/*

 * 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.concurrent.locks.*;

import java.util.*;

/**

 * 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}/../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 ArrayBlockingQueue<E> extends AbstractQueue<E>

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

    /**

     * 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;

    // Internal helper methods

    /**

     * Circularly increment i.

     */

    final int inc(int i) {

        return (++i == items.length) ? 0 : i;

    }

    /**

     * Circularly decrement i.

     */

    final int dec(int i) {

        return ((i == 0) ? items.length : i) - 1;

    }

    @SuppressWarnings("unchecked")

    static <E> E cast(Object item) {

        return (E) item;

    }

    /**

     * Returns item at index i.

     */

    final E itemAt(int i) {

        return this.<E>cast(items[i]);

    }

    /**

     * Throws NullPointerException if argument is null.

     *

     * @param v the element

     */

    private static void checkNotNull(Object v) {

        if (v == null)

            throw new NullPointerException();

    }

    /**

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

     * Call only when holding lock.

     */

    private void insert(E x) {

        items[putIndex] = x;

        putIndex = inc(putIndex);

        ++count;

        notEmpty.signal();

    }

    /**

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

     * Call only when holding lock.

     */

    private E extract() {

        final Object[] items = this.items;

        E x = this.<E>cast(items[takeIndex]);

        items[takeIndex] = null;

        takeIndex = inc(takeIndex);

        --count;

        notFull.signal();

        return x;

    }

    /**

     * Deletes item at position i.

     * Utility for remove and iterator.remove.

     * Call only when holding lock.

     */

    void removeAt(int i) {

        final Object[] items = this.items;

        // if removing front item, just advance

        if (i == takeIndex) {

            items[takeIndex] = null;

            takeIndex = inc(takeIndex);

        } else {

            // slide over all others up through putIndex.

            for (;;) {

                int nexti = inc(i);

                if (nexti != putIndex) {

                    items[i] = items[nexti];

                    i = nexti;

                } else {

                    items[i] = null;

                    putIndex = i;

                    break;

                }

            }

        }

        --count;

        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 {

            int i = 0;

            try {

                for (E e : c) {

                    checkNotNull(e);

                    items[i++] = 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) {

        checkNotNull(e);

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            if (count == items.length)

                return false;

            else {

                insert(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 {

        checkNotNull(e);

        final ReentrantLock lock = this.lock;

        lock.lockInterruptibly();

        try {

            while (count == items.length)

                notFull.await();

            insert(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 {

        checkNotNull(e);

        long nanos = unit.toNanos(timeout);

        final ReentrantLock lock = this.lock;

        lock.lockInterruptibly();

        try {

            while (count == items.length) {

                if (nanos <= 0)

                    return false;

                nanos = notFull.awaitNanos(nanos);

            }

            insert(e);

            return true;

        } finally {

            lock.unlock();

        }

    }

    public E poll() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

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

        } finally {

            lock.unlock();

        }

    }

    public E take() throws InterruptedException {

        final ReentrantLock lock = this.lock;

        lock.lockInterruptibly();

        try {

            while (count == 0)

                notEmpty.await();

            return extract();

        } 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 <= 0)

                    return null;

                nanos = notEmpty.awaitNanos(nanos);

            }

            return extract();

        } finally {

            lock.unlock();

        }

    }

    public E peek() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return (count == 0) ? null : itemAt(takeIndex);

        } 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

     * threads.

     *

     * @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) {

        if (o == null) return false;

        final Object[] items = this.items;

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) {

                if (o.equals(items[i])) {

                    removeAt(i);

                    return true;

                }

            }

            return false;

        } 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) {

        if (o == null) return false;

        final Object[] items = this.items;

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)

                if (o.equals(items[i]))

                    return true;

            return false;

        } finally {

            lock.unlock();

        }

    }

    /**

     * 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() {

        final Object[] items = this.items;

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            final int count = this.count;

            Object[] a = new Object[count];

            for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)

                a[k] = items[i];

            return a;

        } finally {

            lock.unlock();

        }

    }

    /**

     * 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

     * {@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>

     *     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

     */

    @SuppressWarnings("unchecked")

    public <T> T[] toArray(T[] a) {

        final Object[] items = this.items;

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            final int count = this.count;

            final int len = a.length;

            if (len < count)

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

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

            for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)

                a[k] = (T) items[i];

            if (len > count)

                a[count] = null;

            return a;

        } finally {

            lock.unlock();

        }

    }

    public String toString() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            int k = count;

            if (k == 0)

                return "[]";

            StringBuilder sb = new StringBuilder();

            sb.append('[');

            for (int i = takeIndex; ; i = inc(i)) {

                Object e = items[i];

                sb.append(e == this ? "(this Collection)" : e);

                if (--k == 0)

                    return sb.append(']').toString();

                sb.append(',').append(' ');

            }

        } 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 Object[] items = this.items;

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)

                items[i] = null;

            count = 0;

            putIndex = 0;

            takeIndex = 0;

            notFull.signalAll();

        } finally {

            lock.unlock();

        }

    }

    /**

     * @throws UnsupportedOperationException {@inheritDoc}

     * @throws ClassCastException            {@inheritDoc}

     * @throws NullPointerException          {@inheritDoc}

     * @throws IllegalArgumentException      {@inheritDoc}

     */

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

        checkNotNull(c);

        if (c == this)

            throw new IllegalArgumentException();

        final Object[] items = this.items;

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            int i = takeIndex;

            int n = 0;

            int max = count;

            while (n < max) {

                c.add(this.<E>cast(items[i]));

                items[i] = null;

                i = inc(i);

                ++n;

            }

            if (n > 0) {

                count = 0;

                putIndex = 0;

                takeIndex = 0;

                notFull.signalAll();

            }

            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) {

        checkNotNull(c);

        if (c == this)

            throw new IllegalArgumentException();

        if (maxElements <= 0)

            return 0;

        final Object[] items = this.items;

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            int i = takeIndex;

            int n = 0;

            int max = (maxElements < count) ? maxElements : count;

            while (n < max) {

                c.add(this.<E>cast(items[i]));

                items[i] = null;

                i = inc(i);

                ++n;

            }

            if (n > 0) {

                count -= n;

                takeIndex = i;

                notFull.signalAll();

            }

            return n;

        } finally {

            lock.unlock();

        }

    }

    /**

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

     * The elements will be returned in order from first (head) to last (tail).

     *

     * <p>The returned {@code Iterator} is a "weakly consistent" iterator that

     * will never throw {@link java.util.ConcurrentModificationException

     * 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();

    }

    /**

     * Iterator for ArrayBlockingQueue. To maintain weak consistency

     * with respect to puts and takes, we (1) read ahead one slot, so

     * as to not report hasNext true but then not have an element to

     * return -- however we later recheck this slot to use the most

     * current value; (2) ensure that each array slot is traversed at

     * most once (by tracking "remaining" elements); (3) skip over

     * null slots, which can occur if takes race ahead of iterators.

     * However, for circular array-based queues, we cannot rely on any

     * well established definition of what it means to be weakly

     * consistent with respect to interior removes since these may

     * require slot overwrites in the process of sliding elements to

     * cover gaps. So we settle for resiliency, operating on

     * established apparent nexts, which may miss some elements that

     * have moved between calls to next.

     */

    private class Itr implements Iterator<E> {

        private int remaining; // Number of elements yet to be returned

        private int nextIndex; // Index of element to be returned by next

        private E nextItem;    // Element to be returned by next call to next

        private E lastItem;    // Element returned by last call to next

        private int lastRet;   // Index of last element returned, or -1 if none

        Itr() {

            final ReentrantLock lock = ArrayBlockingQueue.this.lock;

            lock.lock();

            try {

                lastRet = -1;

                if ((remaining = count) > 0)

                    nextItem = itemAt(nextIndex = takeIndex);

            } finally {

                lock.unlock();

            }

        }

        public boolean hasNext() {

            return remaining > 0;

        }

        public E next() {

            final ReentrantLock lock = ArrayBlockingQueue.this.lock;

            lock.lock();

            try {

                if (remaining <= 0)

                    throw new NoSuchElementException();

                lastRet = nextIndex;

                E x = itemAt(nextIndex);  // check for fresher value

                if (x == null) {

                    x = nextItem;         // we are forced to report old value

                    lastItem = null;      // but ensure remove fails

                }

                else

                    lastItem = x;

                while (--remaining > 0 && // skip over nulls

                       (nextItem = itemAt(nextIndex = inc(nextIndex))) == null)

                    ;

                return x;

            } finally {

                lock.unlock();

            }

        }

        public void remove() {

            final ReentrantLock lock = ArrayBlockingQueue.this.lock;

            lock.lock();

            try {

                int i = lastRet;

                if (i == -1)

                    throw new IllegalStateException();

                lastRet = -1;

                E x = lastItem;

                lastItem = null;

                // only remove if item still at index

                if (x != null && x == items[i]) {

                    boolean removingHead = (i == takeIndex);

                    removeAt(i);

                    if (!removingHead)

                        nextIndex = dec(nextIndex);

                }

            } finally {

                lock.unlock();

            }

        }

    }

}