/*

 * 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.ref.WeakReference;

import java.util.AbstractQueue;

import java.util.Arrays;

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.locks.Condition;

import java.util.concurrent.locks.ReentrantLock;

import java.util.function.Consumer;

import java.util.function.Predicate;

/**

 * A bounded {@linkplain BlockingQueue blocking queue} backed by an

 * array.  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.

 *

 * <p>This is a classic &quot;bounded buffer&quot;, in which a

 * fixed-sized array holds elements inserted by producers and

 * extracted by consumers.  Once created, the capacity cannot be

 * changed.  Attempts to {@code put} an element into a full queue

 * will result in the operation blocking; attempts to {@code take} an

 * element from an empty queue will similarly block.

 *

 * <p>This class supports an optional fairness policy for ordering

 * waiting producer and consumer threads.  By default, this ordering

 * is not guaranteed. However, a queue constructed with fairness set

 * to {@code true} grants threads access in FIFO order. Fairness

 * generally decreases throughput but reduces variability and avoids

 * starvation.

 *

 * <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 ArrayBlockingQueue<E> extends AbstractQueue<E>

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

    /*

     * Much of the implementation mechanics, especially the unusual

     * nested loops, are shared and co-maintained with ArrayDeque.

     */

    /**

     * Serialization ID. This class relies on default serialization

     * even for the items array, which is default-serialized, even if

     * it is empty. Otherwise it could not be declared final, which is

     * necessary here.

     */

    private static final long serialVersionUID = -817911632652898426L;

    /** The queued items */

    final Object[] items;

    /** items index for next take, poll, peek or remove */

    int takeIndex;

    /** items index for next put, offer, or add */

    int putIndex;

    /** Number of elements in the queue */

    int count;

    /*

     * Concurrency control uses the classic two-condition algorithm

     * found in any textbook.

     */

    /** Main lock guarding all access */

    final ReentrantLock lock;

    /** Condition for waiting takes */

    private final Condition notEmpty;

    /** Condition for waiting puts */

    private final Condition notFull;

    /**

     * Shared state for currently active iterators, or null if there

     * are known not to be any.  Allows queue operations to update

     * iterator state.

     */

    transient Itrs itrs;

    // Internal helper methods

    /**

     * Increments i, mod modulus.

     * Precondition and postcondition: 0 <= i < modulus.

     */

    static final int inc(int i, int modulus) {

        if (++i >= modulus) i = 0;

        return i;

    }

    /**

     * Decrements i, mod modulus.

     * Precondition and postcondition: 0 <= i < modulus.

     */

    static final int dec(int i, int modulus) {

        if (--i < 0) i = modulus - 1;

        return i;

    }

    /**

     * Returns item at index i.

     */

    @SuppressWarnings("unchecked")

    final E itemAt(int i) {

        return (E) items[i];

    }

    /**

     * Returns element at array index i.

     * This is a slight abuse of generics, accepted by javac.

     */

    @SuppressWarnings("unchecked")

    static <E> E itemAt(Object[] items, int i) {

        return (E) items[i];

    }

    /**

     * Inserts element at current put position, advances, and signals.

     * Call only when holding lock.

     */

    private void enqueue(E e) {

        // assert lock.isHeldByCurrentThread();

        // assert lock.getHoldCount() == 1;

        // assert items[putIndex] == null;

        final Object[] items = this.items;

        items[putIndex] = e;

        if (++putIndex == items.length) putIndex = 0;

        count++;

        notEmpty.signal();

    }

    /**

     * Extracts element at current take position, advances, and signals.

     * Call only when holding lock.

     */

    private E dequeue() {

        // assert lock.isHeldByCurrentThread();

        // assert lock.getHoldCount() == 1;

        // assert items[takeIndex] != null;

        final Object[] items = this.items;

        @SuppressWarnings("unchecked")

        E e = (E) items[takeIndex];

        items[takeIndex] = null;

        if (++takeIndex == items.length) takeIndex = 0;

        count--;

        if (itrs != null)

            itrs.elementDequeued();

        notFull.signal();

        return e;

    }

    /**

     * Deletes item at array index removeIndex.

     * Utility for remove(Object) and iterator.remove.

     * Call only when holding lock.

     */

    void removeAt(final int removeIndex) {

        // assert lock.isHeldByCurrentThread();

        // assert lock.getHoldCount() == 1;

        // assert items[removeIndex] != null;

        // assert removeIndex >= 0 && removeIndex < items.length;

        final Object[] items = this.items;

        if (removeIndex == takeIndex) {

            // removing front item; just advance

            items[takeIndex] = null;

            if (++takeIndex == items.length) takeIndex = 0;

            count--;

            if (itrs != null)

                itrs.elementDequeued();

        } else {

            // an "interior" remove

            // slide over all others up through putIndex.

            for (int i = removeIndex, putIndex = this.putIndex;;) {

                int pred = i;

                if (++i == items.length) i = 0;

                if (i == putIndex) {

                    items[pred] = null;

                    this.putIndex = pred;

                    break;

                }

                items[pred] = items[i];

            }

            count--;

            if (itrs != null)

                itrs.removedAt(removeIndex);

        }

        notFull.signal();

    }

    /**

     * Creates an {@code ArrayBlockingQueue} with the given (fixed)

     * capacity and default access policy.

     *

     * @param capacity the capacity of this queue

     * @throws IllegalArgumentException if {@code capacity < 1}

     */

    public ArrayBlockingQueue(int capacity) {

        this(capacity, false);

    }

    /**

     * Creates an {@code ArrayBlockingQueue} with the given (fixed)

     * capacity and the specified access policy.

     *

     * @param capacity the capacity of this queue

     * @param fair if {@code true} then queue accesses for threads blocked

     *        on insertion or removal, are processed in FIFO order;

     *        if {@code false} the access order is unspecified.

     * @throws IllegalArgumentException if {@code capacity < 1}

     */

    public ArrayBlockingQueue(int capacity, boolean fair) {

        if (capacity <= 0)

            throw new IllegalArgumentException();

        this.items = new Object[capacity];

        lock = new ReentrantLock(fair);

        notEmpty = lock.newCondition();

        notFull =  lock.newCondition();

    }

    /**

     * Creates an {@code ArrayBlockingQueue} with the given (fixed)

     * capacity, the specified access policy and initially containing the

     * elements of the given collection,

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

     *

     * @param capacity the capacity of this queue

     * @param fair if {@code true} then queue accesses for threads blocked

     *        on insertion or removal, are processed in FIFO order;

     *        if {@code false} the access order is unspecified.

     * @param c the collection of elements to initially contain

     * @throws IllegalArgumentException if {@code capacity} is less than

     *         {@code c.size()}, or less than 1.

     * @throws NullPointerException if the specified collection or any

     *         of its elements are null

     */

    public ArrayBlockingQueue(int capacity, boolean fair,

                              Collection<? extends E> c) {

        this(capacity, fair);

        final ReentrantLock lock = this.lock;

        lock.lock(); // Lock only for visibility, not mutual exclusion

        try {

            final Object[] items = this.items;

            int i = 0;

            try {

                for (E e : c)

                    items[i++] = Objects.requireNonNull(e);

            } catch (ArrayIndexOutOfBoundsException ex) {

                throw new IllegalArgumentException();

            }

            count = i;

            putIndex = (i == capacity) ? 0 : i;

        } finally {

            lock.unlock();

        }

    }

    /**

     * 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 throwing an

     * {@code IllegalStateException} if this queue is full.

     *

     * @param e the element to add

     * @return {@code true} (as specified by {@link Collection#add})

     * @throws IllegalStateException if this queue is full

     * @throws NullPointerException if the specified element is null

     */

    public boolean add(E e) {

        return super.add(e);

    }

    /**

     * 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.  This method is generally preferable to method {@link #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) {

        Objects.requireNonNull(e);

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            if (count == items.length)

                return false;

            else {

                enqueue(e);

                return true;

            }

        } finally {

            lock.unlock();

        }

    }

    /**

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

     * for space to become available if the queue is full.

     *

     * @throws InterruptedException {@inheritDoc}

     * @throws NullPointerException {@inheritDoc}

     */

    public void put(E e) throws InterruptedException {

        Objects.requireNonNull(e);

        final ReentrantLock lock = this.lock;

        lock.lockInterruptibly();

        try {

            while (count == items.length)

                notFull.await();

            enqueue(e);

        } finally {

            lock.unlock();

        }

    }

    /**

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

     * up to the specified wait time for space to become available if

     * the queue is full.

     *

     * @throws InterruptedException {@inheritDoc}

     * @throws NullPointerException {@inheritDoc}

     */

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

        throws InterruptedException {

        Objects.requireNonNull(e);

        long nanos = unit.toNanos(timeout);

        final ReentrantLock lock = this.lock;

        lock.lockInterruptibly();

        try {

            while (count == items.length) {

                if (nanos <= 0L)

                    return false;

                nanos = notFull.awaitNanos(nanos);

            }

            enqueue(e);

            return true;

        } finally {

            lock.unlock();

        }

    }

    public E poll() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return (count == 0) ? null : dequeue();

        } finally {

            lock.unlock();

        }

    }

    public E take() throws InterruptedException {

        final ReentrantLock lock = this.lock;

        lock.lockInterruptibly();

        try {

            while (count == 0)

                notEmpty.await();

            return dequeue();

        } 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 {

            while (count == 0) {

                if (nanos <= 0L)

                    return null;

                nanos = notEmpty.awaitNanos(nanos);

            }

            return dequeue();

        } finally {

            lock.unlock();

        }

    }

    public E peek() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return itemAt(takeIndex); // null when queue is empty

        } finally {

            lock.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() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return count;

        } finally {

            lock.unlock();

        }

    }

    // 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() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return items.length - count;

        } finally {

            lock.unlock();

        }

    }

    /**

     * 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 this queue contained the specified element

     * (or equivalently, if this queue changed as a result of the call).

     *

     * <p>Removal of interior elements in circular array based queues

     * is an intrinsically slow and disruptive operation, so should

     * be undertaken only in exceptional circumstances, ideally

     * only when the queue is known not to be accessible by other