Generating factorial numbers with parallel processing using Akka

Factorial.scala

import akka.actor.{Actor, ActorLogging, ActorSystem, Props}

object Factorial extends App {
  val factorials = List(200000, 180000, 320000, 280000, 220000, 420000, 550000, 480000)

  val system = ActorSystem("factorial")

  val collector = system.actorOf(Props(new FactorialCollector(factorials)), "fac-coll")

  system.awaitTermination()
}


class FactorialCollector(list: List[Int]) extends Actor with ActorLogging {
  var result: List[BigInt] = Nil
  var size = list.size

  for (num <- list) {
    context.actorOf(Props(new FactorialCalculator), s"fac-$num") ! num
  }

  def receive = {
    case num: BigInt => {
      result = num :: result
      size -= 1

      if (size == 0) {
        log.info("DONE!")
        context.system.shutdown()
      }
    }
  }
}


class FactorialCalculator extends Actor with ActorLogging {
  def receive = {
    case number: Int => sender ! factor(number)
  }

  private def factor(n: Int) = factorTail(n, 1)

  private def factorTail(num: Int, acc: BigInt): BigInt = {
    (num, acc) match {
      case (0, a) => a
      case (n, a) => factorTail(n - 1, n * a)
    }
  }
}