X-Git-Url: https://git.distorted.org.uk/~mdw/lisp/blobdiff_plain/0ff9df03bb54ba792cefa551face51748ae34259..3dba250d40912df843708c98114a084c731f4132:/factorial.lisp

diff --git a/factorial.lisp b/factorial.lisp
index 59892fe..64c521e 100644
--- a/factorial.lisp
+++ b/factorial.lisp
@@ -11,39 +11,36 @@
 ;;; 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)
-  (:export #: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."
+  "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))
-  (let ((stack nil))
-    (do ((i 2 (1+ i)))
-	((> i n))
-      (let ((f i))
-	(loop
-	  (unless stack (return))
-	  (let ((top (car stack)))
-	    (when (< f top) (return))
-	    (setf f (* f top))
-	    (pop stack)))
-	(push f stack)))
-    (do ((stack stack (cdr stack))
-	 (a 1 (* a (car stack))))
-	((null stack) a))))
+  (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 --------------------------------------------------