PrependAll in preparation for intersperse

jnape · jnape · commit d24dadf2fca0 · 2017-01-29T14:00:18.000-06:00
diff --git a/src/main/java/com/jnape/palatable/lambda/functions/builtin/fn2/PrependAll.java b/src/main/java/com/jnape/palatable/lambda/functions/builtin/fn2/PrependAll.java
@@ -0,0 +1,35 @@
+package com.jnape.palatable.lambda.functions.builtin.fn2;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+import com.jnape.palatable.lambda.functions.Fn2;
+
+import static com.jnape.palatable.lambda.functions.builtin.fn1.Head.head;
+import static com.jnape.palatable.lambda.functions.builtin.fn1.Tail.tail;
+import static com.jnape.palatable.lambda.functions.builtin.fn2.Cons.cons;
+import static java.util.Collections.emptyList;
+
+public final class PrependAll<A> implements Fn2<A, Iterable<A>, Iterable<A>> {
+
+    private static final PrependAll INSTANCE = new PrependAll();
+
+    private PrependAll() {
+    }
+
+    @Override
+    public Iterable<A> apply(A a, Iterable<A> as) {
+        return () -> head(as).map(head -> cons(a, cons(head, prependAll(a, tail(as))))).orElse(emptyList()).iterator();
+    }
+
+    @SuppressWarnings("unchecked")
+    public static <A> PrependAll<A> prependAll() {
+        return INSTANCE;
+    }
+
+    public static <A> Fn1<Iterable<A>, Iterable<A>> prependAll(A a) {
+        return PrependAll.<A>prependAll().apply(a);
+    }
+
+    public static <A> Iterable<A> prependAll(A a, Iterable<A> as) {
+        return prependAll(a).apply(as);
+    }
+}
diff --git a/src/test/java/com/jnape/palatable/lambda/functions/builtin/fn2/PrependAllTest.java b/src/test/java/com/jnape/palatable/lambda/functions/builtin/fn2/PrependAllTest.java
@@ -0,0 +1,38 @@
+package com.jnape.palatable.lambda.functions.builtin.fn2;
+
+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.Laziness;
+
+import static com.jnape.palatable.lambda.functions.builtin.fn2.PrependAll.prependAll;
+import static java.util.Arrays.asList;
+import static java.util.Collections.emptyList;
+import static org.junit.Assert.assertThat;
+import static testsupport.matchers.IterableMatcher.isEmpty;
+import static testsupport.matchers.IterableMatcher.iterates;
+
+@RunWith(Traits.class)
+public class PrependAllTest {
+
+    @TestTraits({Laziness.class, FiniteIteration.class, EmptyIterableSupport.class, ImmutableIteration.class})
+    public Fn1<Iterable<Object>, Iterable<Object>> testSubject() {
+        return prependAll(0);
+    }
+
+    @Test
+    public void prependsValueToAllElementsInIterable() {
+        Iterable<Integer> ints = asList(1, 2, 3);
+        assertThat(prependAll(0, ints), iterates(0, 1, 0, 2, 0, 3));
+    }
+
+    @Test
+    public void prependingToEmptyIterableIsStillEmpty() {
+        assertThat(prependAll(0, emptyList()), isEmpty());
+    }
+}
