org.jctools_voltpatches.queues.atomic.BaseLinkedAtomicQueue Maven / Gradle / Ivy

Show more of this group Show more artifacts with this name
Show all versions of voltdbclient Show documentation
VoltDB client interface libraries
There is a newer version: 13.3.2-preview1
/*
 * 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 org.jctools_voltpatches.queues.atomic;

import java.util.AbstractQueue;
import java.util.Iterator;
import java.util.concurrent.atomic.AtomicReference;

import org.jctools_voltpatches.queues.MessagePassingQueue;

abstract class BaseLinkedAtomicQueue extends AbstractQueue {
    private final AtomicReference> producerNode;
    private final AtomicReference> consumerNode;
    public BaseLinkedAtomicQueue() {
        producerNode = new AtomicReference>();
        consumerNode = new AtomicReference>();
    }
    protected final LinkedQueueAtomicNode lvProducerNode() {
        return producerNode.get();
    }
    protected final LinkedQueueAtomicNode lpProducerNode() {
        return producerNode.get();
    }
    protected final void spProducerNode(LinkedQueueAtomicNode node) {
        producerNode.lazySet(node);
    }
    protected final LinkedQueueAtomicNode xchgProducerNode(LinkedQueueAtomicNode node) {
        return producerNode.getAndSet(node);
    }
    protected final LinkedQueueAtomicNode lvConsumerNode() {
        return consumerNode.get();
    }

    protected final LinkedQueueAtomicNode lpConsumerNode() {
        return consumerNode.get();
    }
    protected final void spConsumerNode(LinkedQueueAtomicNode 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() {
        // Read consumer first, this is important because if the producer is node is 'older' than the consumer the
        // consumer may overtake it (consume past it). This will lead to an infinite loop below.
        LinkedQueueAtomicNode chaserNode = lvConsumerNode();
        final LinkedQueueAtomicNode 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 && chaserNode != null && size < Integer.MAX_VALUE) {
            LinkedQueueAtomicNode next;
            next = chaserNode.lvNext();
            // check if this node has been consumed
            if (next == chaserNode) {
                next = lvConsumerNode();
            }
            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.
     *
     * @see MessagePassingQueue#isEmpty()
     */
    @Override
    public final boolean isEmpty() {
        return lvConsumerNode() == lvProducerNode();
    }

    protected E getSingleConsumerNodeValue(LinkedQueueAtomicNode currConsumerNode, LinkedQueueAtomicNode nextNode) {
        // we have to null out the value because we are going to hang on to the node
        final E nextValue = nextNode.getAndNullValue();
        // fix up the next ref to prevent promoted nodes from keep new ones alive
        currConsumerNode.soNext(currConsumerNode);
        spConsumerNode(nextNode);
        return nextValue;
    }
}