Commit | Line | Data |
---|---|---|
874125c4 MW |
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 | (:export #:factorial)) | |
27 | (in-package #:mdw.factorial) | |
28 | ||
29 | (defun factorial (n) | |
30 | "Compute a factorial. This is a little bit optimized: we try to multiply | |
0ff9df03 | 31 | values which are similar in size." |
874125c4 MW |
32 | (when (minusp n) |
33 | (error "negative factorial argument ~A" n)) | |
34 | (let ((stack nil)) | |
35 | (do ((i 2 (1+ i))) | |
36 | ((> i n)) | |
37 | (let ((f i)) | |
38 | (loop | |
39 | (unless stack (return)) | |
40 | (let ((top (car stack))) | |
41 | (when (< f top) (return)) | |
42 | (setf f (* f top)) | |
43 | (pop stack))) | |
44 | (push f stack))) | |
45 | (do ((stack stack (cdr stack)) | |
46 | (a 1 (* a (car stack)))) | |
47 | ((null stack) a)))) | |
48 | ||
49 | ;;;----- That's all, folks -------------------------------------------------- |