// Copyright (c) Microsoft. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Threading;
using Libuv.Internal;

namespace Libuv
{
  public class LibuvAwaitable<TRequest> : ICriticalNotifyCompletion where TRequest : UvRequest
  {
    private static readonly Action _callbackCompleted = () => { };

    private Action _callback;

    private UvException _exception;

    private int _status;

    public static readonly Action<TRequest, int, UvException, object> Callback = (req, status, error, state) =>
    {
      var awaitable = (LibuvAwaitable<TRequest>)state;

      awaitable._exception = error;
      awaitable._status = status;

      var continuation = Interlocked.Exchange(ref awaitable._callback, _callbackCompleted);

      continuation?.Invoke();
    };

    public LibuvAwaitable<TRequest> GetAwaiter() => this;
    public bool IsCompleted => ReferenceEquals(_callback, _callbackCompleted);

    public UvWriteResult GetResult()
    {
      Debug.Assert(_callback == _callbackCompleted);

      var exception = _exception;
      var status = _status;

      // Reset the awaitable state
      _exception = null;
      _status = 0;
      _callback = null;

      return new UvWriteResult(status, exception);
    }

    public void OnCompleted(Action continuation)
    {
      // There should never be a race between IsCompleted and OnCompleted since both operations
      // should always be on the libuv thread
      if (ReferenceEquals(_callback, _callbackCompleted))
        Debug.Fail($"{typeof(LibuvAwaitable<TRequest>)}.{nameof(OnCompleted)} raced with {nameof(IsCompleted)}, scheduling callback.");

      _callback = continuation;
    }

    public void UnsafeOnCompleted(Action continuation)
    {
      OnCompleted(continuation);
    }
  }

  public struct UvWriteResult
  {
    public int Status { get; }
    public UvException Error { get; }

    public UvWriteResult(int status, UvException error)
    {
      Status = status;
      Error = error;
    }
  }
}