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/**
 * 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.util;

import java.util.Queue;

import rx.Observer;
import rx.Subscription;
import rx.exceptions.MissingBackpressureException;
import rx.internal.operators.NotificationLite;
import rx.internal.util.unsafe.SpmcArrayQueue;
import rx.internal.util.unsafe.SpscArrayQueue;
import rx.internal.util.unsafe.UnsafeAccess;

/**
 * This assumes Spsc or Spmc usage. This means only a single producer calling the on* methods. This is the Rx contract of an Observer.
 * Concurrent invocations of on* methods will not be thread-safe.
 */
public class RxRingBuffer implements Subscription {

    public static RxRingBuffer getSpscInstance() {
        if (UnsafeAccess.isUnsafeAvailable()) {
            // TODO the SpscArrayQueue isn't ready yet so using SpmcArrayQueue for now
            return new RxRingBuffer(SPMC_POOL, SIZE);
        } else {
            return new RxRingBuffer();
        }
    }

    public static RxRingBuffer getSpmcInstance() {
        if (UnsafeAccess.isUnsafeAvailable()) {
            return new RxRingBuffer(SPMC_POOL, SIZE);
        } else {
            return new RxRingBuffer();
        }
    }

    /**
     * Queue implementation testing that led to current choices of data structures:
     * 
     * With synchronized LinkedList
     * 
 {@code
     * Benchmark                                        Mode   Samples        Score  Score error    Units
     * r.i.RxRingBufferPerf.ringBufferAddRemove1       thrpt         5 19118392.046  1002814.238    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1000    thrpt         5    17891.641      252.747    ops/s
     * 
     * With MpscPaddedQueue (single consumer, so failing 1 unit test)
     * 
     * Benchmark                                        Mode   Samples        Score  Score error    Units
     * r.i.RxRingBufferPerf.ringBufferAddRemove1       thrpt         5 22164483.238  3035027.348    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1000    thrpt         5    23154.303      602.548    ops/s
     * 
     * 
     * With ConcurrentLinkedQueue (tracking count separately)
     * 
     * Benchmark                                        Mode   Samples        Score  Score error    Units
     * r.i.RxRingBufferPerf.ringBufferAddRemove1       thrpt         5 17353906.092   378756.411    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1000    thrpt         5    19224.411     1010.610    ops/s
     * 
     * With ConcurrentLinkedQueue (using queue.size() method for count)
     * 
     * Benchmark                                        Mode   Samples        Score  Score error    Units
     * r.i.RxRingBufferPerf.ringBufferAddRemove1       thrpt         5 23951121.098  1982380.330    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1000    thrpt         5     1142.351       33.592    ops/s
     * 
     * With SynchronizedQueue (synchronized LinkedList ... no object pooling)
     * 
     * r.i.RxRingBufferPerf.createUseAndDestroy1       thrpt         5 33231667.136   685757.510    ops/s
     * r.i.RxRingBufferPerf.createUseAndDestroy1000    thrpt         5    74623.614     5493.766    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1       thrpt         5 22907359.257   707026.632    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1000    thrpt         5    22222.410      320.829    ops/s
     * 
     * With ArrayBlockingQueue
     * 
     * Benchmark                                            Mode   Samples        Score  Score error    Units
     * r.i.RxRingBufferPerf.createUseAndDestroy1       thrpt         5  2389804.664    68990.804    ops/s
     * r.i.RxRingBufferPerf.createUseAndDestroy1000    thrpt         5    27384.274     1411.789    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1       thrpt         5 26497037.559    91176.247    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1000    thrpt         5    17985.144      237.771    ops/s
     * 
     * With ArrayBlockingQueue and Object Pool
     * 
     * Benchmark                                            Mode   Samples        Score  Score error    Units
     * r.i.RxRingBufferPerf.createUseAndDestroy1       thrpt         5 12465685.522   399070.770    ops/s
     * r.i.RxRingBufferPerf.createUseAndDestroy1000    thrpt         5    27701.294      395.217    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1       thrpt         5 26399625.086   695639.436    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1000    thrpt         5    17985.427      253.190    ops/s
     * 
     * With SpscArrayQueue (single consumer, so failing 1 unit test)
     *  - requires access to Unsafe
     * 
     * Benchmark                                        Mode   Samples        Score  Score error    Units
     * r.i.RxRingBufferPerf.createUseAndDestroy1       thrpt         5  1922996.035    49183.766    ops/s
     * r.i.RxRingBufferPerf.createUseAndDestroy1000    thrpt         5    70890.186     1382.550    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1       thrpt         5 80637811.605  3509706.954    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1000    thrpt         5    71822.453     4127.660    ops/s
     * 
     * 
     * With SpscArrayQueue and Object Pool (object pool improves createUseAndDestroy1 by 10x)
     * 
     * Benchmark                                        Mode   Samples        Score  Score error    Units
     * r.i.RxRingBufferPerf.createUseAndDestroy1       thrpt         5 25220069.264  1329078.785    ops/s
     * r.i.RxRingBufferPerf.createUseAndDestroy1000    thrpt         5    72313.457     3535.447    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1       thrpt         5 81863840.884  2191416.069    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1000    thrpt         5    73140.822     1528.764    ops/s
     * 
     * With SpmcArrayQueue
     *  - requires access to Unsafe
     *  
     * Benchmark                                            Mode   Samples        Score  Score error    Units
     * r.i.RxRingBufferPerf.spmcCreateUseAndDestroy1       thrpt         5 27630345.474   769219.142    ops/s
     * r.i.RxRingBufferPerf.spmcCreateUseAndDestroy1000    thrpt         5    80052.046     4059.541    ops/s
     * r.i.RxRingBufferPerf.spmcRingBufferAddRemove1       thrpt         5 44449524.222   563068.793    ops/s
     * r.i.RxRingBufferPerf.spmcRingBufferAddRemove1000    thrpt         5    65231.253     1805.732    ops/s
     * 
     * With SpmcArrayQueue and ObjectPool (object pool improves createUseAndDestroy1 by 10x)
     * 
     * Benchmark                                        Mode   Samples        Score  Score error    Units
     * r.i.RxRingBufferPerf.createUseAndDestroy1       thrpt         5 18489343.061  1011872.825    ops/s
     * r.i.RxRingBufferPerf.createUseAndDestroy1000    thrpt         5    46416.434     1439.144    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove        thrpt         5 38280945.847  1071801.279    ops/s
     * r.i.RxRingBufferPerf.ringBufferAddRemove1000    thrpt         5    42337.663     1052.231    ops/s
     * 
     * --------------
     * 
     * When UnsafeAccess.isUnsafeAvailable() == true we can use the Spmc/SpscArrayQueue implementations.
     * 
     * } 

     */

    private static final NotificationLite on = NotificationLite.instance();

    private Queue queue;

    private final int size;
    private final ObjectPool> pool;

    /**
     * We store the terminal state separately so it doesn't count against the size.
     * We don't just +1 the size since some of the queues require sizes that are a power of 2.
     * This is a subjective thing ... wanting to keep the size (ie 1024) the actual number of onNext
     * that can be sent rather than something like 1023 onNext + 1 terminal event. It also simplifies
     * checking that we have received only 1 terminal event, as we don't need to peek at the last item
     * or retain a boolean flag.
     */
    public volatile Object terminalState;

    public static final int SIZE = 1024;

    private static ObjectPool> SPSC_POOL = new ObjectPool>() {

        @Override
        protected SpscArrayQueue createObject() {
            return new SpscArrayQueue(SIZE);
        }

    };

    private static ObjectPool> SPMC_POOL = new ObjectPool>() {

        @Override
        protected SpmcArrayQueue createObject() {
            return new SpmcArrayQueue(SIZE);
        }

    };
    
    private RxRingBuffer(Queue queue, int size) {
        this.queue = queue;
        this.pool = null;
        this.size = size;
    }

    private RxRingBuffer(ObjectPool> pool, int size) {
        this.pool = pool;
        this.queue = pool.borrowObject();
        this.size = size;
    }

    public void release() {
        if (pool != null) {
            Queue q = queue;
            q.clear();
            queue = null;
            pool.returnObject(q);
        }
    }

    @Override
    public void unsubscribe() {
        release();
    }

    /* package accessible for unit tests */RxRingBuffer() {
        this(new SynchronizedQueue(SIZE), SIZE);
    }

    /**
     * 
     * @param o
     * @throws MissingBackpressureException
     *             if more onNext are sent than have been requested
     */
    public void onNext(Object o) throws MissingBackpressureException {
        if (queue == null) {
            throw new IllegalStateException("This instance has been unsubscribed and the queue is no longer usable.");
        }
        if (!queue.offer(on.next(o))) {
            throw new MissingBackpressureException();
        }
    }

    public void onCompleted() {
        // we ignore terminal events if we already have one
        if (terminalState == null) {
            terminalState = on.completed();
        }
    }

    public void onError(Throwable t) {
        // we ignore terminal events if we already have one
        if (terminalState == null) {
            terminalState = on.error(t);
        }
    }

    public int available() {
        return size - count();
    }

    public int capacity() {
        return size;
    }

    public int count() {
        if (queue == null) {
            return 0;
        }
        return queue.size();
    }

    public boolean isEmpty() {
        if (queue == null) {
            return true;
        }
        return queue.isEmpty();
    }

    public Object poll() {
        if (queue == null) {
            // we are unsubscribed and have released the undelrying queue
            return null;
        }
        Object o;
        o = queue.poll();
        /*
         * benjchristensen July 10 2014 => The check for 'queue.isEmpty()' came from a very rare concurrency bug where poll()
         * is invoked, then an "onNext + onCompleted/onError" arrives before hitting the if check below. In that case,
         * "o == null" and there is a terminal state, but now "queue.isEmpty()" and we should NOT return the terminalState.
         * 
         * The queue.size() check is a double-check that works to handle this, without needing to synchronize poll with on*
         * or needing to enqueue terminalState.
         * 
         * This did make me consider eliminating the 'terminalState' ref and enqueuing it ... but then that requires
         * a +1 of the size, or -1 of how many onNext can be sent. See comment on 'terminalState' above for why it
         * is currently the way it is.
         */
        if (o == null && terminalState != null && queue.isEmpty()) {
            o = terminalState;
            // once emitted we clear so a poll loop will finish
            terminalState = null;
        }
        return o;
    }

    public Object peek() {
        if (queue == null) {
            // we are unsubscribed and have released the undelrying queue
            return null;
        }
        Object o;
        o = queue.peek();
        if (o == null && terminalState != null && queue.isEmpty()) {
            o = terminalState;
        }
        return o;
    }

    public boolean isCompleted(Object o) {
        return on.isCompleted(o);
    }

    public boolean isError(Object o) {
        return on.isError(o);
    }

    public Object getValue(Object o) {
        return on.getValue(o);
    }

    @SuppressWarnings({ "unchecked", "rawtypes" })
    public boolean accept(Object o, Observer child) {
        return on.accept(child, o);
    }

    public Throwable asError(Object o) {
        return on.getError(o);
    }

    @Override
    public boolean isUnsubscribed() {
        return queue == null;
    }

}
 















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