bogo.rb

class Calculator::Bogo < Calculator
  preference :number_to_buy, :integer, :default => 1
  preference :number_to_get, :integer, :default => 1

  def self.description
    "Buy One Get One"
  end

  def self.register
    super                                
    Promotion.register_calculator(self)
  end

  def compute(order)
    
    line_items = applicable_line_items(order)

    prices = prices_from_line_items(line_items).sort
    
    amount = prices.first(number_free(prices.size)).sum

    amount == 0 ? nil : amount
  end

  private 

  def free_ratio
    self.preferred_number_to_get.to_f / ( self.preferred_number_to_get + self.preferred_number_to_buy ) 
  end

  def applicable_line_items(order)
    order.line_items.select{|li|  li.product.respond_to?(:is_gift_card?) ? (not li.product.is_gift_card?) : true }
  end

  def prices_from_line_items(line_items)
    line_items.inject([]) {|prices, li| prices.concat [li.price]*li.quantity }
  end

  def number_free(number_of_items)
    (number_of_items * free_ratio).to_i
  end

end