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	/*
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	/*
	* This file is available under and governed by the GNU General Public
	* License version 2 only, as published by the Free Software Foundation.
	* However, the following notice accompanied the original version of this
	* file:
	*
	* Written by Doug Lea with assistance from members of JCP JSR-166
	* Expert Group and released to the public domain, as explained at
	* http://creativecommons.org/publicdomain/zero/1.0/
	*/

	package java.util.concurrent;

	import java.lang.invoke.MethodHandles;
	import java.lang.invoke.VarHandle;
	import java.util.AbstractQueue;
	import java.util.Arrays;
	import java.util.Collection;
	import java.util.Comparator;
	import java.util.Iterator;
	import java.util.NoSuchElementException;
	import java.util.Objects;
	import java.util.PriorityQueue;
	import java.util.Queue;
	import java.util.SortedSet;
	import java.util.Spliterator;
	import java.util.concurrent.locks.Condition;
	import java.util.concurrent.locks.ReentrantLock;
	import java.util.function.Consumer;
	import java.util.function.Predicate;
	import jdk.internal.access.SharedSecrets;
	import jdk.internal.util.ArraysSupport;

	/**
	* An unbounded {@linkplain BlockingQueue blocking queue} that uses
	* the same ordering rules as class {@link PriorityQueue} and supplies
	* blocking retrieval operations. While this queue is logically
	* unbounded, attempted additions may fail due to resource exhaustion
	* (causing {@code OutOfMemoryError}). This class does not permit
	* {@code null} elements. A priority queue relying on {@linkplain
	* Comparable natural ordering} also does not permit insertion of
	* non-comparable objects (doing so results in
	* {@code ClassCastException}).
	*
	* <p>This class and its iterator implement all of the <em>optional</em>
	* methods of the {@link Collection} and {@link Iterator} interfaces.
	* The Iterator provided in method {@link #iterator()} and the
	* Spliterator provided in method {@link #spliterator()} are <em>not</em>
	* guaranteed to traverse the elements of the PriorityBlockingQueue in
	* any particular order. If you need ordered traversal, consider using
	* {@code Arrays.sort(pq.toArray())}. Also, method {@code drainTo} can
	* be used to <em>remove</em> some or all elements in priority order and
	* place them in another collection.
	*
	* <p>Operations on this class make no guarantees about the ordering
	* of elements with equal priority. If you need to enforce an
	* ordering, you can define custom classes or comparators that use a
	* secondary key to break ties in primary priority values. For
	* example, here is a class that applies first-in-first-out
	* tie-breaking to comparable elements. To use it, you would insert a
	* {@code new FIFOEntry(anEntry)} instead of a plain entry object.
	*
	* <pre> {@code
	* class FIFOEntry<E extends Comparable<? super E>>
	* implements Comparable<FIFOEntry<E>> {
	* static final AtomicLong seq = new AtomicLong();
	* final long seqNum;
	* final E entry;
	* public FIFOEntry(E entry) {
	* seqNum = seq.getAndIncrement();
	* this.entry = entry;
	* }
	* public E getEntry() { return entry; }
	* public int compareTo(FIFOEntry<E> other) {
	* int res = entry.compareTo(other.entry);
	* if (res == 0 && other.entry != this.entry)
	* res = (seqNum < other.seqNum ? -1 : 1);
	* return res;
	* }
	* }}</pre>
	*
	* <p>This class is a member of the
	* <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
	* Java Collections Framework</a>.
	*
	* @since 1.5
	* @author Doug Lea
	* @param <E> the type of elements held in this queue
	*/
	@SuppressWarnings("unchecked")
	public class PriorityBlockingQueue<E> extends AbstractQueue<E>
	implements BlockingQueue<E>, java.io.Serializable {
	private static final long serialVersionUID = 5595510919245408276L;

	/*
	* The implementation uses an array-based binary heap, with public
	* operations protected with a single lock. However, allocation
	* during resizing uses a simple spinlock (used only while not
	* holding main lock) in order to allow takes to operate
	* concurrently with allocation. This avoids repeated
	* postponement of waiting consumers and consequent element
	* build-up. The need to back away from lock during allocation
	* makes it impossible to simply wrap delegated
	* java.util.PriorityQueue operations within a lock, as was done
	* in a previous version of this class. To maintain
	* interoperability, a plain PriorityQueue is still used during
	* serialization, which maintains compatibility at the expense of
	* transiently doubling overhead.
	*/

	/**
	* Default array capacity.
	*/
	private static final int DEFAULT_INITIAL_CAPACITY = 11;

	/**
	* Priority queue represented as a balanced binary heap: the two
	* children of queue[n] are queue[2n+1] and queue[2(n+1)]. The
	* priority queue is ordered by comparator, or by the elements'
	* natural ordering, if comparator is null: For each node n in the
	* heap and each descendant d of n, n <= d. The element with the
	* lowest value is in queue[0], assuming the queue is nonempty.
	*/
	private transient Object[] queue;

	/**
	* The number of elements in the priority queue.
	*/
	private transient int size;

	/**
	* The comparator, or null if priority queue uses elements'
	* natural ordering.
	*/
	private transient Comparator<? super E> comparator;

	/**
	* Lock used for all public operations.
	*/
	private final ReentrantLock lock = new ReentrantLock();

	/**
	* Condition for blocking when empty.
	*/
	@SuppressWarnings("serial") // Classes implementing Condition may be serializable.
	private final Condition notEmpty = lock.newCondition();

	/**
	* Spinlock for allocation, acquired via CAS.
	*/
	private transient volatile int allocationSpinLock;

	/**
	* A plain PriorityQueue used only for serialization,
	* to maintain compatibility with previous versions
	* of this class. Non-null only during serialization/deserialization.
	*/
	private PriorityQueue<E> q;

	/**
	* Creates a {@code PriorityBlockingQueue} with the default
	* initial capacity (11) that orders its elements according to
	* their {@linkplain Comparable natural ordering}.
	*/
	public PriorityBlockingQueue() {
	this(DEFAULT_INITIAL_CAPACITY, null);
	}

	/**
	* Creates a {@code PriorityBlockingQueue} with the specified
	* initial capacity that orders its elements according to their
	* {@linkplain Comparable natural ordering}.
	*
	* @param initialCapacity the initial capacity for this priority queue
	* @throws IllegalArgumentException if {@code initialCapacity} is less
	* than 1
	*/
	public PriorityBlockingQueue(int initialCapacity) {
	this(initialCapacity, null);
	}

	/**
	* Creates a {@code PriorityBlockingQueue} with the specified initial
	* capacity that orders its elements according to the specified
	* comparator.
	*
	* @param initialCapacity the initial capacity for this priority queue
	* @param comparator the comparator that will be used to order this
	* priority queue. If {@code null}, the {@linkplain Comparable
	* natural ordering} of the elements will be used.
	* @throws IllegalArgumentException if {@code initialCapacity} is less
	* than 1
	*/
	public PriorityBlockingQueue(int initialCapacity,
	Comparator<? super E> comparator) {
	if (initialCapacity < 1)
	throw new IllegalArgumentException();
	this.comparator = comparator;
	this.queue = new Object[Math.max(1, initialCapacity)];
	}

	/**
	* Creates a {@code PriorityBlockingQueue} containing the elements
	* in the specified collection. If the specified collection is a
	* {@link SortedSet} or a {@link PriorityBlockingQueue}, this
	* priority queue will be ordered according to the same ordering.
	* Otherwise, this priority queue will be ordered according to the
	* {@linkplain Comparable natural ordering} of its elements.
	*
	* @param c the collection whose elements are to be placed
	* into this priority queue
	* @throws ClassCastException if elements of the specified collection
	* cannot be compared to one another according to the priority
	* queue's ordering
	* @throws NullPointerException if the specified collection or any
	* of its elements are null
	*/
	public PriorityBlockingQueue(Collection<? extends E> c) {
	boolean heapify = true; // true if not known to be in heap order
	boolean screen = true; // true if must screen for nulls
	if (c instanceof SortedSet<?>) {
	SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
	this.comparator = (Comparator<? super E>) ss.comparator();
	heapify = false;
	}
	else if (c instanceof PriorityBlockingQueue<?>) {
	PriorityBlockingQueue<? extends E> pq =
	(PriorityBlockingQueue<? extends E>) c;
	this.comparator = (Comparator<? super E>) pq.comparator();
	screen = false;
	if (pq.getClass() == PriorityBlockingQueue.class) // exact match
	heapify = false;
	}
	Object[] es = c.toArray();
	int n = es.length;
	if (c.getClass() != java.util.ArrayList.class)
	es = Arrays.copyOf(es, n, Object[].class);
	if (screen && (n == 1 \|\| this.comparator != null)) {
	for (Object e : es)
	if (e == null)
	throw new NullPointerException();
	}
	this.queue = ensureNonEmpty(es);
	this.size = n;
	if (heapify)
	heapify();
	}

	/** Ensures that queue[0] exists, helping peek() and poll(). */
	private static Object[] ensureNonEmpty(Object[] es) {
	return (es.length > 0) ? es : new Object[1];
	}

	/**
	* Tries to grow array to accommodate at least one more element
	* (but normally expand by about 50%), giving up (allowing retry)
	* on contention (which we expect to be rare). Call only while
	* holding lock.
	*
	* @param array the heap array
	* @param oldCap the length of the array
	*/
	private void tryGrow(Object[] array, int oldCap) {
	lock.unlock(); // must release and then re-acquire main lock
	Object[] newArray = null;
	if (allocationSpinLock == 0 &&
	ALLOCATIONSPINLOCK.compareAndSet(this, 0, 1)) {
	try {
	int growth = (oldCap < 64)
	? (oldCap + 2) // grow faster if small
	: (oldCap >> 1);
	int newCap = ArraysSupport.newLength(oldCap, 1, growth);
	if (queue == array)
	newArray = new Object[newCap];
	} finally {
	allocationSpinLock = 0;
	}
	}
	if (newArray == null) // back off if another thread is allocating
	Thread.yield();
	lock.lock();
	if (newArray != null && queue == array) {
	queue = newArray;
	System.arraycopy(array, 0, newArray, 0, oldCap);
	}
	}

	/**
	* Mechanics for poll(). Call only while holding lock.
	*/
	private E dequeue() {
	// assert lock.isHeldByCurrentThread();
	final Object[] es;
	final E result;

	if ((result = (E) ((es = queue)[0])) != null) {
	final int n;
	final E x = (E) es[(n = --size)];
	es[n] = null;
	if (n > 0) {
	final Comparator<? super E> cmp;
	if ((cmp = comparator) == null)
	siftDownComparable(0, x, es, n);
	else
	siftDownUsingComparator(0, x, es, n, cmp);
	}
	}
	return result;
	}

	/**
	* Inserts item x at position k, maintaining heap invariant by
	* promoting x up the tree until it is greater than or equal to
	* its parent, or is the root.
	*
	* To simplify and speed up coercions and comparisons, the
	* Comparable and Comparator versions are separated into different
	* methods that are otherwise identical. (Similarly for siftDown.)
	*
	* @param k the position to fill
	* @param x the item to insert
	* @param es the heap array
	*/
	private static <T> void siftUpComparable(int k, T x, Object[] es) {
	Comparable<? super T> key = (Comparable<? super T>) x;
	while (k > 0) {
	int parent = (k - 1) >>> 1;
	Object e = es[parent];
	if (key.compareTo((T) e) >= 0)
	break;
	es[k] = e;
	k = parent;
	}
	es[k] = key;
	}

	private static <T> void siftUpUsingComparator(
	int k, T x, Object[] es, Comparator<? super T> cmp) {
	while (k > 0) {
	int parent = (k - 1) >>> 1;
	Object e = es[parent];
	if (cmp.compare(x, (T) e) >= 0)
	break;
	es[k] = e;
	k = parent;
	}
	es[k] = x;
	}

	/**
	* Inserts item x at position k, maintaining heap invariant by
	* demoting x down the tree repeatedly until it is less than or
	* equal to its children or is a leaf.
	*
	* @param k the position to fill
	* @param x the item to insert
	* @param es the heap array
	* @param n heap size
	*/
	private static <T> void siftDownComparable(int k, T x, Object[] es, int n) {
	// assert n > 0;
	Comparable<? super T> key = (Comparable<? super T>)x;
	int half = n >>> 1; // loop while a non-leaf
	while (k < half) {
	int child = (k << 1) + 1; // assume left child is least
	Object c = es[child];
	int right = child + 1;
	if (right < n &&
	((Comparable<? super T>) c).compareTo((T) es[right]) > 0)
	c = es[child = right];
	if (key.compareTo((T) c) <= 0)
	break;
	es[k] = c;
	k = child;
	}
	es[k] = key;
	}

	private static <T> void siftDownUsingComparator(
	int k, T x, Object[] es, int n, Comparator<? super T> cmp) {
	// assert n > 0;
	int half = n >>> 1;
	while (k < half) {
	int child = (k << 1) + 1;
	Object c = es[child];
	int right = child + 1;
	if (right < n && cmp.compare((T) c, (T) es[right]) > 0)
	c = es[child = right];
	if (cmp.compare(x, (T) c) <= 0)
	break;
	es[k] = c;
	k = child;
	}
	es[k] = x;
	}

	/**
	* Establishes the heap invariant (described above) in the entire tree,
	* assuming nothing about the order of the elements prior to the call.
	* This classic algorithm due to Floyd (1964) is known to be O(size).
	*/
	private void heapify() {
	final Object[] es = queue;
	int n = size, i = (n >>> 1) - 1;
	final Comparator<? super E> cmp;
	if ((cmp = comparator) == null)
	for (; i >= 0; i--)
	siftDownComparable(i, (E) es[i], es, n);
	else
	for (; i >= 0; i--)
	siftDownUsingComparator(i, (E) es[i], es, n, cmp);
	}

	/**
	* Inserts the specified element into this priority queue.
	*
	* @param e the element to add
	* @return {@code true} (as specified by {@link Collection#add})
	* @throws ClassCastException if the specified element cannot be compared
	* with elements currently in the priority queue according to the
	* priority queue's ordering
	* @throws NullPointerException if the specified element is null
	*/
	public boolean add(E e) {
	return offer(e);
	}

	/**
	* Inserts the specified element into this priority queue.
	* As the queue is unbounded, this method will never return {@code false}.
	*
	* @param e the element to add
	* @return {@code true} (as specified by {@link Queue#offer})
	* @throws ClassCastException if the specified element cannot be compared
	* with elements currently in the priority queue according to the
	* priority queue's ordering
	* @throws NullPointerException if the specified element is null
	*/
	public boolean offer(E e) {
	if (e == null)
	throw new NullPointerException();
	final ReentrantLock lock = this.lock;
	lock.lock();
	int n, cap;
	Object[] es;
	while ((n = size) >= (cap = (es = queue).length))
	tryGrow(es, cap);
	try {
	final Comparator<? super E> cmp;
	if ((cmp = comparator) == null)
	siftUpComparable(n, e, es);
	else
	siftUpUsingComparator(n, e, es, cmp);
	size = n + 1;
	notEmpty.signal();
	} finally {
	lock.unlock();
	}
	return true;
	}

	/**
	* Inserts the specified element into this priority queue.
	* As the queue is unbounded, this method will never block.
	*
	* @param e the element to add
	* @throws ClassCastException if the specified element cannot be compared
	* with elements currently in the priority queue according to the
	* priority queue's ordering
	* @throws NullPointerException if the specified element is null
	*/
	public void put(E e) {
	offer(e); // never need to block
	}

	/**
	* Inserts the specified element into this priority queue.
	* As the queue is unbounded, this method will never block or
	* return {@code false}.
	*
	* @param e the element to add
	* @param timeout This parameter is ignored as the method never blocks
	* @param unit This parameter is ignored as the method never blocks
	* @return {@code true} (as specified by
	* {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
	* @throws ClassCastException if the specified element cannot be compared
	* with elements currently in the priority queue according to the
	* priority queue's ordering
	* @throws NullPointerException if the specified element is null
	*/
	public boolean offer(E e, long timeout, TimeUnit unit) {
	return offer(e); // never need to block
	}

	public E poll() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return dequeue();
	} finally {
	lock.unlock();
	}
	}

	public E take() throws InterruptedException {
	final ReentrantLock lock = this.lock;
	lock.lockInterruptibly();
	E result;
	try {
	while ( (result = dequeue()) == null)
	notEmpty.await();
	} finally {
	lock.unlock();
	}
	return result;
	}

	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
	long nanos = unit.toNanos(timeout);
	final ReentrantLock lock = this.lock;
	lock.lockInterruptibly();
	E result;
	try {
	while ( (result = dequeue()) == null && nanos > 0)
	nanos = notEmpty.awaitNanos(nanos);
	} finally {
	lock.unlock();
	}
	return result;
	}

	public E peek() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return (E) queue[0];
	} finally {
	lock.unlock();
	}
	}

	/**
	* Returns the comparator used to order the elements in this queue,
	* or {@code null} if this queue uses the {@linkplain Comparable
	* natural ordering} of its elements.
	*
	* @return the comparator used to order the elements in this queue,
	* or {@code null} if this queue uses the natural
	* ordering of its elements
	*/
	public Comparator<? super E> comparator() {
	return comparator;
	}

	public int size() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return size;
	} finally {
	lock.unlock();
	}
	}

	/**
	* Always returns {@code Integer.MAX_VALUE} because
	* a {@code PriorityBlockingQueue} is not capacity constrained.
	* @return {@code Integer.MAX_VALUE} always
	*/
	public int remainingCapacity() {
	return Integer.MAX_VALUE;
	}

	private int indexOf(Object o) {
	if (o != null) {
	final Object[] es = queue;
	for (int i = 0, n = size; i < n; i++)
	if (o.equals(es[i]))
	return i;
	}
	return -1;
	}

	/**
	* Removes the ith element from queue.
	*/
	private void removeAt(int i) {
	final Object[] es = queue;
	final int n = size - 1;
	if (n == i) // removed last element
	es[i] = null;
	else {
	E moved = (E) es[n];
	es[n] = null;
	final Comparator<? super E> cmp;
	if ((cmp = comparator) == null)
	siftDownComparable(i, moved, es, n);
	else
	siftDownUsingComparator(i, moved, es, n, cmp);
	if (es[i] == moved) {
	if (cmp == null)
	siftUpComparable(i, moved, es);
	else
	siftUpUsingComparator(i, moved, es, cmp);
	}
	}
	size = n;
	}

	/**
	* Removes a single instance of the specified element from this queue,
	* if it is present. More formally, removes an element {@code e} such
	* that {@code o.equals(e)}, if this queue contains one or more such
	* elements. Returns {@code true} if and only if this queue contained
	* the specified element (or equivalently, if this queue changed as a
	* result of the call).
	*
	* @param o element to be removed from this queue, if present
	* @return {@code true} if this queue changed as a result of the call
	*/
	public boolean remove(Object o) {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	int i = indexOf(o);
	if (i == -1)
	return false;
	removeAt(i);
	return true;
	} finally {
	lock.unlock();
	}
	}

	/**
	* Identity-based version for use in Itr.remove.
	*
	* @param o element to be removed from this queue, if present
	*/
	void removeEq(Object o) {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	final Object[] es = queue;
	for (int i = 0, n = size; i < n; i++) {
	if (o == es[i]) {
	removeAt(i);
	break;
	}
	}
	} finally {
	lock.unlock();
	}
	}

	/**
	* Returns {@code true} if this queue contains the specified element.
	* More formally, returns {@code true} if and only if this queue contains
	* at least one element {@code e} such that {@code o.equals(e)}.
	*
	* @param o object to be checked for containment in this queue
	* @return {@code true} if this queue contains the specified element
	*/
	public boolean contains(Object o) {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return indexOf(o) != -1;
	} finally {
	lock.unlock();
	}
	}

	public String toString() {
	return Helpers.collectionToString(this);
	}

	/**
	* @throws UnsupportedOperationException {@inheritDoc}
	* @throws ClassCastException {@inheritDoc}
	* @throws NullPointerException {@inheritDoc}
	* @throws IllegalArgumentException {@inheritDoc}
	*/
	public int drainTo(Collection<? super E> c) {
	return drainTo(c, Integer.MAX_VALUE);
	}

	/**
	* @throws UnsupportedOperationException {@inheritDoc}
	* @throws ClassCastException {@inheritDoc}
	* @throws NullPointerException {@inheritDoc}
	* @throws IllegalArgumentException {@inheritDoc}
	*/
	public int drainTo(Collection<? super E> c, int maxElements) {
	Objects.requireNonNull(c);
	if (c == this)
	throw new IllegalArgumentException();
	if (maxElements <= 0)
	return 0;
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	int n = Math.min(size, maxElements);
	for (int i = 0; i < n; i++) {
	c.add((E) queue[0]); // In this order, in case add() throws.
	dequeue();
	}
	return n;
	} finally {
	lock.unlock();
	}
	}

	/**
	* Atomically removes all of the elements from this queue.
	* The queue will be empty after this call returns.
	*/
	public void clear() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	final Object[] es = queue;
	for (int i = 0, n = size; i < n; i++)
	es[i] = null;
	size = 0;
	} finally {
	lock.unlock();
	}
	}

	/**
	* Returns an array containing all of the elements in this queue.
	* The returned array elements are in no particular order.
	*
	* <p>The returned array will be "safe" in that no references to it are
	* maintained by this queue. (In other words, this method must allocate
	* a new array). The caller is thus free to modify the returned array.
	*
	* <p>This method acts as bridge between array-based and collection-based
	* APIs.
	*
	* @return an array containing all of the elements in this queue
	*/
	public Object[] toArray() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return Arrays.copyOf(queue, size);
	} finally {
	lock.unlock();
	}
	}

	/**
	* Returns an array containing all of the elements in this queue; the
	* runtime type of the returned array is that of the specified array.
	* The returned array elements are in no particular order.
	* If the queue fits in the specified array, it is returned therein.
	* Otherwise, a new array is allocated with the runtime type of the
	* specified array and the size of this queue.
	*
	* <p>If this queue fits in the specified array with room to spare
	* (i.e., the array has more elements than this queue), the element in
	* the array immediately following the end of the queue is set to
	* {@code null}.
	*
	* <p>Like the {@link #toArray()} method, this method acts as bridge between
	* array-based and collection-based APIs. Further, this method allows
	* precise control over the runtime type of the output array, and may,
	* under certain circumstances, be used to save allocation costs.
	*
	* <p>Suppose {@code x} is a queue known to contain only strings.
	* The following code can be used to dump the queue into a newly
	* allocated array of {@code String}:
	*
	* <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
	*
	* Note that {@code toArray(new Object[0])} is identical in function to
	* {@code toArray()}.
	*
	* @param a the array into which the elements of the queue are to
	* be stored, if it is big enough; otherwise, a new array of the
	* same runtime type is allocated for this purpose
	* @return an array containing all of the elements in this queue
	* @throws ArrayStoreException if the runtime type of the specified array
	* is not a supertype of the runtime type of every element in
	* this queue
	* @throws NullPointerException if the specified array is null
	*/
	public <T> T[] toArray(T[] a) {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	int n = size;
	if (a.length < n)
	// Make a new array of a's runtime type, but my contents:
	return (T[]) Arrays.copyOf(queue, size, a.getClass());
	System.arraycopy(queue, 0, a, 0, n);
	if (a.length > n)
	a[n] = null;
	return a;
	} finally {
	lock.unlock();
	}
	}

	/**
	* Returns an iterator over the elements in this queue. The
	* iterator does not return the elements in any particular order.
	*
	* <p>The returned iterator is
	* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
	*
	* @return an iterator over the elements in this queue
	*/
	public Iterator<E> iterator() {
	return new Itr(toArray());
	}

	/**
	* Snapshot iterator that works off copy of underlying q array.
	*/
	final class Itr implements Iterator<E> {
	final Object[] array; // Array of all elements
	int cursor; // index of next element to return
	int lastRet = -1; // index of last element, or -1 if no such

	Itr(Object[] array) {
	this.array = array;
	}

	public boolean hasNext() {
	return cursor < array.length;
	}

	public E next() {
	if (cursor >= array.length)
	throw new NoSuchElementException();
	return (E)array[lastRet = cursor++];
	}

	public void remove() {
	if (lastRet < 0)
	throw new IllegalStateException();
	removeEq(array[lastRet]);
	lastRet = -1;
	}

	public void forEachRemaining(Consumer<? super E> action) {
	Objects.requireNonNull(action);
	final Object[] es = array;
	int i;
	if ((i = cursor) < es.length) {
	lastRet = -1;
	cursor = es.length;
	for (; i < es.length; i++)
	action.accept((E) es[i]);
	lastRet = es.length - 1;
	}
	}
	}

	/**
	* Saves this queue to a stream (that is, serializes it).
	*
	* For compatibility with previous version of this class, elements
	* are first copied to a java.util.PriorityQueue, which is then
	* serialized.
	*
	* @param s the stream
	* @throws java.io.IOException if an I/O error occurs
	*/
	private void writeObject(java.io.ObjectOutputStream s)
	throws java.io.IOException {
	lock.lock();
	try {
	// avoid zero capacity argument
	q = new PriorityQueue<E>(Math.max(size, 1), comparator);
	q.addAll(this);
	s.defaultWriteObject();
	} finally {
	q = null;
	lock.unlock();
	}
	}

	/**
	* Reconstitutes this queue from a stream (that is, deserializes it).
	* @param s the stream
	* @throws ClassNotFoundException if the class of a serialized object
	* could not be found
	* @throws java.io.IOException if an I/O error occurs
	*/
	private void readObject(java.io.ObjectInputStream s)
	throws java.io.IOException, ClassNotFoundException {
	try {
	s.defaultReadObject();
	int sz = q.size();
	SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, sz);
	this.queue = new Object[Math.max(1, sz)];
	comparator = q.comparator();
	addAll(q);
	} finally {
	q = null;
	}
	}

	/**
	* Immutable snapshot spliterator that binds to elements "late".
	*/
	final class PBQSpliterator implements Spliterator<E> {
	Object[] array; // null until late-bound-initialized
	int index;
	int fence;

	PBQSpliterator() {}

	PBQSpliterator(Object[] array, int index, int fence) {
	this.array = array;
	this.index = index;
	this.fence = fence;
	}

	private int getFence() {
	if (array == null)
	fence = (array = toArray()).length;
	return fence;
	}

	public PBQSpliterator trySplit() {
	int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
	return (lo >= mid) ? null :
	new PBQSpliterator(array, lo, index = mid);
	}

	public void forEachRemaining(Consumer<? super E> action) {
	Objects.requireNonNull(action);
	final int hi = getFence(), lo = index;
	final Object[] es = array;
	index = hi; // ensure exhaustion
	for (int i = lo; i < hi; i++)
	action.accept((E) es[i]);
	}

	public boolean tryAdvance(Consumer<? super E> action) {
	Objects.requireNonNull(action);
	if (getFence() > index && index >= 0) {
	action.accept((E) array[index++]);
	return true;
	}
	return false;
	}

	public long estimateSize() { return getFence() - index; }

	public int characteristics() {
	return (Spliterator.NONNULL \|
	Spliterator.SIZED \|
	Spliterator.SUBSIZED);
	}
	}

	/**
	* Returns a {@link Spliterator} over the elements in this queue.
	* The spliterator does not traverse elements in any particular order
	* (the {@link Spliterator#ORDERED ORDERED} characteristic is not reported).
	*
	* <p>The returned spliterator is
	* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
	*
	* <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
	* {@link Spliterator#NONNULL}.
	*
	* @implNote
	* The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}.
	*
	* @return a {@code Spliterator} over the elements in this queue
	* @since 1.8
	*/
	public Spliterator<E> spliterator() {
	return new PBQSpliterator();
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	*/
	public boolean removeIf(Predicate<? super E> filter) {
	Objects.requireNonNull(filter);
	return bulkRemove(filter);
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	*/
	public boolean removeAll(Collection<?> c) {
	Objects.requireNonNull(c);
	return bulkRemove(e -> c.contains(e));
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	*/
	public boolean retainAll(Collection<?> c) {
	Objects.requireNonNull(c);
	return bulkRemove(e -> !c.contains(e));
	}

	// A tiny bit set implementation

	private static long[] nBits(int n) {
	return new long[((n - 1) >> 6) + 1];
	}
	private static void setBit(long[] bits, int i) {
	bits[i >> 6] \|= 1L << i;
	}
	private static boolean isClear(long[] bits, int i) {
	return (bits[i >> 6] & (1L << i)) == 0;
	}

	/** Implementation of bulk remove methods. */
	private boolean bulkRemove(Predicate<? super E> filter) {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	final Object[] es = queue;
	final int end = size;
	int i;
	// Optimize for initial run of survivors
	for (i = 0; i < end && !filter.test((E) es[i]); i++)
	;
	if (i >= end)
	return false;
	// Tolerate predicates that reentrantly access the
	// collection for read, so traverse once to find elements
	// to delete, a second pass to physically expunge.
	final int beg = i;
	final long[] deathRow = nBits(end - beg);
	deathRow[0] = 1L; // set bit 0
	for (i = beg + 1; i < end; i++)
	if (filter.test((E) es[i]))
	setBit(deathRow, i - beg);
	int w = beg;
	for (i = beg; i < end; i++)
	if (isClear(deathRow, i - beg))
	es[w++] = es[i];
	for (i = size = w; i < end; i++)
	es[i] = null;
	heapify();
	return true;
	} finally {
	lock.unlock();
	}
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	*/
	public void forEach(Consumer<? super E> action) {
	Objects.requireNonNull(action);
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	final Object[] es = queue;
	for (int i = 0, n = size; i < n; i++)
	action.accept((E) es[i]);
	} finally {
	lock.unlock();
	}
	}

	// VarHandle mechanics
	private static final VarHandle ALLOCATIONSPINLOCK;
	static {
	try {
	MethodHandles.Lookup l = MethodHandles.lookup();
	ALLOCATIONSPINLOCK = l.findVarHandle(PriorityBlockingQueue.class,
	"allocationSpinLock",
	int.class);
	} catch (ReflectiveOperationException e) {
	throw new ExceptionInInitializerError(e);
	}
	}
	}

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