improve Array.permutations() performance #1995 (#1996)

* improve Array.permutations() performance #1995 * revert to very old algorithm #1995
vavr-io · Sep 16, 2017 · af215c6 · af215c6
1 parent 4a6ef47
commit af215c6
Showing 1 changed file with 5 additions and 7 deletions.
diff --git a/vavr/src/main/java/io/vavr/collection/Array.java b/vavr/src/main/java/io/vavr/collection/Array.java
@@ -989,13 +989,11 @@ public Array<Array<T>> permutations() {
         } else if (delegate.length == 1) {
             return of(this);
         } else {
-            Array<Array<T>> results = empty();
-            for (T t : distinct()) {
-                for (Array<T> ts : remove(t).permutations()) {
-                    results = results.append(of(t).appendAll(ts));
-                }
-            }
-            return results;
+            final Array<Array<T>> zero = empty();
+            return distinct().foldLeft(zero, (xs, x) -> {
+                final Function<Array<T>, Array<T>> prepend = l -> l.prepend(x);
+                return xs.appendAll(remove(x).permutations().map(prepend));
+            });
         }
     }