math.combinatorics: faster version of factorial.

basis/math/combinatorics/combinatorics-tests.factor

@@ -1,6 +1,7 @@
 USING: kernel math math.combinatorics math.combinatorics.private tools.test sequences ;
 IN: math.combinatorics.tests
 
+[ 1 ] [ -1 factorial ] unit-test ! required by other math.combinatorics words
 [ 1 ] [ 0 factorial ] unit-test
 [ 1 ] [ 1 factorial ] unit-test
 [ 3628800 ] [ 10 factorial ] unit-test

basis/math/combinatorics/combinatorics.factor

@@ -18,7 +18,7 @@ IN: math.combinatorics
 PRIVATE>
 
 : factorial ( n -- n! )
-    iota 1 [ 1 + * ] reduce ;
+    dup 1 > [ [1,b] product ] [ drop 1 ] if ;
 
 : nPk ( n k -- nPk )
     2dup possible? [ dupd - [a,b) product ] [ 2drop 0 ] if ;