using System;

using System.Threading;

using System.Collections;

namespace SomeNamespace

{

    public delegate void ThreadActivationCallback() ;

    public delegate void JobFinishCallback ( object state ) ;

    public delegate bool OnErrorCallback ( Exception exc ) ;

    public class CustomThreadPool

    {

        public event ThreadStart ThreadStarted ;

        public event ThreadStart ThreadFinished ;

        public event WaitCallback JobStarted ;

        public event WaitCallback JobFinished ;

        public event OnErrorCallback OnError ;

        public event ThreadStart Idle ;

        public uint MaxThreads ;

        internal int activeThreads = 0 ;

        internal Stack workItems ;

        internal int threadTimeout = 30000; // 30 seconds

        private Timer timer ;

        public readonly string Name ;

        public ClalThreadPool ( string name , uint maxThreads )

        {

            Name = name ;

            MaxThreads = maxThreads;

            workItems = Stack.Synchronized ( new Stack( (int) ( 10 * maxThreads ))) ;

            timer = new Timer ( new TimerCallback ( OnTimer ) , null , 0 , 1000 ) ;

        }

        public bool QueueUserWorkItem ( WaitCallback callback , object state )

        {

            if ( callback == null )

            return false ;

            lock ( this )

            {

                workItems.Push ( new WorkItem ( callback , state )) ;

                if ( activeThreads == 0 )

                WorkThread.AddWorker ( this ) ;

                return true;

            }

        }

        public void OnTimer ( object state )

        {

            lock ( this )

            {

            if ( workItems.Count ==0 )

                return ;

            if ( activeThreads < MaxThreads )

                WorkThread.AddWorker ( this ) ;

            }

        }

        internal void ThreadStartedEvent()

        {

            if( ThreadStarted != null )

                ThreadStarted() ;

        }

        internal void ThreadFinishedEvent()

        {

            if ( ThreadFinished != null )

                ThreadFinished() ;

        }

        internal void JobStartedEvent( object parameter )

        {

            if ( JobStarted != null )

                JobStarted ( parameter ) ;

        }

        internal void JobFinishedEvent ( object parameter )

        {

            if ( JobFinished != null )

                JobFinished ( parameter ) ;

        }

        internal void IdleEvent ()

        {

            if ( Idle != null )

                Idle () ;

        }

        internal bool OnErrorEvent ( Exception exc )

        {

            if ( OnError != null )

                return OnError ( exc ) ;

            return false ;

        }

        public void Stop()

        {

            lock ( this )

            {

                timer.Change ( Timeout.Infinite , Timeout.Infinite ) ;

                workItems.Clear() ;

                MaxThreads = 0 ;

            }

        }

    }

    internal class WorkItem

    {

        public object parameter ;

        public WaitCallback callback ;

        public WorkItem ( WaitCallback callback , object state )

        {

            this.parameter = state ;

            this.callback = callback ;

        }

        }

        internal class WorkThread

        {

        CustomThreadPool pool ;

        internal WorkThread ( CustomThreadPool pool )

        {

            this.pool = pool ;

        }

        static public void AddWorker ( ClalThreadPool pool )

        {

            Interlocked.Increment ( ref pool.activeThreads ) ;

            WorkThread worker = new WorkThread ( pool ) ;

            Thread thread = new Thread ( new ThreadStart ( worker.Loop )) ;

            thread.Start() ;

        }

        private void Loop()

        {

            try

            {

                pool.ThreadStartedEvent() ;

                while ( true )

                {

                    WorkItem item = null;

                    try

                    {

                        item = (WorkItem) pool.workItems.Pop() ;

                    }

                    catch

                    {

                        Thread.Sleep ( pool.threadTimeout ) ;

                        item = (WorkItem) pool.workItems.Pop() ;

                        if ( item == null )

                            break ;

                    }

                    try

                    {

                        pool.JobStartedEvent ( item.parameter ) ;

                        item.callback ( item.parameter ) ;

                        pool.JobFinishedEvent ( item.parameter ) ;

                    }

                    catch(Exception exc)

                    {

                    try

                    {

                        pool.OnErrorEvent ( exc ) ;

                    }

                    catch ( Exception critical )

                    {

                        // Check for critical exception -- pseudo code

                        if( critical is CRITICAL)

                            throw ext;

                    }

                }

                if ( pool.activeThreads > pool.MaxThreads )

                    return;

                }

            }

            catch{}

            finally

            {

                Interlocked.Decrement ( ref pool.activeThreads ) ;

            }

            pool.ThreadFinishedEvent() ;

            if ( pool.activeThreads == 0 )

                pool.IdleEvent();

        }

    }

}