Object Mentor Blog: Scala Bowling Kata - somewhere in the middle...

Scala Bowling Kata - somewhere in the middle...

Brett Schuchert — Mon, 05 Oct 2009 23:33:00 -0500

I need to do some work with Scala to update our Concurrency in Java class. We want to demonstrate some other approaches to concurrency, e.g., Scala Actors (among others). I began by shaving yaks:

Installed the Eclipse Scala Plugin
Installed Scala using Mac Ports
Figured out how to get those things playing nice (the plugin page pretty much did that, but in a nutshell, add a few jar files to the classpath)

Next, I wanted to use some BDD tool. I’m going to try and stop using the term TDD simply because the T, which stands for Test in TDD, really means “desired behavior”. I considered calling it Trait Driven Development, but:

We don’t need YADT – Yet another damn term
Trait is a heavily overloaded word
I like the term BDD better and it fits.

Anyway, one such choice was Specs, which is what I decided to use.

So back to yak shaving:

I added another jar to my classpath in Eclipse
Then read how to get it running in Eclipse. Not too bad, I suppose.

So now I need to learn Scala. Sure, I’ve used it, but far less than Ruby. So it took me several hours to get specs running along with writing some Scala code to score a game – I’m glad I know the domain at least.

I wanted to make similar behaviors to the ones I wrote for the Ruby version, which I did.

However, unlike the Ruby version, I was curious what would happen if I:

Took an approach similar to Uncle Bob – strikes take one slot in an array
Added input validation

On the one hand, there are some interesting things I managed to create. On the other hand, I’ve got a bit of a mess. I have a stateful object to avoid passing parameters so that I can write some of the code cleanly. I know I need to add in an intermediate computational object, and I’m going to get to that. However, I wanted to get feedback on what I’ve put out there so far.

Specifically,

What do you think of the (bdd-style) examples from specc?
What is the correct way to write the Times(20).Do( ...) thing I came up with, there has be a better way?
For the part of the bowling scoring code that is not stateful (read this as, does not violate the SRP), what do you think of it?
How would you remove most/all of the state (other than the individual rolls) out of the Bowling scorer class? (Or would you choose to have the roll() method return a new instance of BowlingScorer with the new score recorded?)
Notice that the class maintains a mini state machine in the form of tracking whether the first ball of he current frame (not tracked) has or has not been thrown. That’s only there to be able to perform input validation. I considered:
- Walking the array
- Going to 2 slots for every frame (making it easy to find the frame)
- Storing a frame object (ok, I didn’t really consider it, but I did think about it)
- The mini state machine
nextFrameScore uses the index instance variable, and changes it. This both violates command-query separation and demonstrates a violation of the SRP, but it made the scoreAt method look nice.

An interesting side effect is that scoring marks (strikes and spares) uses the same approach, sum up three rolls total.

I know this needs work. What I’ve got works according to its current specification (its examples), so in a sense, that’s a good thing because I’ve already stared experimenting with trying out different solutions. However, I am painfully aware of how unaware I am of Scala at the moment, so your (hopefully gentle) feedback will tell me what I need to learn next.

Looking forward to the virtual beating …

Brett

Here are the two files I’ve created so far (and to be clear, all of the examples pass):

BowlingScorerExampleGroup.scala

package com.om.example

import org.specs._

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

  def roll(value:Int) {
    scorer.roll(value) 
  }

  def rollMany(rolls:Int, value:Int) {
    0.until(rolls).foreach { arg => scorer.roll(value) }
  }

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

  def strike {
    roll(10)
  }

  def spare {
    rollMany(2, 5) 
  }

  abstract class IDo {
    def Do(block: => Unit) 
  }

  def Times(count:Int): IDo = {
    return new IDo {
      def Do(block: => Unit) {
        1.to(count).foreach( arg => block )
      }
    }
  }

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

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

  "A game with all 1's" should {
    Times(20).Do { 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 {
    Times(10).Do( 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
    Times(2).Do( roll(3) )
    haveAScoreOf(39)
  }

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

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

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

  "A Perfect game" should {
    Times(12).Do( strike ) 
    haveAScoreOf(300)
  }

  "The score for each frame of a Perfect game, each frame" should {
    Times(12).Do( 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]
  }
}

BowlingScorer.scala

package com.om.example

class BowlingScorer {
  var rolls:Array[Int] = Array()
  var index:Int = 0
  var firstBallInFrameThrown: Boolean = false;

  def roll(roll:Int) = {
    validate(roll)
    record(roll)
  }

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

    if(openScoreAt(indexToValidate) + roll > 10)
      throw new IllegalArgumentException("Total of rolls for frame must not exceed 10");
  }

  def record(roll: Int) {
    rolls = rolls ++ Array(roll)
    firstBallInFrameThrown = firstBallInFrameThrown == false && roll != 10
  }

  def indexToValidate = {
    if(firstBallInFrameThrown) rolls.length - 1 else rolls.length
  }

  def scoreAt(frame:Int) = {
    1.to(frame).foldLeft(0) { (total, frame) =>  total + nextFrameScore  }
  }

  def score = {
    scoreAt(10)
  }

  def nextFrameScore = {
    var result = 0;
    if(isStrike(index)) {
      result += markScoreAt(index)
      index += 1
    } else if(isSpare(index)) {
      result += markScoreAt(index);
      index += 2
    } else {
      result += openScoreAt(index);
      index += 2
    }
    result
  }

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

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

  def isSpare(index:Int) = {
    openScoreAt(index) == 10
  }

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

  def sumNext(index:Int, count:Int) = {
    index.until(index+count).foldLeft(0)(_ + valueAt(_))
  }

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

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