import java.io.*;
import java.util.*;

// Fun with observers

// A generic observer
interface Observer {
    val id: Int
        get() = 0
    fun notify(event: Int)
}

// A list of observers, supports:
// - adding observers
// - removing observers
// - notifying all observers
class ObserverList {
    private var observers = ArrayList<Observer>()

    fun addObserver(observer: Observer) {
        observers.add(observer)
    }

    fun removeObserver(observer: Observer) {
        for (i in 0 until observers.count()) {
          
            if (observer.id == observers[i].id) {
                observers.removeAt(i)
                break
            }
        }
    }

    fun notifyAll(event: Int) {
        for (i in 0 until observers.count()) {
            observers[i].notify(event)
        }
    }
}

//---

var observerList = ObserverList()

val STARTUP_EVENT: Int = 1
val INTERESTING_EVENT: Int = 2
val SHUTDOWN_EVENT: Int = 3

class MyObserver: Observer {
    companion object {
        var counter: Int = 1
    }

    private val identifier: Int

    constructor() {
        identifier = counter++
    }

    override val id: Int
        get() = identifier

    override fun notify(event: Int) {
        when (event) {
            STARTUP_EVENT -> {
                println("$identifier: Got startup event!")
            }
            INTERESTING_EVENT -> {
                println("$identifier: Got interesting event!")
            }
            SHUTDOWN_EVENT -> {
                println("$identifier: Got shutdown event!")
                // clean up time!
                observerList.removeObserver(this)
            }
            else -> {
                print("Unknown event!")
            }
        }
    }
}

//---

fun main() {
  println("Welcome to the wonderful world of observer lists!")

  val numObservers: Int = 9
  for (i in 0 until numObservers) {
      observerList.addObserver(MyObserver())
  }

  observerList.notifyAll(STARTUP_EVENT)

  // Do a bunch of interesting work and let the observers know about it!
  observerList.notifyAll(INTERESTING_EVENT)

  // Time to shutdown
  observerList.notifyAll(SHUTDOWN_EVENT)
  
  println("Done")
}