#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

#if !NO_CONCURRENT_COLLECTIONS

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace Nuclex.Support.Threading {

  /// <summary>Processes tasks in parallel using many threads</summary>
  /// <typeparam name="TTask">Type of tasks the class will process</typeparam>
  [Obsolete("Use ThreadRunner instead; UI view models should inherit ThreadedViewModel")]
  public abstract class ParallelBackgroundWorker<TTask> : IDisposable {

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

    /// <summary>
    ///   Timeout after which Dispose() will stop waiting for unfinished tasks and
    ///   free resources anyway
    /// </summary>
    private static readonly int DefaultDisposeTimeoutMilliseconds = 1500; // milliseconds

    /// <summary>Initializes a new parallel background worker with unlimited threads</summary>
    public ParallelBackgroundWorker() : this(int.MaxValue) { }

    /// <summary>
    ///   Initializes a new parallel background worker running the specified number
    ///   of tasks in parallel
    /// </summary>
    /// <param name="threadCount">
    ///   Number of tasks to run in parallel (if positive) or number of CPU cores to leave
    ///   unused (if negative).
    /// </param>
    /// <remarks>
    ///   If a negative number of threads is used, at least one thread will be always
    ///   be created, so specifying -2 on a single-core system will still occupy
    ///   the only core.
    /// </remarks>
    public ParallelBackgroundWorker(int threadCount) {
      if(threadCount > 0) {
        this.threadCount = threadCount;
      } else {
        this.threadCount = Math.Max(1, ProcessorCount + threadCount);
      }

      this.queueSynchronizationRoot = new object();
      this.runQueuedTasksInThreadDelegate = new Action(runQueuedTasksInThread);
      this.tasks = new Queue<TTask>();
      this.threadTerminatedEvent = new AutoResetEvent(false);
      this.cancellationTokenSource = new CancellationTokenSource();
      this.exceptions = new ConcurrentBag<Exception>();
    }

    /// <summary>
    ///   Initializes a new parallel background worker that uses the specified name for
    ///   its worker threads.
    /// </summary>
    /// <param name="name">Name that will be assigned to the worker threads</param>
    public ParallelBackgroundWorker(string name) :
      this(int.MaxValue) {
      this.threadName = name;
    }

    /// <summary>
    ///   Initializes a new parallel background worker that uses the specified name for
    ///   its worker threads and running the specified number of tasks in parallel.
    /// </summary>
    /// <param name="name">Name that will be assigned to the worker threads</param>
    /// <param name="threadCount">
    ///   Number of tasks to run in parallel (if positive) or number of CPU cores to leave
    ///   unused (if negative).
    /// </param>
    /// <remarks>
    ///   If a negative number of threads is used, at least one thread will be always
    ///   be created, so specifying -2 on a single-core system will still occupy
    ///   the only core.
    /// </remarks>
    public ParallelBackgroundWorker(string name, int threadCount) :
      this(threadCount) {
      this.threadName = name;
    }

    /// <summary>Immediately releases all resources owned by the instance</summary>
    public void Dispose() {
      if(this.threadTerminatedEvent != null) {
        CancelPendingTasks();
        CancelRunningTasks();

        Wait(DefaultDisposeTimeoutMilliseconds);

        this.threadTerminatedEvent.Dispose();
        this.threadTerminatedEvent = null;
      }
      lock(this.queueSynchronizationRoot) {
        if(this.cancellationTokenSource != null) {
          this.cancellationTokenSource.Dispose();
          this.cancellationTokenSource = null;
        }
      }
    }

    /// <summary>Adds a task for processing by the background worker threads</summary>
    /// <param name="task">Task that will be processed in the background</param>
    public void AddTask(TTask task) {
      if(this.cancellationTokenSource.IsCancellationRequested) {
        return;
      }

      bool needNewThread;

      lock(this.queueSynchronizationRoot) {
        this.tasks.Enqueue(task);

        needNewThread = (this.runningThreadCount < this.threadCount);
        if(needNewThread) {
          ++this.runningThreadCount;
        }
      }

      if(needNewThread) {
        Task newThread = new Task(
          this.runQueuedTasksInThreadDelegate,
          // this.cancellationTokenSource.Token, // DO NOT PASS THIS!
          // Passing the cancellation token makes tasks that have been queued but
          // not started yet cease to execute at all - meaning our runningThreadCount
          // goes out of sync and Dispose() / Wait() / Join() sit around endlessly!
          TaskCreationOptions.LongRunning
        );
        newThread.Start();
      }
    }

    /// <summary>Cancels all tasks that are currently executing</summary>
    /// <remarks>
    ///   It is valid to call this method after Dispose()
    /// </remarks>
    public void CancelRunningTasks() {
      lock(this.queueSynchronizationRoot) {
        if(this.cancellationTokenSource != null) {
          this.cancellationTokenSource.Cancel();
        }
      }
    }

    /// <summary>Cancels all queued tasks waiting to be executed</summary>
    /// <remarks>
    ///   It is valid to call this method after Dispose()
    /// </remarks>
    public void CancelPendingTasks() {
      lock(this.queueSynchronizationRoot) {
        this.tasks.Clear();
      }
    }

    /// <summary>
    ///   Waits until all executing and queued tasks have been processed and throws an
    ///   exception if any errors have occurred
    /// </summary>
    public void Join() {
      while(Thread.VolatileRead(ref this.runningThreadCount) > 0) {
        this.threadTerminatedEvent.WaitOne();
      }

      if(this.exceptions.Count > 0) {
        throw new AggregateException(this.exceptions);
      }
    }

    /// <summary>
    ///   Waits until all executing and queued tasks have been processed or
    ///   the timeout is reached
    /// </summary>
    /// <param name="timeoutMilliseconds">Milliseconds after which the wait times out</param>
    /// <returns>
    ///   True if all tasks have been processed, false if the timeout was reached
    /// </returns>
    public bool Wait(int timeoutMilliseconds) {

      // Wait until the task queue has become empty
      while(queuedTaskCount > 0) {
        if(this.threadTerminatedEvent.WaitOne(timeoutMilliseconds) == false) {
          return false;
        }
      }

      // Now wait until all running tasks have finished
      while(Thread.VolatileRead(ref this.runningThreadCount) > 0) {
        if(this.threadTerminatedEvent.WaitOne(timeoutMilliseconds) == false) {
          return false;
        }
      }

      return true;

    }

    /// <summary>Called in a thread to execute a single task</summary>
    /// <param name="task">Task that should be executed</param>
    /// <param name="cancellationToken">
    ///   Cancellation token through which the method can be signalled to cancel
    /// </param>
    protected abstract void Run(TTask task, CancellationToken cancellationToken);

    /// <summary>
    ///   Runs queued tasks of the parallel background worker until the queue is empty
    /// </summary>
    private void runQueuedTasksInThread() {
      string previousThreadName = null;
      if(!string.IsNullOrEmpty(this.threadName)) {
        previousThreadName = Thread.CurrentThread.Name;
        Thread.CurrentThread.Name = this.threadName;
      }
      try {
        for(;;) {
          TTask task;
          lock(this.queueSynchronizationRoot) {
            if(this.tasks.Count == 0) {
              --this.runningThreadCount;
              break;
            }
            task = this.tasks.Dequeue();
          }

          try {
            Run(task, this.cancellationTokenSource.Token);
          }
          catch(Exception exception) {
            this.exceptions.Add(exception);
          }
        }

        this.threadTerminatedEvent.Set();
      }
      finally {
        if(!string.IsNullOrEmpty(this.threadName)) {
          Thread.CurrentThread.Name = previousThreadName;
        }
      }
    }

    /// <summary>Number of task still waiting to be executed</summary>
    private int queuedTaskCount {
      get {
        lock(this.queueSynchronizationRoot) {
          return this.tasks.Count;
        }
      }
    }

    /// <summary>
    ///   Name that will be assigned to the worker threads while they're processing tasks
    ///   for the parallel background worker
    /// </summary>
    private string threadName;
    /// <summary>Number of threads to use simultaneously</summary>
    private int threadCount;

    /// <summary>Synchronization root used to access the task queue and thread list</summary>
    /// <remarks>
    ///   <para>
    ///     Both lists are intentionally using a shared synchronization root because this will
    ///     guarantee that there's no race condition between adding work and a thread finishing:
    ///   </para>
    ///   <para>
    ///     Main thread
    ///     <code>
    ///       lock(this.queueSynchronizationRoot) {
    ///         this.tasks.Add(newTask);
    ///         
    ///         if(this.runningThreads.Count &lt; maximumThreadCount) {
    ///           startAdditionalTaskThread();
    ///         }
    ///       }
    ///     </code>
    ///   </para>
    ///   <para>
    ///     Task thread
    ///     <code>
    ///       TTask myTask;
    ///       lock(this.queueSynchronizationRoot) {
    ///         this.tasks.TryGet(out myTask);
    ///         
    ///         // RACE CONDITION WITHOUT LOCK: if this wasn't inside the same lock as the task
    ///         // checking, the main thread might check at this point, see that there are already
    ///         // enough threads running!
    ///         if(myTask == null) {
    ///           this.runningThreads.Remove(thisThread);
    ///           return;
    ///         }
    ///       }
    ///     </code>
    ///   </para>
    ///   <para>
    ///     The race condition mentioned above could completely stall the background worker by
    ///     leaving tasks in the queue but no threads active to execute them, so it is vital to
    ///     use this method of synchronization!
    ///   </para>
    /// </remarks>
    private object queueSynchronizationRoot;

    /// <summary>Delegate for the runQueuedTasksInThread() method</summary>
    private Action runQueuedTasksInThreadDelegate;
    /// <summary>Tasks remaining to be processed</summary>
    private Queue<TTask> tasks;
    /// <summary>Threads that are currently executing tasks</summary>
    private int runningThreadCount;

    /// <summary>Exceptions that have occurred while executing tasks</summary>
    private ConcurrentBag<Exception> exceptions;
    /// <summary>Event that is triggered whenever a task gets completed</summary>
    private AutoResetEvent threadTerminatedEvent;

    /// <summary>
    ///   Provides a cancellation token that can be used to signal a thread to terminate
    /// </summary>
    private CancellationTokenSource cancellationTokenSource;

  }

} // namespace Nuclex.Support.Threading

#endif // !NO_CONCURRENT_COLLECTIONS