Merge pull request ReactiveX#414 from zsxwing/skip-last

Implemented the 'SkipLast' operator

rxjava-core/src/main/java/rx/Observable.java

@@ -63,6 +63,7 @@
 import rx.operators.OperationSample;
 import rx.operators.OperationScan;
 import rx.operators.OperationSkip;
+import rx.operators.OperationSkipLast;
 import rx.operators.OperationSkipWhile;
 import rx.operators.OperationSubscribeOn;
 import rx.operators.OperationSum;
@@ -4079,6 +4080,30 @@ public Observable<T> skipWhile(Func1<? super T, Boolean> predicate) {
         return create(OperationSkipWhile.skipWhile(this, predicate));
     }
 
+    /**
+     * Bypasses a specified number of elements at the end of an observable
+     * sequence.
+     * <p>
+     * This operator accumulates a queue with a length enough to store the first
+     * count elements. As more elements are received, elements are taken from
+     * the front of the queue and produced on the result sequence. This causes
+     * elements to be delayed.
+     *
+     * @param count
+     *            number of elements to bypass at the end of the source
+     *            sequence.
+     * @return An observable sequence containing the source sequence elements
+     *         except for the bypassed ones at the end.
+     *
+     * @throws IndexOutOfBoundsException
+     *             count is less than zero.
+     *
+     * @see <a href="http://msdn.microsoft.com/en-us/library/hh211750(v=vs.103).aspx">MSDN: Observable.SkipLast</a>
+     */
+    public Observable<T> skipLast(int count) {
+        return create(OperationSkipLast.skipLast(this, count));
+    }
+
     /**
      * Returns an Observable that emits a single item, a list composed of all the items emitted by
      * the source Observable.

rxjava-core/src/main/java/rx/operators/OperationSkipLast.java

@@ -0,0 +1,224 @@
+/**
+ * Copyright 2013 Netflix, Inc.
+ * 
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package rx.operators;
+
+import static org.mockito.Matchers.any;
+import static org.mockito.Mockito.inOrder;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+
+import java.util.Deque;
+import java.util.LinkedList;
+import java.util.concurrent.locks.ReentrantLock;
+
+import org.junit.Test;
+import org.mockito.InOrder;
+
+import rx.Observable;
+import rx.Observable.OnSubscribeFunc;
+import rx.Observer;
+import rx.Subscription;
+
+/**
+ * Bypasses a specified number of elements at the end of an observable sequence.
+ */
+public class OperationSkipLast {
+
+    /**
+     * Bypasses a specified number of elements at the end of an observable
+     * sequence.
+     * <p>
+     * This operator accumulates a queue with a length enough to store the first
+     * count elements. As more elements are received, elements are taken from
+     * the front of the queue and produced on the result sequence. This causes
+     * elements to be delayed.
+     *
+     * @param source
+     *            the source sequence.
+     * @param count
+     *            number of elements to bypass at the end of the source
+     *            sequence.
+     * @return An observable sequence containing the source sequence elements
+     *         except for the bypassed ones at the end.
+     *
+     * @throws IndexOutOfBoundsException
+     *             count is less than zero.
+     */
+    public static <T> OnSubscribeFunc<T> skipLast(
+            Observable<? extends T> source, int count) {
+        return new SkipLast<T>(source, count);
+    }
+
+    private static class SkipLast<T> implements OnSubscribeFunc<T> {
+        private final int count;
+        private final Observable<? extends T> source;
+
+        private SkipLast(Observable<? extends T> source, int count) {
+            this.count = count;
+            this.source = source;
+        }
+
+        public Subscription onSubscribe(final Observer<? super T> observer) {
+            if (count < 0) {
+                throw new IndexOutOfBoundsException(
+                        "count could not be negative");
+            }
+            final SafeObservableSubscription subscription = new SafeObservableSubscription();
+            return subscription.wrap(source.subscribe(new Observer<T>() {
+
+                private final ReentrantLock lock = new ReentrantLock();
+
+                /**
+                 * Store the last count elements until now.
+                 */
+                private final Deque<T> deque = new LinkedList<T>();
+
+                @Override
+                public void onCompleted() {
+                    observer.onCompleted();
+                }
+
+                @Override
+                public void onError(Throwable e) {
+                    observer.onError(e);
+                }
+
+                @Override
+                public void onNext(T value) {
+                    if (count == 0) {
+                        // If count == 0, we do not need to put value into deque
+                        // and remove it at once. We can emit the value
+                        // directly.
+                        try {
+                            observer.onNext(value);
+                        } catch (Throwable ex) {
+                            observer.onError(ex);
+                            subscription.unsubscribe();
+                        }
+                        return;
+                    }
+                    lock.lock();
+                    try {
+                        deque.offerLast(value);
+                        if (deque.size() > count) {
+                            // Now deque has count + 1 elements, so the first
+                            // element in the deque definitely does not belong
+                            // to the last count elements of the source
+                            // sequence. We can emit it now.
+                            observer.onNext(deque.removeFirst());
+                        }
+                    } catch (Throwable ex) {
+                        observer.onError(ex);
+                        subscription.unsubscribe();
+                    } finally {
+                        lock.unlock();
+                    }
+                }
+
+            }));
+        }
+    }
+
+    public static class UnitTest {
+
+        @Test
+        public void testSkipLastEmpty() {
+            Observable<String> w = Observable.empty();
+            Observable<String> observable = Observable.create(skipLast(w, 2));
+
+            @SuppressWarnings("unchecked")
+            Observer<String> aObserver = mock(Observer.class);
+            observable.subscribe(aObserver);
+            verify(aObserver, never()).onNext(any(String.class));
+            verify(aObserver, never()).onError(any(Throwable.class));
+            verify(aObserver, times(1)).onCompleted();
+        }
+
+        @Test
+        public void testSkipLast1() {
+            Observable<String> w = Observable.from("one", "two", "three");
+            Observable<String> observable = Observable.create(skipLast(w, 2));
+
+            @SuppressWarnings("unchecked")
+            Observer<String> aObserver = mock(Observer.class);
+            InOrder inOrder = inOrder(aObserver);
+            observable.subscribe(aObserver);
+            inOrder.verify(aObserver, never()).onNext("two");
+            inOrder.verify(aObserver, never()).onNext("three");
+            verify(aObserver, times(1)).onNext("one");
+            verify(aObserver, never()).onError(any(Throwable.class));
+            verify(aObserver, times(1)).onCompleted();
+        }
+
+        @Test
+        public void testSkipLast2() {
+            Observable<String> w = Observable.from("one", "two");
+            Observable<String> observable = Observable.create(skipLast(w, 2));
+
+            @SuppressWarnings("unchecked")
+            Observer<String> aObserver = mock(Observer.class);
+            observable.subscribe(aObserver);
+            verify(aObserver, never()).onNext(any(String.class));
+            verify(aObserver, never()).onError(any(Throwable.class));
+            verify(aObserver, times(1)).onCompleted();
+        }
+
+        @Test
+        public void testSkipLastWithZeroCount() {
+            Observable<String> w = Observable.from("one", "two");
+            Observable<String> observable = Observable.create(skipLast(w, 0));
+
+            @SuppressWarnings("unchecked")
+            Observer<String> aObserver = mock(Observer.class);
+            observable.subscribe(aObserver);
+            verify(aObserver, times(1)).onNext("one");
+            verify(aObserver, times(1)).onNext("two");
+            verify(aObserver, never()).onError(any(Throwable.class));
+            verify(aObserver, times(1)).onCompleted();
+        }
+
+        @Test
+        public void testSkipLastWithNull() {
+            Observable<String> w = Observable.from("one", null, "two");
+            Observable<String> observable = Observable.create(skipLast(w, 1));
+
+            @SuppressWarnings("unchecked")
+            Observer<String> aObserver = mock(Observer.class);
+            observable.subscribe(aObserver);
+            verify(aObserver, times(1)).onNext("one");
+            verify(aObserver, times(1)).onNext(null);
+            verify(aObserver, never()).onNext("two");
+            verify(aObserver, never()).onError(any(Throwable.class));
+            verify(aObserver, times(1)).onCompleted();
+        }
+
+        @Test
+        public void testSkipLastWithNegativeCount() {
+            Observable<String> w = Observable.from("one");
+            Observable<String> observable = Observable.create(skipLast(w, -1));
+
+            @SuppressWarnings("unchecked")
+            Observer<String> aObserver = mock(Observer.class);
+            observable.subscribe(aObserver);
+            verify(aObserver, never()).onNext(any(String.class));
+            verify(aObserver, times(1)).onError(
+                    any(IndexOutOfBoundsException.class));
+            verify(aObserver, never()).onCompleted();
+        }
+    }
+}