package lecture13

import akka.actor.{Actor, ActorContext, ActorRef, ActorSystem, Props}
import akka.event.LoggingReceive
import akka.testkit.{ImplicitSender, TestKit}

class CounterRight extends Actor:
  def counter(n: Int): Receive = {
    case "increment" => context.become(counter(n + 1), discardOld = false)
    //                                                 ^^^^^^^^^^^^^^^^^^
    //                                                 This is needed.
    case "get"       => sender() ! n
    case "decrement" => if n != 0 then context.unbecome()
  }
  def receive = counter(0)

class CounterWrong extends Actor:
  def counter(n: Int): Receive = {
    case "increment" => context.become(counter(n + 1))
    case "get"       => sender() ! n
    case "decrement" => if n != 0 then context.unbecome()
  }
  def receive = counter(0)

@main def becomeDemo() =
  new TestKit(ActorSystem("DebugSystem")) with ImplicitSender:
    try
      val counter = system.actorOf(Props(CounterRight()), "counter")

      counter ! "increment"
      counter ! "get"
      expectMsg(1)

      counter ! "increment"
      counter ! "get"
      expectMsg(2)

      counter ! "decrement"
      counter ! "get"
      expectMsg(1)
      // This is wrong if we use CounterWrong because it doesn't set the
      // discardOld parameter to false. Therefore, it will not remember the
      // previous state and reset the behavior to the initial one, which is
      // `counter(0)`.

      counter ! "decrement"
      counter ! "get"
      expectMsg(0)
    finally shutdown(system)