bogo.rb

module Spree
  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 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
end


# register it inside application.rb with:

    config.after_initialize  do
      config.spree.calculators.promotion_actions_create_adjustments << Spree::Calculator::Bogo
    end