using System;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;

namespace Swan.Threading {
  /// <summary>
  /// Represents a class that implements delay logic for thread workers.
  /// </summary>
  public static class WorkerDelayProvider {
    /// <summary>
    /// Gets the default delay provider.
    /// </summary>
    public static IWorkerDelayProvider Default => TokenTimeout;

    /// <summary>
    /// Provides a delay implementation which simply waits on the task and cancels on
    /// the cancellation token.
    /// </summary>
    public static IWorkerDelayProvider Token => new TokenCancellableDelay();

    /// <summary>
    /// Provides a delay implementation which waits on the task and cancels on both,
    /// the cancellation token and a wanted delay timeout.
    /// </summary>
    public static IWorkerDelayProvider TokenTimeout => new TokenTimeoutCancellableDelay();

    /// <summary>
    /// Provides a delay implementation which uses short sleep intervals of 5ms.
    /// </summary>
    public static IWorkerDelayProvider TokenSleep => new TokenSleepDelay();

    /// <summary>
    /// Provides a delay implementation which uses short delay intervals of 5ms and
    /// a wait on the delay task in the final loop.
    /// </summary>
    public static IWorkerDelayProvider SteppedToken => new SteppedTokenDelay();

    private class TokenCancellableDelay : IWorkerDelayProvider {
      public void ExecuteCycleDelay(Int32 wantedDelay, Task delayTask, CancellationToken token) {
        if(wantedDelay == 0 || wantedDelay < -1) {
          return;
        }

        // for wanted delays of less than 30ms it is not worth
        // passing a timeout or a token as it only adds unnecessary
        // overhead.
        if(wantedDelay <= 30) {
          try {
            delayTask.Wait(token);
          } catch { /* ignore */ }
          return;
        }

        // only wait on the cancellation token
        // or until the task completes normally
        try {
          delayTask.Wait(token);
        } catch { /* ignore */ }
      }
    }

    private class TokenTimeoutCancellableDelay : IWorkerDelayProvider {
      public void ExecuteCycleDelay(Int32 wantedDelay, Task delayTask, CancellationToken token) {
        if(wantedDelay == 0 || wantedDelay < -1) {
          return;
        }

        // for wanted delays of less than 30ms it is not worth
        // passing a timeout or a token as it only adds unnecessary
        // overhead.
        if(wantedDelay <= 30) {
          try {
            delayTask.Wait(token);
          } catch { /* ignore */ }
          return;
        }

        try {
          _ = delayTask.Wait(wantedDelay, token);
        } catch { /* ignore */ }
      }
    }

    private class TokenSleepDelay : IWorkerDelayProvider {
      private readonly Stopwatch _elapsedWait = new Stopwatch();

      public void ExecuteCycleDelay(Int32 wantedDelay, Task delayTask, CancellationToken token) {
        this._elapsedWait.Restart();

        if(wantedDelay == 0 || wantedDelay < -1) {
          return;
        }

        while(!token.IsCancellationRequested) {
          Thread.Sleep(5);

          if(wantedDelay != Timeout.Infinite && this._elapsedWait.ElapsedMilliseconds >= wantedDelay) {
            break;
          }
        }
      }
    }

    private class SteppedTokenDelay : IWorkerDelayProvider {
      private const Int32 StepMilliseconds = 15;
      private readonly Stopwatch _elapsedWait = new Stopwatch();

      public void ExecuteCycleDelay(Int32 wantedDelay, Task delayTask, CancellationToken token) {
        this._elapsedWait.Restart();

        if(wantedDelay == 0 || wantedDelay < -1) {
          return;
        }

        if(wantedDelay == Timeout.Infinite) {
          try {
            _ = delayTask.Wait(wantedDelay, token);
          } catch { /* Ignore cancelled tasks */ }
          return;
        }

        while(!token.IsCancellationRequested) {
          Int32 remainingWaitTime = wantedDelay - Convert.ToInt32(this._elapsedWait.ElapsedMilliseconds);

          // Exit for no remaining wait time
          if(remainingWaitTime <= 0) {
            break;
          }

          if(remainingWaitTime >= StepMilliseconds) {
            Task.Delay(StepMilliseconds, token).Wait(token);
          } else {
            try {
              _ = delayTask.Wait(remainingWaitTime);
            } catch { /* ignore cancellation of task exception */ }
          }

          if(this._elapsedWait.ElapsedMilliseconds >= wantedDelay) {
            break;
          }
        }
      }
    }
  }
}