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rxjava2-backport developed by David Karnok
There is a newer version: 2.0.0-RC3
/**
 * Copyright 2015 David Karnok and 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.
 */

/*
 * 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 hu.akarnokd.rxjava2.internal.queue;

import java.util.*;
import java.util.concurrent.atomic.AtomicReference;

abstract class BaseLinkedQueue extends AbstractQueue {
    private final AtomicReference> producerNode;
    private final AtomicReference> consumerNode;
    public BaseLinkedQueue() {
        producerNode = new AtomicReference>();
        consumerNode = new AtomicReference>();
    }
    protected final LinkedQueueNode lvProducerNode() {
        return producerNode.get();
    }
    protected final LinkedQueueNode lpProducerNode() {
        return producerNode.get();
    }
    protected final void spProducerNode(LinkedQueueNode node) {
        producerNode.lazySet(node);
    }
    protected final LinkedQueueNode xchgProducerNode(LinkedQueueNode node) {
        return producerNode.getAndSet(node);
    }
    protected final LinkedQueueNode lvConsumerNode() {
        return consumerNode.get();
    }
    
    protected final LinkedQueueNode lpConsumerNode() {
        return consumerNode.get();
    }
    protected final void spConsumerNode(LinkedQueueNode node) {
        consumerNode.lazySet(node);
    }
    @Override
    public final Iterator iterator() {
        throw new UnsupportedOperationException();
    }

    /**
     * {@inheritDoc} 

     * 
     * IMPLEMENTATION NOTES:

     * This is an O(n) operation as we run through all the nodes and count them.

     * 
     * @see java.util.Queue#size()
     */
    @Override
    public final int size() {
        LinkedQueueNode chaserNode = lvConsumerNode();
        final LinkedQueueNode producerNode = lvProducerNode();
        int size = 0;
        // must chase the nodes all the way to the producer node, but there's no need to chase a moving target.
        while (chaserNode != producerNode && size < Integer.MAX_VALUE) {
            LinkedQueueNode next;
            while((next = chaserNode.lvNext()) == null);
            chaserNode = next;
            size++;
        }
        return size;
    }
    /**
     * {@inheritDoc} 

     * 
     * IMPLEMENTATION NOTES:

     * Queue is empty when producerNode is the same as consumerNode. An alternative implementation would be to observe
     * the producerNode.value is null, which also means an empty queue because only the consumerNode.value is allowed to
     * be null.
     */
    @Override
    public final boolean isEmpty() {
        return lvConsumerNode() == lvProducerNode();
    }
}