/*
 * Copyright (C) 2014-2021 Lightbend Inc. <https://www.lightbend.com>
 */

package akka.stream.impl

import org.reactivestreams.{ Processor, Subscriber, Subscription }

import akka.actor._
import akka.annotation.InternalApi
import akka.event.Logging
import akka.stream.{ AbruptTerminationException, Attributes }
import akka.stream.ActorAttributes
import akka.stream.impl.ActorSubscriberMessage.{ OnComplete, OnError, OnNext, OnSubscribe }
import akka.util.unused

/**
 * INTERNAL API
 */
@InternalApi private[akka] object ActorProcessor {

  def apply[I, O](impl: ActorRef): ActorProcessor[I, O] = {
    val p = new ActorProcessor[I, O](impl)
    // Resolve cyclic dependency with actor. This MUST be the first message no matter what.
    impl ! ExposedPublisher(p.asInstanceOf[ActorPublisher[Any]])
    p
  }
}

/**
 * INTERNAL API
 */
@InternalApi private[akka] class ActorProcessor[I, O](impl: ActorRef)
    extends ActorPublisher[O](impl)
    with Processor[I, O] {
  override def onSubscribe(s: Subscription): Unit = {
    ReactiveStreamsCompliance.requireNonNullSubscription(s)
    impl ! OnSubscribe(s)
  }
  override def onError(t: Throwable): Unit = {
    ReactiveStreamsCompliance.requireNonNullException(t)
    impl ! OnError(t)
  }
  override def onComplete(): Unit = impl ! OnComplete
  override def onNext(elem: I): Unit = {
    ReactiveStreamsCompliance.requireNonNullElement(elem)
    impl ! OnNext(elem)
  }
}

/**
 * INTERNAL API
 */
@InternalApi private[akka] abstract class BatchingInputBuffer(val size: Int, val pump: Pump)
    extends DefaultInputTransferStates {
  if (size < 1) throw new IllegalArgumentException(s"buffer size must be positive (was: $size)")
  if ((size & (size - 1)) != 0) throw new IllegalArgumentException(s"buffer size must be a power of two (was: $size)")

  // TODO: buffer and batch sizing heuristics
  private var upstream: Subscription = _
  private val inputBuffer = new Array[AnyRef](size)
  private var inputBufferElements = 0
  private var nextInputElementCursor = 0
  private var upstreamCompleted = false
  private val IndexMask = size - 1

  private def requestBatchSize = math.max(1, inputBuffer.length / 2)
  private var batchRemaining = requestBatchSize

  override def toString: String =
    s"BatchingInputBuffer(size=$size, elems=$inputBufferElements, completed=$upstreamCompleted, remaining=$batchRemaining)"

  override val subreceive: SubReceive = new SubReceive(waitingForUpstream)

  override def dequeueInputElement(): Any = {
    val elem = inputBuffer(nextInputElementCursor)
    inputBuffer(nextInputElementCursor) = null

    batchRemaining -= 1
    if (batchRemaining == 0 && !upstreamCompleted) {
      upstream.request(requestBatchSize)
      batchRemaining = requestBatchSize
    }

    inputBufferElements -= 1
    nextInputElementCursor += 1
    nextInputElementCursor &= IndexMask
    elem
  }

  protected final def enqueueInputElement(elem: Any): Unit = {
    if (isOpen) {
      if (inputBufferElements == size) throw new IllegalStateException("Input buffer overrun")
      inputBuffer((nextInputElementCursor + inputBufferElements) & IndexMask) = elem.asInstanceOf[AnyRef]
      inputBufferElements += 1
    }
    pump.pump()
  }

  override def cancel(): Unit = {
    if (!upstreamCompleted) {
      upstreamCompleted = true
      if (upstream ne null) upstream.cancel()
      clear()
    }
  }
  override def isClosed: Boolean = upstreamCompleted

  private def clear(): Unit = {
    java.util.Arrays.fill(inputBuffer, 0, inputBuffer.length, null)
    inputBufferElements = 0
  }

  override def inputsDepleted = upstreamCompleted && inputBufferElements == 0
  override def inputsAvailable = inputBufferElements > 0

  protected def onComplete(): Unit = {
    upstreamCompleted = true
    subreceive.become(completed)
    pump.pump()
  }

  protected def onSubscribe(subscription: Subscription): Unit = {
    ReactiveStreamsCompliance.requireNonNullSubscription(subscription)
    if (upstreamCompleted) subscription.cancel()
    else {
      upstream = subscription
      // Prefetch
      upstream.request(inputBuffer.length)
      subreceive.become(upstreamRunning)
    }
    pump.gotUpstreamSubscription()
  }

  protected def onError(e: Throwable): Unit = {
    upstreamCompleted = true
    subreceive.become(completed)
    inputOnError(e)
  }

  protected def waitingForUpstream: Actor.Receive = {
    case OnComplete                => onComplete()
    case OnSubscribe(subscription) => onSubscribe(subscription)
    case OnError(cause)            => onError(cause)
  }

  protected def upstreamRunning: Actor.Receive = {
    case OnNext(element)           => enqueueInputElement(element)
    case OnComplete                => onComplete()
    case OnError(cause)            => onError(cause)
    case OnSubscribe(subscription) => subscription.cancel() // spec rule 2.5
  }

  protected def completed: Actor.Receive = {
    case OnSubscribe(_) => throw new IllegalStateException("onSubscribe called after onError or onComplete")
  }

  protected def inputOnError(@unused e: Throwable): Unit = {
    clear()
  }

}

/**
 * INTERNAL API
 */
@InternalApi private[akka] class SimpleOutputs(val actor: ActorRef, val pump: Pump)
    extends DefaultOutputTransferStates {
  import ReactiveStreamsCompliance._

  protected var exposedPublisher: ActorPublisher[Any] = _

  protected var subscriber: Subscriber[Any] = _
  protected var downstreamDemand: Long = 0L
  protected var downstreamCompleted = false
  override def demandAvailable = downstreamDemand > 0
  override def demandCount: Long = downstreamDemand

  override def subreceive = _subreceive
  private val _subreceive = new SubReceive(waitingExposedPublisher)

  def isSubscribed = subscriber ne null

  def enqueueOutputElement(elem: Any): Unit = {
    ReactiveStreamsCompliance.requireNonNullElement(elem)
    downstreamDemand -= 1
    tryOnNext(subscriber, elem)
  }

  override def complete(): Unit = {
    if (!downstreamCompleted) {
      downstreamCompleted = true
      if (exposedPublisher ne null) exposedPublisher.shutdown(None)
      if (subscriber ne null) tryOnComplete(subscriber)
    }
  }

  override def cancel(): Unit = {
    if (!downstreamCompleted) {
      downstreamCompleted = true
      if (exposedPublisher ne null) exposedPublisher.shutdown(None)
    }
  }

  override def error(e: Throwable): Unit = {
    if (!downstreamCompleted) {
      downstreamCompleted = true
      if (exposedPublisher ne null) exposedPublisher.shutdown(Some(e))
      if ((subscriber ne null) && !e.isInstanceOf[SpecViolation]) tryOnError(subscriber, e)
    }
  }

  override def isClosed: Boolean = downstreamCompleted && (subscriber ne null)

  protected def createSubscription(): Subscription = new ActorSubscription(actor, subscriber)

  private def subscribePending(subscribers: Seq[Subscriber[Any]]): Unit =
    subscribers.foreach { sub =>
      if (subscriber eq null) {
        subscriber = sub
        tryOnSubscribe(subscriber, createSubscription())
      } else
        rejectAdditionalSubscriber(sub, s"${Logging.simpleName(this)}")
    }

  protected def waitingExposedPublisher: Actor.Receive = {
    case ExposedPublisher(publisher) =>
      exposedPublisher = publisher
      subreceive.become(downstreamRunning)
    case other =>
      throw new IllegalStateException(s"The first message must be ExposedPublisher but was [$other]")
  }

  protected def downstreamRunning: Actor.Receive = {
    case SubscribePending =>
      subscribePending(exposedPublisher.takePendingSubscribers())
    case RequestMore(_, elements) =>
      if (elements < 1) {
        error(ReactiveStreamsCompliance.numberOfElementsInRequestMustBePositiveException)
      } else {
        downstreamDemand += elements
        if (downstreamDemand < 1)
          downstreamDemand = Long.MaxValue // Long overflow, Reactive Streams Spec 3:17: effectively unbounded
        pump.pump()
      }
    case Cancel(_) =>
      downstreamCompleted = true
      exposedPublisher.shutdown(Some(new ActorPublisher.NormalShutdownException))
      pump.pump()
  }

}

private[akka] object ActorProcessorImpl {
  case object SubscriptionTimeout
}

/**
 * INTERNAL API
 */
@InternalApi private[akka] abstract class ActorProcessorImpl(attributes: Attributes)
    extends Actor
    with ActorLogging
    with Pump {

  private val debugLoggingEnabled = attributes.mandatoryAttribute[ActorAttributes.DebugLogging].enabled

  protected val primaryInputs: Inputs = {
    val initialInputBufferSize = attributes.mandatoryAttribute[Attributes.InputBuffer].initial
    new BatchingInputBuffer(initialInputBufferSize, this) {
      override def inputOnError(e: Throwable): Unit = ActorProcessorImpl.this.onError(e)
    }
  }

  protected val primaryOutputs: Outputs = new SimpleOutputs(self, this)
  def subTimeoutHandling: Receive

  /**
   * Subclass may override [[#activeReceive]]
   */
  final override def receive = new ExposedPublisherReceive(activeReceive, unhandled) {
    override def receiveExposedPublisher(ep: ExposedPublisher): Unit = {
      primaryOutputs.subreceive(ep)
      context.become(activeReceive)
    }
  }

  def activeReceive: Receive =
    primaryInputs.subreceive.orElse[Any, Unit](primaryOutputs.subreceive).orElse(subTimeoutHandling)

  protected def onError(e: Throwable): Unit = fail(e)

  protected def fail(e: Throwable): Unit = {
    if (debugLoggingEnabled)
      log.debug("fail due to: {}", e.getMessage)
    primaryInputs.cancel()
    primaryOutputs.error(e)
    context.stop(self)
  }

  override def pumpFinished(): Unit = {
    primaryInputs.cancel()
    primaryOutputs.complete()
    context.stop(self)
  }

  override def pumpFailed(e: Throwable): Unit = fail(e)

  override def postStop(): Unit = {
    primaryInputs.cancel()
    primaryOutputs.error(AbruptTerminationException(self))
  }

  override def postRestart(reason: Throwable): Unit = {
    super.postRestart(reason)
    throw new IllegalStateException("This actor cannot be restarted", reason)
  }

}
