using System;
using System.Collections.Generic;
using System.Threading;
using ln.logging;

namespace ln.threading
{
    public class DynamicThreadPool : IDisposable
    {
        public static DynamicThreadPool DefaultPool { get; } = new DynamicThreadPool();

        public int MinThreads { get; set; } = Environment.ProcessorCount * 2;
        public int MaxThreads { get; set; } = Environment.ProcessorCount * 32;        

        public int IdleWaitTime { get; set; } = 5000;

        public int QueuedTasks { get { lock(queuedTasks) return queuedTasks.Count; }}
        public int CurrentThreads { get { lock(poolThreads) return poolThreads.Count; }}
        public int IdleThreads { get { lock(idleThreads) return idleThreads.Count; }}

 
        Queue<Action> queuedTasks = new Queue<Action>();
        HashSet<Thread> poolThreads = new HashSet<Thread>();
        HashSet<Thread> idleThreads = new HashSet<Thread>();

        double avgIdleThreads;
        public double AverageIdleThreads => avgIdleThreads;

        Thread threadController;
        bool stopController;


        public DynamicThreadPool()
        {
            threadController = new Thread(Controller);
            threadController.IsBackground = true;
            threadController.Start();
        }

        void Controller()
        {
            while (!stopController)
            {
                Thread.Sleep(100);
                lock (threadController)
                {
                    avgIdleThreads = (0.9 * avgIdleThreads) + (0.1 * IdleThreads);

                    if ((avgIdleThreads - MinThreads) <= 0.0)
                    {
                        avgIdleThreads += 1.0;
                        StartThread();
                    }
                }
            }
        }

        public void Enqueue(Action action)
        {
            lock (idleThreads)
            {
                lock (queuedTasks)
                    queuedTasks.Enqueue(action);

                if (idleThreads.Count > 0)
                    Monitor.Pulse(idleThreads);
                else
                {
                    lock (poolThreads)
                    if (poolThreads.Count < MaxThreads)
                    {
                        StartThread();
                    }
                }
            }
        }

        void StartThread()
        {
            Thread t = new Thread(PoolThread);
            t.Start();
        }

        bool TryDequeueTask(out Action action)
        {
            lock (queuedTasks)
                return queuedTasks.TryDequeue(out action);
        }

        public void PoolThread()
        {
            Thread currentThread = Thread.CurrentThread;
            try
            {                
                Action action;

                currentThread.IsBackground = true;
                lock (poolThreads)
                    poolThreads.Add(currentThread);

                while (true)
                {
                    while (TryDequeueTask(out action))
                    {
                        try
                        {
                            action();
                        } catch (Exception e)
                        {
                            Logging.Log(LogLevel.ERROR, "ThreadPool: task threw exception: {0}", e.ToString());
                            Logging.Log(e);
                        }
                    }

                    lock (idleThreads)
                    {
                        idleThreads.Add(currentThread);
                        bool wasPulsed = Monitor.Wait(idleThreads, IdleWaitTime);
                        idleThreads.Remove(currentThread);

                        lock (threadController)
                        if (!wasPulsed && (avgIdleThreads - MinThreads)>1.0)
                        {
                            avgIdleThreads-=1.0;
                            break;
                        }
                    }
                }
            } catch (Exception e)
            {
                Logging.Log(e);
            } finally
            {
                lock (poolThreads)
                    poolThreads.Remove(currentThread);
            }
        }

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