#region Apache License 2.0
/*
Nuclex .NET Framework
Copyright (C) 2002-2024 Markus Ewald / Nuclex Development Labs

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/
#endregion // Apache License 2.0

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Threading;

namespace Nuclex.Support.Threading {

  /// <summary>Alternative Thread pool providing one thread for each core</summary>
  /// <remarks>
  ///   <para>
  ///     Unlike the normal thread pool, the affine thread pool provides only as many
  ///     threads as there are CPU cores available on the current platform. This makes
  ///     it more suitable for tasks you want to spread across all available cpu cores
  ///     explicitly.
  ///   </para>
  ///   <para>
  ///     However, it's not a good match if you want to run blocking or waiting tasks
  ///     inside the thread pool because the limited available threads will become
  ///     congested quickly. It is encouraged to use this class in parallel with
  ///     .NET's own thread pool, putting tasks that can block into the .NET thread
  ///     pool and tasks that perform pure processing into the affine thread pool.
  ///   </para>
  ///   <para>
  ///     Implementation based on original code provided by Stephen Toub
  ///     (stoub at microsoft ignorethis dot com)
  ///   </para>
  /// </remarks>
  public static class AffineThreadPool {

    /// <summary>Number of CPU cores available on the system</summary>
    public static readonly int Processors = Environment.ProcessorCount;

    /// <summary>Delegate used by the thread pool to report unhandled exceptions</summary>
    /// <param name="exception">Exception that has not been handled</param>
    public delegate void ExceptionDelegate(Exception exception);

    #region class UserWorkItem

    /// <summary>Used to hold a callback delegate and the state for that delegate.</summary>
    private struct UserWorkItem {

      /// <summary>Initialize the callback holding object.</summary>
      /// <param name="callback">Callback delegate for the callback.</param>
      /// <param name="state">State with which to call the callback delegate.</param>
      public UserWorkItem(WaitCallback callback, object state) {
        this.Callback = callback;
        this.State = state;
      }

      /// <summary>Callback delegate for the callback.</summary>
      public WaitCallback Callback;
      /// <summary>State with which to call the callback delegate.</summary>
      public object State;

    }

    #endregion // class UserWorkItem

    /// <summary>Initializes the thread pool</summary>
    static AffineThreadPool() {

      // Create our thread stores; we handle synchronization ourself
      // as we may run into situations where multiple operations need to be atomic.
      // We keep track of the threads we've created just for good measure; not actually
      // needed for any core functionality.
      workAvailable = new System.Threading.Semaphore(0, int.MaxValue);
      userWorkItems = new Queue<UserWorkItem>(Processors * 4);
      workerThreads = new List<Thread>(Processors);
      inUseThreads = 0;

      // We can use all cores on a PC, starting from index 1
      hardwareThreads = new Queue<int>(Processors);
      for(int core = Processors; core >= 1; --core) {
        hardwareThreads.Enqueue(core);
      }

      // Create all of the worker threads
      for(int index = 0; index < Processors; index++) {

        // Create a new thread and add it to the list of threads.
        Thread newThread = new Thread(new ThreadStart(ProcessQueuedItems));
        workerThreads.Add(newThread);

        // Configure the new thread and start it
        newThread.Name = "Nuclex.Support.AffineThreadPool Thread #" + index.ToString();
        newThread.IsBackground = true;
        newThread.Start();

      }

    }

    /// <summary>Queues a user work item to the thread pool</summary>
    /// <param name="callback">
    ///   A WaitCallback representing the delegate to invoke when a thread in the 
    ///   thread pool picks up the work item
    /// </param>
    public static void QueueUserWorkItem(WaitCallback callback) {

      // Queue the delegate with no state
      QueueUserWorkItem(callback, null);

    }

    /// <summary>Queues a user work item to the thread pool.</summary>
    /// <param name="callback">
    ///   A WaitCallback representing the delegate to invoke when a thread in the 
    ///   thread pool picks up the work item
    /// </param>
    /// <param name="state">
    ///   The object that is passed to the delegate when serviced from the thread pool
    /// </param>
    public static void QueueUserWorkItem(WaitCallback callback, object state) {

      // Create a waiting callback that contains the delegate and its state.
      // Add it to the processing queue, and signal that data is waiting.
      UserWorkItem waiting = new UserWorkItem(callback, state);
      lock(userWorkItems) {
        userWorkItems.Enqueue(waiting);
      }

      // Wake up one of the worker threads so this task will be processed
      workAvailable.Release();

    }

    /// <summary>Gets the number of threads at the disposal of the thread pool</summary>
    public static int MaxThreads { get { return Processors; } }

    /// <summary>Gets the number of currently active threads in the thread pool</summary>
    public static int ActiveThreads { get { return inUseThreads; } }

    /// <summary>
    ///   Gets the number of callback delegates currently waiting in the thread pool
    /// </summary>
    public static int WaitingWorkItems {
      get {
        lock(userWorkItems) {
          return userWorkItems.Count;
        }
      }
    }

    /// <summary>
    ///   Default handler used to respond to unhandled exceptions in ThreadPool threads
    /// </summary>
    /// <param name="exception">Exception that has occurred</param>
    internal static void DefaultExceptionHandler(Exception exception) {
      throw exception;
    }

#if WINDOWS
    /// <summary>Retrieves the ProcessThread for the calling thread</summary>
    /// <returns>The ProcessThread for the calling thread</returns>
    internal static ProcessThread GetProcessThread(int threadId) {
      ProcessThreadCollection threads = Process.GetCurrentProcess().Threads;
      for(int index = 0; index < threads.Count; ++index) {
        if(threads[index].Id == threadId) {
          return threads[index];
        }
      }

      return null;
    }
#endif

    /// <summary>A thread worker function that processes items from the work queue</summary>
    private static void ProcessQueuedItems() {

      // Get the system/hardware thread index this thread is going to use. We hope that
      // the threads more or less start after each other, but there is no guarantee that
      // tasks will be handled by the CPU cores in the order the queue was filled with.
      // This could be added, though, by using a WaitHandle so the thread creator could
      // wait for each thread to take one entry out of the queue.
      int hardwareThreadIndex;
      lock(hardwareThreads) {
        hardwareThreadIndex = hardwareThreads.Dequeue();
      }

#if WINDOWS
      if(Environment.OSVersion.Platform == PlatformID.Win32NT) {
        // Prevent this managed thread from impersonating another system thread.
        // In .NET, managed threads can supposedly be moved to different system threads
        // and, more worryingly, even fibers. This should make sure we're sitting on
        // a normal system thread and stay with that thread during our lifetime.
        Thread.BeginThreadAffinity();

        // Assign the ideal processor, but don't force it. It's not a good idea to
        // circumvent the thread scheduler of a desktop machine, so we try to play nice.
        int threadId = GetCurrentThreadId();
        ProcessThread thread = GetProcessThread(threadId);
        if(thread != null) {
          thread.IdealProcessor = hardwareThreadIndex;
        }
      }
#endif

      // Keep processing tasks indefinitely
      for(; ; ) {
        UserWorkItem workItem = getNextWorkItem();

        // Execute the work item we just picked up. Make sure to accurately
        // record how many callbacks are currently executing.
        Interlocked.Increment(ref inUseThreads);
        try {
          workItem.Callback(workItem.State);
        }
        catch(Exception exception) { // Make sure we don't throw here.
          ExceptionDelegate exceptionHandler = ExceptionHandler;
          if(exceptionHandler != null) {
            exceptionHandler(exception);
          }
        }
        finally {
          Interlocked.Decrement(ref inUseThreads);
        }
      }
    }

    /// <summary>Obtains the next work item from the queue</summary>
    /// <returns>The next work item in the queue</returns>
    /// <remarks>
    ///   If the queue is empty, the call will block until an item is added to
    ///   the queue and the calling thread was the one picking it up.
    /// </remarks>
    private static UserWorkItem getNextWorkItem() {

      // Get the next item in the queue. If there is nothing there, go to sleep
      // for a while until we're woken up when a callback is waiting.
      for(; ; ) {

        // Try to get the next callback available.  We need to lock on the 
        // queue in order to make our count check and retrieval atomic.
        lock(userWorkItems) {
          if(userWorkItems.Count > 0) {
            return userWorkItems.Dequeue();
          }
        }

        // If we can't get one, go to sleep. The semaphore blocks until work
        // becomes available (then acting like an AutoResetEvent that counts
        // how often it has been triggered and letting that number of threads
        // pass through.)
        workAvailable.WaitOne();

      }

    }

    /// <summary>Delegate used to handle assertion checks in the code</summary>
    public static volatile ExceptionDelegate ExceptionHandler = DefaultExceptionHandler;

#if WINDOWS
    /// <summary>Retrieves the calling thread's thread id</summary>
    /// <returns>The thread is of the calling thread</returns>
    [DllImport("kernel32.dll")]
    internal static extern int GetCurrentThreadId();
#endif

    /// <summary>Available hardware threads the thread pool threads pick from</summary>
    private static Queue<int> hardwareThreads;
    /// <summary>Queue of all the callbacks waiting to be executed.</summary>
    private static Queue<UserWorkItem> userWorkItems;
    /// <summary>
    ///   Used to let the threads in the thread pool wait for new work to appear.
    /// </summary>
    private static System.Threading.Semaphore workAvailable;
    /// <summary>List of all worker threads at the disposal of the thread pool.</summary>
    private static List<Thread> workerThreads;
    /// <summary>Number of threads currently active.</summary>
    private static int inUseThreads;

  }

} // namespace Nuclex.Support.Threading