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/**
 * Copyright (c) 2016-present, RxJava Contributors.
 *
 * 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.
 */

/*
 * The code was inspired by the similarly named JCTools class:
 * https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/atomic
 */

package io.reactivex.internal.queue;

import java.util.concurrent.atomic.*;

import io.reactivex.annotations.Nullable;
import io.reactivex.internal.fuseable.SimplePlainQueue;
import io.reactivex.internal.util.Pow2;

/**
 * A Single-Producer-Single-Consumer queue backed by a pre-allocated buffer.
 * 

* This implementation is a mashup of the Fast Flow * algorithm with an optimization of the offer method taken from the BQueue algorithm (a variation on Fast * Flow), and adjusted to comply with Queue.offer semantics with regards to capacity.
* For convenience the relevant papers are available in the resources folder:
* 2010 - Pisa - SPSC Queues on Shared Cache Multi-Core Systems.pdf
* 2012 - Junchang- BQueue- Efficient and Practical Queuing.pdf
*
This implementation is wait free. * * @param the element type of the queue */ public final class SpscArrayQueue extends AtomicReferenceArray implements SimplePlainQueue { private static final long serialVersionUID = -1296597691183856449L; private static final Integer MAX_LOOK_AHEAD_STEP = Integer.getInteger("jctools.spsc.max.lookahead.step", 4096); final int mask; final AtomicLong producerIndex; long producerLookAhead; final AtomicLong consumerIndex; final int lookAheadStep; public SpscArrayQueue(int capacity) { super(Pow2.roundToPowerOfTwo(capacity)); this.mask = length() - 1; this.producerIndex = new AtomicLong(); this.consumerIndex = new AtomicLong(); lookAheadStep = Math.min(capacity / 4, MAX_LOOK_AHEAD_STEP); } @Override public boolean offer(E e) { if (null == e) { throw new NullPointerException("Null is not a valid element"); } // local load of field to avoid repeated loads after volatile reads final int mask = this.mask; final long index = producerIndex.get(); final int offset = calcElementOffset(index, mask); if (index >= producerLookAhead) { int step = lookAheadStep; if (null == lvElement(calcElementOffset(index + step, mask))) { // LoadLoad producerLookAhead = index + step; } else if (null != lvElement(offset)) { return false; } } soElement(offset, e); // StoreStore soProducerIndex(index + 1); // ordered store -> atomic and ordered for size() return true; } @Override public boolean offer(E v1, E v2) { // FIXME return offer(v1) && offer(v2); } @Nullable @Override public E poll() { final long index = consumerIndex.get(); final int offset = calcElementOffset(index); // local load of field to avoid repeated loads after volatile reads final E e = lvElement(offset); // LoadLoad if (null == e) { return null; } soConsumerIndex(index + 1); // ordered store -> atomic and ordered for size() soElement(offset, null); // StoreStore return e; } @Override public boolean isEmpty() { return producerIndex.get() == consumerIndex.get(); } void soProducerIndex(long newIndex) { producerIndex.lazySet(newIndex); } void soConsumerIndex(long newIndex) { consumerIndex.lazySet(newIndex); } @Override public void clear() { // we have to test isEmpty because of the weaker poll() guarantee while (poll() != null || !isEmpty()) { } // NOPMD } int calcElementOffset(long index, int mask) { return (int)index & mask; } int calcElementOffset(long index) { return (int)index & mask; } void soElement(int offset, E value) { lazySet(offset, value); } E lvElement(int offset) { return get(offset); } }





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