; array rotation, again
 
(define (rotate x rotdist)
  (define (swap a b m)
    (do ((i 0 (+ i 1))) ((= i m) x)
      (let ((t (vector-ref x (+ a i))))
        (vector-set! x (+ a i)
          (vector-ref x (+ b i)))
        (vector-set! x (+ b i) t))))
  (let ((n (vector-length x)) (p rotdist))
    (let loop ((i p) (j (- n p)))
      (cond ((= i j) (swap (- p i) p i) x)
            ((> i j) (swap (- p i) p j)
                     (loop (- i j) j))
            (else (swap (- p i) (+ p j (- i)) i)
                  (loop i (- j i)))))))
 
(display (rotate '#(a b c d e f g h) 3)) (newline)
(display (rotate '#(d e f g h a b c) 5)) (newline)
 
(define (gcd i j)
  (let loop ((i i) (j j))
    (if (= i j) i
      (if (> i j)
          (loop (- i j) j)
          (loop i (- j i))))))
 
(display (gcd 28 35)) (newline)

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