math.factorials: adding subfactorial.

factor · Dec 8, 2024 · dcaf7a0 · dcaf7a0
1 parent 00b1cff
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diff --git a/extra/math/factorials/factorials-tests.factor b/extra/math/factorials/factorials-tests.factor
@@ -80,3 +80,27 @@ USING: kernel math.factorials math.functions ranges sequences tools.test ;
 { 3 } [ 6 reverse-factorial ] unit-test
 { 1 } [ 1 reverse-factorial ] unit-test
 { f } [ 18 reverse-factorial ] unit-test
+
+{
+    {
+        1
+        0
+        1
+        2
+        9
+        44
+        265
+        1854
+        14833
+        133496
+        1334961
+        14684570
+        176214841
+        2290792932
+        32071101049
+        481066515734
+        7697064251745
+        130850092279664
+        2355301661033953
+    }
+} [ 19 <iota> [ subfactorial ] map ] unit-test
diff --git a/extra/math/factorials/factorials.factor b/extra/math/factorials/factorials.factor
@@ -2,7 +2,7 @@
 ! See https://factorcode.org/license.txt for BSD license
 
 USING: combinators combinators.short-circuit inverse kernel
-math math.functions math.primes ranges sequences ;
+math math.constants math.functions math.primes ranges sequences ;
 
 IN: math.factorials
 
@@ -123,3 +123,8 @@ PRIVATE>
     1 1 [ 2over > ] [ 1 + [ * ] keep ] while [ = ] dip and ;
 
 \ factorial [ reverse-factorial ] define-inverse
+
+: subfactorial ( n -- ? )
+    [ 1 ] [ factorial 1 + e /i ] if-zero ;
+
+ALIAS: !n subfactorial