/**
 * 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 rx.Observable.Operator;
import rx.Subscriber;

/**
 * Returns an Observable that emits the at most the last <code>count</code> items emitted by the source Observable.
 * <p>
 * <img width="640" src="https://github.com/ReactiveX/RxJava/wiki/images/rx-operators/last.png" alt="">
 */
public final class OperatorTakeLast<T> implements Operator<T, T> {

    final int count;

    public OperatorTakeLast(int count) {
        if (count < 0) {
            throw new IndexOutOfBoundsException("count cannot 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 TakeLastQueueProducer<T> producer = new TakeLastQueueProducer<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));
            }
        };
    }
}
