#|
 
utility.lisp
 
> Extra functions not defined in the CL standard that I find helpful
 
Copyright (C) 2008 Jake Voytko
 
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The above copyright notice and this permission notice shall be
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(defun coin-flip ()
  "Returns true or false with equal probability on the assumption
  that Lisp's RNG has an equal distribution."
  (eql 0 (random 2)))
 
 
(defun at-least-once (f)
  "Calls 'f' at least once and returns a list of the results.
  Additional function calls determined by 'coin-flip'."
  (labels ((at-least-once-helper (f)
             (cons (funcall f)
                   (if (coin-flip)
                       (at-least-once-helper f)
                       nil))))
    (at-least-once-helper f)))
 
 
(defun random-array-element (array)
  "Returns a random element of 'array'."
  (aref array (random (length array))))
 
 
(defmacro swap (item1 item2)
  "Swaps the contents of item1 and item2."
  (let ((tmp (gensym)))
    `(progn
       (setf ,tmp ,item1)
       (setf ,item1 ,item2)
       (setf ,item2 ,tmp))))
 
(defun random-subsequence (list)
  "Returns a random subsequence from list 'list'. Should work on any sequence,
  but the tests aren't in place yet, so I won't guarantee this."
  (cond ((null list) nil)
        ((eql 1 (length list)) list)
        (t ; Start: can't be last element. End: picked from start+1->end
         (let* ((start (random (length list)))
                (end
                 (+ (random (- (length list) start))
                    1 start)))
    (subseq list start  end)))))
 
 
(defun splice (list1 list2 start end)
  "Replaces the elements of 'list1' in indices [start,end) with the contents
  of list2"
  (cond ((>= start (length list1))
         (append list1 list2))
        ((eql 0 start) (append list2 (subseq list1 end)))
        (t (append (subseq list1 0 start)
                   list2
                   (subseq list1 end)))))