/*
 * Copyright (C) 2018-2021 Lightbend Inc. <https://www.lightbend.com>
 */

package akka.stream.scaladsl

import scala.collection.immutable

import akka.stream.{ Attributes, Inlet, Outlet, UniformFanInShape }
import akka.stream.stage.{ GraphStage, GraphStageLogic, InHandler, OutHandler }

/**
 * MergeLatest joins elements from N input streams into stream of lists of size N.
 * i-th element in list is the latest emitted element from i-th input stream.
 * MergeLatest emits list for each element emitted from some input stream,
 * but only after each stream emitted at least one element
 *
 * '''Emits when''' element is available from some input and each input emits at least one element from stream start
 *
 * '''Completes when''' all upstreams complete (eagerClose=false) or one upstream completes (eagerClose=true)
 *
 * '''Cancels when''' downstream cancels
 *
 */
object MergeLatest {

  /**
   * Create a new `MergeLatest` with the specified number of input ports.
   *
   * @param inputPorts number of input ports
   * @param eagerComplete if true, the merge latest will complete as soon as one of its inputs completes.
   */
  def apply[T](inputPorts: Int, eagerComplete: Boolean = false): GraphStage[UniformFanInShape[T, List[T]]] =
    new MergeLatest[T, List[T]](inputPorts, eagerComplete)(_.toList)

}

final class MergeLatest[T, M](val inputPorts: Int, val eagerClose: Boolean)(buildElem: Array[T] => M)
    extends GraphStage[UniformFanInShape[T, M]] {
  require(inputPorts >= 1, "input ports must be >= 1")

  val in: immutable.IndexedSeq[Inlet[T]] = Vector.tabulate(inputPorts)(i => Inlet[T]("MergeLatest.in" + i))
  val out: Outlet[M] = Outlet[M]("MergeLatest.out")
  override val shape: UniformFanInShape[T, M] = UniformFanInShape(out, in: _*)

  override def createLogic(inheritedAttributes: Attributes): GraphStageLogic =
    new GraphStageLogic(shape) with OutHandler {
      private val activeStreams: java.util.HashSet[Int] = new java.util.HashSet[Int]()
      private var runningUpstreams: Int = inputPorts
      private def upstreamsClosed: Boolean = runningUpstreams == 0
      private def allMessagesReady: Boolean = activeStreams.size == inputPorts
      private val messages: Array[Any] = new Array[Any](inputPorts)

      override def preStart(): Unit = in.foreach(tryPull)

      in.zipWithIndex.foreach {
        case (input, index) =>
          setHandler(
            input,
            new InHandler {
              override def onPush(): Unit = {
                messages.update(index, grab(input))
                activeStreams.add(index)
                if (allMessagesReady) emit(out, buildElem(messages.asInstanceOf[Array[T]]))
                tryPull(input)
              }

              override def onUpstreamFinish(): Unit = {
                if (!eagerClose) {
                  runningUpstreams -= 1
                  if (upstreamsClosed) completeStage()
                } else completeStage()
              }
            })
      }

      override def onPull(): Unit = {
        var i = 0
        while (i < inputPorts) {
          if (!hasBeenPulled(in(i))) tryPull(in(i))
          i += 1
        }
      }

      setHandler(out, this)
    }

  override def toString = "MergeLatest"
}
