Implement a rule base class

Implement a potential base class for future rules

lib/planet_express/rule.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+module PlanetExpress
+  class Rule
+    attr_reader :discount_budget_for_month_repository,
+                :shipping_option_repository,
+                :shipment_repository
+
+    def initialize(discount_budget_for_month_repository:,
+                   shipping_option_repository:,
+                   shipment_repository: )
+      @discount_budget_for_month_repository = discount_budget_for_month_repository
+      @shipping_option_repository = shipping_option_repository
+      @shipment_repository = shipment_repository
+    end
+
+    def apply_to(shipment)
+      return unless discount_budget(shipment).remaining_budget?
+      return unless applicable_to?(shipment)
+
+      register_discount(shipment, discount_for(shipment))
+    end
+
+    def inspect
+      self.class.name
+    end
+
+    protected
+
+    def register_discount(shipment, discount)
+      shipment.add_discount(discount)
+      discount_budget(shipment).register_applied_discount(discount.amount)
+    end
+
+    def applicable_to?(shipment)
+      raise NotImplementedError
+    end
+
+    def discount_for(shipment)
+      raise NotImplementedError
+    end
+
+    def price(shipment)
+      shipment.shipping_option.price_in_cents
+    end
+
+    def discount_budget(shipment)
+      discount_budget_for_month_repository.for_month(shipment.date)
+    end
+  end
+end