io.reactivex.rxjava3.operators.SpscArrayQueue Maven / Gradle / Ivy
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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.rxjava3.operators;
import java.util.concurrent.atomic.*;
import io.reactivex.rxjava3.annotations.Nullable;
import io.reactivex.rxjava3.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
* @since 3.1.1
*/
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;
/**
* Constructs an array-backed queue with the given capacity rounded
* up to the next power of 2 size.
* @param capacity the maximum number of elements the queue would hold,
* rounded up to the next power of 2
*/
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);
}
}