score.rb

# encoding: UTF-8

class Score

  attr_accessor :rolls

  def initialize(rolls)
    @rolls = rolls.each_char
  end

  def total
    @special_trick_bonus = 1
    rolls.inject 0 do | total, current |
      roll_score = score_as_points current
      roll_score = roll_score * @special_trick_bonus if in_special_trick?

      @previous_score = roll_score
      update_special_trick_bonus! current
      total + roll_score
    end
  end

  private

  def score_as_points score
    return 10 if strike? score
    return 10 - @previous_score if spare? score
    score.to_i
  end

  def strike?(score) score == "X" end
  def spare?(score) score == "/" end

  def in_special_trick?() @special_trick_bonus > 1 end

  def update_special_trick_bonus! score
    if(strike?(score) || spare?(score))
      @special_trick_bonus += 1
    else
      @special_trick_bonus = 1
    end
  end
end

score_test.rb

# encoding: UTF-8

require_relative "../lib/score"
require "test/unit"

class ScoreTest < Test::Unit::TestCase

  def test_score_can_have_rolls_given
    rolls = "--------------------"
    score = Score.new(rolls)

    assert_not_nil score.rolls
  end

  def test_all_zero_rolls_should_return_zero
    rolls = "--------------------"
    score = Score.new(rolls)
    expected = 0

    assert_equal expected, score.total
  end

  def test_one_5_score_and_nine_zeros_should_return_5
    rolls = "5-------------------"
    score = Score.new(rolls)

    assert_equal 5, score.total
  end

  def test_one_strike_followed_by_a_5_and_8_zeros_should_return_20
    rolls = "X5-----------------"
    score = Score.new(rolls)

    assert_equal 20, score.total
  end

  def test_one_spare_followed_by_a_5_and_8_zeros_should_return_20
    rolls = "9/5----------------"
    score = Score.new(rolls)

    assert_equal 20, score.total
  end

  def test_one_spare_of_8_followed_by_a_5_and_8_zeros_should_return_20
    rolls = "8/5----------------"
    score = Score.new(rolls)

    assert_equal 20, score.total
  end

  def test_two_strikes_followed_by_8_zeros_should_return_30
    rolls = "XX-----------------"
    score = Score.new(rolls)

    assert_equal 30, score.total
  end

  def test_two_strikes_followed_by_3_and_7_zeros_should_return_39
    rolls = "XX3----------------"
    score = Score.new(rolls)

    assert_equal 39, score.total
  end

  def test_7_zeros_should_return_39
    rolls = "---------------/XX3"
    score = Score.new(rolls)

    assert_equal 59, score.total
  end
end