module Returnly
  module Discounts
    class LineItem
      attr_reader :adjustment, :order

      def initialize(order, adjustment)
        @adjustment = adjustment
        @order      = order
      end

      def discount_amount(line_item, units = 0)
        return 0.0 if units <= 0
        return 0.0 if adjustment.adjustable != line_item

        units = line_item.quantity if units > line_item.quantity
        adjustment.amount * weight_of(line_item, units.to_d)
      end

      private

      def weight_of(line_item, units)
        (units / line_item.quantity).round(2, :down)
      end
    end
  end
end