adding tail()

jnape · jnape · commit de00ae94aa2a · 2016-11-19T13:23:13.000-06:00
diff --git a/src/main/java/com/jnape/palatable/lambda/functions/builtin/fn1/Tail.java b/src/main/java/com/jnape/palatable/lambda/functions/builtin/fn1/Tail.java
@@ -0,0 +1,35 @@
+package com.jnape.palatable.lambda.functions.builtin.fn1;
+
+import com.jnape.palatable.lambda.functions.Fn1;
+
+import java.util.Iterator;
+
+/**
+ * Returns the tail of an <code>Iterable</code>; the is, an <code>Iterable</code> of all the elements except for the
+ * head element. If the input <code>Iterable</code> is empty, the result is also an empty <code>Iterable</code>;
+ *
+ * @param <A> the Iterable element type
+ */
+public final class Tail<A> implements Fn1<Iterable<A>, Iterable<A>> {
+
+    private Tail() {
+    }
+
+    @Override
+    public Iterable<A> apply(Iterable<A> as) {
+        return () -> {
+            Iterator<A> iterator = as.iterator();
+            if (iterator.hasNext())
+                iterator.next();
+            return iterator;
+        };
+    }
+
+    public static <A> Tail<A> tail() {
+        return new Tail<>();
+    }
+
+    public static <A> Iterable<A> tail(Iterable<A> as) {
+        return Tail.<A>tail().apply(as);
+    }
+}
diff --git a/src/test/java/com/jnape/palatable/lambda/functions/builtin/fn1/TailTest.java b/src/test/java/com/jnape/palatable/lambda/functions/builtin/fn1/TailTest.java
@@ -0,0 +1,36 @@
+package com.jnape.palatable.lambda.functions.builtin.fn1;
+
+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.fn1.Tail.tail;
+import static java.util.Arrays.asList;
+import static java.util.Collections.emptyList;
+import static org.junit.Assert.assertThat;
+import static testsupport.matchers.IterableMatcher.iterates;
+
+@RunWith(Traits.class)
+public class TailTest {
+
+    @TestTraits({Laziness.class, EmptyIterableSupport.class, FiniteIteration.class, ImmutableIteration.class})
+    public Fn1<? extends Iterable, ?> createTraitsTestSubject() {
+        return tail();
+    }
+
+    @Test
+    public void skipsFirstElementOfNonEmptyList() {
+        assertThat(tail(asList(1, 2, 3)), iterates(2, 3));
+    }
+
+    @Test
+    public void isEmptyIfEmptyIterable() {
+        assertThat(tail(emptyList()), iterates());
+    }
+}
