implement intersection monoid for sets

toxyduck · toxyduck · commit fce0df7b4f6f · 2019-10-20T18:59:38.000+03:00
diff --git a/core/src/main/java/fj/Monoid.java b/core/src/main/java/fj/Monoid.java
@@ -2,14 +2,7 @@
 
 import static fj.F1Functions.dimap;
 
-import fj.data.Array;
-import fj.data.DList;
-import fj.data.List;
-import fj.data.IO;
-import fj.data.Natural;
-import fj.data.Option;
-import fj.data.Set;
-import fj.data.Stream;
+import fj.data.*;
 
 import static fj.Function.*;
 import static fj.Semigroup.semigroupDef;
@@ -1109,6 +1102,20 @@ public Set<A> append(Set<A> a1, Set<A> a2) {
     });
   }
 
+  public static <A> Monoid<Set<A>> setIntersectionMonoid(final Bounded<A> bounded, final Enumerator<A> enumerator, final Ord<A> o) {
+    return monoidDef(new Definition<Set<A>>() {
+      @Override
+      public Set<A> empty() {
+        return Set.iteratorSet(o, enumerator.toStream(bounded).iterator());
+      }
+
+      @Override
+      public Set<A> append(Set<A> a1, Set<A> a2) {
+        return a1.intersect(a2);
+      }
+    });
+  }
+
   /**
    * A monoid for the maximum of elements with ordering o.
    * @deprecated since 4.7. Use {@link Ord#maxMonoid(Object)}
