/*

 * 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.util.AbstractQueue;

import java.util.Collection;

import java.util.Iterator;

import java.util.NoSuchElementException;

import java.util.Objects;

import java.util.Spliterator;

import java.util.Spliterators;

import java.util.concurrent.atomic.AtomicInteger;

import java.util.concurrent.locks.Condition;

import java.util.concurrent.locks.ReentrantLock;

import java.util.function.Consumer;

import java.util.function.Predicate;

/**

 * 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}/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

 */

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.

     *

     * Visibility between writers and readers is provided as follows:

     *

     * Whenever an element is enqueued, the putLock is acquired and

     * count updated.  A subsequent reader guarantees visibility to the

     * enqueued Node by either acquiring the putLock (via fullyLock)

     * or by acquiring the takeLock, and then reading n = count.get();

     * this gives visibility to the first n items.

     *

     * To implement weakly consistent iterators, it appears we need to

     * keep all Nodes GC-reachable from a predecessor dequeued Node.

     * That would cause two problems:

     * - allow a rogue Iterator to cause unbounded memory retention

     * - cause cross-generational linking of old Nodes to new Nodes if

     *   a Node was tenured while live, which generational GCs have a

     *   hard time dealing with, causing repeated major collections.

     * However, only non-deleted Nodes need to be reachable from

     * dequeued Nodes, and reachability does not necessarily have to

     * be of the kind understood by the GC.  We use the trick of

     * linking a Node that has just been dequeued to itself.  Such a

     * self-link implicitly means to advance to head.next.

     */

    /**

     * Linked list node class.

     */

    static class Node<E> {

        E item;

        /**

         * One of:

         * - the real successor Node

         * - this Node, meaning the successor is head.next

         * - null, meaning there is no successor (this is the last node)

         */

        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();

    /**

     * Head of linked list.

     * Invariant: head.item == null

     */

    transient Node<E> head;

    /**

     * Tail of linked list.

     * Invariant: last.next == null

     */

    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();

        }

    }

    /**

     * Links node at end of queue.

     *

     * @param node the node

     */

    private void enqueue(Node<E> node) {

        // assert putLock.isHeldByCurrentThread();

        // assert last.next == null;

        last = last.next = node;

    }

    /**

     * Removes a node from head of queue.

     *

     * @return the node

     */

    private E dequeue() {

        // assert takeLock.isHeldByCurrentThread();

        // assert head.item == null;

        Node<E> h = head;

        Node<E> first = h.next;

        h.next = h; // help GC

        head = first;

        E x = first.item;

        first.item = null;

        return x;

    }

    /**

     * Locks to prevent both puts and takes.

     */

    void fullyLock() {

        putLock.lock();

        takeLock.lock();

    }

    /**

     * Unlocks to allow both puts and takes.

     */

    void fullyUnlock() {

        takeLock.unlock();

        putLock.unlock();

    }

    /**

     * Creates a {@code LinkedBlockingQueue} with a capacity of

     * {@link Integer#MAX_VALUE}.

     */

    public LinkedBlockingQueue() {

        this(Integer.MAX_VALUE);

    }

    /**

     * Creates a {@code LinkedBlockingQueue} with the given (fixed) capacity.

     *

     * @param capacity the capacity of this queue

     * @throws IllegalArgumentException if {@code capacity} 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 {@code LinkedBlockingQueue} 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);

        final ReentrantLock putLock = this.putLock;

        putLock.lock(); // Never contended, but necessary for visibility

        try {

            int n = 0;

            for (E e : c) {

                if (e == null)

                    throw new NullPointerException();

                if (n == capacity)

                    throw new IllegalStateException("Queue full");

                enqueue(new Node<E>(e));

                ++n;

            }

            count.set(n);

        } finally {

            putLock.unlock();

        }

    }

    // 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 {@code size} of this queue.

     *

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

     * an element will succeed by inspecting {@code remainingCapacity}

     * 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();

        final int c;

        final Node<E> node = new Node<E>(e);

        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.

             */

            while (count.get() == capacity) {

                notFull.await();

            }

            enqueue(node);

            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 {@code true} if successful, or {@code false} 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);

        final int c;

        final ReentrantLock putLock = this.putLock;

        final AtomicInteger count = this.count;

        putLock.lockInterruptibly();

        try {

            while (count.get() == capacity) {

                if (nanos <= 0L)

                    return false;

                nanos = notFull.awaitNanos(nanos);

            }

            enqueue(new Node<E>(e));

            c = count.getAndIncrement();

            if (c + 1 < capacity)

                notFull.signal();

        } 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 {@code true} upon success and {@code false} 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;

        final int c;

        final Node<E> node = new Node<E>(e);

        final ReentrantLock putLock = this.putLock;

        putLock.lock();

        try {

            if (count.get() == capacity)

                return false;

            enqueue(node);

            c = count.getAndIncrement();

            if (c + 1 < capacity)

                notFull.signal();

        } finally {

            putLock.unlock();

        }

        if (c == 0)

            signalNotEmpty();

        return true;

    }

    public E take() throws InterruptedException {

        final E x;

        final int c;

        final AtomicInteger count = this.count;

        final ReentrantLock takeLock = this.takeLock;

        takeLock.lockInterruptibly();

        try {

            while (count.get() == 0) {

                notEmpty.await();

            }

            x = dequeue();

            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 {

        final E x;

        final int c;

        long nanos = unit.toNanos(timeout);

        final AtomicInteger count = this.count;

        final ReentrantLock takeLock = this.takeLock;

        takeLock.lockInterruptibly();

        try {

            while (count.get() == 0) {

                if (nanos <= 0L)

                    return null;

                nanos = notEmpty.awaitNanos(nanos);

            }

            x = dequeue();

            c = count.getAndDecrement();

            if (c > 1)

                notEmpty.signal();

        } finally {

            takeLock.unlock();

        }

        if (c == capacity)

            signalNotFull();

        return x;

    }

    public E poll() {

        final AtomicInteger count = this.count;

        if (count.get() == 0)

            return null;

        final E x;

        final int c;

        final ReentrantLock takeLock = this.takeLock;

        takeLock.lock();

        try {

            if (count.get() == 0)

                return null;

            x = dequeue();

            c = count.getAndDecrement();

            if (c > 1)

                notEmpty.signal();

        } finally {

            takeLock.unlock();

        }

        if (c == capacity)

            signalNotFull();

        return x;

    }

    public E peek() {

        final AtomicInteger count = this.count;

        if (count.get() == 0)

            return null;

        final ReentrantLock takeLock = this.takeLock;

        takeLock.lock();

        try {

            return (count.get() > 0) ? head.next.item : null;

        } finally {

            takeLock.unlock();

        }

    }