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

namespace ZeroLevel.Services.Async.Internal
{
	internal struct CompletionSourceAwaiterFactory<T> 
		: IAwaiterFactory<T>, IEquatable<CompletionSourceAwaiterFactory<T>>
	{
		private readonly CancellationToken _cancellationToken;

		public CompletionSourceAwaiterFactory(CancellationToken cancellationToken)
		{
			_cancellationToken = cancellationToken;
		}

		public IAwaiter<T> CreateAwaiter() => new CompletionSourceAwaiter(_cancellationToken);

		/// <summary>
		/// A simple <see cref="TaskCompletionSource{T}"/> wrapper that implements <see cref="IAwaiter{T}"/>.
		/// </summary>
		private class CompletionSourceAwaiter : IAwaiter<T>
		{
			private readonly TaskCompletionSource<T> _completionSource;
			private readonly CancellationTokenRegistration _registration;

			public CompletionSourceAwaiter(CancellationToken cancellationToken)
			{
				_completionSource = new TaskCompletionSource<T>();
				Task = new ValueTask<T>(_completionSource.Task.WithYield());

				_registration = cancellationToken.Register(
					state =>
					{
						TaskCompletionSource<T> awaiter = state as TaskCompletionSource<T>;
						awaiter.TrySetCanceled();
					},
					_completionSource,
					useSynchronizationContext: false);
			}

			public bool TrySetResult(T result)
			{
				_registration.Dispose();
				return _completionSource.TrySetResult(result);
			}

			public ValueTask<T> Task { get; }
		}

		#region IEquatable<CompletionSourceAwaiterFactory<T>>

		public override int GetHashCode() => EqualityComparer<CancellationToken>.Default.GetHashCode(_cancellationToken);
		public bool Equals(CompletionSourceAwaiterFactory<T> other) => _cancellationToken == other._cancellationToken;
		public override bool Equals(object obj) => obj is CompletionSourceAwaiterFactory<T> && Equals((CompletionSourceAwaiterFactory<T>)obj);

		public static bool operator ==(CompletionSourceAwaiterFactory<T> x, CompletionSourceAwaiterFactory<T> y) => x.Equals(y);
		public static bool operator !=(CompletionSourceAwaiterFactory<T> x, CompletionSourceAwaiterFactory<T> y) => !x.Equals(y);

		#endregion
	}
}