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using System;
using System.Threading;

namespace Enyim.Collections
{
/// <summary>
/// Implements a non-locking queue.
/// </summary>
/// <typeparam name="T"></typeparam>
public class InterlockedQueue<T>
{
private Node headNode;
private Node tailNode;

/// <summary>
/// Initializes a new instance of the <see cref="T:InterlockedQueue"/> class.
/// </summary>
public InterlockedQueue()
{
Node node = new Node(default(T));

this.headNode = node;
this.tailNode = node;
}

/// <summary>
/// Removes and returns the item at the beginning of the <see cref="T:InterlockedQueue"/>.
/// </summary>
/// <param name="value">The object that is removed from the beginning of the <see cref="T:InterlockedQueue"/>.</param>
/// <returns><value>true</value> if an item was successfully dequeued; otherwise <value>false</value>.</returns>
public bool Dequeue(out T value)
{
Node head;
Node tail;
Node next;

while (true)
{
// read head
head = this.headNode;
tail = this.tailNode;
next = head.Next;

// Are head, tail, and next consistent?
if (Object.ReferenceEquals(this.headNode, head))
{
// is tail falling behind
if (Object.ReferenceEquals(head.Next, tail.Next))
{
// is the queue empty?
if (Object.ReferenceEquals(next, null))
{
value = default(T);

// queue is empty and cannot dequeue
return false;
}

Interlocked.CompareExchange<Node>(
ref this.tailNode,
next.Next,
tail);
}
else // No need to deal with tail
{
// read value before CAS otherwise another deque might try to free the next node
value = next.Value;

// try to swing the head to the next node
if (Interlocked.CompareExchange<Node>(
ref this.headNode,
next,
head) == head)
{
return true;
}
}
}
}
}

/// <summary>
/// Adds an object to the end of the <see cref="T:InterlockedQueue"/>.
/// </summary>
/// <param name="value">The item to be added to the <see cref="T:InterlockedQueue"/>. The value can be <value>null</value>.</param>
public void Enqueue(T value)
{
// Allocate a new node from the free list
Node valueNode = new Node(value);

while (true)
{
Node tail = this.tailNode;
Node next = tail.Next;

// are tail and next consistent
if (Object.ReferenceEquals(tail, this.tailNode))
{
// was tail pointing to the last node?
if (Object.ReferenceEquals(next, null))
{
if (Object.ReferenceEquals(
Interlocked.CompareExchange(ref tail.Next, valueNode, next),
next
)
)
{
Interlocked.CompareExchange(ref this.tailNode, valueNode, tail);
break;
}
}
else // tail was not pointing to last node
{
// try to swing Tail to the next node
Interlocked.CompareExchange<Node>(ref this.tailNode, next, tail);
}
}
}
}

#region [ Node ]
private class Node
{
public T Value;
public Node Next;

public Node(T value)
{
this.Value = value;
}
}
#endregion
}
}

#region [ License information ]
/* ************************************************************
*
* Copyright (c) 2010 Attila Kiskó, enyim.com
*
* Licensed 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
*
* http://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.
*
* ************************************************************/
#endregion
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