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rxjava-core developed by Netflix
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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 static java.util.concurrent.atomic.AtomicReferenceFieldUpdater.newUpdater;

import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;

import rx.internal.util.MpscPaddedQueue.Node;

abstract class MpscLinkedQueuePad0 {
    long p00, p01, p02, p03, p04, p05, p06, p07;
    long p30, p31, p32, p33, p34, p35, p36, p37;
}

abstract class MpscLinkedQueueHeadRef extends MpscLinkedQueuePad0 {
    @SuppressWarnings("rawtypes")
    private static final AtomicReferenceFieldUpdater UPDATER =
        newUpdater(MpscLinkedQueueHeadRef.class, Node.class, "headRef");
    private volatile Node headRef;

    protected final Node headRef() {
        return headRef;
    }
    protected final void headRef(Node val) {
        headRef = val;
    }
    protected final void lazySetHeadRef(Node newVal) {
        UPDATER.lazySet(this, newVal);
    }
}

abstract class MpscLinkedQueuePad1 extends MpscLinkedQueueHeadRef {
    long p00, p01, p02, p03, p04, p05, p06, p07;
    long p30, p31, p32, p33, p34, p35, p36, p37;
}

abstract class MpscLinkedQueueTailRef extends MpscLinkedQueuePad1 {
    @SuppressWarnings("rawtypes")
    private static final AtomicReferenceFieldUpdater UPDATER =
        newUpdater(MpscLinkedQueueTailRef.class, Node.class, "tailRef");
    private volatile Node tailRef;
    protected final Node tailRef() {
        return tailRef;
    }
    protected final void tailRef(Node val) {
        tailRef = val;
    }
    @SuppressWarnings("unchecked")
    protected final Node getAndSetTailRef(Node newVal) {
        return (Node) UPDATER.getAndSet(this, newVal);
    }
}
/**
 * A multiple-producer single consumer queue implementation with padded reference to tail to avoid cache-line
 * thrashing. Based on Netty's MpscQueue implementation
 * but using {@code AtomicReferenceFieldUpdater} instead of {@code Unsafe}.

 * Original algorithm presented  on 1024
 * Cores by D. Vyukov.

 * Data structure modified to avoid false sharing between head and tail references as per implementation of
 * MpscLinkedQueue on JCTools project.
 * 
 * @param  the element type
 */
public final class MpscPaddedQueue extends MpscLinkedQueueTailRef {
    long p00, p01, p02, p03, p04, p05, p06, p07;
    long p30, p31, p32, p33, p34, p35, p36, p37;
    /**
     * Initializes the empty queue.
     */
    public MpscPaddedQueue() {
        Node stub = new Node(null);
        headRef(stub);
        tailRef(stub);
    }

    /**
     * Offer a new value.
     *
     * @param v the value to offer
     */
    public void offer(E v) {
        Node n = new Node(v);
        getAndSetTailRef(n).next(n);
    }

    /**
     * @warn method description missing
     * @return Poll a value from the head of the queue or return null if the queue is empty.
     */
    public E poll() {
        Node n = peekNode();
        if (n == null) {
            return null;
        }
        E v = n.value;
        n.value = null; // do not retain this value as the node still stays in the queue
        lazySetHeadRef(n);
        return v;
    }
    
    /**
     * Check if there is a node available without changing anything.
     * @return
     */
    private Node peekNode() {
        for (;;) {
            Node t = headRef();
            Node n = t.next();
            if (n != null || headRef() == t) {
                return n;
            }
        }
    }
    /**
     * Clears the queue.
     */
    public void clear() {
        for (;;) {
            if (poll() == null) {
                break;
            }
        }
    }

    /**
     * Regular node with value and reference to the next node.
     */
    static final class Node {
        E value;
        @SuppressWarnings(value = "rawtypes")
        static final AtomicReferenceFieldUpdater TAIL_UPDATER = AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "next");
        private volatile Node next;

        Node(E value) {
            this.value = value;
        }

        void next(Node newNext) {
            TAIL_UPDATER.lazySet(this, newNext);
        }

        Node next() {
            return next;
        }
    }
    
}