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  1. ; length of a cycle
  2.  
  3. (define (f n)
  4.   (set! counter (+ counter 1))
  5.   (case n
  6.     ((0 1) 6)
  7.     ((2 8) 0)
  8.     ((3)   1)
  9.     ((4 7) 4)
  10.     ((5 6) 3)))
  11.  
  12. (define counter 0)
  13.  
  14. (define (floyd f x0)
  15.   (let loop ((tortoise (f x0)) (hare (f (f x0))))
  16.     (if (not (= tortoise hare))
  17.         (loop (f tortoise) (f (f hare)))
  18.         (let loop ((mu 0) (tortoise x0) (hare hare))
  19.           (if (not (= tortoise hare))
  20.               (loop (+ mu 1) (f tortoise) (f hare))
  21.               (let loop ((lam 1) (hare (f tortoise)))
  22.                 (if (not (= tortoise hare))
  23.                     (loop (+ lam 1) (f hare))
  24.                     (values lam mu))))))))
  25.  
  26. (call-with-values
  27.   (lambda () (floyd f 2))
  28.   (lambda (lam mu)
  29.     (display lam) (newline)
  30.     (display mu) (newline)))
  31.  
  32. (display counter) (newline)
  33.  
  34. (define (brent f x0)
  35.   (let loop ((power 1) (lam 1) (tortoise x0) (hare (f x0)))
  36.     (if (not (= tortoise hare))
  37.         (if (= power lam)
  38.             (loop (* power 2) 1 hare (f hare))
  39.             (loop power (+ lam 1) tortoise (f hare)))
  40.         (let loop ((i 0) (hare x0))
  41.           (if (< i lam)
  42.               (loop (+ i 1) (f hare))
  43.               (let loop ((mu 0) (tortoise x0) (hare hare))
  44.                 (if (not (= tortoise hare))
  45.                     (loop (+ mu 1) (f tortoise) (f hare))
  46.                     (values lam mu))))))))
  47.  
  48. (set! counter 0)
  49.  
  50. (call-with-values
  51.   (lambda () (brent f 2))
  52.   (lambda (lam mu)
  53.     (display lam) (newline)
  54.     (display mu) (newline)))
  55.  
  56. (display counter) (newline)
Success #stdin #stdout 0.02s 42848KB
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3
2
16
3
2
13
https://ideone.com/3z8ycq
language:
Scheme (guile 2.2.4)
created:
6 years ago
visibility:
public

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