(module fisher-yates mzscheme
  (require (lib "contract.ss"))
  (provide/contract [shuffle! ((vectorof any/c) . -> . (vectorof any/c))])
  
  ;; shuffle!: (vectorof X) -> (vectorof X)
  ;; fisher-yates shuffle.
  (define (shuffle! von)
    (let loop ([n (sub1 (vector-length von))])
      (cond
        [(< n 0)
         von]
        [else
         (let* ([k (random (add1 n))]
                [t (vector-ref von k)])
           (vector-set! von k (vector-ref von n))
           (vector-set! von n t)
           (loop (sub1 n)))]))))