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

namespace System.Extensions
{
    public static class TaskExtensions
    {
        /// <summary>
        /// Mark task as <see cref="TaskCreationOptions.LongRunning"/>.
        /// </summary>
        /// <typeparam name="T">Result type.</typeparam>
        /// <param name="task">Task.</param>
        /// <returns>The started <see cref="Task{TResult}"/></returns>
        public static Task<T> LongRunning<T>(this Task<T> task)
        {
            return Task.Factory.StartNew(async () =>
            {
                return await task;
            }, TaskCreationOptions.LongRunning).Unwrap();
        }

        /// <summary>
        /// Mark task as <see cref="TaskCreationOptions.LongRunning"/>.
        /// </summary>
        /// <param name="task">Task.</param>
        /// <returns>The started <see cref="Task"/></returns>
        public static Task LongRunning(this Task task)
        {
            return Task.Factory.StartNew(async () =>
            {
                await task;
            }, TaskCreationOptions.LongRunning).Unwrap();
        }

        /// <summary>
        /// Execute action periodically and asynchronously.
        /// </summary>
        /// <param name="onTick">Action to be executed.</param>
        /// <param name="dueTime">Time to pass before starting to execute.</param>
        /// <param name="interval">Interval between procs.</param>
        /// <param name="token">Cancellation token used to cancel the cycle.</param>
        /// <returns></returns>
        public static async Task RunPeriodicAsync(this Action onTick, TimeSpan dueTime, TimeSpan interval, CancellationToken token)
        {
            if (dueTime > TimeSpan.Zero)
            {
                await Task.Delay(dueTime, token).ConfigureAwait(false);
            }

            while (!token.IsCancellationRequested)
            {
                onTick?.Invoke();
                if (interval > TimeSpan.Zero)
                {
                    await Task.Delay(interval, token).ConfigureAwait(false);
                }
            }
        }

        /// <summary>
        /// Execute's an async Task<T> method which has a void return value synchronously
        /// </summary>
        /// <param name="task">Task<T> method to execute</param>
        public static void RunSync(this Func<Task> task)
        {
            var oldContext = SynchronizationContext.Current;
            var synch = new ExclusiveSynchronizationContext();
            SynchronizationContext.SetSynchronizationContext(synch);
            synch.Post(async _ =>
            {
                try
                {
                    await task();
                }
                catch (Exception e)
                {
                    synch.InnerException = e;
                    throw;
                }
                finally
                {
                    synch.EndMessageLoop();
                }
            }, null);
            synch.BeginMessageLoop();

            SynchronizationContext.SetSynchronizationContext(oldContext);
        }

        /// <summary>
        /// Execute's an async Task<T> method which has a T return type synchronously
        /// </summary>
        /// <typeparam name="T">Return Type</typeparam>
        /// <param name="task">Task<T> method to execute</param>
        /// <returns></returns>
        public static T RunSync<T>(this Func<Task<T>> task)
        {
            var oldContext = SynchronizationContext.Current;
            var synch = new ExclusiveSynchronizationContext();
            SynchronizationContext.SetSynchronizationContext(synch);
            var ret = default(T);
            synch.Post(async _ =>
            {
                try
                {
                    ret = await task();
                }
                catch (Exception e)
                {
                    synch.InnerException = e;
                    throw;
                }
                finally
                {
                    synch.EndMessageLoop();
                }
            }, null);
            synch.BeginMessageLoop();
            SynchronizationContext.SetSynchronizationContext(oldContext);
            return ret;
        }
        
        private class ExclusiveSynchronizationContext : SynchronizationContext
        {
            private bool done;
            public Exception InnerException { get; set; }
            readonly AutoResetEvent workItemsWaiting = new AutoResetEvent(false);
            readonly Queue<Tuple<SendOrPostCallback, object>> items =
                new Queue<Tuple<SendOrPostCallback, object>>();

            public override void Send(SendOrPostCallback d, object state)
            {
                throw new NotSupportedException("We cannot send to our same thread");
            }

            public override void Post(SendOrPostCallback d, object state)
            {
                lock (items)
                {
                    items.Enqueue(Tuple.Create(d, state));
                }
                workItemsWaiting.Set();
            }

            public void EndMessageLoop()
            {
                Post(_ => done = true, null);
            }

            public void BeginMessageLoop()
            {
                while (!done)
                {
                    Tuple<SendOrPostCallback, object> task = null;
                    lock (items)
                    {
                        if (items.Count > 0)
                        {
                            task = items.Dequeue();
                        }
                    }
                    if (task != null)
                    {
                        task.Item1(task.Item2);
                        if (InnerException != null) // the method threw an exeption
                        {
                            throw new AggregateException("AsyncHelpers.Run method threw an exception.", InnerException);
                        }
                    }
                    else
                    {
                        workItemsWaiting.WaitOne();
                    }
                }
            }

            public override SynchronizationContext CreateCopy()
            {
                return this;
            }
        }
    }
}