/*
 * 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
 *
 *     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.
 */

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.ServiceModel;
using System.Threading;

namespace Apache.NMS.WCF
{
	// ItemDequeuedCallback is called as an item is dequeued from the InputQueue.  The 
	// InputQueue lock is not held during the callback.  However, the user code will
	// not be notified of the item being available until the callback returns.  If you
	// are not sure if the callback will block for a long time, then first call 
	// IOThreadScheduler.ScheduleCallback to get to a "safe" thread.
	delegate void ItemDequeuedCallback();

	/// <summary>
	/// Handles asynchronous interactions between producers and consumers. 
	/// Producers can dispatch available data to the input queue, 
	/// where it will be dispatched to a waiting consumer or stored until a
	/// consumer becomes available. Consumers can synchronously or asynchronously
	/// request data from the queue, which will be returned when data becomes
	/// available.
	/// </summary>
	/// <typeparam name="T">The concrete type of the consumer objects that are waiting for data.</typeparam>
	class InputQueue<T> : IDisposable where T : class
	{
		//Stores items that are waiting to be consumed.
		ItemQueue itemQueue;

		//Each IQueueReader represents some consumer that is waiting for
		//items to appear in the queue. The readerQueue stores them
		//in an ordered list so consumers get serviced in a FIFO manner.
		Queue<IQueueReader> readerQueue;

		//Each IQueueWaiter represents some waiter that is waiting for
		//items to appear in the queue.  When any item appears, all
		//waiters are signalled.
		List<IQueueWaiter> waiterList;

		static WaitCallback onInvokeDequeuedCallback;
		static WaitCallback onDispatchCallback;
		static WaitCallback completeOutstandingReadersCallback;
		static WaitCallback completeWaitersFalseCallback;
		static WaitCallback completeWaitersTrueCallback;

		//Represents the current state of the InputQueue
		//as it transitions through its lifecycle.
		QueueState queueState;
		enum QueueState
		{
			Open,
			Shutdown,
			Closed
		}

		public InputQueue()
		{
			this.itemQueue = new ItemQueue();
			this.readerQueue = new Queue<IQueueReader>();
			this.waiterList = new List<IQueueWaiter>();
			this.queueState = QueueState.Open;
		}

		public int PendingCount
		{
			get
			{
				lock(ThisLock)
				{
					return itemQueue.ItemCount;
				}
			}
		}

		// added by Roman
		public int NumberOfReaders
		{
			get
			{
				lock(ThisLock)
				{
					return readerQueue.Count;
				}
			}
		}
		public void Open()
		{
			lock(ThisLock)
			{
				if(queueState == QueueState.Open)
				{
					return;
				}

				if(queueState == QueueState.Closed)
				{
					throw new ObjectDisposedException(this.GetType().ToString());
				}
			}
		}

		object ThisLock
		{
			get { return itemQueue; }
		}

		public IAsyncResult BeginDequeue(TimeSpan timeout, AsyncCallback callback, object state)
		{
			Item item = default(Item);

			lock(ThisLock)
			{
				if(queueState == QueueState.Open)
				{
					if(itemQueue.HasAvailableItem)
					{
						item = itemQueue.DequeueAvailableItem();
					}
					else
					{
						AsyncQueueReader reader = new AsyncQueueReader(this, timeout, callback, state);
						readerQueue.Enqueue(reader);
						return reader;
					}
				}
				else if(queueState == QueueState.Shutdown)
				{
					if(itemQueue.HasAvailableItem)
					{
						item = itemQueue.DequeueAvailableItem();
					}
					else if(itemQueue.HasAnyItem)
					{
						AsyncQueueReader reader = new AsyncQueueReader(this, timeout, callback, state);
						readerQueue.Enqueue(reader);
						return reader;
					}
				}
			}

			InvokeDequeuedCallback(item.DequeuedCallback);
			return new TypedCompletedAsyncResult<T>(item.GetValue(), callback, state);
		}

		public IAsyncResult BeginWaitForItem(TimeSpan timeout, AsyncCallback callback, object state)
		{
			lock(ThisLock)
			{
				if(queueState == QueueState.Open)
				{
					if(!itemQueue.HasAvailableItem)
					{
						AsyncQueueWaiter waiter = new AsyncQueueWaiter(timeout, callback, state);
						waiterList.Add(waiter);
						return waiter;
					}
				}
				else if(queueState == QueueState.Shutdown)
				{
					if(!itemQueue.HasAvailableItem && itemQueue.HasAnyItem)
					{
						AsyncQueueWaiter waiter = new AsyncQueueWaiter(timeout, callback, state);
						waiterList.Add(waiter);
						return waiter;
					}
				}
			}

			return new TypedCompletedAsyncResult<bool>(true, callback, state);
		}

		static void CompleteOutstandingReadersCallback(object state)
		{
			IQueueReader[] outstandingReaders = (IQueueReader[]) state;

			for(int i = 0; i < outstandingReaders.Length; i++)
			{
				outstandingReaders[i].Set(default(Item));
			}
		}

		static void CompleteWaitersFalseCallback(object state)
		{
			CompleteWaiters(false, (IQueueWaiter[]) state);
		}

		static void CompleteWaitersTrueCallback(object state)
		{
			CompleteWaiters(true, (IQueueWaiter[]) state);
		}

		static void CompleteWaiters(bool itemAvailable, IQueueWaiter[] waiters)
		{
			for(int i = 0; i < waiters.Length; i++)
			{
				waiters[i].Set(itemAvailable);
			}
		}

		static void CompleteWaitersLater(bool itemAvailable, IQueueWaiter[] waiters)
		{
			if(itemAvailable)
			{
				if(completeWaitersTrueCallback == null)
				{
					completeWaitersTrueCallback = new WaitCallback(CompleteWaitersTrueCallback);
				}

				ThreadPool.QueueUserWorkItem(completeWaitersTrueCallback, waiters);
			}
			else
			{
				if(completeWaitersFalseCallback == null)
				{
					completeWaitersFalseCallback = new WaitCallback(CompleteWaitersFalseCallback);
				}

				ThreadPool.QueueUserWorkItem(completeWaitersFalseCallback, waiters);
			}
		}

		void GetWaiters(out IQueueWaiter[] waiters)
		{
			if(waiterList.Count > 0)
			{
				waiters = waiterList.ToArray();
				waiterList.Clear();
			}
			else
			{
				waiters = null;
			}
		}

		public void Close()
		{
			((IDisposable) this).Dispose();
		}

		public void Shutdown()
		{
			IQueueReader[] outstandingReaders = null;
			lock(ThisLock)
			{
				if(queueState == QueueState.Shutdown)
				{
					return;
				}

				if(queueState == QueueState.Closed)
				{
					return;
				}

				this.queueState = QueueState.Shutdown;

				if(readerQueue.Count > 0 && this.itemQueue.ItemCount == 0)
				{
					outstandingReaders = new IQueueReader[readerQueue.Count];
					readerQueue.CopyTo(outstandingReaders, 0);
					readerQueue.Clear();
				}
			}

			if(outstandingReaders != null)
			{
				for(int i = 0; i < outstandingReaders.Length; i++)
				{
					outstandingReaders[i].Set(new Item((Exception) null, null));
				}
			}
		}

		public T Dequeue(TimeSpan timeout)
		{
			T value;

			if(!this.Dequeue(timeout, out value))
			{
				throw new TimeoutException(string.Format("Dequeue timed out in {0}.", timeout));
			}

			return value;
		}

		public bool Dequeue(TimeSpan timeout, out T value)
		{
			WaitQueueReader reader = null;
			Item item = new Item();

			lock(ThisLock)
			{
				if(queueState == QueueState.Open)
				{
					if(itemQueue.HasAvailableItem)
					{
						item = itemQueue.DequeueAvailableItem();
					}
					else
					{
						reader = new WaitQueueReader(this);
						readerQueue.Enqueue(reader);
					}
				}
				else if(queueState == QueueState.Shutdown)
				{
					if(itemQueue.HasAvailableItem)
					{
						item = itemQueue.DequeueAvailableItem();
					}
					else if(itemQueue.HasAnyItem)
					{
						reader = new WaitQueueReader(this);
						readerQueue.Enqueue(reader);
					}
					else
					{
						value = default(T);
						return true;
					}
				}
				else // queueState == QueueState.Closed
				{
					value = default(T);
					return true;
				}
			}

			if(reader != null)
			{
				return reader.Wait(timeout, out value);
			}
			else
			{
				InvokeDequeuedCallback(item.DequeuedCallback);
				value = item.GetValue();
				return true;
			}
		}

		public void Dispose()
		{
			Dispose(true);

			GC.SuppressFinalize(this);
		}

		protected void Dispose(bool disposing)
		{
			if(disposing)
			{
				bool dispose = false;

				lock(ThisLock)
				{
					if(queueState != QueueState.Closed)
					{
						queueState = QueueState.Closed;
						dispose = true;
					}
				}

				if(dispose)
				{
					while(readerQueue.Count > 0)
					{
						IQueueReader reader = readerQueue.Dequeue();
						reader.Set(default(Item));
					}

					while(itemQueue.HasAnyItem)
					{
						Item item = itemQueue.DequeueAnyItem();
						item.Dispose();
						InvokeDequeuedCallback(item.DequeuedCallback);
					}
				}
			}
		}

		public void Dispatch()
		{
			IQueueReader reader = null;
			Item item = new Item();
			IQueueReader[] outstandingReaders = null;
			IQueueWaiter[] waiters = null;
			bool itemAvailable = true;

			lock(ThisLock)
			{
				itemAvailable = !((queueState == QueueState.Closed) || (queueState == QueueState.Shutdown));
				this.GetWaiters(out waiters);

				if(queueState != QueueState.Closed)
				{
					itemQueue.MakePendingItemAvailable();

					if(readerQueue.Count > 0)
					{
						item = itemQueue.DequeueAvailableItem();
						reader = readerQueue.Dequeue();

						if(queueState == QueueState.Shutdown && readerQueue.Count > 0 && itemQueue.ItemCount == 0)
						{
							outstandingReaders = new IQueueReader[readerQueue.Count];
							readerQueue.CopyTo(outstandingReaders, 0);
							readerQueue.Clear();

							itemAvailable = false;
						}
					}
				}
			}

			if(outstandingReaders != null)
			{
				if(completeOutstandingReadersCallback == null)
				{
					completeOutstandingReadersCallback = new WaitCallback(CompleteOutstandingReadersCallback);
				}

				ThreadPool.QueueUserWorkItem(completeOutstandingReadersCallback, outstandingReaders);
			}

			if(waiters != null)
			{
				CompleteWaitersLater(itemAvailable, waiters);
			}

			if(reader != null)
			{
				InvokeDequeuedCallback(item.DequeuedCallback);
				reader.Set(item);
			}
		}

		//Ends an asynchronous Dequeue operation.
		public T EndDequeue(IAsyncResult result)
		{
			T value;

			if(!this.EndDequeue(result, out value))
			{
				throw new TimeoutException("Asynchronous Dequeue operation timed out.");
			}

			return value;
		}

		public bool EndDequeue(IAsyncResult result, out T value)
		{
			TypedCompletedAsyncResult<T> typedResult = result as TypedCompletedAsyncResult<T>;

			if(typedResult != null)
			{
				value = TypedCompletedAsyncResult<T>.End(result);
				return true;
			}

			return AsyncQueueReader.End(result, out value);
		}

		public bool EndWaitForItem(IAsyncResult result)
		{
			TypedCompletedAsyncResult<bool> typedResult = result as TypedCompletedAsyncResult<bool>;
			if(typedResult != null)
			{
				return TypedCompletedAsyncResult<bool>.End(result);
			}

			return AsyncQueueWaiter.End(result);
		}

		public void EnqueueAndDispatch(T item)
		{
			EnqueueAndDispatch(item, null);
		}

		public void EnqueueAndDispatch(T item, ItemDequeuedCallback dequeuedCallback)
		{
			EnqueueAndDispatch(item, dequeuedCallback, true);
			//EnqueueAndDispatch(item, dequeuedCallback, false);
		}

		public void EnqueueAndDispatch(Exception exception, ItemDequeuedCallback dequeuedCallback, bool canDispatchOnThisThread)
		{
			Debug.Assert(exception != null, "exception parameter should not be null");
			EnqueueAndDispatch(new Item(exception, dequeuedCallback), canDispatchOnThisThread);
		}

		public void EnqueueAndDispatch(T item, ItemDequeuedCallback dequeuedCallback, bool canDispatchOnThisThread)
		{
			Debug.Assert(item != null, "item parameter should not be null");
			EnqueueAndDispatch(new Item(item, dequeuedCallback), canDispatchOnThisThread);
		}

		void EnqueueAndDispatch(Item item, bool canDispatchOnThisThread)
		{
			bool disposeItem = false;
			IQueueReader reader = null;
			bool dispatchLater = false;
			IQueueWaiter[] waiters = null;
			bool itemAvailable = true;

			lock(ThisLock)
			{
				itemAvailable = !((queueState == QueueState.Closed) || (queueState == QueueState.Shutdown));
				this.GetWaiters(out waiters);

				if(queueState == QueueState.Open)
				{
					if(canDispatchOnThisThread)
					{
						if(readerQueue.Count == 0)
						{
							itemQueue.EnqueueAvailableItem(item);
						}
						else
						{
							reader = readerQueue.Dequeue();
						}
					}
					else
					{
						if(readerQueue.Count == 0)
						{
							itemQueue.EnqueueAvailableItem(item);
						}
						else
						{
							itemQueue.EnqueuePendingItem(item);
							dispatchLater = true;
						}
					}
				}
				else // queueState == QueueState.Closed || queueState == QueueState.Shutdown
				{
					disposeItem = true;
				}
			}

			if(waiters != null)
			{
				if(canDispatchOnThisThread)
				{
					CompleteWaiters(itemAvailable, waiters);
				}
				else
				{
					CompleteWaitersLater(itemAvailable, waiters);
				}
			}

			if(reader != null)
			{
				InvokeDequeuedCallback(item.DequeuedCallback);
				reader.Set(item);
			}

			if(dispatchLater)
			{
				if(onDispatchCallback == null)
				{
					onDispatchCallback = new WaitCallback(OnDispatchCallback);
				}

				ThreadPool.QueueUserWorkItem(onDispatchCallback, this);
			}
			else if(disposeItem)
			{
				InvokeDequeuedCallback(item.DequeuedCallback);
				item.Dispose();
			}
		}

		public bool EnqueueWithoutDispatch(T item, ItemDequeuedCallback dequeuedCallback)
		{
			Debug.Assert(item != null, "EnqueueWithoutDispatch: item parameter should not be null");
			return EnqueueWithoutDispatch(new Item(item, dequeuedCallback));
		}

		public bool EnqueueWithoutDispatch(Exception exception, ItemDequeuedCallback dequeuedCallback)
		{
			Debug.Assert(exception != null, "EnqueueWithoutDispatch: exception parameter should not be null");
			return EnqueueWithoutDispatch(new Item(exception, dequeuedCallback));
		}

		// This will not block, however, Dispatch() must be called later if this function
		// returns true.
		bool EnqueueWithoutDispatch(Item item)
		{
			lock(ThisLock)
			{
				// Open
				if(queueState != QueueState.Closed && queueState != QueueState.Shutdown)
				{
					if(readerQueue.Count == 0)
					{
						itemQueue.EnqueueAvailableItem(item);
						return false;
					}
					else
					{
						itemQueue.EnqueuePendingItem(item);
						return true;
					}
				}
			}

			item.Dispose();
			InvokeDequeuedCallbackLater(item.DequeuedCallback);
			return false;
		}

		static void OnDispatchCallback(object state)
		{
			((InputQueue<T>) state).Dispatch();
		}

		static void InvokeDequeuedCallbackLater(ItemDequeuedCallback dequeuedCallback)
		{
			if(dequeuedCallback != null)
			{
				if(onInvokeDequeuedCallback == null)
				{
					onInvokeDequeuedCallback = OnInvokeDequeuedCallback;
				}

				ThreadPool.QueueUserWorkItem(onInvokeDequeuedCallback, dequeuedCallback);
			}
		}

		static void InvokeDequeuedCallback(ItemDequeuedCallback dequeuedCallback)
		{
			if(dequeuedCallback != null)
			{
				dequeuedCallback();
			}
		}

		static void OnInvokeDequeuedCallback(object state)
		{
			ItemDequeuedCallback dequeuedCallback = (ItemDequeuedCallback) state;
			dequeuedCallback();
		}

		bool RemoveReader(IQueueReader reader)
		{
			lock(ThisLock)
			{
				if(queueState == QueueState.Open || queueState == QueueState.Shutdown)
				{
					bool removed = false;

					for(int i = readerQueue.Count; i > 0; i--)
					{
						IQueueReader temp = readerQueue.Dequeue();
						if(Object.ReferenceEquals(temp, reader))
						{
							removed = true;
						}
						else
						{
							readerQueue.Enqueue(temp);
						}
					}

					return removed;
				}
			}

			return false;
		}

		public bool WaitForItem(TimeSpan timeout)
		{
			WaitQueueWaiter waiter = null;
			bool itemAvailable = false;

			lock(ThisLock)
			{
				if(queueState == QueueState.Open)
				{
					if(itemQueue.HasAvailableItem)
					{
						itemAvailable = true;
					}
					else
					{
						waiter = new WaitQueueWaiter();
						waiterList.Add(waiter);
					}
				}
				else if(queueState == QueueState.Shutdown)
				{
					if(itemQueue.HasAvailableItem)
					{
						itemAvailable = true;
					}
					else if(itemQueue.HasAnyItem)
					{
						waiter = new WaitQueueWaiter();
						waiterList.Add(waiter);
					}
					else
					{
						return false;
					}
				}
				else // queueState == QueueState.Closed
				{
					return true;
				}
			}

			if(waiter != null)
			{
				return waiter.Wait(timeout);
			}
			else
			{
				return itemAvailable;
			}
		}

		interface IQueueReader
		{
			void Set(Item item);
		}

		interface IQueueWaiter
		{
			void Set(bool itemAvailable);
		}

		class WaitQueueReader : IQueueReader
		{
			Exception exception;
			InputQueue<T> inputQueue;
			T item;
			ManualResetEvent waitEvent;
			object thisLock = new object();

			public WaitQueueReader(InputQueue<T> inputQueue)
			{
				this.inputQueue = inputQueue;
				waitEvent = new ManualResetEvent(false);
			}

			object ThisLock
			{
				get
				{
					return this.thisLock;
				}
			}

			public void Set(Item item)
			{
				lock(ThisLock)
				{
					Debug.Assert(this.item == null, "InputQueue.WaitQueueReader.Set: (this.item == null)");
					Debug.Assert(this.exception == null, "InputQueue.WaitQueueReader.Set: (this.exception == null)");

					this.exception = item.Exception;
					this.item = item.Value;
					waitEvent.Set();
				}
			}

			public bool Wait(TimeSpan timeout, out T value)
			{
				bool isSafeToClose = false;
				try
				{
					if(timeout == TimeSpan.MaxValue)
					{
						waitEvent.WaitOne();
					}
					else if(!waitEvent.WaitOne(timeout, false))
					{
						if(this.inputQueue.RemoveReader(this))
						{
							value = default(T);
							isSafeToClose = true;
							return false;
						}
						else
						{
							waitEvent.WaitOne();
						}
					}

					isSafeToClose = true;
				}
				finally
				{
					if(isSafeToClose)
					{
						waitEvent.Close();
					}
				}

				value = item;
				return true;
			}
		}

		public class AsyncQueueReader : AsyncResult, IQueueReader
		{
			static TimerCallback timerCallback = new TimerCallback(AsyncQueueReader.TimerCallback);

			bool expired;
			InputQueue<T> inputQueue;
			T item;
			Timer timer;

			public AsyncQueueReader(InputQueue<T> inputQueue, TimeSpan timeout, AsyncCallback callback, object state)
				: base(callback, state)
			{
				this.inputQueue = inputQueue;
				if(timeout != TimeSpan.MaxValue)
				{
					this.timer = new Timer(timerCallback, this, timeout, TimeSpan.FromMilliseconds(-1));
				}
			}

			public static bool End(IAsyncResult result, out T value)
			{
				AsyncQueueReader readerResult = AsyncResult.End<AsyncQueueReader>(result);

				if(readerResult.expired)
				{
					value = default(T);
					return false;
				}
				else
				{
					value = readerResult.item;
					return true;
				}
			}

			static void TimerCallback(object state)
			{
				AsyncQueueReader thisPtr = (AsyncQueueReader) state;
				if(thisPtr.inputQueue.RemoveReader(thisPtr))
				{
					thisPtr.expired = true;
					thisPtr.Complete(false);
				}
			}

			public void Set(Item item)
			{
				this.item = item.Value;
				if(this.timer != null)
				{
					this.timer.Change(-1, -1);
				}
				Complete(false, item.Exception);
			}
		}

		public struct Item
		{
			T value;
			Exception exception;
			ItemDequeuedCallback dequeuedCallback;

			public Item(T value, ItemDequeuedCallback dequeuedCallback)
				: this(value, null, dequeuedCallback)
			{
			}

			public Item(Exception exception, ItemDequeuedCallback dequeuedCallback)
				: this(null, exception, dequeuedCallback)
			{
			}

			Item(T value, Exception exception, ItemDequeuedCallback dequeuedCallback)
			{
				this.value = value;
				this.exception = exception;
				this.dequeuedCallback = dequeuedCallback;
			}

			public Exception Exception
			{
				get { return this.exception; }
			}

			public T Value
			{
				get { return value; }
			}

			public ItemDequeuedCallback DequeuedCallback
			{
				get { return dequeuedCallback; }
			}

			public void Dispose()
			{
				if(value != null)
				{
					if(value is IDisposable)
					{
						((IDisposable) value).Dispose();
					}
					else if(value is ICommunicationObject)
					{
						((ICommunicationObject) value).Abort();
					}
				}
			}

			public T GetValue()
			{
				if(this.exception != null)
				{
					throw this.exception;
				}

				return this.value;
			}
		}

		class WaitQueueWaiter : IQueueWaiter
		{
			bool itemAvailable;
			ManualResetEvent waitEvent;
			object thisLock = new object();

			public WaitQueueWaiter()
			{
				waitEvent = new ManualResetEvent(false);
			}

			object ThisLock
			{
				get
				{
					return this.thisLock;
				}
			}

			public void Set(bool itemAvailable)
			{
				lock(ThisLock)
				{
					this.itemAvailable = itemAvailable;
					waitEvent.Set();
				}
			}

			public bool Wait(TimeSpan timeout)
			{
				if(timeout == TimeSpan.MaxValue)
				{
					waitEvent.WaitOne();
				}
				else if(!waitEvent.WaitOne(timeout, false))
				{
					return false;
				}

				return this.itemAvailable;
			}
		}

		class AsyncQueueWaiter : AsyncResult, IQueueWaiter
		{
			static TimerCallback timerCallback = new TimerCallback(AsyncQueueWaiter.TimerCallback);
			Timer timer;
			bool itemAvailable;
			object thisLock = new object();

			public AsyncQueueWaiter(TimeSpan timeout, AsyncCallback callback, object state)
				: base(callback, state)
			{
				if(timeout != TimeSpan.MaxValue)
				{
					this.timer = new Timer(timerCallback, this, timeout, TimeSpan.FromMilliseconds(-1));
				}
			}

			object ThisLock
			{
				get
				{
					return this.thisLock;
				}
			}

			public static bool End(IAsyncResult result)
			{
				AsyncQueueWaiter waiterResult = AsyncResult.End<AsyncQueueWaiter>(result);
				return waiterResult.itemAvailable;
			}

			static void TimerCallback(object state)
			{
				AsyncQueueWaiter thisPtr = (AsyncQueueWaiter) state;
				thisPtr.Complete(false);
			}

			public void Set(bool itemAvailable)
			{
				bool timely;

				lock(ThisLock)
				{
					timely = (this.timer == null) || this.timer.Change(-1, -1);
					this.itemAvailable = itemAvailable;
				}

				if(timely)
				{
					Complete(false);
				}
			}
		}

		class ItemQueue
		{
			Item[] items;
			int head;
			int pendingCount;
			int totalCount;

			public ItemQueue()
			{
				items = new Item[1];
			}

			public Item DequeueAvailableItem()
			{
				if(totalCount == pendingCount)
				{
					Debug.Assert(false, "ItemQueue does not contain any available items");
					throw new Exception("Internal Error");
				}
				return DequeueItemCore();
			}

			public Item DequeueAnyItem()
			{
				if(pendingCount == totalCount)
				{
					pendingCount--;
				}
				return DequeueItemCore();
			}

			void EnqueueItemCore(Item item)
			{
				if(totalCount == items.Length)
				{
					Item[] newItems = new Item[items.Length * 2];
					for(int i = 0; i < totalCount; i++)
					{
						newItems[i] = items[(head + i) % items.Length];
					}

					head = 0;
					items = newItems;
				}
				int tail = (head + totalCount) % items.Length;
				items[tail] = item;
				totalCount++;
			}

			Item DequeueItemCore()
			{
				if(totalCount == 0)
				{
					Debug.Assert(false, "ItemQueue does not contain any items");
					throw new Exception("Internal Error");
				}
				Item item = items[head];
				items[head] = new Item();
				totalCount--;
				head = (head + 1) % items.Length;
				return item;
			}

			public void EnqueuePendingItem(Item item)
			{
				EnqueueItemCore(item);
				pendingCount++;
			}

			public void EnqueueAvailableItem(Item item)
			{
				EnqueueItemCore(item);
			}

			public void MakePendingItemAvailable()
			{
				if(pendingCount == 0)
				{
					Debug.Assert(false, "ItemQueue does not contain any pending items");
					throw new Exception("Internal Error");
				}
				pendingCount--;
			}

			public bool HasAvailableItem
			{
				get { return totalCount > pendingCount; }
			}

			public bool HasAnyItem
			{
				get { return totalCount > 0; }
			}

			public int ItemCount
			{
				get { return totalCount; }
			}
		}
	}
}