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  1. ; the early bird catches the worm
  2.  
  3. (import (rnrs exceptions (6)))
  4. (import (rnrs sorting (6)))
  5.  
  6. (define (undigits ds . args)
  7.   (let ((b (if (null? args) 10 (car args))))
  8.     (let loop ((ds ds) (n 0))
  9.       (if (null? ds) n
  10.           (loop (cdr ds) (+ (* n b) (car ds)))))))
  11.  
  12. (define (permutations xs)
  13.   (define (rev xs n ys)
  14.     (if (zero? n) ys
  15.       (rev (cdr xs) (- n 1) (cons (car xs) ys))))
  16.   (let ((xs xs) (perms (list xs)))
  17.     (define (perm n)
  18.       (if (> n 1)
  19.           (do ((j (- n 1) (- j 1)))
  20.               ((zero? j) (perm (- n 1)))
  21.             (perm (- n 1))
  22.             (set! xs (rev xs n (list-tail xs n)))
  23.             (set! perms (cons xs perms)))))
  24.     (perm (length xs))
  25.     perms))
  26.  
  27. (define-syntax try
  28.   (syntax-rules (trying)
  29.     ((try trying expr default)
  30.       (call-with-current-continuation
  31.         (lambda (return)
  32.           (with-exception-handler
  33.             (lambda (x) (return default))
  34.             (lambda () expr)))))
  35.     ((try) #f)
  36.     ((try expr) (try trying expr #f))
  37.     ((try expr0 expr1 ...)
  38.       (let ((t (try trying expr0 #f)))
  39.         (if t t (try expr1 ...))))))
  40.  
  41. (define (time? n)
  42.   (and (< (quotient n 100) 24)
  43.        (< (modulo n 100) 60)))
  44.  
  45. (define (earliest xs)
  46.   (try (car
  47.          (list-sort <
  48.            (filter time?
  49.              (map undigits
  50.                (permutations xs)))))
  51.        #f))
  52.  
  53. (display (earliest '(1 2 3 4))) (newline)
  54. (display (earliest '(6 7 8 9))) (newline)
Success #stdin #stdout 0.02s 44792KB
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https://ideone.com/mmrZAD
language:
Scheme (guile 2.2.4)
created:
6 years ago
visibility:
public

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