Shuffle a list

list-shuffler.lisp

;; Easy Daily Programmer https://www.reddit.com/r/dailyprogrammer/comments/3e0hmh/20150720_challenge_224_easy_shuffling_a_list/

(defun list-shuffler-recursive (input-list &optional accumulator)
  "Shuffle a list using tail call recursion."
  (if (eq input-list nil)
      accumulator
      (progn
	(rotatef (car input-list) 
		 (nth (random (length input-list)) input-list))
	(list-shuffler-recursive (cdr input-list) 
				 (append accumulator (list (car input-list)))))))

(defun list-shuffler-iterative (input-list)
  "Shuffle a list iteratively using a loop"
  (loop with l = (length input-list)
     for i below l
     do (rotatef (nth i input-list)
		 (nth (random l) input-list)))
  input-list)

(defun array-shuffler-iterative (input-array)
  "Shuffle a array iteratively using a loop"
  (loop with l = (length input-array)
     for i below l
     do (rotatef (aref input-array i)
		 (aref input-array (random l))))
  input-array)

;; The array based version is atleast 4 times faster than using lists. Both iterative versions do not cons any bytes in SBCL

I saw this someplace years ago, and I always laugh when I think about shuffling and this algorithm.

;; Fast way to shuffle?
(let ((a (list 1 2 3 4 5 6 7 8 9)))
  (setf a (sort a (lambda (a b) (declare (ignorable a b)) (= 1 (random 2)))))
  (format t "~a~%" a))