/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *      https://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.commons.pool2.impl;
  
  import java.io.IOException;
  import java.io.ObjectInputStream;
  import java.io.Serializable;
  import java.time.Duration;
  import java.util.AbstractQueue;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.Objects;
  import java.util.concurrent.BlockingDeque;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.locks.Condition;
  
  /**
   * An optionally-bounded {@linkplain java.util.concurrent.BlockingDeque blocking
   * deque} based on linked nodes.
   *
   * <p> The optional capacity bound constructor argument serves as a
   * way to prevent excessive 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
   * deque above capacity.
   * </p>
   *
   * <p>Most operations run in constant time (ignoring time spent
   * blocking).  Exceptions include {@link #remove(Object) remove},
   * {@link #removeFirstOccurrence removeFirstOccurrence}, {@link
   * #removeLastOccurrence removeLastOccurrence}, {@link #contains
   * contains}, {@link #iterator iterator.remove()}, and the bulk
   * operations, all of which run in linear time.
   * </p>
   *
   * <p>This class and its iterator implement all of the
   * <em>optional</em> methods of the {@link Collection} and {@link
   * Iterator} interfaces.
   * </p>
   *
   * <p>This class is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   * </p>
   *
   * @param <E> the type of elements held in this collection
   *
   * Note: This was copied from Apache Harmony and modified to suit the needs of
   *       Commons Pool.
   *
   * @since 2.0
   */
  final class LinkedBlockingDeque<E> extends AbstractQueue<E>
          implements BlockingDeque<E>, Serializable {
  
      /*
       * Implemented as a simple doubly-linked list protected by a
       * single lock and using conditions to manage blocking.
       *
       * 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 jump to "first" (for next links)
       * or "last" (for prev links).
       */
  
      /*
       * We have "diamond" multiple interface/abstract class inheritance
       * here, and that introduces ambiguities. Often we want the
       * BlockingDeque javadoc combined with the AbstractQueue
       * implementation, so a lot of method specs are duplicated here.
       */
  
      /**
       * Base class for Iterators for LinkedBlockingDeque
       */
     private abstract class AbstractItr implements Iterator<E> {
         /**
          * The next node to return in next()
          */
          Node<E> next;
 
         /**
          * nextItem holds on to item fields because once we claim that
          * an element exists in hasNext(), we must return item read
          * under lock (in advance()) even if it was in the process of
          * being removed when hasNext() was called.
          */
         E nextItem;
 
         /**
          * Node returned by most recent call to next. Needed by remove.
          * Reset to null if this element is deleted by a call to remove.
          */
         private Node<E> lastRet;
 
         /**
          * Constructs a new iterator. Sets the initial position.
          */
         AbstractItr() {
             // set to initial position
             lock.lock();
             try {
                 next = firstNode();
                 nextItem = next == null ? null : next.item;
             } finally {
                 lock.unlock();
             }
         }
 
         /**
          * Advances next.
          */
         void advance() {
             lock.lock();
             try {
                 // assert next != null;
                 next = succ(next);
                 nextItem = next == null ? null : next.item;
             } finally {
                 lock.unlock();
             }
         }
 
         /**
          * Obtain the first node to be returned by the iterator.
          *
          * @return first node
          */
         abstract Node<E> firstNode();
 
         @Override
         public boolean hasNext() {
             return next != null;
         }
 
         @Override
         public E next() {
             if (next == null) {
                 throw new NoSuchElementException();
             }
             lastRet = next;
             final E x = nextItem;
             advance();
             return x;
         }
 
         /**
          * For a given node, obtain the next node to be returned by the
          * iterator.
          *
          * @param n given node
          * @return next node
          */
         abstract Node<E> nextNode(Node<E> n);
 
         @Override
         public void remove() {
             final Node<E> n = lastRet;
             if (n == null) {
                 throw new IllegalStateException();
             }
             lastRet = null;
             lock.lock();
             try {
                 if (n.item != null) {
                     unlink(n);
                 }
             } finally {
                 lock.unlock();
             }
         }
 
         /**
          * Returns the successor node of the given non-null, but
          * possibly previously deleted, node.
          *
          * @param n node whose successor is sought
          * @return successor node
          */
         private Node<E> succ(Node<E> n) {
             // Chains of deleted nodes ending in null or self-links
             // are possible if multiple interior nodes are removed.
             for (;;) {
                 final Node<E> s = nextNode(n);
                 if (s == null) {
                     return null;
                 }
                 if (s.item != null) {
                     return s;
                 }
                 if (s == n) {
                     return firstNode();
                 }
                 n = s;
             }
         }
     }
 
     /** Descending iterator */
     private final class DescendingItr extends AbstractItr {
         @Override
         Node<E> firstNode() {
             return last;
         }
 
         @Override
         Node<E> nextNode(final Node<E> n) {
             return n.prev;
         }
     }
 
     /** Forward iterator */
     private final class Itr extends AbstractItr {
         @Override
         Node<E> firstNode() {
             return first;
         }
 
         @Override
         Node<E> nextNode(final Node<E> n) {
             return n.next;
         }
     }
 
     /**
      * Doubly-linked list node class.
      *
      * @param <E> node item type
      */
     private static final class Node<E> {
         /**
          * The item, or null if this node has been removed.
          */
         E item;
 
         /**
          * One of:
          * - the real predecessor Node
          * - this Node, meaning the predecessor is tail
          * - null, meaning there is no predecessor
          */
         Node<E> prev;
 
         /**
          * One of:
          * - the real successor Node
          * - this Node, meaning the successor is head
          * - null, meaning there is no successor
          */
         Node<E> next;
 
         /**
          * Constructs a new list node.
          *
          * @param x The list item
          * @param p Previous item
          * @param n Next item
          */
         Node(final E x, final Node<E> p, final Node<E> n) {
             item = x;
             prev = p;
             next = n;
         }
     }
 
     private static final long serialVersionUID = -387911632671998426L;
 
     /**
      * Pointer to first node.
      * Invariant: (first == null && last == null) ||
      *            (first.prev == null && first.item != null)
      */
     private transient Node<E> first; // @GuardedBy("lock")
 
     /**
      * Pointer to last node.
      * Invariant: (first == null && last == null) ||
      *            (last.next == null && last.item != null)
      */
     private transient Node<E> last; // @GuardedBy("lock")
 
     /** Number of items in the deque */
     private transient int count; // @GuardedBy("lock")
 
     /** Maximum number of items in the deque */
     private final int capacity;
 
     /** Main lock guarding all access */
     private final InterruptibleReentrantLock lock;
 
     /** Condition for waiting takes */
     private final Condition notEmpty;
 
     /** Condition for waiting puts */
     private final Condition notFull;
 
     /**
      * Creates a {@code LinkedBlockingDeque} with a capacity of
      * {@link Integer#MAX_VALUE}.
      */
     LinkedBlockingDeque() {
         this(Integer.MAX_VALUE);
     }
 
     /**
      * Creates a {@code LinkedBlockingDeque} with a capacity of
      * {@link Integer#MAX_VALUE} and the given fairness policy.
      * @param fairness true means threads waiting on the deque should be served
      * as if waiting in a FIFO request queue
      */
     LinkedBlockingDeque(final boolean fairness) {
         this(Integer.MAX_VALUE, fairness);
     }
 
     // Basic linking and unlinking operations, called only while holding lock
 
     /**
      * Creates a {@code LinkedBlockingDeque} 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
      */
     LinkedBlockingDeque(final Collection<? extends E> c) {
         this(Integer.MAX_VALUE);
         lock.lock(); // Never contended, but necessary for visibility
         try {
             for (final E e : c) {
                 Objects.requireNonNull(e);
                 if (!linkLast(e)) {
                     throw new IllegalStateException("Deque full");
                 }
             }
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity.
      *
      * @param capacity the capacity of this deque
      * @throws IllegalArgumentException if {@code capacity} is less than 1
      */
     LinkedBlockingDeque(final int capacity) {
         this(capacity, false);
     }
 
     /**
      * Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity
      * and fairness policy.
      *
      * @param capacity the capacity of this deque
      * @param fairness true means threads waiting on the deque should be served
      * as if waiting in a FIFO request queue
      * @throws IllegalArgumentException if {@code capacity} is less than 1
      */
     LinkedBlockingDeque(final int capacity, final boolean fairness) {
         if (capacity <= 0) {
             throw new IllegalArgumentException();
         }
         this.capacity = capacity;
         lock = new InterruptibleReentrantLock(fairness);
         notEmpty = lock.newCondition();
         notFull = lock.newCondition();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public boolean add(final E e) {
         addLast(e);
         return true;
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public void addFirst(final E e) {
         if (!offerFirst(e)) {
             throw new IllegalStateException("Deque full");
         }
     }
 
     // BlockingDeque methods
 
     /**
      * {@inheritDoc}
      */
     @Override
     public void addLast(final E e) {
         if (!offerLast(e)) {
             throw new IllegalStateException("Deque full");
         }
     }
 
     /**
      * Atomically removes all of the elements from this deque.
      * The deque will be empty after this call returns.
      */
     @Override
     public void clear() {
         lock.lock();
         try {
             for (Node<E> f = first; f != null;) {
                 f.item = null;
                 final Node<E> n = f.next;
                 f.prev = null;
                 f.next = null;
                 f = n;
             }
             first = last = null;
             count = 0;
             notFull.signalAll();
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Returns {@code true} if this deque contains the specified element.
      * More formally, returns {@code true} if and only if this deque contains
      * at least one element {@code e} such that {@code o.equals(e)}.
      *
      * @param o object to be checked for containment in this deque
      * @return {@code true} if this deque contains the specified element
      */
     @Override
     public boolean contains(final Object o) {
         if (o == null) {
             return false;
         }
         lock.lock();
         try {
             for (Node<E> p = first; p != null; p = p.next) {
                 if (o.equals(p.item)) {
                     return true;
                 }
             }
             return false;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public Iterator<E> descendingIterator() {
         return new DescendingItr();
     }
 
     /**
      * Drains the queue to the specified collection.
      *
      * @param c The collection to add the elements to
      * @return number of elements added to the collection
      * @throws UnsupportedOperationException if the add operation is not
      *         supported by the specified collection
      * @throws ClassCastException if the class of the elements held by this
      *         collection prevents them from being added to the specified
      *         collection
      * @throws NullPointerException if c is null
      * @throws IllegalArgumentException if c is this instance
      */
     public int drainTo(final Collection<? super E> c) {
         return drainTo(c, Integer.MAX_VALUE);
     }
 
     /**
      * Drains no more than the specified number of elements from the queue to the
      * specified collection.
      *
      * @param collection collection to add the elements to
      * @param maxElements maximum number of elements to remove from the queue
      * @return number of elements added to the collection
      * @throws UnsupportedOperationException if the add operation is not
      *         supported by the specified collection
      * @throws ClassCastException if the class of the elements held by this
      *         collection prevents them from being added to the specified
      *         collection
      * @throws NullPointerException if c is null
      * @throws IllegalArgumentException if c is this instance
      */
     public int drainTo(final Collection<? super E> collection, final int maxElements) {
         Objects.requireNonNull(collection, "collection");
         if (collection == this) {
             throw new IllegalArgumentException();
         }
         lock.lock();
         try {
             final int n = Math.min(maxElements, count);
             for (int i = 0; i < n; i++) {
                 collection.add(first.item); // In this order, in case add() throws.
                 unlinkFirst();
             }
             return n;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Retrieves, but does not remove, the head of the queue represented by
      * this deque.  This method differs from {@link #peek peek} only in that
      * it throws an exception if this deque is empty.
      *
      * <p>This method is equivalent to {@link #getFirst() getFirst}.
      *
      * @return the head of the queue represented by this deque
      * @throws NoSuchElementException if this deque is empty
      */
     @Override
     public E element() {
         return getFirst();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public E getFirst() {
         final E x = peekFirst();
         if (x == null) {
             throw new NoSuchElementException();
         }
         return x;
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public E getLast() {
         final E x = peekLast();
         if (x == null) {
             throw new NoSuchElementException();
         }
         return x;
     }
 
     /**
      * Gets the length of the queue of threads waiting to take instances from this deque. See disclaimer on accuracy
      * in {@link java.util.concurrent.locks.ReentrantLock#getWaitQueueLength(Condition)}.
      *
      * @return number of threads waiting on this deque's notEmpty condition.
      */
     int getTakeQueueLength() {
         lock.lock();
         try {
            return lock.getWaitQueueLength(notEmpty);
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Returns true if there are threads waiting to take instances from this deque. See disclaimer on accuracy in
      * {@link java.util.concurrent.locks.ReentrantLock#hasWaiters(Condition)}.
      *
      * @return true if there is at least one thread waiting on this deque's notEmpty condition.
      */
     boolean hasTakeWaiters() {
         lock.lock();
         try {
             return lock.hasWaiters(notEmpty);
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Interrupts the threads currently waiting to take an object from the pool. See disclaimer on accuracy in
      * {@link java.util.concurrent.locks.ReentrantLock#getWaitingThreads(Condition)}.
      */
     void interuptTakeWaiters() {
         lock.lock();
         try {
             lock.interruptWaiters(notEmpty);
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Returns an iterator over the elements in this deque in proper sequence.
      * The elements will be returned in order from first (head) to last (tail).
      * 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 deque in proper sequence
      */
     @Override
     public Iterator<E> iterator() {
         return new Itr();
     }
 
     /**
      * Links provided element as first element, or returns false if full.
      *
      * @param e The element to link as the first element.
      * @return {@code true} if successful, otherwise {@code false}
      */
     private boolean linkFirst(final E e) {
         // assert lock.isHeldByCurrentThread();
         if (count >= capacity) {
             return false;
         }
         final Node<E> f = first;
         final Node<E> x = new Node<>(e, null, f);
         first = x;
         if (last == null) {
             last = x;
         } else {
             f.prev = x;
         }
         ++count;
         notEmpty.signal();
         return true;
     }
 
     /**
      * Links provided element as last element, or returns false if full.
      *
      * @param e The element to link as the last element.
      * @return {@code true} if successful, otherwise {@code false}
      */
     private boolean linkLast(final E e) {
         // assert lock.isHeldByCurrentThread();
         if (count >= capacity) {
             return false;
         }
         final Node<E> l = last;
         final Node<E> x = new Node<>(e, l, null);
         last = x;
         if (first == null) {
             first = x;
         } else {
             l.next = x;
         }
         ++count;
         notEmpty.signal();
         return true;
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public boolean offer(final E e) {
         return offerLast(e);
     }
 
     /**
      * Links the provided element as the last in the queue, waiting up to the
      * specified time to do so if the queue is full.
      * <p>
      * This method is equivalent to {@link #offerLast(Object, long, TimeUnit)}
      *
      * @param e         element to link
      * @param timeout   length of time to wait
      * @return {@code true} if successful, otherwise {@code false}
      * @throws NullPointerException if e is null
      * @throws InterruptedException if the thread is interrupted whilst waiting
      *         for space
      */
     boolean offer(final E e, final Duration timeout) throws InterruptedException {
         return offerLast(e, timeout);
     }
 
     /**
      * Links the provided element as the last in the queue, waiting up to the
      * specified time to do so if the queue is full.
      * <p>
      * This method is equivalent to {@link #offerLast(Object, long, TimeUnit)}
      *
      * @param e         element to link
      * @param timeout   length of time to wait
      * @param unit      units that timeout is expressed in
      * @return {@code true} if successful, otherwise {@code false}
      * @throws NullPointerException if e is null
      * @throws InterruptedException if the thread is interrupted whilst waiting
      *         for space
      */
     public boolean offer(final E e, final long timeout, final TimeUnit unit) throws InterruptedException {
         return offerLast(e, timeout, unit);
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public boolean offerFirst(final E e) {
         Objects.requireNonNull(e, "e");
         lock.lock();
         try {
             return linkFirst(e);
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Links the provided element as the first in the queue, waiting up to the
      * specified time to do so if the queue is full.
      *
      * @param e         element to link
      * @param timeout   length of time to wait
      * @return {@code true} if successful, otherwise {@code false}
      * @throws NullPointerException if e is null
      * @throws InterruptedException if the thread is interrupted whilst waiting
      *         for space
      */
     boolean offerFirst(final E e, final Duration timeout) throws InterruptedException {
         Objects.requireNonNull(e, "e");
         long nanos = timeout.toNanos();
         lock.lockInterruptibly();
         try {
             while (!linkFirst(e)) {
                 if (nanos <= 0) {
                     return false;
                 }
                 nanos = notFull.awaitNanos(nanos);
             }
             return true;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Links the provided element as the first in the queue, waiting up to the
      * specified time to do so if the queue is full.
      *
      * @param e         element to link
      * @param timeout   length of time to wait
      * @param unit      units that timeout is expressed in
      * @return {@code true} if successful, otherwise {@code false}
      * @throws NullPointerException if e is null
      * @throws InterruptedException if the thread is interrupted whilst waiting
      *         for space
      */
     public boolean offerFirst(final E e, final long timeout, final TimeUnit unit) throws InterruptedException {
         return offerFirst(e, PoolImplUtils.toDuration(timeout, unit));
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public boolean offerLast(final E e) {
         Objects.requireNonNull(e, "e");
         lock.lock();
         try {
             return linkLast(e);
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Links the provided element as the last in the queue, waiting up to the
      * specified time to do so if the queue is full.
      *
      * @param e         element to link
      * @param timeout   length of time to wait
      * @return {@code true} if successful, otherwise {@code false}
      * @throws NullPointerException if e is null
      * @throws InterruptedException if the thread is interrupted whist waiting
      *         for space
      */
     boolean offerLast(final E e, final Duration timeout) throws InterruptedException {
         Objects.requireNonNull(e, "e");
         long nanos = timeout.toNanos();
         lock.lockInterruptibly();
         try {
             while (!linkLast(e)) {
                 if (nanos <= 0) {
                     return false;
                 }
                 nanos = notFull.awaitNanos(nanos);
             }
             return true;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Links the provided element as the last in the queue, waiting up to the
      * specified time to do so if the queue is full.
      *
      * @param e         element to link
      * @param timeout   length of time to wait
      * @param unit      units that timeout is expressed in
      * @return {@code true} if successful, otherwise {@code false}
      * @throws NullPointerException if e is null
      * @throws InterruptedException if the thread is interrupted whist waiting
      *         for space
      */
     public boolean offerLast(final E e, final long timeout, final TimeUnit unit) throws InterruptedException {
         return offerLast(e, PoolImplUtils.toDuration(timeout, unit));
     }
 
     @Override
     public E peek() {
         return peekFirst();
     }
 
     // BlockingQueue methods
 
     @Override
     public E peekFirst() {
         lock.lock();
         try {
             return first == null ? null : first.item;
         } finally {
             lock.unlock();
         }
     }
 
     @Override
     public E peekLast() {
         lock.lock();
         try {
             return last == null ? null : last.item;
         } finally {
             lock.unlock();
         }
     }
 
     @Override
     public E poll() {
         return pollFirst();
     }
 
     /**
      * Unlinks the first element in the queue, waiting up to the specified time
      * to do so if the queue is empty.
      *
      * <p>This method is equivalent to {@link #pollFirst(long, TimeUnit)}.
      *
      * @param timeout   length of time to wait
      * @return the unlinked element
      * @throws InterruptedException if the current thread is interrupted
      */
     E poll(final Duration timeout) throws InterruptedException {
         return pollFirst(timeout);
     }
 
     /**
      * Unlinks the first element in the queue, waiting up to the specified time
      * to do so if the queue is empty.
      *
      * <p>This method is equivalent to {@link #pollFirst(long, TimeUnit)}.
      *
      * @param timeout   length of time to wait
      * @param unit      units that timeout is expressed in
      * @return the unlinked element
      * @throws InterruptedException if the current thread is interrupted
      */
     public E poll(final long timeout, final TimeUnit unit) throws InterruptedException {
         return pollFirst(timeout, unit);
     }
 
     @Override
     public E pollFirst() {
         lock.lock();
         try {
             return unlinkFirst();
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Unlinks the first element in the queue, waiting up to the specified time
      * to do so if the queue is empty.
      *
      * @param timeout   length of time to wait
      * @return the unlinked element
      * @throws InterruptedException if the current thread is interrupted
      */
     E pollFirst(final Duration timeout) throws InterruptedException {
         long nanos = timeout.toNanos();
         lock.lockInterruptibly();
         try {
             E x;
             while ((x = unlinkFirst()) == null) {
                 if (nanos <= 0) {
                     return null;
                 }
                 nanos = notEmpty.awaitNanos(nanos);
             }
             return x;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Unlinks the first element in the queue, waiting up to the specified time
      * to do so if the queue is empty.
      *
      * @param timeout   length of time to wait
      * @param unit      units that timeout is expressed in
      * @return the unlinked element
      * @throws InterruptedException if the current thread is interrupted
      */
     public E pollFirst(final long timeout, final TimeUnit unit) throws InterruptedException {
         return pollFirst(PoolImplUtils.toDuration(timeout, unit));
     }
 
     @Override
     public E pollLast() {
         lock.lock();
         try {
             return unlinkLast();
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Unlinks the last element in the queue, waiting up to the specified time
      * to do so if the queue is empty.
      *
      * @param timeout   length of time to wait
      * @return the unlinked element
      * @throws InterruptedException if the current thread is interrupted
      */
     E pollLast(final Duration timeout)
         throws InterruptedException {
         long nanos = timeout.toNanos();
         lock.lockInterruptibly();
         try {
             E x;
             while ((x = unlinkLast()) == null) {
                 if (nanos <= 0) {
                     return null;
                 }
                 nanos = notEmpty.awaitNanos(nanos);
             }
             return x;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Unlinks the last element in the queue, waiting up to the specified time
      * to do so if the queue is empty.
      *
      * @param timeout   length of time to wait
      * @param unit      units that timeout is expressed in
      * @return the unlinked element
      * @throws InterruptedException if the current thread is interrupted
      */
     public E pollLast(final long timeout, final TimeUnit unit)
         throws InterruptedException {
         return pollLast(PoolImplUtils.toDuration(timeout, unit));
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public E pop() {
         return removeFirst();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public void push(final E e) {
         addFirst(e);
     }
 
     /**
      * Links the provided element as the last in the queue, waiting until there
      * is space to do so if the queue is full.
      *
      * <p>
      * This method is equivalent to {@link #putLast(Object)}.
      * </p>
      *
      * @param e element to link
      * @throws NullPointerException if e is null
      * @throws InterruptedException if the thread is interrupted whilst waiting
      *         for space
      */
     public void put(final E e) throws InterruptedException {
         putLast(e);
     }
 
     /**
      * Links the provided element as the first in the queue, waiting until there
      * is space to do so if the queue is full.
      *
      * @param e element to link
      * @throws NullPointerException if e is null
      * @throws InterruptedException if the thread is interrupted whilst waiting
      *         for space
      */
     public void putFirst(final E e) throws InterruptedException {
         Objects.requireNonNull(e, "e");
         lock.lock();
         try {
             while (!linkFirst(e)) {
                 notFull.await();
             }
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Links the provided element as the last in the queue, waiting until there
      * is space to do so if the queue is full.
      *
      * @param e element to link
      * @throws NullPointerException if e is null
      * @throws InterruptedException if the thread is interrupted whilst waiting
      *         for space
      */
     public void putLast(final E e) throws InterruptedException {
         Objects.requireNonNull(e, "e");
         lock.lock();
         try {
             while (!linkLast(e)) {
                 notFull.await();
             }
         } finally {
             lock.unlock();
         }
     }
 
     // Stack methods
 
     /**
      * Reconstitutes this deque from a stream (that is, deserialize it).
      *
      * @param s the stream
      */
     private void readObject(final ObjectInputStream s) throws IOException, ClassNotFoundException {
         s.defaultReadObject();
         count = 0;
         first = null;
         last = null;
         // Read in all elements and place in queue
         for (;;) {
             @SuppressWarnings("unchecked")
             final E item = (E) s.readObject();
             if (item == null) {
                 break;
             }
             add(item);
         }
     }
 
     /**
      * Returns the number of additional elements that this deque can ideally
      * (in the absence of memory or resource constraints) accept without
      * blocking. This is always equal to the initial capacity of this deque
      * less the current {@code size} of this deque.
      *
      * <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.
      * </p>
      *
      * @return The number of additional elements the queue is able to accept
      */
     public int remainingCapacity() {
         lock.lock();
         try {
             return capacity - count;
         } finally {
             lock.unlock();
         }
     }
 
     // Collection methods
 
     /**
      * Retrieves and removes the head of the queue represented by this deque.
      * This method differs from {@link #poll poll} only in that it throws an
      * exception if this deque is empty.
      *
      * <p>
      * This method is equivalent to {@link #removeFirst() removeFirst}.
      * </p>
      *
      * @return the head of the queue represented by this deque
      * @throws NoSuchElementException if this deque is empty
      */
     @Override
     public E remove() {
         return removeFirst();
     }
 
     /**
      * Removes the first occurrence of the specified element from this deque.
      * If the deque does not contain the element, it is unchanged.
      * More formally, removes the first element {@code e} such that
      * {@code o.equals(e)} (if such an element exists).
      * Returns {@code true} if this deque contained the specified element
      * (or equivalently, if this deque changed as a result of the call).
      *
      * <p>
      * This method is equivalent to
      * {@link #removeFirstOccurrence(Object) removeFirstOccurrence}.
      * </p>
      *
      * @param o element to be removed from this deque, if present
      * @return {@code true} if this deque changed as a result of the call
      */
     @Override
     public boolean remove(final Object o) {
         return removeFirstOccurrence(o);
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public E removeFirst() {
         final E x = pollFirst();
         if (x == null) {
             throw new NoSuchElementException();
         }
         return x;
     }
 
     /*
      * TODO: Add support for more efficient bulk operations.
      *
      * We don't want to acquire the lock for every iteration, but we
      * also want other threads a chance to interact with the
      * collection, especially when count is close to capacity.
      */
 
 //     /**
 //      * Adds all of the elements in the specified collection to this
 //      * queue.  Attempts to addAll of a queue to itself result in
 //      * {@code IllegalArgumentException}. Further, the behavior of
 //      * this operation is undefined if the specified collection is
 //      * modified while the operation is in progress.
 //      *
 //      * @param c collection containing elements to be added to this queue
 //      * @return {@code true} if this queue changed as a result of the call
 //      * @throws ClassCastException
 //      * @throws NullPointerException
 //      * @throws IllegalArgumentException
 //      * @throws IllegalStateException
 //      * @see #add(Object)
 //      */
 //     public boolean addAll(Collection<? extends E> c) {
 //         if (c == null)
 //             throw new NullPointerException();
 //         if (c == this)
 //             throw new IllegalArgumentException();
 //         final ReentrantLock lock = this.lock;
 //         lock.lock();
 //         try {
 //             boolean modified = false;
 //             for (E e : c)
 //                 if (linkLast(e))
 //                     modified = true;
 //             return modified;
 //         } finally {
 //             lock.unlock();
 //         }
 //     }
 
     @Override
     public boolean removeFirstOccurrence(final Object o) {
         if (o == null) {
             return false;
         }
         lock.lock();
         try {
             for (Node<E> p = first; p != null; p = p.next) {
                 if (o.equals(p.item)) {
                     unlink(p);
                     return true;
                 }
             }
             return false;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public E removeLast() {
         final E x = pollLast();
         if (x == null) {
             throw new NoSuchElementException();
         }
         return x;
     }
 
     @Override
     public boolean removeLastOccurrence(final Object o) {
         if (o == null) {
             return false;
         }
         lock.lock();
         try {
             for (Node<E> p = last; p != null; p = p.prev) {
                 if (o.equals(p.item)) {
                     unlink(p);
                     return true;
                 }
             }
             return false;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Returns the number of elements in this deque.
      *
      * @return the number of elements in this deque
      */
     @Override
     public int size() {
         lock.lock();
         try {
             return count;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Unlinks the first element in the queue, waiting until there is an element
      * to unlink if the queue is empty.
      *
      * <p>
      * This method is equivalent to {@link #takeFirst()}.
      * </p>
      *
      * @return the unlinked element
      * @throws InterruptedException if the current thread is interrupted
      */
     public E take() throws InterruptedException {
         return takeFirst();
     }
 
     /**
      * Unlinks the first element in the queue, waiting until there is an element
      * to unlink if the queue is empty.
      *
      * @return the unlinked element
      * @throws InterruptedException if the current thread is interrupted
      */
     public E takeFirst() throws InterruptedException {
         lock.lock();
         try {
             E x;
             while ((x = unlinkFirst()) == null) {
                 notEmpty.await();
             }
             return x;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Unlinks the last element in the queue, waiting until there is an element
      * to unlink if the queue is empty.
      *
      * @return the unlinked element
      * @throws InterruptedException if the current thread is interrupted
      */
     public E takeLast() throws InterruptedException {
         lock.lock();
         try {
             E x;
             while ((x = unlinkLast()) == null) {
                 notEmpty.await();
             }
             return x;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Returns an array containing all of the elements in this deque, in
      * proper sequence (from first to last element).
      *
      * <p>
      * The returned array will be "safe" in that no references to it are
      * maintained by this deque.  (In other words, this method must allocate
      * a new array).  The caller is thus free to modify the returned array.
      * </p>
      * <p>
      * This method acts as bridge between array-based and collection-based
      * APIs.
      * </p>
      *
      * @return an array containing all of the elements in this deque
      */
     @Override
     public Object[] toArray() {
         lock.lock();
         try {
             final Object[] a = new Object[count];
             int k = 0;
             for (Node<E> p = first; p != null; p = p.next) {
                 a[k++] = p.item;
             }
             return a;
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * {@inheritDoc}
      */
     @SuppressWarnings("unchecked")
     @Override
     public <T> T[] toArray(T[] a) {
         lock.lock();
         try {
             if (a.length < count) {
                 a = (T[]) java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), count);
             }
             int k = 0;
             for (Node<E> p = first; p != null; p = p.next) {
                 a[k++] = (T) p.item;
             }
             if (a.length > k) {
                 a[k] = null;
             }
             return a;
         } finally {
             lock.unlock();
         }
     }
 
     @Override
     public String toString() {
         lock.lock();
         try {
             return super.toString();
         } finally {
             lock.unlock();
         }
     }
 
     /**
      * Unlinks the provided node.
      *
      * @param x The node to unlink
      */
     private void unlink(final Node<E> x) {
         // assert lock.isHeldByCurrentThread();
         final Node<E> p = x.prev;
         final Node<E> n = x.next;
         if (p == null) {
             unlinkFirst();
         } else if (n == null) {
             unlinkLast();
         } else {
             p.next = n;
             n.prev = p;
             x.item = null;
             // Don't mess with x's links.  They may still be in use by an iterator.
             --count;
             notFull.signal();
         }
     }
 
     // Monitoring methods
 
     /**
      * Removes and returns the first element, or null if empty.
      *
      * @return The first element or {@code null} if empty
      */
     private E unlinkFirst() {
         // assert lock.isHeldByCurrentThread();
         final Node<E> f = first;
         if (f == null) {
             return null;
         }
         final Node<E> n = f.next;
         final E item = f.item;
         f.item = null;
         f.next = f; // help GC
         first = n;
         if (n == null) {
             last = null;
         } else {
             n.prev = null;
         }
         --count;
         notFull.signal();
         return item;
     }
 
     /**
      * Removes and returns the last element, or null if empty.
      *
      * @return The first element or {@code null} if empty
      */
     private E unlinkLast() {
         // assert lock.isHeldByCurrentThread();
         final Node<E> l = last;
         if (l == null) {
             return null;
         }
         final Node<E> p = l.prev;
         final E item = l.item;
         l.item = null;
         l.prev = l; // help GC
         last = p;
         if (p == null) {
             first = null;
         } else {
             p.next = null;
         }
         --count;
         notFull.signal();
         return item;
     }
 
     /**
      * Saves the state of this deque to a stream (that is, serialize it).
      *
      * @serialData The capacity (int), followed by elements (each an
      * {@code Object}) in the proper order, followed by a null
      * @param s the stream
      * @throws  IOException if I/O errors occur while writing to the underlying {@code OutputStream}
      */
     private void writeObject(final java.io.ObjectOutputStream s) throws IOException {
         lock.lock();
         try {
             // Write out capacity and any hidden stuff
             s.defaultWriteObject();
             // Write out all elements in the proper order.
             for (Node<E> p = first; p != null; p = p.next) {
                 s.writeObject(p.item);
             }
             // Use trailing null as sentinel
             s.writeObject(null);
         } finally {
             lock.unlock();
         }
     }
 }