Object Mentor Blog: Scala Bowling Kata - still in the middle I suppose

Scala Bowling Kata - still in the middle I suppose

Brett Schuchert — Wed, 07 Oct 2009 00:51:00 -0500

I had a 3.5 hour flight today. I realized I was missing some of the validation from the Ruby version related to how many rolls a valid game should allow:

  "One more roll on a game with 20 rolls and an open 10th frame" should {
    20 times { roll(1) }
    roll(1) must throwA[IllegalArgumentException]
  }

  "Two more rolls a game with 10 spares" should {
    10 times { spare }
    roll(1)
    roll(1) must throwA[IllegalArgumentException]
  }

  "Two marks in the 10th frame should" should {
    18 times { roll(1) }
    strike
    spare
    roll(1) must throwA[IllegalArgumentException]
  }

On my flight from DFW to SNA, I got these behaviors implemented. The code was pretty ugly!

However, ugly code in hand, passing examples, a slight understanding of some of the problems with my code and a desire to make the BowlingScorer immutable was all I needed to make progress.

I removed the index instance field by rewriting the score method and injecting a tuple into foldLeft (written here using the short-hand notation /:):

  def scoreAt(frame:Int) = 
    ((0,0) /: (1 to frame)) { (t, _) =>  
      (t._1 + scoreAtIndex(t._2), t._2 + incrementAt(t._2))
    }._1

I had a mini state machine to track whether the first ball had been thrown yet or not. I replaced that by walking the list of scores:

  def onFirstThrow = {
    var index = 0
    while(index < rolls.length)
      if(isStrike(index)) index += 1 else index += 2
    index == rolls.length
  }

While I am happy I was able to remove the index variable, which was really a parameter being passed around in a field (ugly), I am not happy with this method.

I changed the roll method to return a new instance of a BowlingScorer, making the bowling scorer immutable:

  def roll(roll:Int) = {
    validate(roll)
    new BowlingScorer(rolls ++ Array(roll))
  }

So I think I’m still somewhere in the middle of working through this code. Again, I’m still learning Scala. I have a lot to learn. I really only barely understand functional programming and, frankly, the Eclipse IDE, while functional, is getting in the way quite a bit. So for a toy example it is OK. Given the choice of this environment or vi and the command line, I’d not pick the former. (I might give the other IDE’s a go, but that’s not really what I’m interested in learning right now.)

So here’s the next version. I plan to work through all of the comments I’ve yet to process from the previous blog posting over the next few days. If you can recommend a better implementation of onFirstThrow, I’d appreciate it.

Other general comments also welcome.

BowlingScorerExampleGroup.scala

package com.om.example

import org.specs._

object BowlingScorerExampleGroup extends SpecificationWithJUnit {
  var scorer = new BowlingScorer(Nil);

  def roll(value:Int) =
    scorer = scorer.roll(value) 

  def haveAScoreOf(expected:Int) =
    scorer.score must_== expected

  def strike =
    roll(10)

  def spare {
    roll(5)
    roll(5)
  }

  implicit def intToDo(count: Int) = {
    new {
      def times(f: => Unit) = {
        1 to count foreach { _ => f }
      }
    }
  }

  "A Newly Created Bowling Scorer" should {
    haveAScoreOf(0)
  }

  "A game with all 0's" should {
    20 times { roll(0) }
    haveAScoreOf(0)
  }

  "A game with all 1's" should {
    20 times { roll(1) }
    haveAScoreOf(20)
  }

  "A game with a single spare followed by a 5" should {
    spare
    roll(5)
    haveAScoreOf(20)
  }

  "A game with all 5's" should {
    10 times { spare }
    roll(5)
    haveAScoreOf(150)
  }

  "A game with a single strike followed by a 4" should {
    strike
    roll(4)
    haveAScoreOf(18)
  }

  "A game with a strike, spare then an open frame with two 3's" should {
    strike
    spare
    2 times { roll(3) }
    haveAScoreOf(39)
  }

  "A game with strike, spare then an open frame with two 3's" should {
    spare
    strike
    2 times { roll(3) }
    haveAScoreOf(42)
  }

  "A Dutch 200 game, Spare-Strike" should {
    5 times {
      spare 
      strike
    }
    spare
    haveAScoreOf(200)
  }

  "A Dutch 200 game, Strike-Spare" should {
    5 times {
      strike
      spare 
    }
    strike
    haveAScoreOf(200)
  } 

  "A Perfect game" should {
    12 times { strike }
    haveAScoreOf(300)
  }

  "The score for each frame of a Perfect game, each frame" should {
    12 times { strike }
    1 to 10 foreach { frame => scorer.scoreAt(frame) must_== 30 * frame }
  }

  "An individaul roll of > 10" should {
    roll(11) must throwA[IllegalArgumentException]
  }

  "An iniviaul roll of < 0" should {
    roll(-1) must throwA[IllegalArgumentException]
  }

  "A frame trying to contain more than 10 pins" should {
    roll(8)
    roll(3) must throwA[IllegalArgumentException]
  }

  "One more roll on a game with 20 rolls and an open 10th frame" should {
    20 times { roll(1) }
    roll(1) must throwA[IllegalArgumentException]
  }

  "Two more rolls a game with 10 spares" should {
    10 times { spare }
    roll(1)
    roll(1) must throwA[IllegalArgumentException]
  }

  "Two marks in the 10th frame should" should {
    18 times { roll(1) }
    strike
    spare
    roll(1) must throwA[IllegalArgumentException]
  }
}

BowlingScorer.scala

package com.om.example

class BowlingScorer(rollsSoFar:List[Int]){
   val rolls:List[Int] = rollsSoFar

  def roll(roll:Int) = {
    validate(roll)
    new BowlingScorer(rolls ++ Array(roll))
  }

  def validate(roll:Int) {
    if(invalidRoll(roll))
      throw new IllegalArgumentException("Individaul rolls must be from 0 .. 10")

    if(frameRollTooHigh(roll))
      throw new IllegalArgumentException("Total of rolls for frame must not exceed 10");

    if(willBeTooManyRolls)
      throw new IllegalArgumentException("Game over, no more rolls allowed")
  }

  def invalidRoll(roll:Int) = 
    (0 to 10 contains(roll)) == false  

  def frameRollTooHigh(roll:Int) =
    openScoreAt(indexToValidate) + roll > 10

  def willBeTooManyRolls = 
    tenthRolled(indexOf10thFrame) && nextRollTooMany(indexOf10thFrame)

  def tenthRolled(tenthIndex:Int) = 
    tenthIndex < rolls.length

  def nextRollTooMany(tenthIndex: Int) = 
    allowedTenthFrameRolls(tenthIndex) < rollsInTenthFrame(tenthIndex) + 1

  def indexOf10thFrame = 
    (0 /: (1 until 10)) {(c, _) => c + incrementAt(c)}

  def allowedTenthFrameRolls(index:Int) =
    if(isMark(index)) 3 else 2

  def rollsInTenthFrame(index: Int) =
    rolls.length - index

  def indexToValidate =
    if(onFirstThrow) rolls.length else rolls.length - 1

  def onFirstThrow = {
    var index = 0
    while(index < rolls.length)
      if(isStrike(index)) index += 1 else index += 2
    index == rolls.length
  }

  def scoreAt(frame:Int) = 
    ((0,0) /: (1 to frame)) { (t, _) =>  
      (t._1 + scoreAtIndex(t._2), t._2 + incrementAt(t._2))
    }._1

  def score = scoreAt(10)

  def scoreAtIndex(index:Int) =
    if(isMark(index)) markScoreAt(index) else openScoreAt(index)

  def incrementAt(index:Int) =
    if(isStrike(index)) 1 else 2

  def isMark(index:Int) =
    isStrike(index) || isSpare(index)

  def isStrike(index:Int) =
    valueAt(index) == 10

  def markScoreAt(index:Int) =
    sumNext(index, 3)

  def isSpare(index:Int) =
    openScoreAt(index) == 10 && valueAt(index) != 10

  def openScoreAt(index:Int) =
    sumNext(index, 2)

  def sumNext(index:Int, count:Int) =
    (0 /: (index until index+count))(_ + valueAt(_))

  def valueAt(index:Int) = 
    if(rolls.length > index) rolls(index) else 0
}

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