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/*

 * 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/licenses/publicdomain

 */

package java.util.concurrent;

import java.util.concurrent.locks.*;

import java.util.*;

/**

 * 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 <tt>OutOfMemoryError</tt>). This class does not permit

 * <tt>null</tt> elements.  A priority queue relying on {@linkplain

 * Comparable natural ordering} also does not permit insertion of

 * non-comparable objects (doing so results in

 * <tt>ClassCastException</tt>).

 *

 * <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()} is <em>not</em> guaranteed to traverse the elements of

 * the PriorityBlockingQueue in any particular order. If you need

 * ordered traversal, consider using

 * <tt>Arrays.sort(pq.toArray())</tt>.  Also, method <tt>drainTo</tt>

 * 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

 * <tt>new FIFOEntry(anEntry)</tt> instead of a plain entry object.

 *

 * <pre>

 * class FIFOEntry<E extends Comparable<? super E>>

 *     implements Comparable<FIFOEntry<E>> {

 *   final static 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}/../technotes/guides/collections/index.html">

 * Java Collections Framework</a>.

 *

 * @since 1.5

 * @author Doug Lea

 * @param <E> the type of elements held in this collection

 */

public class PriorityBlockingQueue<E> extends AbstractQueue<E>

    implements BlockingQueue<E>, java.io.Serializable {

    private static final long serialVersionUID = 5595510919245408276L;

    private final PriorityQueue<E> q;

    private final ReentrantLock lock = new ReentrantLock(true);

    private final Condition notEmpty = lock.newCondition();

    /**

     * Creates a <tt>PriorityBlockingQueue</tt> with the default

     * initial capacity (11) that orders its elements according to

     * their {@linkplain Comparable natural ordering}.

     */

    public PriorityBlockingQueue() {

        q = new PriorityQueue<E>();

    }

    /**

     * Creates a <tt>PriorityBlockingQueue</tt> 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 <tt>initialCapacity</tt> is less

     *         than 1

     */

    public PriorityBlockingQueue(int initialCapacity) {

        q = new PriorityQueue<E>(initialCapacity, null);

    }

    /**

     * Creates a <tt>PriorityBlockingQueue</tt> 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 <tt>initialCapacity</tt> is less

     *         than 1

     */

    public PriorityBlockingQueue(int initialCapacity,

                                 Comparator<? super E> comparator) {

        q = new PriorityQueue<E>(initialCapacity, comparator);

    }

    /**

     * Creates a <tt>PriorityBlockingQueue</tt> containing the elements

     * in the specified collection.  If the specified collection is a

     * {@link SortedSet} or a {@link PriorityQueue},  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) {

        q = new PriorityQueue<E>(c);

    }

    /**

     * Inserts the specified element into this priority queue.

     *

     * @param e the element to add

     * @return <tt>true</tt> (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.

     *

     * @param e the element to add

     * @return <tt>true</tt> (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) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            boolean ok = q.offer(e);

            assert ok;

            notEmpty.signal();

            return true;

        } finally {

            lock.unlock();

        }

    }

    /**

     * 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.

     *

     * @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 <tt>true</tt>

     * @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 q.poll();

        } finally {

            lock.unlock();

        }

    }

    public E take() throws InterruptedException {

        final ReentrantLock lock = this.lock;

        lock.lockInterruptibly();

        try {

            try {

                while (q.size() == 0)

                    notEmpty.await();

            } catch (InterruptedException ie) {

                notEmpty.signal(); // propagate to non-interrupted thread

                throw ie;

            }

            E x = q.poll();

            assert x != null;

            return x;

        } finally {

            lock.unlock();

        }

    }

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {

        long nanos = unit.toNanos(timeout);

        final ReentrantLock lock = this.lock;

        lock.lockInterruptibly();

        try {

            for (;;) {

                E x = q.poll();

                if (x != null)

                    return x;

                if (nanos <= 0)

                    return null;

                try {

                    nanos = notEmpty.awaitNanos(nanos);

                } catch (InterruptedException ie) {

                    notEmpty.signal(); // propagate to non-interrupted thread

                    throw ie;

                }

            }

        } finally {

            lock.unlock();

        }

    }

    public E peek() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return q.peek();

        } finally {

            lock.unlock();

        }

    }

    /**

     * Returns the comparator used to order the elements in this queue,

     * or <tt>null</tt> if this queue uses the {@linkplain Comparable

     * natural ordering} of its elements.

     *

     * @return the comparator used to order the elements in this queue,

     *         or <tt>null</tt> if this queue uses the natural

     *         ordering of its elements

     */

    public Comparator<? super E> comparator() {

        return q.comparator();

    }

    public int size() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return q.size();

        } finally {

            lock.unlock();

        }

    }

    /**

     * Always returns <tt>Integer.MAX_VALUE</tt> because

     * a <tt>PriorityBlockingQueue</tt> is not capacity constrained.

     * @return <tt>Integer.MAX_VALUE</tt>

     */

    public int remainingCapacity() {

        return Integer.MAX_VALUE;

    }

    /**

     * 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 <tt>true</tt> if this queue changed as a result of the call

     */

    public boolean remove(Object o) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return q.remove(o);

        } 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 <tt>true</tt> if this queue contains the specified element

     */

    public boolean contains(Object o) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return q.contains(o);

        } 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 q.toArray();

        } finally {

            lock.unlock();

        }

    }

    public String toString() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return q.toString();

        } finally {

            lock.unlock();

        }

    }

    /**

     * @throws UnsupportedOperationException {@inheritDoc}

     * @throws ClassCastException            {@inheritDoc}

     * @throws NullPointerException          {@inheritDoc}

     * @throws IllegalArgumentException      {@inheritDoc}

     */

    public int drainTo(Collection<? super E> c) {

        if (c == null)

            throw new NullPointerException();

        if (c == this)

            throw new IllegalArgumentException();

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            int n = 0;

            E e;

            while ( (e = q.poll()) != null) {

                c.add(e);

                ++n;

            }

            return n;

        } finally {

            lock.unlock();

        }

    }

    /**

     * @throws UnsupportedOperationException {@inheritDoc}

     * @throws ClassCastException            {@inheritDoc}

     * @throws NullPointerException          {@inheritDoc}

     * @throws IllegalArgumentException      {@inheritDoc}

     */

    public int drainTo(Collection<? super E> c, int maxElements) {

        if (c == null)

            throw new NullPointerException();

        if (c == this)

            throw new IllegalArgumentException();

        if (maxElements <= 0)

            return 0;

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            int n = 0;

            E e;

            while (n < maxElements && (e = q.poll()) != null) {

                c.add(e);

                ++n;

            }

            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 {

            q.clear();

        } 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

     * <tt>null</tt>.

     *

     * <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 <tt>x</tt> is a queue known to contain only strings.

     * The following code can be used to dump the queue into a newly

     * allocated array of <tt>String</tt>:

     *

     * <pre>

     *     String[] y = x.toArray(new String[0]);</pre>

     *

     * Note that <tt>toArray(new Object[0])</tt> is identical in function to

     * <tt>toArray()</tt>.

     *

     * @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 {

            return q.toArray(a);

        } finally {

            lock.unlock();

        }

    }

    /**

     * Returns an iterator over the elements in this queue. The

     * iterator does not return the elements in any particular order.

     * The returned <tt>Iterator</tt> is a "weakly consistent"

     * iterator that will never throw {@link

     * ConcurrentModificationException}, and guarantees to traverse

     * elements as they existed upon construction of the iterator, and

     * may (but is not guaranteed to) reflect any modifications

     * subsequent to construction.

     *

     * @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.

     */

    private class Itr implements Iterator<E> {

        final Object[] array; // Array of all elements

        int cursor;           // index of next element to return;

        int lastRet;          // index of last element, or -1 if no such

        Itr(Object[] array) {

            lastRet = -1;

            this.array = array;

        }

        public boolean hasNext() {

            return cursor < array.length;

        }

        public E next() {

            if (cursor >= array.length)

                throw new NoSuchElementException();

            lastRet = cursor;

            return (E)array[cursor++];

        }

        public void remove() {

            if (lastRet < 0)

                throw new IllegalStateException();

            Object x = array[lastRet];

            lastRet = -1;

            // Traverse underlying queue to find == element,

            // not just a .equals element.

            lock.lock();

            try {

                for (Iterator it = q.iterator(); it.hasNext(); ) {

                    if (it.next() == x) {

                        it.remove();

                        return;

                    }

                }

            } finally {

                lock.unlock();

            }

        }

    }

    /**

     * Saves the state to a stream (that is, serializes it).  This

     * merely wraps default serialization within lock.  The

     * serialization strategy for items is left to underlying

     * Queue. Note that locking is not needed on deserialization, so

     * readObject is not defined, just relying on default.

     */

    private void writeObject(java.io.ObjectOutputStream s)

        throws java.io.IOException {

        lock.lock();

        try {

            s.defaultWriteObject();

        } finally {

            lock.unlock();

        }

    }

}