| 1 | ;;; -*-lisp-*- |
| 2 | ;;; |
| 3 | ;;; Compute factorials |
| 4 | ;;; |
| 5 | ;;; (c) 2006 Mark Wooding |
| 6 | ;;; |
| 7 | |
| 8 | ;;;----- Licensing notice --------------------------------------------------- |
| 9 | ;;; |
| 10 | ;;; This program is free software; you can redistribute it and/or modify |
| 11 | ;;; it under the terms of the GNU General Public License as published by |
| 12 | ;;; the Free Software Foundation; either version 2 of the License, or |
| 13 | ;;; (at your option) any later version. |
| 14 | ;;; |
| 15 | ;;; This program is distributed in the hope that it will be useful, |
| 16 | ;;; but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 17 | ;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 18 | ;;; GNU General Public License for more details. |
| 19 | ;;; |
| 20 | ;;; You should have received a copy of the GNU General Public License |
| 21 | ;;; along with this program; if not, write to the Free Software Foundation, |
| 22 | ;;; Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. |
| 23 | |
| 24 | (defpackage #:mdw.factorial |
| 25 | (:use #:common-lisp)) |
| 26 | (in-package #:mdw.factorial) |
| 27 | |
| 28 | (export 'factorial) |
| 29 | (defun factorial (n) |
| 30 | "Compute a factorial." |
| 31 | |
| 32 | ;; This is a little bit optimized: we try to multiply values which are |
| 33 | ;; similar in size. |
| 34 | (when (minusp n) |
| 35 | (error "negative factorial argument ~A" n)) |
| 36 | (do ((i 2 (1+ i)) |
| 37 | (stack nil (do ((s stack (cdr s)) |
| 38 | (f i (* f (car s)))) |
| 39 | ((or (null s) (< f (car s))) |
| 40 | (cons f s))))) |
| 41 | ((> i n) |
| 42 | (do ((s stack (cdr s)) |
| 43 | (a 1 (* a (car s)))) |
| 44 | ((null s) a))))) |
| 45 | |
| 46 | ;;;----- That's all, folks -------------------------------------------------- |