//
//  Engine.swift
//  ChessKitEngine
//

import ChessKitEngineCore

public final class Engine: Sendable {

  // MARK: Public properties

  /// The type of the engine.
  public let type: EngineType

  /// Whether the engine is currently running.
  ///
  /// - To start the engine, call ``Engine/start(coreCount:multipv:)``.
  /// - To stop the engine, call ``Engine/stop()``.
  ///
  /// Engine must be running for ``Engine/send(command:)`` to work.
  public var isRunning: Bool {
    get async { await engineConfigurationActor.isRunning }
  }

  /// Whether logging should be enabled.
  ///
  /// If set to `true`, engine commands and responses
  /// will be logged to the console. The default value is
  /// `false`.
  ///
  ///  Can be set via ``Engine/set(loggingEnabled:)`` function.
  public var loggingEnabled: Bool {
    get async { await engineConfigurationActor.loggingEnabled }
  }

  /// an AsyncStream that is called when engine responses are received.
  ///
  /// The underlying value ``EngineResponse`` contains the engine
  /// response corresponding to the UCI protocol.
  public var responseStream: AsyncStream<EngineResponse>? {
    get async { await engineConfigurationActor.asyncStream }
  }

  // MARK: Private properties

  /// Actor used to hold mutating data in a thread safe environment.
  private let engineConfigurationActor: EngineConfiguration

  /// Messenger used to communicate with engine.
  private let messenger: EngineMessenger

  /// Queue used to synchronously send commands to engine.
  private let commandQueue = DispatchQueue(
    label: "ck-engine-command-queue",
    qos: .userInitiated
  )

  /// Queue used to synchronously log engine commands and responses.
  private let logQueue = DispatchQueue(
    label: "ck-engine-log-queue",
    qos: .utility
  )

  // MARK: Life cycle

  /// Initializes an engine with the provided ``EngineType`` and optional logging enabled flag.
  ///
  /// - parameter type: The type of engine to use.
  /// - parameter loggingEnabled: If set to `true`, engine commands and responses
  ///   will be logged to the console. The default value is `false`.
  public init(type: EngineType, loggingEnabled: Bool = false) {
    self.type = type
    messenger = EngineMessenger(engineType: type.objc)
    engineConfigurationActor = EngineConfiguration(loggingEnabled: loggingEnabled)
  }

  deinit {
    Task { [commandQueue, engineConfigurationActor, messenger] in
      // actions from `stop()` repeated here to avoid capturing `self` inside `deinit`
      guard await engineConfigurationActor.isRunning else { return }

      commandQueue.sync {
        messenger.sendCommand(EngineCommand.stop.rawValue)
        messenger.sendCommand(EngineCommand.quit.rawValue)
      }
      messenger.stop()

      await engineConfigurationActor.clearAsyncStream()
      await engineConfigurationActor.set(isRunning: false)
      await engineConfigurationActor.set(initialSetupComplete: false)
    }
  }

  // MARK: Public functions

  /// Starts the engine.
  ///
  /// You must call this function and wait for ``EngineResponse/readyok``
  /// before you can ask the engine to perform any work.
  ///
  /// - parameter coreCount: The number of processor cores to use for engine
  /// calculation. The default value is `nil` which uses one less than
  /// the number of cores available on the device.
  /// - parameter multipv: The number of lines the engine should return,
  /// sent via the `"MultiPV"` UCI option.
  ///
  public func start(coreCount: Int? = nil, multipv: Int = 1) async {
    // Setup async stream response if not already set.
    await engineConfigurationActor.setAsyncStream()

    setMessengerResponseHandler(coreCount: coreCount, multipv: multipv)
    messenger.start()

    // start engine setup loop
    await send(command: .uci)
  }

  /// Stops the engine.
  ///
  /// Call this to stop all engine calculation and clean up.
  /// After calling ``Engine/stop()``, ``Engine/start(coreCount:multipv:)`` must be called before
  /// sending any more commands with ``Engine/send(command:)``.
  public func stop() async {
    guard await isRunning else { return }

    await send(command: .stop)
    await send(command: .quit)
    messenger.stop()

    await engineConfigurationActor.clearAsyncStream()
    await engineConfigurationActor.set(isRunning: false)
    await engineConfigurationActor.set(initialSetupComplete: false)
  }

  /// Sends a command to the engine.
  ///
  /// - parameter command: The command to send.
  ///
  /// Commands must be of type ``EngineCommand`` to ensure
  /// validity.
  ///
  /// Any responses will be returned via ``Engine/responseStream``.
  public func send(command: EngineCommand) async {
    guard await isRunning || [.uci, .isready].contains(command) else {
      await log("Engine is not running, call start() first.")
      return
    }

    await log(command.rawValue)

    commandQueue.sync {
      messenger.sendCommand(command.rawValue)
    }
  }

  /// Enable printing logs to console.
  ///
  /// - parameter loggingEnabled: If set to `true`, engine commands and responses
  ///   will be logged to the console. The default value is `false`.
  ///
  @available(*, deprecated, renamed: "set(loggingEnabled:)")
  public func setLoggingEnabled(_ loggingEnabled: Bool) {
    Task {
      await set(loggingEnabled: loggingEnabled)
    }
  }

  /// Enable printing logs to console.
  ///
  /// - parameter loggingEnabled: If set to `true`, engine commands and responses
  ///   will be logged to the console. The default value is `false`.
  ///
  public func set(loggingEnabled: Bool) async {
    await engineConfigurationActor.set(loggingEnabled: loggingEnabled)
  }

  // MARK: Private functions

  /// Logs `message` if ``Engine/loggingEnabled`` is `true`.
  private func log(_ message: String) async {
    if await loggingEnabled {
      logQueue.sync {
        Logging.print(message)
      }
    }
  }

  /// convinience function to set up `messenger.responseHandler`
  private func setMessengerResponseHandler(
    coreCount: Int? = nil,
    multipv: Int = 1
  ) {
    messenger.responseHandler = { [weak self] response in
      Task { [weak self] in
        guard let self,
          let parsed = EngineResponse(rawValue: response)
        else {
          if !response.isEmpty {
            await self?.log(response)
          }
          return
        }

        await self.log(parsed.rawValue)

        if await !self.isRunning {
          if parsed == .readyok {
            await self.performInitialSetup(
              coreCount: coreCount ?? min(1, ProcessInfo.processInfo.processorCount - 1),
              multipv: multipv
            )
          } else if let next = EngineCommand.nextSetupLoopCommand(given: parsed) {
            await self.send(command: next)
          }
        }
        await self.engineConfigurationActor.streamContinuation?.yield(parsed)
      }
    }
  }

  /// Sets initial engine options.
  private func performInitialSetup(coreCount: Int, multipv: Int) async {
    guard await !engineConfigurationActor.initialSetupComplete else { return }

    await engineConfigurationActor.set(isRunning: true)

    // configure engine-specific options
    for command in type.setupCommands {
      await send(command: command)
    }

    // configure common engine options
    await send(
      command: .setoption(
        id: "Threads",
        value: "\(max(coreCount - 1, 1))"
      ))

    await send(command: .setoption(id: "MultiPV", value: "\(multipv)"))
    await engineConfigurationActor.set(initialSetupComplete: true)
  }

}

// MARK: - EngineConfiguration actor

/// An actor to hold the configuration for the engine class.
/// Since ``Engine`` conforms to `Sendable` protocol, we need to
/// move the mutable data into async safe environment.
fileprivate actor EngineConfiguration: Sendable {
  /// Whether the engine is currently running.
  private(set) var isRunning = false

  /// Whether logging should be enabled.
  private(set) var loggingEnabled = false

  /// Whether the initial engine setup was completed
  private(set) var initialSetupComplete = false

  /// An async stream to notify the end user about engine responses
  private(set) var asyncStream: AsyncStream<EngineResponse>?

  /// A reference to AsyncStream's continuation for later access by `EngineMessenger.responseHandler`
  private(set) var streamContinuation: AsyncStream<EngineResponse>.Continuation?

  init(loggingEnabled: Bool = false) {
    self.loggingEnabled = loggingEnabled

    Task {
      await setAsyncStream()
    }
  }

  func set(loggingEnabled: Bool) async {
    self.loggingEnabled = loggingEnabled
  }

  func set(initialSetupComplete: Bool) async {
    self.initialSetupComplete = initialSetupComplete
  }

  func set(isRunning: Bool) async {
    self.isRunning = isRunning
  }

  func setAsyncStream() async {
    guard asyncStream == nil else { return }

    asyncStream = AsyncStream { continuation in
      Task {
        await set(streamContinuation: continuation)
      }
    }
  }

  func clearAsyncStream() async {
    asyncStream = nil
    streamContinuation = nil
  }

  private func set(streamContinuation: AsyncStream<EngineResponse>.Continuation?) async {
    self.streamContinuation = streamContinuation
  }
}