using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Diagnostics;

namespace Implab.Parallels {
    public abstract class DispatchPool<TUnit> : IDisposable {
        readonly int m_minThreads;
        readonly int m_maxThreads;
        int m_runningThreads = 0;
        int m_maxRunningThreads = 0;
        int m_suspended = 0;
        int m_exitRequired = 0;
        AutoResetEvent m_hasTasks = new AutoResetEvent(false);

        protected DispatchPool(int min, int max) {
            if (min < 0)
                throw new ArgumentOutOfRangeException("min");
            if (max <= 0)
                throw new ArgumentOutOfRangeException("max");

            if (min > max)
                min = max;
            m_minThreads = min;
            m_maxThreads = max;
        }

        protected DispatchPool(int threads)
            : this(threads, threads) {
        }

        protected DispatchPool() {
            int maxThreads, maxCP;
            ThreadPool.GetMaxThreads(out maxThreads, out maxCP);

            m_minThreads = 0;
            m_maxThreads = maxThreads;
        }

        protected void InitPool() {
            for (int i = 0; i < m_minThreads; i++)
                StartWorker();
        }

        public int ThreadCount {
            get {
                return m_runningThreads;
            }
        }

        public int MaxRunningThreads {
            get {
                return m_maxRunningThreads;
            }
        }

        protected bool IsDisposed {
            get {
                return m_exitRequired != 0;
            }
        }

        protected abstract bool TryDequeue(out TUnit unit);

        protected virtual bool ExtendPool() {
            if (m_suspended > 0) {
                m_hasTasks.Set();
                return true;
            } else
                return StartWorker();
        }

        /// <summary>
        /// Запускает либо новый поток, если раньше не было ни одного потока, либо устанавливает событие пробуждение одного спящего потока
        /// </summary>
        protected void WakePool() {
            m_hasTasks.Set(); // wake sleeping thread;

            if (AllocateThreadSlot(1)) {
                var worker = new Thread(this.Worker);
                worker.IsBackground = true;
                worker.Start();
            }
        }

        protected virtual void Suspend() {
            m_hasTasks.WaitOne();
        }

        #region thread slots traits

        bool AllocateThreadSlot() {
            int current;
            // use spins to allocate slot for the new thread
            do {
                current = m_runningThreads;
                if (current >= m_maxThreads || m_exitRequired != 0)
                    // no more slots left or the pool has been disposed
                    return false;
            } while (current != Interlocked.CompareExchange(ref m_runningThreads, current + 1, current));

            UpdateMaxThreads(current + 1);

            return true;
        }

        bool AllocateThreadSlot(int desired) {
            if (desired - 1 != Interlocked.CompareExchange(ref m_runningThreads, desired, desired - 1))
                return false;

            UpdateMaxThreads(desired);

            return true;
        }

        bool ReleaseThreadSlot(out bool last) {
            last = false;
            int current;
            // use spins to release slot for the new thread
            do {
                current = m_runningThreads;
                if (current <= m_minThreads && m_exitRequired == 0)
                    // the thread is reserved
                    return false;
            } while (current != Interlocked.CompareExchange(ref m_runningThreads, current - 1, current));

            last = (current == 1);

            return true;
        }

        /// <summary>
        /// releases thread slot unconditionally, used during cleanup
        /// </summary>
        /// <returns>true - no more threads left</returns>
        bool ReleaseThreadSlotAnyway() {
            var left = Interlocked.Decrement(ref m_runningThreads);
            return left == 0;
        }

        void UpdateMaxThreads(int count) {
            int max;
            do {
                max = m_maxRunningThreads;
                if (max >= count)
                    break;
            } while(max != Interlocked.CompareExchange(ref m_maxRunningThreads, count, max));
        }

        #endregion

        bool StartWorker() {
            if (AllocateThreadSlot()) {
                // slot successfully allocated
                var worker = new Thread(this.Worker);
                worker.IsBackground = true;
                worker.Start();

                return true;
            } else {
                return false;
            }
        }

        bool FetchTask(out TUnit unit) {
            do {
                // exit if requested
                if (m_exitRequired != 0) {
                    // release the thread slot
                    if (ReleaseThreadSlotAnyway()) // it was the last worker
                        m_hasTasks.Dispose();
                    else
                        m_hasTasks.Set(); // wake next worker
                    unit = default(TUnit);
                    return false;
                }

                // fetch task
                if (TryDequeue(out unit)) {
                    ExtendPool();
                    return true;
                }

                //no tasks left, exit if the thread is no longer needed
                bool last;
                if (ReleaseThreadSlot(out last)) {
                    if (last && m_hasTasks.WaitOne(0)) {
                        if (AllocateThreadSlot(1))
                            continue; // spin again...
                        else
                            // we failed to reallocate slot for this thread
                            // therefore we need to release the event
                            m_hasTasks.Set(); 
                    }

                    return false;
                }

                // entering suspend state
                Interlocked.Increment(ref m_suspended);
                // keep this thread and wait                
                Suspend();
                Interlocked.Decrement(ref m_suspended);
            } while (true);
        }

        protected abstract void InvokeUnit(TUnit unit);

        void Worker() {
            TUnit unit;
            while (FetchTask(out unit))
                InvokeUnit(unit);
        }

        protected virtual void Dispose(bool disposing) {
            if (disposing) {
                if (m_exitRequired == 0) {
                    if (Interlocked.CompareExchange(ref m_exitRequired, 1, 0) != 0)
                        return;

                    // wake sleeping threads
                    m_hasTasks.Set();
                    GC.SuppressFinalize(this);
                }
            }
        }

        public void Dispose() {
            Dispose(true);
        }

        ~DispatchPool() {
            Dispose(false);
        }
    }
}