bowling.rb

class BowlingGame

  def initialize
    @all_rolls = []
  end

  def roll(pins)
    @all_rolls << pins
  end

  def score
    sum(build_frame_scores(@all_rolls).flatten.slice(0,10))
  end

  def sum(list)
    list.inject(0) { |total, i| total + i }
  end

  def build_frame_scores(rolls)
    return [] if rolls.empty?

    strike = rolls.first == 10
    spare  = rolls[0] + (rolls[1] || 0) == 10

    if strike
      [sum(rolls.slice(0,3)), build_frame_scores(rolls.drop(1))]
    elsif spare
      [sum(rolls.slice(0,3)), build_frame_scores(rolls.drop(2))]
    else
      [sum(rolls.slice(0,2)), build_frame_scores(rolls.drop(2))]
    end
  end
end