Shopify Scripts BuySetGetXOffCampaign example - https://www.shopify.com/partners/blog/109804486-getting-started-with-shopify-scripts-a-practical-walkthrough

shopify-scripts-buy-set-get-x-off-campaign.rb

# The campaign class.
class BuySetGetXOffCampaign

  def initialize(selector, discount, partitioner)
    @selector = selector
    @discount = discount
    @partitioner = partitioner
  end

  def run(cart)
    applicable_items = cart.line_items.select do |line_item|
      @selector.match?(line_item)
    end
    discounted_items = @partitioner.partition(cart, applicable_items)

    discounted_items.each do |line_item|
      @discount.apply(line_item)
    end
  end

end