Object Mentor Blog: Bowling Game Kata in Ruby

Bowling Game Kata in Ruby

Brett Schuchert — Thu, 01 Oct 2009 13:37:00 -0500

I have not really used Ruby much. I’ve written a few tutorials, messed around with RSpec and Test::Unit and even Rails a bit, but I really don’t know Ruby that well. I get Ruby (the MOP, instances, blocks, open classes, ...) but there’s a difference between understanding that stuff and using it day-to-day.

Last night we had a Dojo in Oklahoma City and I wanted to get refreshed with RSpec, so I decided to jump in and do the bowling game kata. I did not follow uncle bob’s lead exactly. For one, I went ahead and stored two throws for each frame. While what he ends up with is a bit shorter, it bothers me a little bit. I’ve also seen people surprised by how bob stores his scores, so in a sense it violates the law of least astonishment.

That’s neither here nor there, I got it working just fine – though handling strikes was a bit more difficult because I decided to store two rolls instead of one (so clearly, there’s no best answer, just ones that suck for different reasons).

After my first go around, I had a single spec with examples all under a single describe (what’s that term they’d use for what the describe expression creates?). I added several examples for partial scores, to make sure I was handling incomplete games correctly. I restructured those a bit and tried to make the names a bit more clear, not sure if I was successful.

In my original version I started with a frame in the score method as a local variable, but it quickly got converted to an index, and the index was mostly passed around after that. The approach was very c-esque. I didn’t like that index all over the place, so I tried to remove it by refactoring. It took several false starts before I bit the bullet and simply duplicated each of the methods, one at a time, using parallel development. The old version using an index, the new one use a 1-based frame number. After I got that working with frames, I removed most of the methods using an index, except for a few.

What follows is the spec file and the ruby class. If you read the names of some of the examples, you might think I used to bowl in league, I did. My average was a paltry 158, my best game ever a 248. Best split I ever picked up? 4, 6, 7, 10.

Comments welcome.

bowling_score_card_spec.rb

require 'bowling_score_card'

describe BowlingScoreCard do
    before(:each) do
        @bowling_game_scorer = BowlingScoreCard.new
    end

    def roll value
        @bowling_game_scorer.roll value
    end

    def roll_many count, value
        count.times { roll value }
    end

    def score_should_be value
        @bowling_game_scorer.score.should == value
    end

    it 'should score 0' do
        score_should_be 0
    end

    describe 'Scores for Complete Games' do
        it 'should score 0 for an all gutter game' do
            roll_many 20, 0
            score_should_be 0
        end

        it 'should show 20 for an all 1 game' do
            roll_many 20, 1
            score_should_be 20
        end

        it 'should score game with single spare correctly' do
            roll_many 3, 5
            roll_many 17, 0
            score_should_be 20
        end

        it 'should score game with single strike correctly' do
            roll 10
            roll 5
            roll 2
            roll_many 17, 0
            score_should_be 24
        end

        it 'should score a dutch-200, spare-strike, correclty' do
            10.times do
                roll_many 2, 5
                roll 10
            end
            roll_many 2, 5

            score_should_be 200
        end

        it 'should score a dutch-200, strike-spare, correctly' do
            10.times do
                roll 10
                roll_many 2, 5
            end
            roll 10
            score_should_be 200
        end

        it "should score all 5's game as 150" do
            roll_many 21, 5
            score_should_be 150
        end

        it 'should score a perfect game correctly' do
            roll_many 12, 10
            score_should_be 300
        end

        it 'should not count a 0, 10 roll as a strike' do
            roll 0
            roll 10
            roll_many 18, 1
            score_should_be 29
        end

    end

    describe 'Scoring for open games' do
        it 'should score just an open frame' do
            roll 4
            roll 3
            score_should_be 7
        end

        it 'should score just a spare' do
            roll_many 2, 5
            score_should_be 10
        end

        it 'should score partial game with spare and following frame only' do
            roll_many 3, 5
            score_should_be 20
        end

        it 'should score an opening turkey correctly' do
            roll_many 3, 10
            score_should_be 60
        end
    end

    describe 'Scoring open game starting with a srike' do
        before(:each) do
            roll 10
        end
        it 'should score partial game with only strike' do
            score_should_be 10
        end

        it 'should score partial game with strike and half-open frame' do
            roll 4
            score_should_be 18
        end

        it 'should score partial game with strike and open frame' do
            roll 3
            roll 6
            score_should_be 28
        end

        it 'should score partial game with strike and spare' do
            roll 3
            roll 7
            score_should_be 30
        end
    end

    describe 'Open game starting with two Strikes' do
        before(:each) do
            roll 10
            roll 10
        end

        it 'should have a score of 30' do
            score_should_be 30
        end

        it 'should score correctly with following non-mark' do
            roll 4
            score_should_be 42
        end

        it 'should score correclty with third frame open' do
            roll 4
            roll 3
            score_should_be 48
        end
    end
end

bowling_score_card.rb

class BowlingScoreCard
    def initialize
        @rolls = []
    end

    def roll value
        @rolls << value
        @rolls << 0 if first_throw_is_strike?
    end

    def score
        (1..10).inject(0) { |score, frame| score += score_for frame }
    end

    def score_for frame
        return strike_score_for frame if is_strike_at? frame
        return spare_score_for frame if is_spare_at? frame
        open_score_for frame
    end

    def first_throw_is_strike?
        is_first_throw_in_frame? && @rolls.last == 10
    end

    def is_first_throw_in_frame?
        @rolls.length.odd?
    end

    def open_score_for frame
        first_throw_for(frame) + second_throw_for(frame);
    end

    def spare_score_for frame
        open_score_for(frame) + first_throw_for(next_frame(frame))
    end

    def strike_score_for frame
        score = open_score_for(frame) + open_score_for(next_frame(frame))
        if is_strike_at? next_frame(frame)
            score += first_throw_for(next_frame(next_frame(frame)))
        end
        score
    end

    def next_frame frame
        frame + 1
    end

    def is_spare_at? frame
        (open_score_for frame) == 10 && is_strike_at?(frame) == false
    end

    def is_strike_at? frame
        first_throw_for(frame) == 10
    end

    def first_throw_for frame
        score_at_throw(index_for(frame))
    end

    def second_throw_for frame
        score_at_throw(index_for(frame) + 1)
    end

    def index_for frame
        (frame - 1) * 2
    end

    def score_at_throw index
        @rolls.length > index ? @rolls[index] : 0
    end
end

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