Merge pull request ReactiveX#140 from mairbek/TakeLast

Implemented TakeLast operator

Author: benjchristensen

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

@@ -46,6 +46,7 @@
 import rx.operators.OperationSkip;
 import rx.operators.OperationSynchronize;
 import rx.operators.OperationTake;
+import rx.operators.OperationTakeLast;
 import rx.operators.OperationToObservableFuture;
 import rx.operators.OperationToObservableIterable;
 import rx.operators.OperationToObservableList;
@@ -1347,6 +1348,22 @@ public static <T> Observable<T> take(final Observable<T> items, final int num) {
         return _create(OperationTake.take(items, num));
     }
 
+    /**
+     * Returns an Observable that emits the last <code>count</code> items emitted by the source
+     * Observable.
+     *
+     * @param items
+     *            the source Observable
+     * @param count
+     *            the number of items from the end of the sequence emitted by the source
+     *            Observable to emit
+     * @return an Observable that only emits the last <code>count</code> items emitted by the source
+     *         Observable
+     */
+    public static <T> Observable<T> takeLast(final Observable<T> items, final int count) {
+        return _create(OperationTakeLast.takeLast(items, count));
+    }
+
     /**
      * Returns an Observable that emits a single item, a list composed of all the items emitted by
      * the source Observable.
@@ -2279,6 +2296,20 @@ public Observable<T> take(final int num) {
         return take(this, num);
     }
 
+    /**
+     * Returns an Observable that emits the last <code>count</code> items emitted by the source
+     * Observable.
+     *
+     * @param count
+     *            the number of items from the end of the sequence emitted by the source
+     *            Observable to emit
+     * @return an Observable that only emits the last <code>count</code> items emitted by the source
+     *         Observable
+     */
+    public Observable<T> takeLast(final int count) {
+        return takeLast(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/OperationTakeLast.java

@@ -0,0 +1,174 @@
+/**
+ * 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 org.junit.Test;
+import rx.Observable;
+import rx.Observer;
+import rx.Subscription;
+import rx.util.AtomicObservableSubscription;
+import rx.util.functions.Func1;
+
+import java.util.Iterator;
+import java.util.concurrent.LinkedBlockingDeque;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.mockito.Matchers.any;
+import static org.mockito.Mockito.*;
+
+/**
+ * Returns a specified number of contiguous elements from the end of an observable sequence.
+ */
+public final class OperationTakeLast {
+
+    public static <T> Func1<Observer<T>, Subscription> takeLast(final Observable<T> items, final int count) {
+        return new Func1<Observer<T>, Subscription>() {
+
+            @Override
+            public Subscription call(Observer<T> observer) {
+                return new TakeLast<T>(items, count).call(observer);
+            }
+
+        };
+    }
+
+    private static class TakeLast<T> implements Func1<Observer<T>, Subscription> {
+        private final int count;
+        private final Observable<T> items;
+        private final AtomicObservableSubscription subscription = new AtomicObservableSubscription();
+
+        TakeLast(final Observable<T> items, final int count) {
+            this.count = count;
+            this.items = items;
+        }
+
+        public Subscription call(Observer<T> observer) {
+            return subscription.wrap(items.subscribe(new ItemObserver(observer)));
+        }
+
+        private class ItemObserver implements Observer<T> {
+
+            private LinkedBlockingDeque<T> deque = new LinkedBlockingDeque<T>(count);
+            private final Observer<T> observer;
+
+            public ItemObserver(Observer<T> observer) {
+                this.observer = observer;
+            }
+
+            @Override
+            public void onCompleted() {
+                Iterator<T> reverse = deque.descendingIterator();
+                while (reverse.hasNext()) {
+                    observer.onNext(reverse.next());
+                }
+                observer.onCompleted();
+            }
+
+            @Override
+            public void onError(Exception e) {
+                observer.onError(e);
+            }
+
+            @Override
+            public void onNext(T args) {
+                while (!deque.offerFirst(args)) {
+                    deque.removeLast();
+                }
+            }
+
+        }
+
+    }
+
+    public static class UnitTest {
+
+        @Test
+        public void testTakeLastEmpty() {
+            Observable<String> w = Observable.toObservable();
+            Observable<String> take = Observable.create(takeLast(w, 2));
+
+            @SuppressWarnings("unchecked")
+            Observer<String> aObserver = mock(Observer.class);
+            take.subscribe(aObserver);
+            verify(aObserver, never()).onNext(any(String.class));
+            verify(aObserver, never()).onError(any(Exception.class));
+            verify(aObserver, times(1)).onCompleted();
+        }
+
+        @Test
+        public void testTakeLast1() {
+            Observable<String> w = Observable.toObservable("one", "two", "three");
+            Observable<String> take = Observable.create(takeLast(w, 2));
+
+            @SuppressWarnings("unchecked")
+            Observer<String> aObserver = mock(Observer.class);
+            take.subscribe(aObserver);
+            verify(aObserver, times(1)).onNext("two");
+            verify(aObserver, times(1)).onNext("three");
+            verify(aObserver, never()).onNext("one");
+            verify(aObserver, never()).onError(any(Exception.class));
+            verify(aObserver, times(1)).onCompleted();
+        }
+
+        @Test
+        public void testTakeLast2() {
+            Observable<String> w = Observable.toObservable("one");
+            Observable<String> take = Observable.create(takeLast(w, 10));
+
+            @SuppressWarnings("unchecked")
+            Observer<String> aObserver = mock(Observer.class);
+            take.subscribe(aObserver);
+            verify(aObserver, times(1)).onNext("one");
+            verify(aObserver, never()).onError(any(Exception.class));
+            verify(aObserver, times(1)).onCompleted();
+        }
+
+        @Test
+        public void testTakeLastOrdering() {
+            Observable<String> w = Observable.toObservable("one", "two", "three");
+            Observable<String> take = Observable.create(takeLast(w, 2));
+
+            @SuppressWarnings("unchecked")
+            Observer<String> aObserver = mock(Observer.class);
+            take.subscribe(countingWrapper(aObserver));
+            verify(aObserver, times(1)).onNext("two_1");
+            verify(aObserver, times(1)).onNext("three_2");
+        }
+
+
+        private static Observer<String> countingWrapper(final Observer<String> underlying) {
+            return new Observer<String>() {
+                private final AtomicInteger counter = new AtomicInteger();
+                @Override
+                public void onCompleted() {
+                    underlying.onCompleted();
+                }
+
+                @Override
+                public void onError(Exception e) {
+                    underlying.onCompleted();
+                }
+
+                @Override
+                public void onNext(String args) {
+                    underlying.onNext(args + "_" + counter.incrementAndGet());
+                }
+            };
+        }
+
+    }
+
+}