;;; -*-lisp-*-
;;;
;;; Compute factorials
;;;
;;; (c) 2006 Mark Wooding
;;;

;;;----- Licensing notice ---------------------------------------------------
;;;
;;; This program is free software; you can redistribute it and/or modify
;;; it under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 2 of the License, or
;;; (at your option) any later version.
;;;
;;; This program is distributed in the hope that it will be useful,
;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with this program; if not, write to the Free Software Foundation,
;;; Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

(defpackage #:mdw.factorial
  (:use #:common-lisp))
(in-package #:mdw.factorial)

(export 'factorial)
(defun factorial (n)
  "Compute a factorial."

  ;; This is a little bit optimized: we try to multiply values which are
  ;; similar in size.
  (when (minusp n)
    (error "negative factorial argument ~A" n))
  (do ((i 2 (1+ i))
       (stack nil (do ((s stack (cdr s))
		       (f i (* f (car s))))
		      ((or (null s) (< f (car s)))
		       (cons f s)))))
      ((> i n)
       (do ((s stack (cdr s))
	    (a 1 (* a (car s))))
	   ((null s) a)))))

;;;----- That's all, folks --------------------------------------------------