Data commit

Task/Permutations-Derangements/Raku/permutations-derangements.raku

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+sub derangements(@l) {
+    @l.permutations.grep(-> @p { none(@p Zeqv @l) })
+}
+
+sub prefix:<!>(Int $n) {
+    (1, 0, 1, -> $a, $b { ($++ + 2) × ($b + $a) } ... *)[$n]
+}
+
+say 'derangements([1, 2, 3, 4])';
+say derangements([1, 2, 3, 4]), "\n";
+
+say 'n == !n == derangements(^n).elems';
+for 0 .. 9 -> $n {
+    say "!$n == { !$n } == { derangements(^$n).elems }"
+}