The Monty Hall kata in scala

Game.scala

import scala.util.{Random}
import scala.collection.mutable.{ListBuffer}

class Game(doors: ListBuffer[Symbol] = Random.shuffle(ListBuffer('Car, 'Goat, 'Goat))) {
  var playerDoor:Symbol = _
  var montyDoor:Symbol = _

  if (doors.count(_ == 'Car) != 1 || doors.count(_ == 'Goat) != 2) {
    throw new IllegalArgumentException("Should be one Car and two Goats")
  }

  def numberOfDoors = doors.size
  def carDoor = doors.indexOf('Car)

  def chooseDoor(doorNo: Int) = {
    playerDoor = doors.remove(doorNo)
    montyDoor = montyPicksDoor
  }

  def montyPicksDoor() = {
    if (doors(0) == 'Goat)
      doors.remove(0)
    else 
      doors.remove(1)
  }

  def switch = playerDoor = doors.remove(0)
  def playerWon = playerDoor == 'Car
  override def toString = doors.mkString(", ")
}

GameSpec.scala

import org.specs._
import scala.util._
import scala.collection.mutable.{ListBuffer}

class MontySpec extends Specification {

  val game = new Game(ListBuffer('Goat, 'Car, 'Goat))

  "The Game" should {
    "have three doors" in {
      game.numberOfDoors must_==  3
    }

    "should have one car and two doors" in {
      new Game(ListBuffer('Goat, 'Goat, 'Goat)) must throwA[IllegalArgumentException]
      new Game(ListBuffer('Car,  'Car,  'Car))  must throwA[IllegalArgumentException]
      new Game(ListBuffer('Goat, 'Car,  'Car))  must throwA[IllegalArgumentException]
      new Game(ListBuffer('Car,  'Car,  'Goat)) must throwA[IllegalArgumentException]
      new Game(ListBuffer('Car,  'Goat, 'Car))  must throwA[IllegalArgumentException]
    }

    "know where the car is" in {
      game.carDoor must_== 1
    }

    "be able to tell if the player was a winner" in {
      game.chooseDoor(1)
      game.playerWon must be(true)
    }
  }

  "A Player" should {
    "be able to choose a door" in {
      game.chooseDoor(0)
      game.playerDoor must be('Goat)
    }

    "be able to switch doors" in {
      game.chooseDoor(0)
      game.switch
      game.playerDoor must beOneOf('Car, 'Goat)
      game.numberOfDoors must_==  0
    }
  }

  "Monty" should {
    "pick a door after the player has chosen one" in {
      game.chooseDoor(0)
      game.montyDoor must be('Goat)
      game.numberOfDoors must_==  1
    }
  }

  "Running a thousand games" should {
    "show that switching wins 2 times out of 3" in {
      (for(i <- 1 to 1000) yield {
        val game = new Game
        game.chooseDoor(Random.nextInt(3))
        game.switch
        game.playerWon
      }).count(_ == true) must be closeTo(667 +/- 67)
    }
  }

}