improve permutations

using @permutations avoids computing permutations($n - 1) several times Signed-off-by: Moritz Lenz <moritz@faui2k3.org>
rakudo · Nov 15, 2015 · 57b83f4 · 57b83f4 · grondilu · Nov 19, 2015
1 parent 8ddc787
commit 57b83f4
Showing 1 changed file with 11 additions and 8 deletions.
diff --git a/src/core/native_array.pm b/src/core/native_array.pm
@@ -519,13 +519,16 @@ class array does Iterable is repr('VMArray') {
 sub permutations(int $n) {
     my int $i;
     $n == 1 ?? ( (0,), ) !!
-    gather while $i < $n {
-        my Int @i;
-        my int $j;
-        @i.push($j++) while $j < $i;
-        $j = $i + 1;
-        @i.push($j++) while $j < $n;
-        take (nqp::clone($i), |@i[@$_]) for permutations($n - 1);
-        $i = $i + 1;
+    gather {
+        my @permutations = permutations($n - 1);
+        while $i < $n {
+            my Int @i;
+            my int $j;
+            @i.push($j++) while $j < $i;
+            $j = $i + 1;
+            @i.push($j++) while $j < $n;
+            take (nqp::clone($i), |@i[@$_]) for @permutations;
+            $i = $i + 1;
+        }
     }
 }