checkout.rb

class CheckOut
  attr_accessor :total

  def initialize(rules)
    @rules = rules
    @total = 0
    @amount_of_units = Hash.new(0)
  end

  def scan(item)
    update_amount_of_units item
    @total += find_price item
  end

  private
  def update_amount_of_units item
    @amount_of_units[item] += 1
  end

  def find_price item
    if has_special_price?(item) && fits_the_special_price_criteria?(item)
      @rules[item][:special_price][:price] - (@rules[item][:special_price][:amount]-1) * @rules[item][:unit_price]
    else
      @rules[item][:unit_price]
    end
  end

  def fits_the_special_price_criteria? item
    (@amount_of_units[item] % @rules[item][:special_price][:amount] == 0) if has_special_price? item
  end

  def has_special_price? item
    @rules[item].has_key? :special_price
  end
end