// Ami Bar
// amibar@gmail.com
//
// Smart thread pool in C#.
// 7 Aug 2004 - Initial release
// 14 Sep 2004 - Bug fixes 
// 15 Oct 2004 - Added new features
//        - Work items return result.
//        - Support waiting synchronization for multiple work items.
//        - Work items can be cancelled.
//        - Passage of the caller thread’s context to the thread in the pool.
//        - Minimal usage of WIN32 handles.
//        - Minor bug fixes.
// 26 Dec 2004 - Changes:
//        - Removed static constructors.
//      - Added finalizers.
//        - Changed Exceptions so they are serializable.
//        - Fixed the bug in one of the SmartThreadPool constructors.
//        - Changed the SmartThreadPool.WaitAll() so it will support any number of waiters. 
//        The SmartThreadPool.WaitAny() is still limited by the .NET Framework.
//        - Added PostExecute with options on which cases to call it.
//      - Added option to dispose of the state objects.
//      - Added a WaitForIdle() method that waits until the work items queue is empty.
//      - Added an STPStartInfo class for the initialization of the thread pool.
//      - Changed exception handling so if a work item throws an exception it 
//        is rethrown at GetResult(), rather then firing an UnhandledException event.
//        Note that PostExecute exception are always ignored.
// 25 Mar 2005 - Changes:
//        - Fixed lost of work items bug
// 3 Jul 2005: Changes.
//      - Fixed bug where Enqueue() throws an exception because PopWaiter() returned null, hardly reconstructed. 
// 16 Aug 2005: Changes.
//        - Fixed bug where the InUseThreads becomes negative when canceling work items. 
//
// 31 Jan 2006 - Changes:
//        - Added work items priority
//        - Removed support of chained delegates in callbacks and post executes (nobody really use this)
//        - Added work items groups
//        - Added work items groups idle event
//        - Changed SmartThreadPool.WaitAll() behavior so when it gets empty array
//          it returns true rather then throwing an exception.
//        - Added option to start the STP and the WIG as suspended
//        - Exception behavior changed, the real exception is returned by an 
//          inner exception
//        - Added option to keep the Http context of the caller thread. (Thanks to Steven T.)
//        - Added performance counters
//        - Added priority to the threads in the pool
//
// 13 Feb 2006 - Changes:
//        - Added a call to the dispose of the Performance Counter so
//          their won't be a Performance Counter leak.
//        - Added exception catch in case the Performance Counters cannot 
//          be created.

using System;
using System.Security;
using System.Threading;
using System.Collections;
using System.Diagnostics;
using System.Runtime.CompilerServices;

using Amib.Threading.Internal;

namespace Amib.Threading
{
    #region SmartThreadPool class
    /// <summary>
    /// Smart thread pool class.
    /// </summary>
    public class SmartThreadPool : IWorkItemsGroup, IDisposable
    {
        #region Default Constants

        /// <summary>
        /// Default minimum number of threads the thread pool contains. (0)
        /// </summary>
        public const int DefaultMinWorkerThreads = 0;

        /// <summary>
        /// Default maximum number of threads the thread pool contains. (25)
        /// </summary>
        public const int DefaultMaxWorkerThreads = 25;

        /// <summary>
        /// Default idle timeout in milliseconds. (One minute)
        /// </summary>
        public const int DefaultIdleTimeout = 60*1000; // One minute

        /// <summary>
        /// Indicate to copy the security context of the caller and then use it in the call. (false)
        /// </summary>
        public const bool DefaultUseCallerCallContext = false; 

        /// <summary>
        /// Indicate to copy the HTTP context of the caller and then use it in the call. (false)
        /// </summary>
        public const bool DefaultUseCallerHttpContext = false;

        /// <summary>
        /// Indicate to dispose of the state objects if they support the IDispose interface. (false)
        /// </summary>
        public const bool DefaultDisposeOfStateObjects = false; 

        /// <summary>
        /// The default option to run the post execute
        /// </summary>
        public const CallToPostExecute DefaultCallToPostExecute = CallToPostExecute.Always;

        /// <summary>
        /// The default post execute method to run. 
        /// When null it means not to call it.
        /// </summary>
        public static readonly PostExecuteWorkItemCallback DefaultPostExecuteWorkItemCallback = null;

        /// <summary>
        /// The default work item priority
        /// </summary>
        public const WorkItemPriority DefaultWorkItemPriority = WorkItemPriority.Normal;

        /// <summary>
        /// The default is to work on work items as soon as they arrive
        /// and not to wait for the start.
        /// </summary>
        public const bool DefaultStartSuspended = false;

        /// <summary>
        /// The default is not to use the performance counters
        /// </summary>
        public static readonly string DefaultPerformanceCounterInstanceName = null;

        public static readonly int DefaultStackSize = 0;

        /// <summary>
        /// The default thread priority
        /// </summary>
        public const ThreadPriority DefaultThreadPriority = ThreadPriority.Normal;

        #endregion

        #region Member Variables

        /// <summary>
        /// Contains the name of this instance of SmartThreadPool.
        /// Can be changed by the user.
        /// </summary>
        private string _name = "SmartThreadPool";

        /// <summary>
        /// Hashtable of all the threads in the thread pool.
        /// </summary>
        private Hashtable _workerThreads = Hashtable.Synchronized(new Hashtable());

        /// <summary>
        /// Queue of work items.
        /// </summary>
        private WorkItemsQueue _workItemsQueue = new WorkItemsQueue();

        /// <summary>
        /// Count the work items handled.
        /// Used by the performance counter.
        /// </summary>
        private long _workItemsProcessed = 0;

        /// <summary>
        /// Number of threads that currently work (not idle).
        /// </summary>
        private int _inUseWorkerThreads = 0;

        /// <summary>
        /// Start information to use. 
        /// It is simpler than providing many constructors.
        /// </summary>
        private STPStartInfo _stpStartInfo = new STPStartInfo();

        /// <summary>
        /// Total number of work items that are stored in the work items queue 
        /// plus the work items that the threads in the pool are working on.
        /// </summary>
        private int _currentWorkItemsCount = 0;

        /// <summary>
        /// Signaled when the thread pool is idle, i.e. no thread is busy
        /// and the work items queue is empty
        /// </summary>
        private ManualResetEvent _isIdleWaitHandle = new ManualResetEvent(true);

        /// <summary>
        /// An event to signal all the threads to quit immediately.
        /// </summary>
        private ManualResetEvent _shuttingDownEvent = new ManualResetEvent(false);

        /// <summary>
        /// A flag to indicate the threads to quit.
        /// </summary>
        private bool _shutdown = false;

        /// <summary>
        /// Counts the threads created in the pool.
        /// It is used to name the threads.
        /// </summary>
        private int _threadCounter = 0;

        /// <summary>
        /// Indicate that the SmartThreadPool has been disposed
        /// </summary>
        private bool _isDisposed = false;

        /// <summary>
        /// Event to send that the thread pool is idle
        /// </summary>
        private event EventHandler _stpIdle;

        /// <summary>
        /// On idle event
        /// </summary>
        //private event WorkItemsGroupIdleHandler _onIdle;

        /// <summary>
        /// Holds all the WorkItemsGroup instaces that have at least one 
        /// work item int the SmartThreadPool
        /// This variable is used in case of Shutdown
        /// </summary>
        private Hashtable _workItemsGroups = Hashtable.Synchronized(new Hashtable());

        /// <summary>
        /// A reference from each thread in the thread pool to its SmartThreadPool
        /// object container.
        /// With this variable a thread can know whatever it belongs to a 
        /// SmartThreadPool.
        /// </summary>
        [ThreadStatic]
        private static SmartThreadPool _smartThreadPool;

        /// <summary>
        /// A reference to the current work item a thread from the thread pool 
        /// is executing.
        /// </summary>
        [ThreadStatic]
        private static WorkItem _currentWorkItem;

        /// <summary>
        /// STP performance counters
        /// </summary>
        private ISTPInstancePerformanceCounters _pcs = NullSTPInstancePerformanceCounters.Instance;

        #endregion

        #region Construction and Finalization

        /// <summary>
        /// Constructor
        /// </summary>
        public SmartThreadPool()
        {
            Initialize();
        }

        /// <summary>
        /// Constructor
        /// </summary>
        /// <param name="idleTimeout">Idle timeout in milliseconds</param>
        public SmartThreadPool(int idleTimeout)
        {
            _stpStartInfo.IdleTimeout = idleTimeout;
            Initialize();
        }

        /// <summary>
        /// Constructor
        /// </summary>
        /// <param name="idleTimeout">Idle timeout in milliseconds</param>
        /// <param name="maxWorkerThreads">Upper limit of threads in the pool</param>
        public SmartThreadPool(
            int idleTimeout,
            int maxWorkerThreads)
        {
            _stpStartInfo.IdleTimeout = idleTimeout;
            _stpStartInfo.MaxWorkerThreads = maxWorkerThreads;
            Initialize();
        }

        /// <summary>
        /// Constructor
        /// </summary>
        /// <param name="idleTimeout">Idle timeout in milliseconds</param>
        /// <param name="maxWorkerThreads">Upper limit of threads in the pool</param>
        /// <param name="minWorkerThreads">Lower limit of threads in the pool</param>
        public SmartThreadPool(
            int idleTimeout,
            int maxWorkerThreads,
            int minWorkerThreads)
        {
            _stpStartInfo.IdleTimeout = idleTimeout;
            _stpStartInfo.MaxWorkerThreads = maxWorkerThreads;
            _stpStartInfo.MinWorkerThreads = minWorkerThreads;
            Initialize();
        }

        /// <summary>
        /// Constructor
        /// </summary>
        public SmartThreadPool(STPStartInfo stpStartInfo)
        {
            _stpStartInfo = new STPStartInfo(stpStartInfo);
            Initialize();
        }

        private void Initialize()
        {
            ValidateSTPStartInfo();

            if (null != _stpStartInfo.PerformanceCounterInstanceName)
            {
                try
                {
                    _pcs = new STPInstancePerformanceCounters(_stpStartInfo.PerformanceCounterInstanceName);
                }
                catch(Exception e)
                {
                    Debug.WriteLine("Unable to create Performance Counters: " + e.ToString());
                    _pcs = NullSTPInstancePerformanceCounters.Instance;
                }
            }

            StartOptimalNumberOfThreads();
        }

        private void StartOptimalNumberOfThreads()
        {
            int threadsCount = Math.Max(_workItemsQueue.Count, _stpStartInfo.MinWorkerThreads);
            threadsCount = Math.Min(threadsCount, _stpStartInfo.MaxWorkerThreads);
            StartThreads(threadsCount);
        }

        private void ValidateSTPStartInfo()
        {
            if (_stpStartInfo.MinWorkerThreads < 0)
            {
                throw new ArgumentOutOfRangeException(
                    "MinWorkerThreads", "MinWorkerThreads cannot be negative");
            }

            if (_stpStartInfo.MaxWorkerThreads <= 0)
            {
                throw new ArgumentOutOfRangeException(
                    "MaxWorkerThreads", "MaxWorkerThreads must be greater than zero");
            }

            if (_stpStartInfo.MinWorkerThreads > _stpStartInfo.MaxWorkerThreads)
            {
                throw new ArgumentOutOfRangeException(
                    "MinWorkerThreads, maxWorkerThreads", 
                    "MaxWorkerThreads must be greater or equal to MinWorkerThreads");
            }
        }

        private void ValidateCallback(Delegate callback)
        {
            if(callback.GetInvocationList().Length > 1)
            {
                throw new NotSupportedException("SmartThreadPool doesn't support delegates chains");
            }
        }

        #endregion

        #region Thread Processing

        /// <summary>
        /// Waits on the queue for a work item, shutdown, or timeout.
        /// </summary>
        /// <returns>
        /// Returns the WaitingCallback or null in case of timeout or shutdown.
        /// </returns>
        private WorkItem Dequeue()
        {
            WorkItem workItem = 
                _workItemsQueue.DequeueWorkItem(_stpStartInfo.IdleTimeout, _shuttingDownEvent);

            return workItem;
        }

        /// <summary>
        /// Put a new work item in the queue
        /// </summary>
        /// <param name="workItem">A work item to queue</param>
        private void Enqueue(WorkItem workItem)
        {
            Enqueue(workItem, true);
        }

        /// <summary>
        /// Put a new work item in the queue
        /// </summary>
        /// <param name="workItem">A work item to queue</param>
        internal void Enqueue(WorkItem workItem, bool incrementWorkItems)
        {
            // Make sure the workItem is not null
            Debug.Assert(null != workItem);

            if (incrementWorkItems)
            {
                IncrementWorkItemsCount();
            }

            _workItemsQueue.EnqueueWorkItem(workItem);
            workItem.WorkItemIsQueued();

            // If all the threads are busy then try to create a new one
            if ((InUseThreads + WaitingCallbacks) > _workerThreads.Count) 
            {
                StartThreads(1);
            }
        }

        private void IncrementWorkItemsCount()
        {
            _pcs.SampleWorkItems(_workItemsQueue.Count, _workItemsProcessed);

            int count = Interlocked.Increment(ref _currentWorkItemsCount);
            //Trace.WriteLine("WorkItemsCount = " + _currentWorkItemsCount.ToString());
            if (count == 1) 
            {
                //Trace.WriteLine("STP is NOT idle");
                _isIdleWaitHandle.Reset();
            }
        }

        private void DecrementWorkItemsCount()
        {
            ++_workItemsProcessed;

            // The counter counts even if the work item was cancelled
            _pcs.SampleWorkItems(_workItemsQueue.Count, _workItemsProcessed);

            int count = Interlocked.Decrement(ref _currentWorkItemsCount);
            //Trace.WriteLine("WorkItemsCount = " + _currentWorkItemsCount.ToString());
            if (count == 0) 
            {
                //Trace.WriteLine("STP is idle");
                _isIdleWaitHandle.Set();
            }
        }

        internal void RegisterWorkItemsGroup(IWorkItemsGroup workItemsGroup)
        {
            _workItemsGroups[workItemsGroup] = workItemsGroup;
        }

        internal void UnregisterWorkItemsGroup(IWorkItemsGroup workItemsGroup)
        {
            if (_workItemsGroups.Contains(workItemsGroup))
            {
                _workItemsGroups.Remove(workItemsGroup);
            }
        }

        /// <summary>
        /// Inform that the current thread is about to quit or quiting.
        /// The same thread may call this method more than once.
        /// </summary>
        private void InformCompleted()
        {
            // There is no need to lock the two methods together 
            // since only the current thread removes itself
            // and the _workerThreads is a synchronized hashtable
            if (_workerThreads.Contains(Thread.CurrentThread))
            {
                _workerThreads.Remove(Thread.CurrentThread);
                _pcs.SampleThreads(_workerThreads.Count, _inUseWorkerThreads);
            }
        }

        /// <summary>
        /// Starts new threads
        /// </summary>
        /// <param name="threadsCount">The number of threads to start</param>
        private void StartThreads(int threadsCount)
        {
            if (_stpStartInfo.StartSuspended)
            {
                return;
            }

            lock(_workerThreads.SyncRoot)
            {
                // Don't start threads on shut down
                if (_shutdown)
                {
                    return;
                }

                for(int i = 0; i < threadsCount; ++i)
                {
                    // Don't create more threads then the upper limit
                    if (_workerThreads.Count >= _stpStartInfo.MaxWorkerThreads)
                    {
                        return;
                    }

                    // Create a new thread
                    Thread workerThread;
                    if (_stpStartInfo.StackSize > 0)
                        workerThread = new Thread(ProcessQueuedItems, _stpStartInfo.StackSize);
                    else
                        workerThread = new Thread(ProcessQueuedItems);

                    // Configure the new thread and start it
                    workerThread.Name = "STP " + Name + " Thread #" + _threadCounter;
                    workerThread.IsBackground = true;
                    workerThread.Priority = _stpStartInfo.ThreadPriority;
                    workerThread.Start();
                    ++_threadCounter;

                    // Add the new thread to the hashtable and update its creation
                    // time.
                    _workerThreads[workerThread] = DateTime.Now;
                    _pcs.SampleThreads(_workerThreads.Count, _inUseWorkerThreads);
                }
            }
        }

        /// <summary>
        /// A worker thread method that processes work items from the work items queue.
        /// </summary>
        private void ProcessQueuedItems()
        {
            // Initialize the _smartThreadPool variable
            _smartThreadPool = this;

            try
            {
                bool bInUseWorkerThreadsWasIncremented = false;

                // Process until shutdown.
                while(!_shutdown)
                {
                    // Update the last time this thread was seen alive.
                    // It's good for debugging.
                    _workerThreads[Thread.CurrentThread] = DateTime.Now;

                    // Wait for a work item, shutdown, or timeout
                    WorkItem workItem = Dequeue();

                    // Update the last time this thread was seen alive.
                    // It's good for debugging.
                    _workerThreads[Thread.CurrentThread] = DateTime.Now;

                    // On timeout or shut down.
                    if (null == workItem)
                    {
                        // Double lock for quit.
                        if (_workerThreads.Count > _stpStartInfo.MinWorkerThreads)
                        {
                            lock(_workerThreads.SyncRoot)
                            {
                                if (_workerThreads.Count > _stpStartInfo.MinWorkerThreads)
                                {
                                    // Inform that the thread is quiting and then quit.
                                    // This method must be called within this lock or else
                                    // more threads will quit and the thread pool will go
                                    // below the lower limit.
                                    InformCompleted();
                                    break;
                                }
                            }
                        }
                    }

                    // If we didn't quit then skip to the next iteration.
                    if (null == workItem)
                    {
                        continue;
                    }

                    try 
                    {
                        // Initialize the value to false
                        bInUseWorkerThreadsWasIncremented = false;

                        // Change the state of the work item to 'in progress' if possible.
                        // We do it here so if the work item has been canceled we won't 
                        // increment the _inUseWorkerThreads.
                        // The cancel mechanism doesn't delete items from the queue,  
                        // it marks the work item as canceled, and when the work item
                        // is dequeued, we just skip it.
                        // If the post execute of work item is set to always or to
                        // call when the work item is canceled then the StartingWorkItem()
                        // will return true, so the post execute can run.
                        if (!workItem.StartingWorkItem())
                        {
                            continue;
                        }

                        // Execute the callback.  Make sure to accurately
                        // record how many callbacks are currently executing.
                        int inUseWorkerThreads = Interlocked.Increment(ref _inUseWorkerThreads);
                        _pcs.SampleThreads(_workerThreads.Count, inUseWorkerThreads);

                        // Mark that the _inUseWorkerThreads incremented, so in the finally{}
                        // statement we will decrement it correctly.
                        bInUseWorkerThreadsWasIncremented = true;

                        // Set the _currentWorkItem to the current work item
                        _currentWorkItem = workItem;

                        lock(workItem)
                        {
                            workItem.currentThread = Thread.CurrentThread;
                        }

                        ExecuteWorkItem(workItem);

                        lock(workItem)
                        {
                            workItem.currentThread = null;
                        }

                    }
                    catch(ThreadAbortException ex)
                    {
                        lock(workItem)
                        {
                            workItem.currentThread = null;
                        }
                        ex.GetHashCode();
                        Thread.ResetAbort();
                    }
                    catch(Exception ex)
                    {
                        ex.GetHashCode();
                        // Do nothing
                    }
                    finally
                    {
                        lock(workItem)
                        {
                            workItem.currentThread = null;
                        }

                        if (null != workItem)
                        {
                            workItem.DisposeOfState();
                        }

                        // Set the _currentWorkItem to null, since we 
                        // no longer run user's code.
                        _currentWorkItem = null;

                        // Decrement the _inUseWorkerThreads only if we had 
                        // incremented it. Note the cancelled work items don't
                        // increment _inUseWorkerThreads.
                        if (bInUseWorkerThreadsWasIncremented)
                        {
                            int inUseWorkerThreads = Interlocked.Decrement(ref _inUseWorkerThreads);
                            _pcs.SampleThreads(_workerThreads.Count, inUseWorkerThreads);
                        }

                        // Notify that the work item has been completed.
                        // WorkItemsGroup may enqueue their next work item.
                        workItem.FireWorkItemCompleted();

                        // Decrement the number of work items here so the idle 
                        // ManualResetEvent won't fluctuate.
                        DecrementWorkItemsCount();
                    }
                }
            } 
            catch(ThreadAbortException tae)
            {
                tae.GetHashCode();
                // Handle the abort exception gracfully.
                Thread.ResetAbort();
            }
            catch(Exception e)
            {
                Debug.Assert(null != e);
            }
            finally
            {
                InformCompleted();
            }
        }

        private void ExecuteWorkItem(WorkItem workItem)
        {
            _pcs.SampleWorkItemsWaitTime(workItem.WaitingTime);
            try
            {
                workItem.Execute();
            }
            catch
            {
                throw;
            }
            finally
            {
                _pcs.SampleWorkItemsProcessTime(workItem.ProcessTime);
            }
        }


        #endregion

        #region Public Methods

        /// <summary>
        /// Queue a work item
        /// </summary>
        /// <param name="callback">A callback to execute</param>
        /// <returns>Returns a work item result</returns>
        public IWorkItemResult QueueWorkItem(WorkItemCallback callback)
        {
            ValidateNotDisposed();
            ValidateCallback(callback);
            WorkItem workItem = WorkItemFactory.CreateWorkItem(this, _stpStartInfo, callback);
            Enqueue(workItem);
            return workItem.GetWorkItemResult();
        }

        /// <summary>
        /// Queue a work item
        /// </summary>
        /// <param name="callback">A callback to execute</param>
        /// <param name="workItemPriority">The priority of the work item</param>
        /// <returns>Returns a work item result</returns>
        public IWorkItemResult QueueWorkItem(WorkItemCallback callback, WorkItemPriority workItemPriority)
        {
            ValidateNotDisposed();
            ValidateCallback(callback);
            WorkItem workItem = WorkItemFactory.CreateWorkItem(this, _stpStartInfo, callback, workItemPriority);
            Enqueue(workItem);
            return workItem.GetWorkItemResult();
        }

        /// <summary>
        /// Queue a work item
        /// </summary>
        /// <param name="workItemInfo">Work item info</param>
        /// <param name="callback">A callback to execute</param>
        /// <returns>Returns a work item result</returns>
        public IWorkItemResult QueueWorkItem(WorkItemInfo workItemInfo, WorkItemCallback callback)
        {
            ValidateNotDisposed();
            ValidateCallback(callback);
            WorkItem workItem = WorkItemFactory.CreateWorkItem(this, _stpStartInfo, workItemInfo, callback);
            Enqueue(workItem);
            return workItem.GetWorkItemResult();
        }

        /// <summary>
        /// Queue a work item
        /// </summary>
        /// <param name="callback">A callback to execute</param>
        /// <param name="state">
        /// The context object of the work item. Used for passing arguments to the work item. 
        /// </param>
        /// <returns>Returns a work item result</returns>
        public IWorkItemResult QueueWorkItem(WorkItemCallback callback, object state)
        {
            ValidateNotDisposed();
            ValidateCallback(callback);
            WorkItem workItem = WorkItemFactory.CreateWorkItem(this, _stpStartInfo, callback, state);
            Enqueue(workItem);
            return workItem.GetWorkItemResult();
        }

        /// <summary>
        /// Queue a work item
        /// </summary>
        /// <param name="callback">A callback to execute</param>
        /// <param name="state">
        /// The context object of the work item. Used for passing arguments to the work item. 
        /// </param>
        /// <param name="workItemPriority">The work item priority</param>
        /// <returns>Returns a work item result</returns>
        public IWorkItemResult QueueWorkItem(WorkItemCallback callback, object state, WorkItemPriority workItemPriority)
        {
            ValidateNotDisposed();
            ValidateCallback(callback);
            WorkItem workItem = WorkItemFactory.CreateWorkItem(this, _stpStartInfo, callback, state, workItemPriority);
            Enqueue(workItem);
            return workItem.GetWorkItemResult();
        }

        /// <summary>
        /// Queue a work item
        /// </summary>
        /// <param name="workItemInfo">Work item information</param>
        /// <param name="callback">A callback to execute</param>
        /// <param name="state">
        /// The context object of the work item. Used for passing arguments to the work item. 
        /// </param>
        /// <returns>Returns a work item result</returns>
        public IWorkItemResult QueueWorkItem(WorkItemInfo workItemInfo, WorkItemCallback callback, object state)
        {
            ValidateNotDisposed();
            ValidateCallback(callback);
            WorkItem workItem = WorkItemFactory.CreateWorkItem(this, _stpStartInfo, workItemInfo, callback, state);
            Enqueue(workItem);
            return workItem.GetWorkItemResult();
        }

        /// <summary>
        /// Queue a work item
        /// </summary>
        /// <param name="callback">A callback to execute</param>
        /// <param name="state">
        /// The context object of the work item. Used for passing arguments to the work item. 
        /// </param>
        /// <param name="postExecuteWorkItemCallback">
        /// A delegate to call after the callback completion
        /// </param>
        /// <returns>Returns a work item result</returns>
        public IWorkItemResult QueueWorkItem(
            WorkItemCallback callback, 
            object state,
            PostExecuteWorkItemCallback postExecuteWorkItemCallback)
        {
            ValidateNotDisposed();
            ValidateCallback(callback);
            WorkItem workItem = WorkItemFactory.CreateWorkItem(this, _stpStartInfo, callback, state, postExecuteWorkItemCallback);
            Enqueue(workItem);
            return workItem.GetWorkItemResult();
        }

        /// <summary>
        /// Queue a work item
        /// </summary>
        /// <param name="callback">A callback to execute</param>
        /// <param name="state">
        /// The context object of the work item. Used for passing arguments to the work item. 
        /// </param>
        /// <param name="postExecuteWorkItemCallback">
        /// A delegate to call after the callback completion
        /// </param>
        /// <param name="workItemPriority">The work item priority</param>
        /// <returns>Returns a work item result</returns>
        public IWorkItemResult QueueWorkItem(
            WorkItemCallback callback, 
            object state,
            PostExecuteWorkItemCallback postExecuteWorkItemCallback,
            WorkItemPriority workItemPriority)
        {
            ValidateNotDisposed();
            ValidateCallback(callback);
            WorkItem workItem = WorkItemFactory.CreateWorkItem(this, _stpStartInfo, callback, state, postExecuteWorkItemCallback, workItemPriority);
            Enqueue(workItem);
            return workItem.GetWorkItemResult();
        }

        /// <summary>
        /// Queue a work item
        /// </summary>
        /// <param name="callback">A callback to execute</param>
        /// <param name="state">
        /// The context object of the work item. Used for passing arguments to the work item. 
        /// </param>
        /// <param name="postExecuteWorkItemCallback">
        /// A delegate to call after the callback completion
        /// </param>
        /// <param name="callToPostExecute">Indicates on which cases to call to the post execute callback</param>
        /// <returns>Returns a work item result</returns>
        public IWorkItemResult QueueWorkItem(
            WorkItemCallback callback, 
            object state,
            PostExecuteWorkItemCallback postExecuteWorkItemCallback,
            CallToPostExecute callToPostExecute)
        {
            ValidateNotDisposed();
            ValidateCallback(callback);
            WorkItem workItem = WorkItemFactory.CreateWorkItem(this, _stpStartInfo, callback, state, postExecuteWorkItemCallback, callToPostExecute);
            Enqueue(workItem);
            return workItem.GetWorkItemResult();
        }

        /// <summary>
        /// Queue a work item
        /// </summary>
        /// <param name="callback">A callback to execute</param>
        /// <param name="state">
        /// The context object of the work item. Used for passing arguments to the work item. 
        /// </param>
        /// <param name="postExecuteWorkItemCallback">
        /// A delegate to call after the callback completion
        /// </param>
        /// <param name="callToPostExecute">Indicates on which cases to call to the post execute callback</param>
        /// <param name="workItemPriority">The work item priority</param>
        /// <returns>Returns a work item result</returns>
        public IWorkItemResult QueueWorkItem(
            WorkItemCallback callback, 
            object state,
            PostExecuteWorkItemCallback postExecuteWorkItemCallback,
            CallToPostExecute callToPostExecute,
            WorkItemPriority workItemPriority)
        {
            ValidateNotDisposed();
            ValidateCallback(callback);
            WorkItem workItem = WorkItemFactory.CreateWorkItem(this, _stpStartInfo, callback, state, postExecuteWorkItemCallback, callToPostExecute, workItemPriority);
            Enqueue(workItem);
            return workItem.GetWorkItemResult();
        }

        /// <summary>
        /// Wait for the thread pool to be idle
        /// </summary>
        public void WaitForIdle()
        {
            WaitForIdle(Timeout.Infinite);
        }

        /// <summary>
        /// Wait for the thread pool to be idle
        /// </summary>
        public bool WaitForIdle(TimeSpan timeout)
        {
            return WaitForIdle((int)timeout.TotalMilliseconds);
        }

        /// <summary>
        /// Wait for the thread pool to be idle
        /// </summary>
        public bool WaitForIdle(int millisecondsTimeout)
        {
            ValidateWaitForIdle();
            return _isIdleWaitHandle.WaitOne(millisecondsTimeout, false);
        }

        private void ValidateWaitForIdle()
        {
            if(_smartThreadPool == this)
            {
                throw new NotSupportedException(
                    "WaitForIdle cannot be called from a thread on its SmartThreadPool, it will cause may cause a deadlock");
            }
        }

        internal void ValidateWorkItemsGroupWaitForIdle(IWorkItemsGroup workItemsGroup)
        {
            ValidateWorkItemsGroupWaitForIdleImpl(workItemsGroup, SmartThreadPool._currentWorkItem);
            if ((null != workItemsGroup) && 
                (null != SmartThreadPool._currentWorkItem) &&
                SmartThreadPool._currentWorkItem.WasQueuedBy(workItemsGroup))
            {
                throw new NotSupportedException("WaitForIdle cannot be called from a thread on its SmartThreadPool, it will cause may cause a deadlock");
            }
        }

        [MethodImpl(MethodImplOptions.NoInlining)]
        private void ValidateWorkItemsGroupWaitForIdleImpl(IWorkItemsGroup workItemsGroup, WorkItem workItem)
        {
            if ((null != workItemsGroup) && 
                (null != workItem) &&
                workItem.WasQueuedBy(workItemsGroup))
            {
                throw new NotSupportedException("WaitForIdle cannot be called from a thread on its SmartThreadPool, it will cause may cause a deadlock");
            }
        }



        /// <summary>
        /// Force the SmartThreadPool to shutdown
        /// </summary>
        public void Shutdown()
        {
            Shutdown(true, 0);
        }

        public void Shutdown(bool forceAbort, TimeSpan timeout)
        {
            Shutdown(forceAbort, (int)timeout.TotalMilliseconds);
        }

        /// <summary>
        /// Empties the queue of work items and abort the threads in the pool.
        /// </summary>
        public void Shutdown(bool forceAbort, int millisecondsTimeout)
        {
            ValidateNotDisposed();

            ISTPInstancePerformanceCounters pcs = _pcs;

            if (NullSTPInstancePerformanceCounters.Instance != _pcs)
            {
                _pcs.Dispose();
                // Set the _pcs to "null" to stop updating the performance
                // counters
                _pcs = NullSTPInstancePerformanceCounters.Instance;
            }

            Thread [] threads = null;
            lock(_workerThreads.SyncRoot)
            {
                // Shutdown the work items queue
                _workItemsQueue.Dispose();

                // Signal the threads to exit
                _shutdown = true;
                _shuttingDownEvent.Set();

                // Make a copy of the threads' references in the pool
                threads = new Thread [_workerThreads.Count];
                _workerThreads.Keys.CopyTo(threads, 0);
            }

            int millisecondsLeft = millisecondsTimeout;
            DateTime start = DateTime.Now;
            bool waitInfinitely = (Timeout.Infinite == millisecondsTimeout);
            bool timeout = false;

            // Each iteration we update the time left for the timeout.
            foreach(Thread thread in threads)
            {
                // Join don't work with negative numbers
                if (!waitInfinitely && (millisecondsLeft < 0))
                {
                    timeout = true;
                    break;
                }

                // Wait for the thread to terminate
                bool success = thread.Join(millisecondsLeft);
                if(!success)
                {
                    timeout = true;
                    break;
                }

                if(!waitInfinitely)
                {
                    // Update the time left to wait
                    TimeSpan ts = DateTime.Now - start;
                    millisecondsLeft = millisecondsTimeout - (int)ts.TotalMilliseconds;
                }
            }

            if (timeout && forceAbort)
            {
                // Abort the threads in the pool
                foreach(Thread thread in threads)
                {
                    if ((thread != null) && thread.IsAlive) 
                    {
                        try 
                        {
                            thread.Abort("Shutdown");
                        }
                        catch(SecurityException e)
                        {
                            e.GetHashCode();
                        }
                        catch(ThreadStateException ex)
                        {
                            ex.GetHashCode();
                            // In case the thread has been terminated 
                            // after the check if it is alive.
                        }
                    }
                }
            }

            // Dispose of the performance counters
            pcs.Dispose();
        }

        /// <summary>
        /// Wait for all work items to complete
        /// </summary>
        /// <param name="workItemResults">Array of work item result objects</param>
        /// <returns>
        /// true when every work item in workItemResults has completed; otherwise false.
        /// </returns>
        public static bool WaitAll(
            IWorkItemResult [] workItemResults)
        {
            return WaitAll(workItemResults, Timeout.Infinite, true);
        }

        /// <summary>
        /// Wait for all work items to complete
        /// </summary>
        /// <param name="workItemResults">Array of work item result objects</param>
        /// <param name="timeout">The number of milliseconds to wait, or a TimeSpan that represents -1 milliseconds to wait indefinitely. </param>
        /// <param name="exitContext">
        /// true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it; otherwise, false. 
        /// </param>
        /// <returns>
        /// true when every work item in workItemResults has completed; otherwise false.
        /// </returns>
        public static bool WaitAll(
            IWorkItemResult [] workItemResults,
            TimeSpan timeout,
            bool exitContext)
        {
            return WaitAll(workItemResults, (int)timeout.TotalMilliseconds, exitContext);
        }

        /// <summary>
        /// Wait for all work items to complete
        /// </summary>
        /// <param name="workItemResults">Array of work item result objects</param>
        /// <param name="timeout">The number of milliseconds to wait, or a TimeSpan that represents -1 milliseconds to wait indefinitely. </param>
        /// <param name="exitContext">
        /// true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it; otherwise, false. 
        /// </param>
        /// <param name="cancelWaitHandle">A cancel wait handle to interrupt the wait if needed</param>
        /// <returns>
        /// true when every work item in workItemResults has completed; otherwise false.
        /// </returns>
        public static bool WaitAll(
            IWorkItemResult [] workItemResults,  
            TimeSpan timeout,
            bool exitContext,
            WaitHandle cancelWaitHandle)
        {
            return WaitAll(workItemResults, (int)timeout.TotalMilliseconds, exitContext, cancelWaitHandle);
        }

        /// <summary>
        /// Wait for all work items to complete
        /// </summary>
        /// <param name="workItemResults">Array of work item result objects</param>
        /// <param name="millisecondsTimeout">The number of milliseconds to wait, or Timeout.Infinite (-1) to wait indefinitely.</param>
        /// <param name="exitContext">
        /// true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it; otherwise, false. 
        /// </param>
        /// <returns>
        /// true when every work item in workItemResults has completed; otherwise false.
        /// </returns>
        public static bool WaitAll(
            IWorkItemResult [] workItemResults,  
            int millisecondsTimeout,
            bool exitContext)
        {
            return WorkItem.WaitAll(workItemResults, millisecondsTimeout, exitContext, null);
        }

        /// <summary>
        /// Wait for all work items to complete
        /// </summary>
        /// <param name="workItemResults">Array of work item result objects</param>
        /// <param name="millisecondsTimeout">The number of milliseconds to wait, or Timeout.Infinite (-1) to wait indefinitely.</param>
        /// <param name="exitContext">
        /// true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it; otherwise, false. 
        /// </param>
        /// <param name="cancelWaitHandle">A cancel wait handle to interrupt the wait if needed</param>
        /// <returns>
        /// true when every work item in workItemResults has completed; otherwise false.
        /// </returns>
        public static bool WaitAll(
            IWorkItemResult [] workItemResults,  
            int millisecondsTimeout,
            bool exitContext,
            WaitHandle cancelWaitHandle)
        {
            return WorkItem.WaitAll(workItemResults, millisecondsTimeout, exitContext, cancelWaitHandle);
        }


        /// <summary>
        /// Waits for any of the work items in the specified array to complete, cancel, or timeout
        /// </summary>
        /// <param name="workItemResults">Array of work item result objects</param>
        /// <returns>
        /// The array index of the work item result that satisfied the wait, or WaitTimeout if any of the work items has been canceled.
        /// </returns>
        public static int WaitAny(
            IWorkItemResult [] workItemResults)
        {
            return WaitAny(workItemResults, Timeout.Infinite, true);
        }

        /// <summary>
        /// Waits for any of the work items in the specified array to complete, cancel, or timeout
        /// </summary>
        /// <param name="workItemResults">Array of work item result objects</param>
        /// <param name="timeout">The number of milliseconds to wait, or a TimeSpan that represents -1 milliseconds to wait indefinitely. </param>
        /// <param name="exitContext">
        /// true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it; otherwise, false. 
        /// </param>
        /// <returns>
        /// The array index of the work item result that satisfied the wait, or WaitTimeout if no work item result satisfied the wait and a time interval equivalent to millisecondsTimeout has passed or the work item has been canceled.
        /// </returns>
        public static int WaitAny(
            IWorkItemResult [] workItemResults,
            TimeSpan timeout,
            bool exitContext)
        {
            return WaitAny(workItemResults, (int)timeout.TotalMilliseconds, exitContext);
        }

        /// <summary>
        /// Waits for any of the work items in the specified array to complete, cancel, or timeout
        /// </summary>
        /// <param name="workItemResults">Array of work item result objects</param>
        /// <param name="timeout">The number of milliseconds to wait, or a TimeSpan that represents -1 milliseconds to wait indefinitely. </param>
        /// <param name="exitContext">
        /// true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it; otherwise, false. 
        /// </param>
        /// <param name="cancelWaitHandle">A cancel wait handle to interrupt the wait if needed</param>
        /// <returns>
        /// The array index of the work item result that satisfied the wait, or WaitTimeout if no work item result satisfied the wait and a time interval equivalent to millisecondsTimeout has passed or the work item has been canceled.
        /// </returns>
        public static int WaitAny(
            IWorkItemResult [] workItemResults,
            TimeSpan timeout,
            bool exitContext,
            WaitHandle cancelWaitHandle)
        {
            return WaitAny(workItemResults, (int)timeout.TotalMilliseconds, exitContext, cancelWaitHandle);
        }

        /// <summary>
        /// Waits for any of the work items in the specified array to complete, cancel, or timeout
        /// </summary>
        /// <param name="workItemResults">Array of work item result objects</param>
        /// <param name="millisecondsTimeout">The number of milliseconds to wait, or Timeout.Infinite (-1) to wait indefinitely.</param>
        /// <param name="exitContext">
        /// true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it; otherwise, false. 
        /// </param>
        /// <returns>
        /// The array index of the work item result that satisfied the wait, or WaitTimeout if no work item result satisfied the wait and a time interval equivalent to millisecondsTimeout has passed or the work item has been canceled.
        /// </returns>
        public static int WaitAny(
            IWorkItemResult [] workItemResults,  
            int millisecondsTimeout,
            bool exitContext)
        {
            return WorkItem.WaitAny(workItemResults, millisecondsTimeout, exitContext, null);
        }

        /// <summary>
        /// Waits for any of the work items in the specified array to complete, cancel, or timeout
        /// </summary>
        /// <param name="workItemResults">Array of work item result objects</param>
        /// <param name="millisecondsTimeout">The number of milliseconds to wait, or Timeout.Infinite (-1) to wait indefinitely.</param>
        /// <param name="exitContext">
        /// true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it; otherwise, false. 
        /// </param>
        /// <param name="cancelWaitHandle">A cancel wait handle to interrupt the wait if needed</param>
        /// <returns>
        /// The array index of the work item result that satisfied the wait, or WaitTimeout if no work item result satisfied the wait and a time interval equivalent to millisecondsTimeout has passed or the work item has been canceled.
        /// </returns>
        public static int WaitAny(
            IWorkItemResult [] workItemResults,  
            int millisecondsTimeout,
            bool exitContext,
            WaitHandle cancelWaitHandle)
        {
            return WorkItem.WaitAny(workItemResults, millisecondsTimeout, exitContext, cancelWaitHandle);
        }

        public IWorkItemsGroup CreateWorkItemsGroup(int concurrency)
        {
            IWorkItemsGroup workItemsGroup = new WorkItemsGroup(this, concurrency, _stpStartInfo);
            return workItemsGroup;
        }

        public IWorkItemsGroup CreateWorkItemsGroup(int concurrency, WIGStartInfo wigStartInfo)
        {
            IWorkItemsGroup workItemsGroup = new WorkItemsGroup(this, concurrency, wigStartInfo);
            return workItemsGroup;
        }

        public event WorkItemsGroupIdleHandler OnIdle
        {
            add
            {
                throw new NotImplementedException("This event is not implemented in the SmartThreadPool class. Please create a WorkItemsGroup in order to use this feature.");
                //_onIdle += value;
            }
            remove
            {
                throw new NotImplementedException("This event is not implemented in the SmartThreadPool class. Please create a WorkItemsGroup in order to use this feature.");
                //_onIdle -= value;
            }
        }

        public void Cancel()
        {
            ICollection workItemsGroups = _workItemsGroups.Values;
            foreach(WorkItemsGroup workItemsGroup in workItemsGroups)
            {
                workItemsGroup.Cancel();
            }
        }

        public void Start()
        {
            lock (this)
            {
                if (!this._stpStartInfo.StartSuspended)
                {
                    return;
                }
                _stpStartInfo.StartSuspended = false;
            }
            
            ICollection workItemsGroups = _workItemsGroups.Values;
            foreach(WorkItemsGroup workItemsGroup in workItemsGroups)
            {
                workItemsGroup.OnSTPIsStarting();
            }

            StartOptimalNumberOfThreads();
        }

        #endregion

        #region Properties

        /// <summary>
        /// Get/Set the name of the SmartThreadPool instance
        /// </summary>
        public string Name 
        { 
            get
            {
                return _name;
            }

            set
            {
                _name = value;
            }
        }

        /// <summary>
        /// Get the lower limit of threads in the pool.
        /// </summary>
        public int MinThreads 
        { 
            get 
            {
                ValidateNotDisposed();
                return _stpStartInfo.MinWorkerThreads; 
            }
        }

        /// <summary>
        /// Get the upper limit of threads in the pool.
        /// </summary>
        public int MaxThreads 
        { 
            get 
            {
                ValidateNotDisposed();
                return _stpStartInfo.MaxWorkerThreads; 
            } 
        }
        /// <summary>
        /// Get the number of threads in the thread pool.
        /// Should be between the lower and the upper limits.
        /// </summary>
        public int ActiveThreads 
        { 
            get 
            {
                ValidateNotDisposed();
                return _workerThreads.Count; 
            } 
        }

        /// <summary>
        /// Get the number of busy (not idle) threads in the thread pool.
        /// </summary>
        public int InUseThreads 
        { 
            get 
            { 
                ValidateNotDisposed();
                return _inUseWorkerThreads; 
            } 
        }

        /// <summary>
        /// Get the number of work items in the queue.
        /// </summary>
        public int WaitingCallbacks 
        { 
            get 
            { 
                ValidateNotDisposed();
                return _workItemsQueue.Count;
            } 
        }


        public event EventHandler Idle
        {
            add
            {
                _stpIdle += value;
            }

            remove
            {
                _stpIdle -= value;
            }
        }

        #endregion

        #region IDisposable Members

//        ~SmartThreadPool()
//        {
//            Dispose();
//        }

        public void Dispose()
        {
            if (!_isDisposed)
            {
                if (!_shutdown)
                {
                    Shutdown();
                }

                if (null != _shuttingDownEvent)
                {
                    _shuttingDownEvent.Close();
                    _shuttingDownEvent = null;
                }
                _workerThreads.Clear();
                _isDisposed = true;
                GC.SuppressFinalize(this);
            }
        }

        private void ValidateNotDisposed()
        {
            if(_isDisposed)
            {
                throw new ObjectDisposedException(GetType().ToString(), "The SmartThreadPool has been shutdown");
            }
        }
        #endregion
    }
    #endregion
}
