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;
            }
        }

        bool StartWorker() {
            var current = m_runningThreads;
            // use spins to allocate slot for the new thread
            do {
                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));

            m_maxRunningThreads = Math.Max(m_maxRunningThreads, current + 1);

            // slot successfully allocated

            var worker = new Thread(this.Worker);
            worker.IsBackground = true;
            worker.Start();

            return true;
        }

        protected abstract bool TryDequeue(out TUnit unit);

        protected virtual void WakeNewWorker() {
            if (m_suspended > 0)
                m_hasTasks.Set();
            else
                StartWorker();
        }

        bool FetchTask(out TUnit unit) {
            do {
                // exit if requested
                if (m_exitRequired != 0) {
                    // release the thread slot
                    int running;
                    do {
                        running = m_runningThreads;
                    } while (running != Interlocked.CompareExchange(ref m_runningThreads, running - 1, running));
                    running--;

                    if (running == 0) // it was the last worker
                        m_hasTasks.Dispose();
                    else
                        m_hasTasks.Set(); // release next worker
                    unit = default(TUnit);
                    return false;
                }

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

                //no tasks left, exit if the thread is no longer needed
                int runningThreads;
                bool exit = true;
                do {
                    runningThreads = m_runningThreads;
                    if (runningThreads <= m_minThreads) {
                        exit = false;
                        break;
                    }
                } while (runningThreads != Interlocked.CompareExchange(ref m_runningThreads, runningThreads - 1, runningThreads));

                if (exit) {
                    Interlocked.Decrement(ref m_runningThreads);
                    return false;
                }

                // keep this thread and wait
                Interlocked.Increment(ref m_suspended);
                m_hasTasks.WaitOne();
                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);
        }
    }
}