Bowling game 4th iteration

bowling_game.rb

class BowlingGame
  def initialize
    @rolls = []
  end

  def roll(pins)
    @rolls << pins
  end

  def score
    calculator = Score.new(@rolls)
    calculator.calculate
  end

  class Score
    def initialize(rolls)
      @rolls = rolls
      @first_ball_of_frame = 0
    end

    def calculate()
      score = 0

      0.upto(9) do
        if strike?
          score += 10 + next_two_balls_for_strike
          @first_ball_of_frame += 1
        elsif spare?
          score += 10 + extra_ball_for_square
          @first_ball_of_frame += 2
        else
          score += two_balls_in_frame
          @first_ball_of_frame += 2
        end
      end
      score
    end

    def strike?
      @rolls[@first_ball_of_frame] == 10
    end

    def spare?
      @rolls[@first_ball_of_frame] + @rolls[@first_ball_of_frame + 1] == 10
    end

    def two_balls_in_frame
      @rolls[@first_ball_of_frame] + @rolls[@first_ball_of_frame + 1]
    end

    def extra_ball_for_square
      @rolls[@first_ball_of_frame + 2]
    end

    def next_two_balls_for_strike
      @rolls[@first_ball_of_frame + 1] + @rolls[@first_ball_of_frame + 2]
    end
  end
end

bowling_game_spec.rb

describe BowlingGame do
  let(:game){ BowlingGame.new }

  it "calculates the score of a gutter game" do
    20.times { game.roll(0) }
    game.score.should be == 0
  end

  it "calculates the score of 3 pins followed by gutter balls" do
    game.roll(3)
    19.times { game.roll(0) }
    game.score.should be == 3
  end

  it "calculates the score of a spare followed by some pins + gutter balls" do
    roll_spare()
    game.roll(3)
    17.times { game.roll(0) }
    game.score.should be == 16
  end

  it "calculates the perfect game" do
    12.times { roll_strike() }
    game.score.should be == 300
  end

  it "calculates a strike at the end" do
    18.times { game.roll(0) }
    roll_strike
    2.times { game.roll(2) }
    game.score.should be == 14
  end

  def roll_strike
    game.roll(10)
  end

  def roll_spare()
    game.roll(5)
    game.roll(5)
  end
end

spec output

BowlingGame
  calculates the score of a gutter game
  calculates the score of 3 pins followed by gutter balls
  calculates the score of a spare followed by some pins + gutter balls
  calculates the perfect game
  calculates a strike at the end

Finished in 0.00527 seconds
5 examples, 0 failures

It looks quite verbose. Not happy with it - but at least it finally seems to work..