) (prev seed) (next 1))
  16.       (vector-set! a 55 prev)
  17.       (do ((i 21 (modulo (+ i 21) 55))) ((zero? i))
  18.         (vector-set! a i next) (set! next (mod-diff prev next))
  19.         (set! seed (+ (quotient seed 2) (if (odd? seed) #x40000000 0)))
  20.         (set! next (mod-diff next seed)) (set! prev (vector-ref a i)))
  21.       (flip-cycle) (flip-cycle) (flip-cycle) (flip-cycle) (flip-cycle)))
  22.   (define (next-rand)
  23.     (if (negative? (vector-ref a fptr)) (flip-cycle)
  24.       (let ((next (vector-ref a fptr))) (set! fptr (- fptr 1)) next)))
  25.   (define (unif-rand m)
  26.     (let ((t (- two31 (modulo two31 m))))
  27.       (let loop ((r (next-rand)))
  28.         (if (<= t r) (loop (next-rand)) (modulo r m)))))
  29.   (init-rand 19380110) ; happy birthday donald e knuth
  30.   (set! rand (lambda seed
  31.     (cond ((null? seed) (/ (next-rand) two31))
  32.           ((eq? (car seed) 'get) (cons fptr (vector->list a)))
  33.           ((eq? (car seed) 'set) (set! fptr (caadr seed))
  34.                                  (set! a (list->vector (cdadr seed))))
  35.           (else (/ (init-rand (modulo (numerator
  36.                   (inexact->exact (car seed))) two31)) two31)))))
  37.   (set! randint (lambda args
  38.     (cond ((null? (cdr args))
  39.             (if (< (car args) two31) (unif-rand (car args))
  40.               (floor (* (next-rand) (car args)))))
  41.           ((< (car args) (cadr args))
  42.             (let ((span (- (cadr args) (car args))))
  43.               (+ (car args)
  44.                  (if (< span two31) (unif-rand span)
  45.                    (floor (* (next-rand) span))))))
  46.           (else (let ((span (- (car args) (cadr args))))
  47.                   (- (car args)
  48.                      (if (< span two31) (unif-rand span)
  49.                        (floor (* (next-rand) span))))))))))
  50.  
  51. (define (floyd xs)
  52.   (let loop ((xs xs) (n 1) (x #f))
  53.     (if (null? xs) x
  54.       (loop (cdr xs) (+ n 1)
  55.             (if (< (rand) (/ n)) (car xs) x)))))
  56.  
  57. (display (floyd '(1 2 3 4 5)))
Success #stdin #stdout 0.02s 8952KB
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https://ideone.com/Hh4IfH
language:
Scheme (guile 2.2.4)
created:
7 years ago
visibility:
public

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