//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2020 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

import Swift

#if !SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY
@available(SwiftStdlib 5.7, *)
@_unavailableInEmbedded
extension Task where Success == Never, Failure == Never {
  @available(SwiftStdlib 5.7, *)
  internal static func _sleep(
    until seconds: Int64, _ nanoseconds: Int64,
    tolerance: Duration?,
    clock: _ClockID
  ) async throws {
    // Create a token which will initially have the value "not started", which
    // means the continuation has neither been created nor completed.
    let token = unsafe UnsafeSleepStateToken()

    do {
      // Install a cancellation handler to resume the continuation by
      // throwing CancellationError.
      try await withTaskCancellationHandler {
        let _: () = try unsafe await withUnsafeThrowingContinuation { continuation in
          while true {
            let state = unsafe token.load()
            switch unsafe state {
            case .notStarted:
              // Try to swap in the continuation word.
              let newState = unsafe SleepState.activeContinuation(continuation)
              if unsafe !token.exchange(expected: state, desired: newState) {
                // Keep trying!
                continue
              }

              // Create a task that resumes the continuation normally if it
              // finishes first. Enqueue it directly with the delay, so it fires
              // when we're done sleeping.
              let sleepTaskFlags = taskCreateFlags(
                priority: nil, isChildTask: false, copyTaskLocals: false,
                inheritContext: false, enqueueJob: false,
                addPendingGroupTaskUnconditionally: false,
                isDiscardingTask: false, isSynchronousStart: false)
              let (sleepTask, _) = Builtin.createAsyncTask(sleepTaskFlags) {
                unsafe onSleepWake(token)
              }
              let toleranceSeconds: Int64
              let toleranceNanoseconds: Int64
              if let components = tolerance?.components {
                toleranceSeconds = components.seconds
                toleranceNanoseconds = components.attoseconds / 1_000_000_000
              } else {
                toleranceSeconds = 0
                toleranceNanoseconds = -1
              }

              _enqueueJobGlobalWithDeadline(
                  seconds, nanoseconds,
                  toleranceSeconds, toleranceNanoseconds,
                  clock.rawValue, Builtin.convertTaskToJob(sleepTask))
              return

            case .activeContinuation, .finished:
              fatalError("Impossible to have multiple active continuations")

            case .cancelled:
              fatalError("Impossible to have cancelled before we began")

            case .cancelledBeforeStarted:
              // Finish the continuation normally. We'll throw later, after
              // we clean up.
              unsafe continuation.resume()
              return
          }
        }
        }
      } onCancel: {
        unsafe onSleepCancel(token)
      }

      // Determine whether we got cancelled before we even started.
      let cancelledBeforeStarted: Bool
      switch unsafe token.load() {
      case .notStarted, .activeContinuation, .cancelled:
        fatalError("Invalid state for non-cancelled sleep task")

      case .cancelledBeforeStarted:
        cancelledBeforeStarted = true

      case .finished:
        cancelledBeforeStarted = false
      }

      // We got here without being cancelled, so deallocate the storage for
      // the flag word and continuation.
      unsafe token.deallocate()

      // If we got cancelled before we even started, through the cancellation
      // error now.
      if cancelledBeforeStarted {
        throw _Concurrency.CancellationError()
      }
    } catch {
      // The task was cancelled; propagate the error. The "on wake" task is
      // responsible for deallocating the flag word and continuation, if it's
      // still running.
      throw error
    }
  }

  /// Suspends the current task until the given deadline within a tolerance.
  ///
  /// If the task is canceled before the time ends, this function throws 
  /// `CancellationError`.
  ///
  /// This function doesn't block the underlying thread.
  ///
  ///       try await Task.sleep(until: .now + .seconds(3))
  ///
  @available(SwiftStdlib 5.7, *)
  public static func sleep<C: Clock>(
    until deadline: C.Instant,
    tolerance: C.Instant.Duration? = nil,
    clock: C = ContinuousClock()
  ) async throws {
    try await clock.sleep(until: deadline, tolerance: tolerance)
  }
  
  /// Suspends the current task for the given duration.
  ///
  /// If the task is cancelled before the time ends, this function throws 
  /// `CancellationError`.
  ///
  /// This function doesn't block the underlying thread.
  ///
  ///       try await Task.sleep(for: .seconds(3))
  ///
  @available(SwiftStdlib 5.7, *)
  @_alwaysEmitIntoClient
  public static func sleep<C: Clock>(
    for duration: C.Instant.Duration,
    tolerance: C.Instant.Duration? = nil,
    clock: C = ContinuousClock()
  ) async throws {
    try await clock.sleep(for: duration, tolerance: tolerance)
  }
}
#else
@available(SwiftStdlib 5.7, *)
@available(*, unavailable, message: "Unavailable in task-to-thread concurrency model")
extension Task where Success == Never, Failure == Never {
  @available(SwiftStdlib 5.7, *)
  @available(*, unavailable, message: "Unavailable in task-to-thread concurrency model")
  public static func sleep<C: Clock>(
    until deadline: C.Instant,
    tolerance: C.Instant.Duration? = nil,
    clock: C = ContinuousClock()
  ) async throws {
    fatalError("Unavailable in task-to-thread concurrency model")
  }
  @available(SwiftStdlib 5.7, *)
  @available(*, unavailable, message: "Unavailable in task-to-thread concurrency model")
  @_alwaysEmitIntoClient
  public static func sleep<C: Clock>(
    for duration: C.Instant.Duration,
    tolerance: C.Instant.Duration? = nil,
    clock: C = ContinuousClock()
  ) async throws {
    fatalError("Unavailable in task-to-thread concurrency model")
  }
}
#endif