bowling kata [http://codingdojo.org/cgi-bin/wiki.pl?KataBowling]

TenPinBowling.scala

object TenPinBowling {
  
  /**
   * Returns the score for each frame of a 10-pin bowling game.
   * 
   * @param rolls
   *    List of pins knocked down by each roll
   * @return
   *    List of scores for each frame
   */
  def score(rolls: List[Int]): List[Int] = {
    
    def badInput() = { throw new IllegalArgumentException("Bad input") }
    
    def calculate(rolls: List[Int], frame: Int, pastScores: List[Int]): List[Int] = {
      
      val lastScore = if (pastScores.isEmpty) 0 else pastScores.head
      
      val (frameScore, extraBallsCounted, rest) = rolls match {
        case 10 :: a :: b :: xs => // Strike
          (10 + a + b + lastScore, 2, a :: b :: xs)
        case a :: b :: c :: xs if a + b == 10 => // Spare
          (10 + c + lastScore, 1, c :: xs)
        case a :: b :: xs if a + b < 10 => // Normal 2-roll frame
          (a + b + lastScore, 0, xs)
        case _ =>
          badInput()
      }
      
      if (frame == 10)
        // Any balls left should be accounted for by a strike or spare
        if (rest.length == extraBallsCounted)
          frameScore :: pastScores
        else
          badInput()
      else
        calculate(rest, frame + 1, frameScore :: pastScores)
    }
    
    if (rolls.exists(x => x < 0 || x > 10))
      badInput()
    else
      calculate(rolls, 1, Nil).reverse
  }
  
  def main(args: Array[String]): Unit = {
    
    if (args.length < 10 || args.length > 21) {
      println("Specify the rolls as arguments, " 
              + "e.g. for the perfect game 10 10 10 10 10 10 10 10 10 10 10 10")
      exit(1)
    }
    
    val rolls = args.map(_.toInt).toList
    println("Scores: " + score(rolls))
    
  }
  
}