math.factorials: adding reverse-factorial.

factor · Nov 29, 2016 · 1e2e2dc · 1e2e2dc
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diff --git a/extra/math/factorials/factorials-tests.factor b/extra/math/factorials/factorials-tests.factor
@@ -75,3 +75,9 @@ IN: math.factorials
 
 { V{ 3 5 7 29 31 211 2309 2311 } }
 [ 10,000 iota [ primorial-prime? ] filter ] unit-test
+
+{ 10 } [ 3628800 reverse-factorial ] unit-test
+{ 12 } [ 479001600 reverse-factorial ] unit-test
+{ 3 } [ 6 reverse-factorial ] unit-test
+{ 1 } [ 1 reverse-factorial ] unit-test
+{ f } [ 18 reverse-factorial ] unit-test
diff --git a/extra/math/factorials/factorials.factor b/extra/math/factorials/factorials.factor
@@ -118,3 +118,6 @@ PRIVATE>
 
 : primorial-prime? ( n -- ? )
     { [ prime? ] [ 2 > ] [ [ primorial ] -prime? ] } 1&& ;
+
+: reverse-factorial ( m -- n )
+    1 1 [ 2over > ] [ 1 + [ * ] keep ] while [ = ] dip and ;