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C00002 00002	 librar.lsp[f81,jmc]	library of useful lisp functions
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;;; librar.lsp[f81,jmc]	library of useful lisp functions


;;; These IF macros should be initialized with maclisp, but they seem to lose
;;; sometimes, so here they are. 1981 december.

(defun if macro (l)(cons 'cond (ifxxx1 (cdr l))))

(defun ifxxx1 (u) (cond ((null u) nil) ((null (cdr u)) (list (list t (car u))))
(t (cons (list (car u) (cadr u)) (ifxxx1 (cddr u))))))

;;; Shuffles a list in random order by assigning each element randomly to
;;; one of two output lists  w1  and  w2  and then appending the results
;;; of shuffling  w1  and  w2.  It assigns equal probability to each of
;;; the  n!  permutations of  u.  It only terminates with probability one,
;;; since if the random number generator gave always  0  or always  1  it
;;; wouldn't terminate.
(defun shuffle (u)
       (if (or (null u) (null (cdr u)))
	   u
	   (shuffle1 u nil nil)))
(defun shuffle1 (u w1 w2)
       (if (null u)
	   (append (shuffle w1) (shuffle w2))
	   (= 0 (random 2))
	   (shuffle1 (cdr u) (cons (car u) w1) w2)
	   (shuffle1 (cdr u) w1 (cons (car u) w2))
	   ))