Simplify selectDeploymentTarget

* Break the loop immediately when no more targets

akka-cluster/src/main/scala/akka/cluster/routing/ClusterRouterConfig.scala

@@ -32,6 +32,7 @@ import akka.routing.RemoteRouterConfig
 import akka.actor.RootActorPath
 import akka.actor.ActorCell
 import akka.actor.RelativeActorPath
+import scala.annotation.tailrec
 
 /**
  * [[akka.routing.RouterConfig]] implementation for deployment on cluster nodes.
@@ -168,7 +169,10 @@ private[akka] class ClusterRouteeProvider(
    * to use for cluster routers.
    */
   override def createRoutees(nrOfInstances: Int): Unit = {
-    for (i ← 1 to settings.totalInstances; target ← selectDeploymentTarget) {
+    @tailrec
+    def doCreateRoutees(): Unit = selectDeploymentTarget match {
+      case None ⇒ // done
+      case Some(target) ⇒
         val ref =
           if (settings.isRouteesPathDefined) {
             context.actorFor(RootActorPath(target) / settings.routeesPathElements)
@@ -179,7 +183,12 @@ private[akka] class ClusterRouteeProvider(
           }
         // must register each one, since registered routees are used in selectDeploymentTarget
         registerRoutees(Some(ref))
+
+        // recursion until all created
+        doCreateRoutees()
     }
+
+    doCreateRoutees()
   }
 
   private[routing] def createRoutees(): Unit = createRoutees(settings.totalInstances)
@@ -187,13 +196,17 @@ private[akka] class ClusterRouteeProvider(
   private def selectDeploymentTarget: Option[Address] = {
     val currentRoutees = routees
     val currentNodes = availbleNodes
-    if (currentRoutees.size >= settings.totalInstances || currentNodes.isEmpty) {
+    if (currentNodes.isEmpty || currentRoutees.size >= settings.totalInstances) {
       None
     } else {
+      // find the node with least routees
       val numberOfRouteesPerNode: Map[Address, Int] =
-        Map.empty[Address, Int] ++ currentNodes.toSeq.map(_ -> 0) ++
-          currentRoutees.groupBy(fullAddress).map {
-            case (address, refs) ⇒ address -> refs.size
+        currentRoutees.foldLeft(currentNodes.map(_ -> 0).toMap) { (acc, x) ⇒
+          val address = fullAddress(x)
+          acc.get(address) match {
+            case Some(count) ⇒ acc + (address -> (count + 1))
+            case None        ⇒ acc + (address -> 1)
+          }
         }
 
       val (address, count) = numberOfRouteesPerNode.minBy(_._2)