/*

  * 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

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  */

 /*

  * 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();

             }

         }

     }

 }