/**
* Copyright 2014 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.internal.operators;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.concurrent.atomic.AtomicLongFieldUpdater;
import rx.Observable.Operator;
import rx.Producer;
import rx.Subscriber;
/**
* Returns an Observable that emits the last <code>count</code> items emitted by the source Observable.
* <p>
* <img width="640" src="https://github.com/Netflix/RxJava/wiki/images/rx-operators/last.png" alt="">
*/
public final class OperatorTakeLast<T> implements Operator<T, T> {
private final int count;
public OperatorTakeLast(int count) {
if (count < 0) {
throw new IndexOutOfBoundsException("count could not be negative");
}
this.count = count;
}
@Override
public Subscriber<? super T> call(final Subscriber<? super T> subscriber) {
final Deque<Object> deque = new ArrayDeque<Object>();
final NotificationLite<T> notification = NotificationLite.instance();
final QueueProducer<T> producer = new QueueProducer<T>(notification, deque, subscriber);
subscriber.setProducer(producer);
return new Subscriber<T>(subscriber) {
// no backpressure up as it wants to receive and discard all but the last
@Override
public void onStart() {
// we do this to break the chain of the child subscriber being passed through
request(Long.MAX_VALUE);
}
@Override
public void onCompleted() {
deque.offer(notification.completed());
producer.startEmitting();
}
@Override
public void onError(Throwable e) {
deque.clear();
subscriber.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 ignore the value directly.
return;
}
if (deque.size() == count) {
deque.removeFirst();
}
deque.offerLast(notification.next(value));
}
};
}
private static final class QueueProducer<T> implements Producer {
private final NotificationLite<T> notification;
private final Deque<Object> deque;
private final Subscriber<? super T> subscriber;
private volatile boolean emittingStarted = false;
public QueueProducer(NotificationLite<T> n, Deque<Object> q, Subscriber<? super T> subscriber) {
this.notification = n;
this.deque = q;
this.subscriber = subscriber;
}
private volatile long requested = 0;
@SuppressWarnings("rawtypes")
private static final AtomicLongFieldUpdater<QueueProducer> REQUESTED_UPDATER = AtomicLongFieldUpdater.newUpdater(QueueProducer.class, "requested");
void startEmitting() {
if (!emittingStarted) {
emittingStarted = true;
emit(0); // start emitting
}
}
@Override
public void request(long n) {
if (requested == Long.MAX_VALUE) {
return;
}
long _c;
if (n == Long.MAX_VALUE) {
_c = REQUESTED_UPDATER.getAndSet(this, Long.MAX_VALUE);
} else {
_c = REQUESTED_UPDATER.getAndAdd(this, n);
}
if (!emittingStarted) {
// we haven't started yet, so record what was requested and return
return;
}
emit(_c);
}
void emit(long previousRequested) {
if (requested == Long.MAX_VALUE) {
// fast-path without backpressure
if (previousRequested == 0) {
try {
for (Object value : deque) {
notification.accept(subscriber, value);
}
} catch (Throwable e) {
subscriber.onError(e);
} finally {
deque.clear();
}
} else {
// backpressure path will handle Long.MAX_VALUE and emit the rest events.
}
} else {
// backpressure is requested
if (previousRequested == 0) {
while (true) {
/*
* This complicated logic is done to avoid touching the volatile `requested` value
* during the loop itself. If it is touched during the loop the performance is impacted significantly.
*/
long numToEmit = requested;
int emitted = 0;
Object o;
while (--numToEmit >= 0 && (o = deque.poll()) != null) {
if (subscriber.isUnsubscribed()) {
return;
}
if (notification.accept(subscriber, o)) {
// terminal event
return;
} else {
emitted++;
}
}
for (;;) {
long oldRequested = requested;
long newRequested = oldRequested - emitted;
if (oldRequested == Long.MAX_VALUE) {
// became unbounded during the loop
// continue the outer loop to emit the rest events.
break;
}
if (REQUESTED_UPDATER.compareAndSet(this, oldRequested, newRequested)) {
if (newRequested == 0) {
// we're done emitting the number requested so return
return;
}
break;
}
}
}
}
}
}
}
}