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#!/usr/local/bin/sbcl --script
;;;; craps.lisp
;;;; My submission to
;;;; Simulates games of Craps, outputs statistics; REQUIRES SBCL (*posix-argv*)
;;;; GRE, 11/4/11
(defun two-dice ()
"The sum of rolling two dice"
(+ (1+ (random 6)) (1+ (random 6))))
(defun crap (&optional (rounds 1) (rolls nil) (point 0))
"A single round of craps"
(let* ((roll (two-dice)) (rolls (cons roll rolls)))
(if (zerop point)
(cond ((member roll '(7 11)) (list 'win rounds rolls))
((member roll '(2 3 12)) (list 'lose rounds rolls))
(t (crap (1+ rounds) rolls roll)))
(cond ((= roll point) (list 'win rounds (reverse rolls)))
((= roll 7) (list 'lose rounds (reverse rolls)))
(t (crap (1+ rounds) rolls point))))))
(defun max-index (seq)
"Index with maximum in sequence"
(let ((m (reduce #'max seq)))
(position m seq)))
(defun craps (n)
"Stats for n rounds of craps"
(loop repeat n with rolls = (make-array 13 :initial-element 0)
for c = (crap) then (crap)
counting (eq 'win (first c)) into wins
counting (eq 'lose (first c)) into losses
summing (second c) into n-rounds
maximizing (second c) into max-rounds
do (loop for r in (third c) do (incf (elt rolls r)))
finally (return (list wins losses n-rounds max-rounds (max-index rolls)))))
(defun craps-output (n)
"Output statistics for n craps games"
(let ((cs (coerce (craps n) 'vector)))
(format t "~{~a: ~f~%~}" (list "Win percentage" (/ (elt cs 0) n)
"Loss percentage" (/ (elt cs 1) n)
"Average number of rounds" (/ (elt cs 2) n)))
(format t "~{~a: ~d~%~}" (list "Maximum number of rounds" (elt cs 3)
"Most common roll" (elt cs 4)))))
;;; Main event
(craps-output (read-from-string (second *posix-argv*)))
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