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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.atomic.*;

import java.util.concurrent.locks.*;

import java.util.*;

/**

 * An optionally-bounded {@linkplain BlockingQueue blocking queue} based on

 * linked nodes.

 * This queue orders elements FIFO (first-in-first-out).

 * The <em>head</em> of the queue is that element that has been on the

 * queue the longest time.

 * The <em>tail</em> of the queue is that element that has been on the

 * queue the shortest time. New elements

 * are inserted at the tail of the queue, and the queue retrieval

 * operations obtain elements at the head of the queue.

 * Linked queues typically have higher throughput than array-based queues but

 * less predictable performance in most concurrent applications.

 *

 * <p> The optional capacity bound constructor argument serves as a

 * way to prevent excessive queue expansion. The capacity, if unspecified,

 * is equal to {@link Integer#MAX_VALUE}.  Linked nodes are

 * dynamically created upon each insertion unless this would bring the

 * queue above capacity.

 *

 * <p>This class and its iterator implement all of the

 * <em>optional</em> methods of the {@link Collection} and {@link

 * Iterator} interfaces.

 *

 * <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 LinkedBlockingQueue<E> extends AbstractQueue<E>

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

    private static final long serialVersionUID = -6903933977591709194L;

    /*

     * A variant of the "two lock queue" algorithm.  The putLock gates

     * entry to put (and offer), and has an associated condition for

     * waiting puts.  Similarly for the takeLock.  The "count" field

     * that they both rely on is maintained as an atomic to avoid

     * needing to get both locks in most cases. Also, to minimize need

     * for puts to get takeLock and vice-versa, cascading notifies are

     * used. When a put notices that it has enabled at least one take,

     * it signals taker. That taker in turn signals others if more

     * items have been entered since the signal. And symmetrically for

     * takes signalling puts. Operations such as remove(Object) and

     * iterators acquire both locks.

     */

    /**

     * Linked list node class

     */

    static class Node<E> {

        /** The item, volatile to ensure barrier separating write and read */

        volatile E item;

        Node<E> next;

        Node(E x) { item = x; }

    }

    /** The capacity bound, or Integer.MAX_VALUE if none */

    private final int capacity;

    /** Current number of elements */

    private final AtomicInteger count = new AtomicInteger(0);

    /** Head of linked list */

    private transient Node<E> head;

    /** Tail of linked list */

    private transient Node<E> last;

    /** Lock held by take, poll, etc */

    private final ReentrantLock takeLock = new ReentrantLock();

    /** Wait queue for waiting takes */

    private final Condition notEmpty = takeLock.newCondition();

    /** Lock held by put, offer, etc */

    private final ReentrantLock putLock = new ReentrantLock();

    /** Wait queue for waiting puts */

    private final Condition notFull = putLock.newCondition();

    /**

     * Signals a waiting take. Called only from put/offer (which do not

     * otherwise ordinarily lock takeLock.)

     */

    private void signalNotEmpty() {

        final ReentrantLock takeLock = this.takeLock;

        takeLock.lock();

        try {

            notEmpty.signal();

        } finally {

            takeLock.unlock();

        }

    }

    /**

     * Signals a waiting put. Called only from take/poll.

     */

    private void signalNotFull() {

        final ReentrantLock putLock = this.putLock;

        putLock.lock();

        try {

            notFull.signal();

        } finally {

            putLock.unlock();

        }

    }

    /**

     * Creates a node and links it at end of queue.

     * @param x the item

     */

    private void insert(E x) {

        last = last.next = new Node<E>(x);

    }

    /**

     * Removes a node from head of queue,

     * @return the node

     */

    private E extract() {

        Node<E> first = head.next;

        head = first;

        E x = first.item;

        first.item = null;

        return x;

    }

    /**

     * Lock to prevent both puts and takes.

     */

    private void fullyLock() {

        putLock.lock();

        takeLock.lock();

    }

    /**

     * Unlock to allow both puts and takes.

     */

    private void fullyUnlock() {

        takeLock.unlock();

        putLock.unlock();

    }

    /**

     * Creates a <tt>LinkedBlockingQueue</tt> with a capacity of

     * {@link Integer#MAX_VALUE}.

     */

    public LinkedBlockingQueue() {

        this(Integer.MAX_VALUE);

    }

    /**

     * Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity.

     *

     * @param capacity the capacity of this queue

     * @throws IllegalArgumentException if <tt>capacity</tt> is not greater

     *         than zero

     */

    public LinkedBlockingQueue(int capacity) {

        if (capacity <= 0) throw new IllegalArgumentException();

        this.capacity = capacity;

        last = head = new Node<E>(null);

    }

    /**

     * Creates a <tt>LinkedBlockingQueue</tt> with a capacity of

     * {@link Integer#MAX_VALUE}, initially containing the elements of the

     * given collection,

     * added in traversal order of the collection's iterator.

     *

     * @param c the collection of elements to initially contain

     * @throws NullPointerException if the specified collection or any

     *         of its elements are null

     */

    public LinkedBlockingQueue(Collection<? extends E> c) {

        this(Integer.MAX_VALUE);

        for (E e : c)

            add(e);

    }

    // this doc comment is overridden to remove the reference to collections

    // greater in size than Integer.MAX_VALUE

    /**

     * Returns the number of elements in this queue.

     *

     * @return the number of elements in this queue

     */

    public int size() {

        return count.get();

    }

    // this doc comment is a modified copy of the inherited doc comment,

    // without the reference to unlimited queues.

    /**

     * Returns the number of additional elements that this queue can ideally

     * (in the absence of memory or resource constraints) accept without

     * blocking. This is always equal to the initial capacity of this queue

     * less the current <tt>size</tt> of this queue.

     *

     * <p>Note that you <em>cannot</em> always tell if an attempt to insert

     * an element will succeed by inspecting <tt>remainingCapacity</tt>

     * because it may be the case that another thread is about to

     * insert or remove an element.

     */

    public int remainingCapacity() {

        return capacity - count.get();

    }

    /**

     * Inserts the specified element at the tail of this queue, waiting if

     * necessary for space to become available.

     *

     * @throws InterruptedException {@inheritDoc}

     * @throws NullPointerException {@inheritDoc}

     */

    public void put(E e) throws InterruptedException {

        if (e == null) throw new NullPointerException();

        // Note: convention in all put/take/etc is to preset

        // local var holding count  negative to indicate failure unless set.

        int c = -1;

        final ReentrantLock putLock = this.putLock;

        final AtomicInteger count = this.count;

        putLock.lockInterruptibly();

        try {

            /*

             * Note that count is used in wait guard even though it is

             * not protected by lock. This works because count can

             * only decrease at this point (all other puts are shut

             * out by lock), and we (or some other waiting put) are

             * signalled if it ever changes from

             * capacity. Similarly for all other uses of count in

             * other wait guards.

             */

            try {

                while (count.get() == capacity)

                    notFull.await();

            } catch (InterruptedException ie) {

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

                throw ie;

            }

            insert(e);

            c = count.getAndIncrement();

            if (c + 1 < capacity)

                notFull.signal();

        } finally {

            putLock.unlock();

        }

        if (c == 0)

            signalNotEmpty();

    }

    /**

     * Inserts the specified element at the tail of this queue, waiting if

     * necessary up to the specified wait time for space to become available.

     *

     * @return <tt>true</tt> if successful, or <tt>false</tt> if

     *         the specified waiting time elapses before space is available.

     * @throws InterruptedException {@inheritDoc}

     * @throws NullPointerException {@inheritDoc}

     */

    public boolean offer(E e, long timeout, TimeUnit unit)

        throws InterruptedException {

        if (e == null) throw new NullPointerException();

        long nanos = unit.toNanos(timeout);

        int c = -1;

        final ReentrantLock putLock = this.putLock;

        final AtomicInteger count = this.count;

        putLock.lockInterruptibly();

        try {

            for (;;) {

                if (count.get() < capacity) {

                    insert(e);

                    c = count.getAndIncrement();

                    if (c + 1 < capacity)

                        notFull.signal();

                    break;

                }

                if (nanos <= 0)

                    return false;

                try {

                    nanos = notFull.awaitNanos(nanos);

                } catch (InterruptedException ie) {

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

                    throw ie;

                }

            }

        } finally {

            putLock.unlock();

        }

        if (c == 0)

            signalNotEmpty();

        return true;

    }

    /**

     * Inserts the specified element at the tail of this queue if it is

     * possible to do so immediately without exceeding the queue's capacity,

     * returning <tt>true</tt> upon success and <tt>false</tt> if this queue

     * is full.

     * When using a capacity-restricted queue, this method is generally

     * preferable to method {@link BlockingQueue#add add}, which can fail to

     * insert an element only by throwing an exception.

     *

     * @throws NullPointerException if the specified element is null

     */

    public boolean offer(E e) {

        if (e == null) throw new NullPointerException();

        final AtomicInteger count = this.count;

        if (count.get() == capacity)

            return false;

        int c = -1;

        final ReentrantLock putLock = this.putLock;

        putLock.lock();

        try {

            if (count.get() < capacity) {

                insert(e);

                c = count.getAndIncrement();

                if (c + 1 < capacity)

                    notFull.signal();

            }

        } finally {

            putLock.unlock();

        }

        if (c == 0)

            signalNotEmpty();

        return c >= 0;

    }

    public E take() throws InterruptedException {

        E x;

        int c = -1;

        final AtomicInteger count = this.count;

        final ReentrantLock takeLock = this.takeLock;

        takeLock.lockInterruptibly();

        try {

            try {

                while (count.get() == 0)

                    notEmpty.await();

            } catch (InterruptedException ie) {

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

                throw ie;

            }

            x = extract();

            c = count.getAndDecrement();

            if (c > 1)

                notEmpty.signal();

        } finally {

            takeLock.unlock();

        }

        if (c == capacity)

            signalNotFull();

        return x;

    }

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

        E x = null;

        int c = -1;

        long nanos = unit.toNanos(timeout);

        final AtomicInteger count = this.count;

        final ReentrantLock takeLock = this.takeLock;

        takeLock.lockInterruptibly();

        try {

            for (;;) {

                if (count.get() > 0) {

                    x = extract();

                    c = count.getAndDecrement();

                    if (c > 1)

                        notEmpty.signal();

                    break;

                }

                if (nanos <= 0)

                    return null;

                try {

                    nanos = notEmpty.awaitNanos(nanos);

                } catch (InterruptedException ie) {

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

                    throw ie;

                }

            }

        } finally {

            takeLock.unlock();

        }

        if (c == capacity)

            signalNotFull();

        return x;

    }

    public E poll() {

        final AtomicInteger count = this.count;

        if (count.get() == 0)

            return null;

        E x = null;

        int c = -1;

        final ReentrantLock takeLock = this.takeLock;

        takeLock.lock();

        try {

            if (count.get() > 0) {

                x = extract();

                c = count.getAndDecrement();

                if (c > 1)

                    notEmpty.signal();

            }

        } finally {

            takeLock.unlock();

        }

        if (c == capacity)

            signalNotFull();

        return x;

    }

    public E peek() {

        if (count.get() == 0)

            return null;

        final ReentrantLock takeLock = this.takeLock;

        takeLock.lock();

        try {

            Node<E> first = head.next;

            if (first == null)

                return null;

            else

                return first.item;

        } finally {

            takeLock.unlock();

        }

    }

    /**

     * Removes a single instance of the specified element from this queue,

     * if it is present.  More formally, removes an element <tt>e</tt> such

     * that <tt>o.equals(e)</tt>, if this queue contains one or more such

     * elements.

     * Returns <tt>true</tt> 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) {

        if (o == null) return false;

        boolean removed = false;

        fullyLock();

        try {

            Node<E> trail = head;

            Node<E> p = head.next;

            while (p != null) {

                if (o.equals(p.item)) {

                    removed = true;

                    break;

                }

                trail = p;

                p = p.next;

            }

            if (removed) {

                p.item = null;

                trail.next = p.next;

                if (last == p)

                    last = trail;

                if (count.getAndDecrement() == capacity)

                    notFull.signalAll();

            }

        } finally {

            fullyUnlock();

        }

        return removed;

    }

    /**

     * Returns an array containing all of the elements in this queue, in

     * proper sequence.

     *

     * <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() {

        fullyLock();

        try {

            int size = count.get();

            Object[] a = new Object[size];

            int k = 0;

            for (Node<E> p = head.next; p != null; p = p.next)

                a[k++] = p.item;

            return a;

        } finally {

            fullyUnlock();

        }

    }

    /**

     * Returns an array containing all of the elements in this queue, in

     * proper sequence; the runtime type of the returned array is that of

     * the specified array.  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) {

        fullyLock();

        try {

            int size = count.get();

            if (a.length < size)

                a = (T[])java.lang.reflect.Array.newInstance

                    (a.getClass().getComponentType(), size);

            int k = 0;

            for (Node p = head.next; p != null; p = p.next)

                a[k++] = (T)p.item;

            if (a.length > k)

                a[k] = null;

            return a;

        } finally {

            fullyUnlock();

        }

    }

    public String toString() {

        fullyLock();

        try {

            return super.toString();

        } finally {

            fullyUnlock();

        }

    }

    /**

     * Atomically removes all of the elements from this queue.

     * The queue will be empty after this call returns.

     */

    public void clear() {

        fullyLock();

        try {

            head.next = null;

            assert head.item == null;

            last = head;

            if (count.getAndSet(0) == capacity)

                notFull.signalAll();

        } finally {

            fullyUnlock();

        }

    }

    /**

     * @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();

        Node<E> first;

        fullyLock();

        try {

            first = head.next;

            head.next = null;

            assert head.item == null;

            last = head;

            if (count.getAndSet(0) == capacity)

                notFull.signalAll();

        } finally {

            fullyUnlock();

        }

        // Transfer the elements outside of locks

        int n = 0;

        for (Node<E> p = first; p != null; p = p.next) {

            c.add(p.item);

            p.item = null;

            ++n;

        }

        return n;

    }

    /**

     * @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();

        fullyLock();

        try {

            int n = 0;

            Node<E> p = head.next;

            while (p != null && n < maxElements) {

                c.add(p.item);

                p.item = null;

                p = p.next;

                ++n;

            }

            if (n != 0) {

                head.next = p;

                assert head.item == null;

                if (p == null)

                    last = head;

                if (count.getAndAdd(-n) == capacity)

                    notFull.signalAll();

            }

            return n;

        } finally {

            fullyUnlock();

        }

    }

    /**

     * Returns an iterator over the elements in this queue in proper sequence.

     * 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 in proper sequence

     */

    public Iterator<E> iterator() {

      return new Itr();

    }

    private class Itr implements Iterator<E> {

        /*

         * Basic weak-consistent iterator.  At all times hold the next

         * item to hand out so that if hasNext() reports true, we will

         * still have it to return even if lost race with a take etc.

         */

        private Node<E> current;

        private Node<E> lastRet;

        private E currentElement;

        Itr() {

            final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;

            final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;

            putLock.lock();

            takeLock.lock();

            try {

                current = head.next;

                if (current != null)

                    currentElement = current.item;

            } finally {

                takeLock.unlock();

                putLock.unlock();

            }

        }

        public boolean hasNext() {