September Morn Update

Task/Permutations-Derangements/Factor/permutations-derangements.factor

@@ -0,0 +1,22 @@
+USING: combinators formatting io kernel math math.combinatorics
+prettyprint sequences ;
+IN: rosetta-code.derangements
+
+: !n ( n -- m )
+    {
+        { 0 [ 1 ] }
+        { 1 [ 0 ] }
+        [ [ 1 - !n ] [ 2 - !n + ] [ 1 - * ] tri ]
+    } case ;
+
+: derangements ( n -- seq )
+    <iota> dup [ [ = ] 2map [ f = ] all? ] with
+    filter-permutations ;
+
+"4 derangements" print 4 derangements . nl
+"n   count    calc\n=  ======  ======" print
+10 <iota> [
+    dup [ derangements length ] [ !n ] bi
+    "%d%8d%8d\n" printf
+] each nl
+"!20 = " write 20 !n .