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/*
* 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.
*
* Original License: https://github.com/JCTools/JCTools/blob/master/LICENSE
* Original location: https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/SpscLinkedQueue.java
*/
package rx.internal.util.unsafe;
import rx.internal.util.atomic.LinkedQueueNode;
/**
* This is a weakened version of the MPSC algorithm as presented on 1024
* Cores by D. Vyukov. The original has been adapted to Java and it's quirks with regards to memory model and
* layout:
*
* - Use inheritance to ensure no false sharing occurs between producer/consumer node reference fields.
*
- As this is an SPSC we have no need for XCHG, an ordered store is enough.
*
* The queue is initialized with a stub node which is set to both the producer and consumer node references. From this
* point follow the notes on offer/poll.
*
* @author nitsanw
*
* @param
*/
public final class SpscLinkedQueue extends BaseLinkedQueue {
public SpscLinkedQueue() {
spProducerNode(new LinkedQueueNode());
spConsumerNode(producerNode);
consumerNode.soNext(null); // this ensures correct construction: StoreStore
}
/**
* {@inheritDoc}
*
* IMPLEMENTATION NOTES:
* Offer is allowed from a SINGLE thread.
* Offer allocates a new node (holding the offered value) and:
*
* - Sets that node as the producerNode.next
*
- Sets the new node as the producerNode
*
* From this follows that producerNode.next is always null and for all other nodes node.next is not null.
*
* @see MessagePassingQueue#offer(Object)
* @see java.util.Queue#offer(java.lang.Object)
*/
@Override
public boolean offer(final E nextValue) {
if (nextValue == null) {
throw new NullPointerException("null elements not allowed");
}
final LinkedQueueNode nextNode = new LinkedQueueNode(nextValue);
producerNode.soNext(nextNode);
producerNode = nextNode;
return true;
}
/**
* {@inheritDoc}
*
* IMPLEMENTATION NOTES:
* Poll is allowed from a SINGLE thread.
* Poll reads the next node from the consumerNode and:
*
* - If it is null, the queue is empty.
*
- If it is not null set it as the consumer node and return it's now evacuated value.
*
* This means the consumerNode.value is always null, which is also the starting point for the queue. Because null
* values are not allowed to be offered this is the only node with it's value set to null at any one time.
*
*/
@Override
public E poll() {
final LinkedQueueNode nextNode = consumerNode.lvNext();
if (nextNode != null) {
// we have to null out the value because we are going to hang on to the node
final E nextValue = nextNode.getAndNullValue();
consumerNode = nextNode;
return nextValue;
}
return null;
}
@Override
public E peek() {
final LinkedQueueNode nextNode = consumerNode.lvNext();
if (nextNode != null) {
return nextNode.lpValue();
} else {
return null;
}
}
}