Union

jnape · jnape · commit 77f8e02d289d · 2018-06-10T16:14:02.000-05:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 - `CmpEqBy`, `CmpEq`, `GTBy`, `GT`, `LTBy`, `LT`, `GTEBy`, `GTE`, `LTEBy`, and `LTE` inequality checks
 - `MinBy`, `MaxBy`, `Min`, and `Max` semigroups
 - `Product2-8` interfaces, representing general product types
+- `Union`, a semigroup that behaves like a lazy set union on `Iterable`s 
 
 ### Changed
 - `Tuple2-8` now implement `Product2-8`
diff --git a/src/main/java/com/jnape/palatable/lambda/monoid/builtin/Union.java b/src/main/java/com/jnape/palatable/lambda/monoid/builtin/Union.java
@@ -0,0 +1,47 @@
+package com.jnape.palatable.lambda.monoid.builtin;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+import com.jnape.palatable.lambda.monoid.Monoid;
+
+import java.util.Collections;
+
+import static com.jnape.palatable.lambda.functions.builtin.fn1.Distinct.distinct;
+import static com.jnape.palatable.lambda.monoid.builtin.Concat.concat;
+
+/**
+ * Given two {@link Iterable}s, return the union of all unique occurrences of elements between them. Note that this
+ * operation preserves order, so the unique elements of the first {@link Iterable} are iterated before the unique
+ * elements of the second {@link Iterable}.
+ *
+ * @param <A> the {@link Iterable} element type)
+ */
+public final class Union<A> implements Monoid<Iterable<A>> {
+
+    private static final Union INSTANCE = new Union();
+
+    private Union() {
+    }
+
+    @Override
+    public Iterable<A> identity() {
+        return Collections::emptyIterator;
+    }
+
+    @Override
+    public Iterable<A> apply(Iterable<A> xs, Iterable<A> ys) {
+        return distinct(concat(xs, ys));
+    }
+
+    @SuppressWarnings("unchecked")
+    public static <A> Union<A> union() {
+        return INSTANCE;
+    }
+
+    public static <A> Fn1<Iterable<A>, Iterable<A>> union(Iterable<A> xs) {
+        return Union.<A>union().apply(xs);
+    }
+
+    public static <A> Iterable<A> union(Iterable<A> xs, Iterable<A> ys) {
+        return union(xs).apply(ys);
+    }
+}
diff --git a/src/test/java/com/jnape/palatable/lambda/monoid/builtin/UnionTest.java b/src/test/java/com/jnape/palatable/lambda/monoid/builtin/UnionTest.java
@@ -0,0 +1,44 @@
+package com.jnape.palatable.lambda.monoid.builtin;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+import com.jnape.palatable.traitor.annotations.TestTraits;
+import com.jnape.palatable.traitor.runners.Traits;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import testsupport.traits.EmptyIterableSupport;
+import testsupport.traits.FiniteIteration;
+import testsupport.traits.ImmutableIteration;
+import testsupport.traits.InfiniteIterableSupport;
+import testsupport.traits.Laziness;
+
+import static com.jnape.palatable.lambda.monoid.builtin.Union.union;
+import static java.util.Arrays.asList;
+import static java.util.Collections.emptyList;
+import static java.util.Collections.singletonList;
+import static org.junit.Assert.assertThat;
+import static testsupport.matchers.IterableMatcher.isEmpty;
+import static testsupport.matchers.IterableMatcher.iterates;
+
+@RunWith(Traits.class)
+public class UnionTest {
+
+    @TestTraits({Laziness.class, InfiniteIterableSupport.class, EmptyIterableSupport.class, FiniteIteration.class, ImmutableIteration.class})
+    public Fn1<Iterable<Integer>, Iterable<Integer>> testSubject() {
+        return union(asList(1, 2, 3));
+    }
+
+    @Test
+    public void identity() {
+        assertThat(union().identity(), isEmpty());
+    }
+
+    @Test
+    public void semigroup() {
+        assertThat(union(emptyList(), emptyList()), isEmpty());
+        assertThat(union(asList(1, 2), emptyList()), iterates(1, 2));
+        assertThat(union(emptyList(), singletonList(3)), iterates(3));
+        assertThat(union(asList(1, 2), singletonList(3)), iterates(1, 2, 3));
+        assertThat(union(asList(1, 2, 2), singletonList(3)), iterates(1, 2, 3));
+        assertThat(union(asList(1, 2), asList(1, 2, 3)), iterates(1, 2, 3));
+    }
+}
