content: /*
   * Copyright (C) 2016-2023 ActionTech.
   * License: http://www.gnu.org/licenses/gpl.html GPL version 2 or higher.
   */
  
  package com.actiontech.dble.util;
  
  import java.util.*;
  import java.util.concurrent.BlockingDeque;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.locks.Condition;
  import java.util.concurrent.locks.ReentrantLock;
  
  public class FairLinkedBlockingDeque<E> extends AbstractQueue<E> implements BlockingDeque<E>, java.io.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 advance to head.
       */
  
      /*
       * 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.
       */
  
      private static final long serialVersionUID = -387911632671998426L;
  
      /**
       * Pointer to first node
       */
      transient Node<E> first;
      /**
       * Pointer to last node
       */
      transient Node<E> last;
      /**
       * Number of items in the deque
       */
      private transient int count;
      /**
       * Maximum number of items in the deque
       */
      private final int capacity;
      /**
       * Main lock guarding all access
       */
      final ReentrantLock lock = new ReentrantLock(true);
      /**
       * Condition for waiting takes
       */
      private final Condition notEmpty = lock.newCondition();
      /**
       * Condition for waiting puts
       */
      private final Condition notFull = lock.newCondition();
  
      /**
       * Creates a <tt>LinkedBlockingDeque</tt> with a capacity of
       * {@link Integer#MAX_VALUE}.
       */
      public FairLinkedBlockingDeque() {
          this(Integer.MAX_VALUE);
      }
  
      /**
       * Creates a <tt>LinkedBlockingDeque</tt> with the given (fixed) capacity.
       *
       * @param capacity the capacity of this deque
       * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
       */
      public FairLinkedBlockingDeque(int capacity) {
          if (capacity <= 0)
              throw new IllegalArgumentException();
          this.capacity = capacity;
      }
  
      /**
       * Creates a <tt>LinkedBlockingDeque</tt> with a capacity of
       * {@link Integer#MAX_VALUE}, initially containing the elements of the given
       * collection, added in traversal order of the collection's iterator.
       *
       * @param c the collection of elements to initially contain
       * @throws NullPointerException if the specified collection or any of its elements are null
       */
      public FairLinkedBlockingDeque(Collection<? extends E> c) {
          this(Integer.MAX_VALUE);
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock(); // Never contended, but necessary for visibility
          try {
              for (E e : c) {
                  if (e == null)
                      throw new NullPointerException();
                  if (!linkLast(e))
                      throw new IllegalStateException("Deque full");
              }
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * wait until the queue's size >= size
       */
      public void waitUtilCount(int size) {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              while (count < size) {
                  try {
                      notEmpty.await();
                  } catch (InterruptedException e) {
                      notEmpty.signalAll();
                  }
              }
          } finally {
              reentrantLock.unlock();
          }
      }
  
      // Basic linking and unlinking operations, called only while holding lock
  
      /**
       * Links e as first element, or returns false if full.
       */
      private boolean linkFirst(E e) {
          // assert lock.isHeldByCurrentThread();
          if (count >= capacity)
              return false;
          Node<E> f = first;
          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 e as last element, or returns false if full.
       */
      private boolean linkLast(E e) {
          // assert lock.isHeldByCurrentThread();
          if (count >= capacity)
              return false;
          Node<E> l = last;
          Node<E> x = new Node<>(e, l, null);
          last = x;
          if (first == null)
              first = x;
          else
              l.next = x;
          ++count;
          notEmpty.signal();
          return true;
      }
  
      /**
       * Removes and returns first element, or null if empty.
       */
      private E unlinkFirst() {
          // assert lock.isHeldByCurrentThread();
          Node<E> f = first;
          if (f == null)
              return null;
          Node<E> n = f.next;
          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 last element, or null if empty.
       */
      private E unlinkLast() {
          // assert lock.isHeldByCurrentThread();
          Node<E> l = last;
          if (l == null)
              return null;
          Node<E> p = l.prev;
          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;
      }
  
      /**
       * Unlinks x
       */
      void unlink(Node<E> x) {
          // assert lock.isHeldByCurrentThread();
          Node<E> p = x.prev;
          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();
          }
      }
  
      // BlockingDeque methods
  
      /**
       * @throws IllegalStateException {@inheritDoc}
       * @throws NullPointerException  {@inheritDoc}
       */
      public void addFirst(E e) {
          if (!offerFirst(e))
              throw new IllegalStateException("Deque full");
      }
  
      /**
       * @throws IllegalStateException {@inheritDoc}
       * @throws NullPointerException  {@inheritDoc}
       */
      public void addLast(E e) {
          if (!offerLast(e))
              throw new IllegalStateException("Deque full");
      }
  
      /**
       * @throws NullPointerException {@inheritDoc}
       */
      public boolean offerFirst(E e) {
          if (e == null)
              throw new NullPointerException();
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              return linkFirst(e);
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * @throws NullPointerException {@inheritDoc}
       * @throws InterruptedException {@inheritDoc}
       */
      public boolean offerFirst(E e, long timeout, TimeUnit unit) throws InterruptedException {
          if (e == null)
              throw new NullPointerException();
          long nanos = unit.toNanos(timeout);
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lockInterruptibly();
          try {
              while (!linkFirst(e)) {
                  if (nanos <= 0)
                      return false;
                  nanos = notFull.awaitNanos(nanos);
              }
              return true;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * @throws NullPointerException {@inheritDoc}
       */
      public boolean offerLast(E e) {
          if (e == null)
              throw new NullPointerException();
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              return linkLast(e);
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * @throws NullPointerException {@inheritDoc}
       * @throws InterruptedException {@inheritDoc}
       */
      public boolean offerLast(E e, long timeout, TimeUnit unit) throws InterruptedException {
          if (e == null)
              throw new NullPointerException();
          long nanos = unit.toNanos(timeout);
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lockInterruptibly();
          try {
              while (!linkLast(e)) {
                  if (nanos <= 0)
                      return false;
                  nanos = notFull.awaitNanos(nanos);
              }
              return true;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * @throws NullPointerException {@inheritDoc}
       * @throws InterruptedException {@inheritDoc}
       */
      public void putFirst(E e) throws InterruptedException {
          if (e == null)
              throw new NullPointerException();
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              while (!linkFirst(e))
                  notFull.await();
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * @throws NullPointerException {@inheritDoc}
       * @throws InterruptedException {@inheritDoc}
       */
      public void putLast(E e) throws InterruptedException {
          if (e == null)
              throw new NullPointerException();
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              while (!linkLast(e))
                  notFull.await();
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E removeFirst() {
          E x = pollFirst();
          if (x == null)
              throw new NoSuchElementException();
          return x;
      }
  
      /**
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E removeLast() {
          E x = pollLast();
          if (x == null)
              throw new NoSuchElementException();
          return x;
      }
  
      /**
       * add to last, if full, replace it
       */
      public E addOrReplaceLast(E e) throws InterruptedException {
          if (e == null)
              throw new NullPointerException();
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lockInterruptibly();
          try {
              boolean added = linkLast(e);
              if (!added) {
                  Node<E> l = last;
                  if (l == null)
                      return null;
                  E item = l.item;
                  l.item = e;
                  return item;
              } else {
                  return null;
              }
          } finally {
              reentrantLock.unlock();
          }
      }
  
      public E pollFirst() {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              return unlinkFirst();
          } finally {
              reentrantLock.unlock();
          }
      }
  
      public E pollFirst(long timeout, TimeUnit unit) throws InterruptedException {
          long nanos = unit.toNanos(timeout);
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lockInterruptibly();
          try {
              E x;
              while ((x = unlinkFirst()) == null) {
                  if (nanos <= 0)
                      return null;
                  nanos = notEmpty.awaitNanos(nanos);
              }
              return x;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      public E pollLast() {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              return unlinkLast();
          } finally {
              reentrantLock.unlock();
          }
      }
  
      public E pollLast(long timeout, TimeUnit unit) throws InterruptedException {
          long nanos = unit.toNanos(timeout);
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lockInterruptibly();
          try {
              E x;
              while ((x = unlinkLast()) == null) {
                  if (nanos <= 0)
                      return null;
                  nanos = notEmpty.awaitNanos(nanos);
              }
              return x;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      public E takeFirst() throws InterruptedException {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              E x;
              while ((x = unlinkFirst()) == null)
                  notEmpty.await();
              return x;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      public E takeLast() throws InterruptedException {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              E x;
              while ((x = unlinkLast()) == null)
                  notEmpty.await();
              return x;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E getFirst() {
          E x = peekFirst();
          if (x == null)
              throw new NoSuchElementException();
          return x;
      }
  
      /**
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E getLast() {
          E x = peekLast();
          if (x == null)
              throw new NoSuchElementException();
          return x;
      }
  
      public E peekFirst() {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              return (first == null) ? null : first.item;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      public E peekLast() {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              return (last == null) ? null : last.item;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      public boolean removeFirstOccurrence(Object o) {
          if (o == null)
              return false;
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              for (Node<E> p = first; p != null; p = p.next) {
                  if (o.equals(p.item)) {
                      unlink(p);
                      return true;
                  }
              }
              return false;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      public boolean removeLastOccurrence(Object o) {
          if (o == null)
              return false;
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              for (Node<E> p = last; p != null; p = p.prev) {
                  if (o.equals(p.item)) {
                      unlink(p);
                      return true;
                  }
              }
              return false;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      // BlockingQueue methods
  
      /**
       * Inserts the specified element at the end of this deque unless it would
       * violate capacity restrictions. When using a capacity-restricted deque, it
       * is generally preferable to use method {@link #offer(Object) offer}.
       * <p>
       * <p>
       * This method is equivalent to {@link #addLast}.
       *
       * @throws IllegalStateException if the element cannot be added at this time due to capacity
       *                               restrictions
       * @throws NullPointerException  if the specified element is null
       */
      public boolean add(E e) {
          addLast(e);
          return true;
      }
  
      /**
       * @throws NullPointerException if the specified element is null
       */
      public boolean offer(E e) {
          return offerLast(e);
      }
  
      /**
       * @throws NullPointerException {@inheritDoc}
       * @throws InterruptedException {@inheritDoc}
       */
      public boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException {
          return offerLast(e, timeout, unit);
      }
  
      /**
       * @throws NullPointerException {@inheritDoc}
       * @throws InterruptedException {@inheritDoc}
       */
      public void put(E e) throws InterruptedException {
          putLast(e);
      }
  
      /**
       * 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>
       * <p>
       * This method is equivalent to {@link #removeFirst() removeFirst}.
       *
       * @return the head of the queue represented by this deque
       * @throws NoSuchElementException if this deque is empty
       */
      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 <tt>e</tt> such that <tt>o.equals(e)</tt> (if
       * such an element exists). Returns <tt>true</tt> if this deque contained
       * the specified element (or equivalently, if this deque changed as a result
       * of the call).
       * <p>
       * <p>
       * This method is equivalent to {@link #removeFirstOccurrence(Object)
       * removeFirstOccurrence}.
       *
       * @param o element to be removed from this deque, if present
       * @return <tt>true</tt> if this deque changed as a result of the call
       */
      public boolean remove(Object o) {
          return removeFirstOccurrence(o);
      }
  
      public E poll() {
          return pollFirst();
      }
  
      public E poll(long timeout, TimeUnit unit) throws InterruptedException {
          return pollFirst(timeout, unit);
      }
  
      public E take() throws InterruptedException {
          return takeFirst();
      }
  
      /**
       * 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>
       * <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
       */
      public E element() {
          return getFirst();
      }
  
      public E peek() {
          return peekFirst();
      }
  
      /**
       * 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 <tt>size</tt> of this deque.
       * <p>
       * <p>
       * Note that you <em>cannot</em> always tell if an attempt to insert an
       * element will succeed by inspecting <tt>remainingCapacity</tt> because it
       * may be the case that another thread is about to insert or remove an
       * element.
       */
      public int remainingCapacity() {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              return capacity - count;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * @throws UnsupportedOperationException {@inheritDoc}
       * @throws ClassCastException            {@inheritDoc}
       * @throws NullPointerException          {@inheritDoc}
       * @throws IllegalArgumentException      {@inheritDoc}
       */
      public int drainTo(Collection<? super E> c) {
          return drainTo(c, Integer.MAX_VALUE);
      }
  
      /**
       * @throws UnsupportedOperationException {@inheritDoc}
       * @throws ClassCastException            {@inheritDoc}
       * @throws NullPointerException          {@inheritDoc}
       * @throws IllegalArgumentException      {@inheritDoc}
       */
      public int drainTo(Collection<? super E> c, int maxElements) {
          if (c == null)
              throw new NullPointerException();
          if (c == this)
              throw new IllegalArgumentException();
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              int n = Math.min(maxElements, count);
              for (int i = 0; i < n; i++) {
                  c.add(first.item); // In this order, in case add() throws.
                  unlinkFirst();
              }
              return n;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      // Stack methods
  
      /**
       * @throws IllegalStateException {@inheritDoc}
       * @throws NullPointerException  {@inheritDoc}
       */
      public void push(E e) {
          addFirst(e);
      }
  
      /**
       * @throws NoSuchElementException {@inheritDoc}
       */
      public E pop() {
          return removeFirst();
      }
  
      // Collection methods
  
      /**
       * Returns the number of elements in this deque.
       *
       * @return the number of elements in this deque
       */
      public int size() {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              return count;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * Returns <tt>true</tt> if this deque contains the specified element. More
       * formally, returns <tt>true</tt> if and only if this deque contains at
       * least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
       *
       * @param o object to be checked for containment in this deque
       * @return <tt>true</tt> if this deque contains the specified element
       */
      public boolean contains(Object o) {
          if (o == null)
              return false;
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              for (Node<E> p = first; p != null; p = p.next)
                  if (o.equals(p.item))
                      return true;
              return false;
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /*
       * 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 {@inheritDoc}
      // * @throws NullPointerException {@inheritDoc}
      // * @throws IllegalArgumentException {@inheritDoc}
      // * @throws IllegalStateException {@inheritDoc}
      // * @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();
      // }
      // }
  
      /**
       * Returns an array containing all of the elements in this deque, in proper
       * sequence (from first to last element).
       * <p>
       * <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.
       *
       * @return an array containing all of the elements in this deque
       */
      // @SuppressWarnings("unchecked")
      public Object[] toArray() {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              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 {
              reentrantLock.unlock();
          }
      }
  
      /**
       * Returns an array containing all of the elements in this deque, in proper
       * sequence; the runtime type of the returned array is that of the specified
       * array. If the deque 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 deque.
       * <p>
       * <p>
       * If this deque fits in the specified array with room to spare (i.e., the
       * array has more elements than this deque), the element in the array
       * immediately following the end of the deque is set to <tt>null</tt>.
       * <p>
       * <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>
       * <p>
       * Suppose <tt>x</tt> is a deque known to contain only strings. The
       * following code can be used to dump the deque into a newly allocated array
       * of <tt>String</tt>:
       * <p>
       * <pre>
       * String[] y = x.toArray(new String[0]);
       * </pre>
       * <p>
       * Note that <tt>toArray(new Object[0])</tt> is identical in function to
       * <tt>toArray()</tt>.
       *
       * @param a the array into which the elements of the deque 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 deque
       * @throws ArrayStoreException  if the runtime type of the specified array is not a supertype
       *                              of the runtime type of every element in this deque
       * @throws NullPointerException if the specified array is null
       */
      public <T> T[] toArray(T[] a) {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.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 {
              reentrantLock.unlock();
          }
      }
  
      public String toString() {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              return super.toString();
          } finally {
              reentrantLock.unlock();
          }
      }
  
      /**
       * Atomically removes all of the elements from this deque. The deque will be
       * empty after this call returns.
       */
      public void clear() {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.lock();
          try {
              for (Node<E> f = first; f != null; ) {
                  f.item = null;
                  Node<E> n = f.next;
                  f.prev = null;
                  f.next = null;
                  f = n;
              }
              first = last = null;
              count = 0;
              notFull.signalAll();
          } finally {
              reentrantLock.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 <tt>Iterator</tt> is a "weakly consistent" iterator that
       * will never throw {@link 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
       */
      public Iterator<E> iterator() {
          return new Itr();
      }
  
      /**
       * Returns an iterator over the elements in this deque in reverse sequential
       * order. The elements will be returned in order from last (tail) to first
       * (head). The returned <tt>Iterator</tt> is a "weakly consistent" iterator
       * that will never throw {@link 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.
       */
      public Iterator<E> descendingIterator() {
          return new DescendingItr();
      }
  
      /**
       * Save the state of this deque to a stream (that is, serialize it).
       *
       * @param s the stream
       * @serialData The capacity (int), followed by elements (each an
       * <tt>Object</tt>) in the proper order, followed by a null
       */
      private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
          final ReentrantLock reentrantLock = this.lock;
          reentrantLock.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 {
              reentrantLock.unlock();
          }
      }
  
      /**
       * Reconstitute this deque from a stream (that is, deserialize it).
       *
       * @param s the stream
       */
      private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
          s.defaultReadObject();
          count = 0;
          first = null;
          last = null;
          // Read in all elements and place in queue
          for (; ; ) {
              // @SuppressWarnings("unchecked")
              E item = (E) s.readObject();
              if (item == null)
                  break;
              add(item);
          }
      }
  
      /**
       * 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;
  
          abstract Node<E> firstNode();
  
          abstract Node<E> nextNode(Node<E> n);
  
          AbstractItr() {
              // set to initial position
              final ReentrantLock reentrantLock = FairLinkedBlockingDeque.this.lock;
              reentrantLock.lock();
              try {
                  next = firstNode();
                  nextItem = (next == null) ? null : next.item;
              } finally {
                  reentrantLock.unlock();
              }
          }
  
          /**
           * Advances next.
           */
  
          void advance() {
              final ReentrantLock reentrantLock = FairLinkedBlockingDeque.this.lock;
              reentrantLock.lock();
              try {
                  // assert next != null;
                  Node<E> s = nextNode(next);
                  if (s == next) {
                      next = firstNode();
                  } else {
                      // Skip over removed nodes.
                      // May be necessary if multiple interior Nodes are removed.
                      while (s != null && s.item == null)
                          s = nextNode(s);
                      next = s;
                  }
                  nextItem = (next == null) ? null : next.item;
              } finally {
                  reentrantLock.unlock();
              }
          }
  
          public boolean hasNext() {
              return next != null;
          }
  
          public E next() {
              if (next == null)
                  throw new NoSuchElementException();
              lastRet = next;
              E x = nextItem;
              advance();
              return x;
          }
  
          public void remove() {
              Node<E> n = lastRet;
              if (n == null)
                  throw new IllegalStateException();
              lastRet = null;
  
              final ReentrantLock reentrantLock = FairLinkedBlockingDeque.this.lock;
              reentrantLock.lock();
              try {
                  if (n.item != null)
                      unlink(n);
              } finally {
                  reentrantLock.unlock();
              }
          }
      }
  
      /**
       * Forward iterator
       */
      private class Itr extends AbstractItr {
          Node<E> firstNode() {
              return first;
          }
  
          Node<E> nextNode(Node<E> n) {
              return n.next;
          }
      }
  
      /**
       * Descending iterator
       */
      private class DescendingItr extends AbstractItr {
          Node<E> firstNode() {
              return last;
          }
  
          Node<E> nextNode(Node<E> n) {
              return n.prev;
          }
      }
  
      /**
       * Doubly-linked list node class
       */
      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;
  
          Node(E x, Node<E> p, Node<E> n) {
              item = x;
              prev = p;
              next = n;
          }
      }
  }
  