Migrating a percentage off shipping script

Danger pair D16. This file and its partner look almost identical and behave differently, so read both before you decide which one you have.

# ================================ Customizable Settings ================================
DISCOUNT_TYPE = :percent
DISCOUNT_AMOUNT = 10
DISCOUNT_MESSAGE = '10% off shipping'

# ================================ Script Code (do not edit) ================================
class DiscountApplicator
  def initialize(discount_type, discount_amount, discount_message)
    @discount_type = discount_type
    @discount_message = discount_message

    @discount_amount = if discount_type == :percent
      discount_amount * 0.01
    else
      Money.new(cents: 100) * discount_amount
    end
  end

  def apply(shipping_rate)
    discount_amount = if @discount_type == :percent
      shipping_rate.price * @discount_amount
    else
      @discount_amount > shipping_rate.price ? shipping_rate.price : @discount_amount
    end

    shipping_rate.apply_discount(discount_amount, message: @discount_message)
  end
end

class DiscountRatesCampaign
  def initialize(discount_applicator)
    @discount_applicator = discount_applicator
  end

  def run(cart, shipping_rates)
    shipping_rates.each do |shipping_rate|
      @discount_applicator.apply(shipping_rate)
    end
  end
end

CAMPAIGNS = [
  DiscountRatesCampaign.new(
    DiscountApplicator.new(DISCOUNT_TYPE, DISCOUNT_AMOUNT, DISCOUNT_MESSAGE),
  ),
]

CAMPAIGNS.each do |campaign|
  campaign.run(Input.cart, Input.shipping_rates)
end

Output.shipping_rates = Input.shipping_rates

Recognised as Shipping discount and rebuilt as a free_shipping offer. These are the real configuration keys the offer is saved with, so you can check your own rebuild against them field by field.

Built on our public demo store, Meridian Goods, so every price here can be reproduced rather than taken on trust. The discount below is what the same engine that runs at checkout computes for this exact cart.

This rule carries no condition at all, so it fires on any cart holding anything and there is no quiet cart to build. The negative check is an empty cart, which the app runs for you. That is correct behaviour, not a failure.