// Copyright 2024-present 650 Industries. All rights reserved.
import BackgroundTasks

public class BackgroundTaskScheduler {
  /**
   * Keep track of number of registered task consumers
   */
  static var numberOfRegisteredTasksOfThisType: Int = 0

  /**
   * Interval for task scheduler. The iOS BGTaskScheduler does not guarantee that the number of minutes will be
   * exact, but it indicates when we'd like the task to start. This will be set to at least 12 hours
   */
  private static var intervalSeconds: TimeInterval = 12 * 60 * 60

  /**
   A one-time async gate that becomes ready after BGTaskScheduler registration finishes.
   Multiple awaiters will be resumed when ready; subsequent awaiters return immediately.
   */
  private actor RegistrationGate {
    private var ready: Bool = false
    private var waiters: [CheckedContinuation<Void, Never>] = []

    func awaitReady() async {
      if ready { return }
      await withCheckedContinuation { (continuation: CheckedContinuation<Void, Never>) in
        if ready {
          continuation.resume()
        } else {
          waiters.append(continuation)
        }
      }
    }

    func signalReady() {
      guard !ready else { return }
      ready = true
      let continuations = waiters
      waiters.removeAll()
      for c in continuations {
        c.resume()
      }
    }
  }

  /**
   Registration gate that will only be signaled when the bgTaskSchedulerDidFinishRegister is called.
   */
  private static let registrationGate = RegistrationGate()

  /**
   Call from the BackgroundTaskAppDelegate after the call to BGTaskScheduler.shared.register has finished
   so that we can hold back any tryScheduleWorker calls, especially the call to BGTaskScheduler.shared.submit
   that will fail if called before the BGTaskScheduler has successfully registered its handler.
   */
  public static func bgTaskSchedulerDidFinishRegister() {
    Task {
      await registrationGate.signalReady()
    }
  }

  /**
   * Call when a task is registered to keep track of how many background task consumers we have
   */
  public static func didRegisterTask(minutes: Int?) {
    if let minutes = minutes {
      intervalSeconds = Double(minutes) * 60
    }
    numberOfRegisteredTasksOfThisType += 1

    if numberOfRegisteredTasksOfThisType == 1 {
      Task {
        try await tryScheduleWorker()
      }
    }
  }

  /**
   * Call when a task is unregistered to keep track of how many background task consumers we have
   */
  public static func didUnregisterTask() {
    numberOfRegisteredTasksOfThisType -= 1
    if numberOfRegisteredTasksOfThisType == 0 {
      Task {
        await stopWorker()
      }
    }
  }

  /**
   * Tries to schedule the worker task to run
   */
  public static func tryScheduleWorker() async throws {
    if numberOfRegisteredTasksOfThisType == 0 {
      print("Background Task: skipping scheduling. No registered tasks")
      return
    }

    // Wait until BGTaskScheduler registration has completed.
    await registrationGate.awaitReady()

    // Stop existing tasks
    await stopWorker()

    // Create request
    let request = BGProcessingTaskRequest(identifier: BackgroundTaskConstants.BackgroundWorkerIdentifier)

    // We'll require network but accept running on battery power.
    request.requiresNetworkConnectivity = true
    request.requiresExternalPower = false

    // Set up mimimum start date
    request.earliestBeginDate = Date().addingTimeInterval(intervalSeconds)

    do {
      try BGTaskScheduler.shared.submit(request)
    } catch let error as BGTaskScheduler.Error {
      switch error.code {
      case .unavailable:
        throw CouldNotRegisterWorkerTask("Background task scheduling is unavailable.")
      case .tooManyPendingTaskRequests:
        throw CouldNotRegisterWorkerTask("Too many pending task requests.")
      case .notPermitted:
        throw CouldNotRegisterWorkerTask("Task request not permitted.")
      @unknown default:
        print("An unknown BGTaskScheduler error occurred.")
        // Handle any future cases added by Apple
      }
    } catch {
      // All other errors
      throw CouldNotRegisterWorkerTask("Unknown error occurred.")
    }
  }

  /**
   Cancels the worker task
   */
  public static func stopWorker() async {
    BGTaskScheduler.shared.cancel(taskRequestWithIdentifier: BackgroundTaskConstants.BackgroundWorkerIdentifier)
  }

  /**
   Returns true if the worker task is pending
   */
  public static func isWorkerRunning() async -> Bool {
    let requests = await BGTaskScheduler.shared.pendingTaskRequests()
    return requests.contains(where: { $0.identifier == BackgroundTaskConstants.BackgroundWorkerIdentifier })
  }

  /**
   Returns true if we're on a device that supports background tasks
   */
  public static func supportsBackgroundTasks() -> Bool {
#if targetEnvironment(simulator)
    // If we're on emulator we should definetly return restricted
    return false
#else
    return true
#endif
  }
}