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197 lines (160 loc) · 4.46 KB
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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.
*/
package javaopt.queue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.concurrent.atomic.AtomicLong;
/**
* <ul>
* <li>Lock free, observing single writer principal.
* <li>Replacing the long fields with AtomicLong and using lazySet instead of
* volatile assignment.
* <li>Using the power of 2 mask, forcing the capacity to next power of 2.
* <li>Adding head and tail cache fields. Avoiding redundant volatile reads.
* <li>Padding head/tail AtomicLong fields. Avoiding false sharing.
* <li>Padding head/tail cache fields. Avoiding false sharing.
* </ul>
*/
public final class P1C1QueueStep4<E> implements Queue<E> {
private final int capacity;
private final int mask;
private final E[] buffer;
private final AtomicLong tail = new PaddedAtomicLong(0);
private final AtomicLong head = new PaddedAtomicLong(0);
public static class PaddedLong {
public long value = 0, p1, p2, p3, p4, p5, p6;
}
private final PaddedLong tailCache = new PaddedLong();
private final PaddedLong headCache = new PaddedLong();
@SuppressWarnings("unchecked")
public P1C1QueueStep4(final int capacity) {
this.capacity = findNextPositivePowerOfTwo(capacity);
mask = this.capacity - 1;
buffer = (E[]) new Object[this.capacity];
}
public static int findNextPositivePowerOfTwo(final int value) {
return 1 << (32 - Integer.numberOfLeadingZeros(value - 1));
}
public boolean add(final E e) {
if (offer(e)) {
return true;
}
throw new IllegalStateException("Queue is full");
}
public boolean offer(final E e) {
if (null == e) {
throw new NullPointerException("Null is not a valid element");
}
final long currentTail = tail.get();
final long wrapPoint = currentTail - capacity;
if (headCache.value <= wrapPoint) {
headCache.value = head.get();
if (headCache.value <= wrapPoint) {
return false;
}
}
buffer[(int) currentTail & mask] = e;
tail.lazySet(currentTail + 1);
return true;
}
public E poll() {
final long currentHead = head.get();
if (currentHead >= tailCache.value) {
tailCache.value = tail.get();
if (currentHead >= tailCache.value) {
return null;
}
}
final int index = (int) currentHead & mask;
final E e = buffer[index];
buffer[index] = null;
head.lazySet(currentHead + 1);
return e;
}
public E remove() {
final E e = poll();
if (null == e) {
throw new NoSuchElementException("Queue is empty");
}
return e;
}
public E element() {
final E e = peek();
if (null == e) {
throw new NoSuchElementException("Queue is empty");
}
return e;
}
public E peek() {
return buffer[(int) head.get() & mask];
}
public int size() {
return (int) (tail.get() - head.get());
}
public boolean isEmpty() {
return tail.get() == head.get();
}
public boolean contains(final Object o) {
if (null == o) {
return false;
}
for (long i = head.get(), limit = tail.get(); i < limit; i++) {
final E e = buffer[(int) i & mask];
if (o.equals(e)) {
return true;
}
}
return false;
}
public Iterator<E> iterator() {
throw new UnsupportedOperationException();
}
public Object[] toArray() {
throw new UnsupportedOperationException();
}
public <T> T[] toArray(final T[] a) {
throw new UnsupportedOperationException();
}
public boolean remove(final Object o) {
throw new UnsupportedOperationException();
}
public boolean containsAll(final Collection<?> c) {
for (final Object o : c) {
if (!contains(o)) {
return false;
}
}
return true;
}
public boolean addAll(final Collection<? extends E> c) {
for (final E e : c) {
add(e);
}
return true;
}
public boolean removeAll(final Collection<?> c) {
throw new UnsupportedOperationException();
}
public boolean retainAll(final Collection<?> c) {
throw new UnsupportedOperationException();
}
public void clear() {
Object value;
do {
value = poll();
} while (null != value);
}
}