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  1. ; one-swappable array
  2.  
  3. (define (swappable? xs)
  4.   (define (x i) (vector-ref xs i))
  5.   (define (swap! i j)
  6.     (let ((t (vector-ref xs i)))
  7.       (vector-set! xs i
  8.         (vector-ref xs j))
  9.       (vector-set! xs j t)))
  10.   (define (sorted? i j)
  11.     (cond ((= i j) #t)
  12.           ((not (< (x i) (x (+ i 1)))) #f)
  13.           (else (sorted? (+ i 1) j))))
  14.   (let* ((len (vector-length xs))
  15.          (left (let loop ((i 0))
  16.                  (if (= i (- len 1)) #f
  17.                    (if (< (x (+ i 1)) (x i)) i
  18.                      (loop (+ i 1))))))
  19.          (right (let loop ((j (- len 1)))
  20.                   (if (= j 1) #f
  21.                     (if (< (x j) (x (- j 1))) j
  22.                       (loop (- j 1)))))))
  23.     (if (not (and left right)) #f
  24.       (begin
  25.         (swap! left right)
  26.         (if (sorted? (max (- left 1) 0)
  27.                      (min (+ right 1) (- len 1)))
  28.             (list left right)
  29.             #f)))))
  30.  
  31. (display (swappable? '#(1 2 6 4 5 3 7))) (newline)
  32. (display (swappable? '#(7 6 5 4 3 2 1))) (newline)
  33. (display (swappable? '#(1 2 3 4 5 6 7))) (newline)
  34. (display (swappable? '#(7 2 3 4 5 6 1))) (newline)
  35. (display (swappable? '#(1 2 4 3 5 6 7))) (newline)
  36. (display (swappable? '#(2 7 3 4 5 1 6))) (newline)
Success #stdin #stdout 0.01s 7268KB
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(2 5)
#f
#f
(0 6)
(2 3)
#f
https://ideone.com/YvssYR
language:
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created:
8 years ago
visibility:
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