Crawler.scala

private def crawlUrl(url: Url): Unit = {
  if (!visitedLinks.contains(url)) {
    visitedLinks += url
    inProgress += url
    actorFor(url.host) ! Crawl(url)
  }
}

private def actorFor(host: Host): ActorRef = {
  workers.get(host) match {
    case None =>
      val workerActor = context.actorOf(Props(new Worker(http, parseLinks, self)))
      workers += host -> workerActor
      workerActor

    case Some(ar) => ar
  }
}