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

namespace MaddoShared
{
    public static class ThreadingHelper
    {
        /// <summary>
        /// Starts the given tasks and waits for them to complete. This will run, at most, the specified number of tasks in parallel.
        /// <para>NOTE: If one of the given tasks has already been started, an exception will be thrown.</para>
        /// </summary>
        /// <param name="tasksToRun">The tasks to run.</param>
        /// <param name="maxTasksToRunInParallel">The maximum number of tasks to run in parallel.</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        public static void StartAndWaitAllThrottled(IEnumerable<Task> tasksToRun, int maxTasksToRunInParallel, CancellationToken cancellationToken = new CancellationToken())
        {
            StartAndWaitAllThrottled(tasksToRun, maxTasksToRunInParallel, -1, cancellationToken);
        }

        /// <summary>
        /// Starts the given tasks and waits for them to complete. This will run, at most, the specified number of tasks in parallel.
        /// <para>NOTE: If one of the given tasks has already been started, an exception will be thrown.</para>
        /// </summary>
        /// <param name="tasksToRun">The tasks to run.</param>
        /// <param name="maxTasksToRunInParallel">The maximum number of tasks to run in parallel.</param>
        /// <param name="timeoutInMilliseconds">The maximum milliseconds we should allow the max tasks to run in parallel before allowing another task to start. Specify -1 to wait indefinitely.</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        public static void StartAndWaitAllThrottled(IEnumerable<Task> tasksToRun, int maxTasksToRunInParallel, int timeoutInMilliseconds, CancellationToken cancellationToken = new CancellationToken())
        {
            // Convert to a list of tasks so that we don&#39;t enumerate over it multiple times needlessly.
            var tasks = tasksToRun.ToList();

            using (var throttler = new SemaphoreSlim(maxTasksToRunInParallel))
            {
                var postTaskTasks = new List<Task>();

                // Have each task notify the throttler when it completes so that it decrements the number of tasks currently running.
                tasks.ForEach(t => postTaskTasks.Add(t.ContinueWith(tsk => throttler.Release())));

                // Start running each task.
                foreach (var task in tasks)
                {
                    // Increment the number of tasks currently running and wait if too many are running.
                    throttler.Wait(timeoutInMilliseconds, cancellationToken);

                    cancellationToken.ThrowIfCancellationRequested();
                    task.Start();
                }

                // Wait for all of the provided tasks to complete.
                // We wait on the list of "post" tasks instead of the original tasks, otherwise there is a potential race condition where the throttler&#39;s using block is exited before some Tasks have had their "post" action completed, which references the throttler, resulting in an exception due to accessing a disposed object.
                Task.WaitAll(postTaskTasks.ToArray(), cancellationToken);
            }
        }
    }
}
