Implement S Shipments Discount rule

lib/planet_express.rb

@@ -18,7 +18,7 @@ require 'planet_express/shipping_option'
 require 'planet_express/shipping_option_repository'
 
 require 'planet_express/rule'
-# require 'planet_express/s_shipment_discount_rule'
+require 'planet_express/s_shipment_discount_rule'
 # require 'planet_express/l_shipment_discount_rule'
 
 require 'planet_express/shipment_discount_calculator'

lib/planet_express/s_shipment_discount_rule.rb

@@ -0,0 +1,36 @@
+module PlanetExpress
+  class SShipmentDiscountRule < Rule
+    protected
+
+    def applicable_to?(shipment)
+      s_shipment?(shipment) && more_expensive_than_cheapest_shipping_option?(shipment)
+    end
+
+    def discount_for(shipment)
+      Discount.new(discount_amount(shipment), self)
+    end
+
+    private
+
+    def cheapest_available_shipping_price(shipment)
+      shipping_option_repository.lowest_price_for_package_size(package_size(shipment))
+    end
+
+    def more_expensive_than_cheapest_shipping_option?(shipment)
+      price(shipment) > cheapest_available_shipping_price(shipment)
+    end
+
+    def package_size(shipment)
+      shipment.shipping_option.package_size
+    end
+
+    def s_shipment?(shipment)
+      shipment.shipping_option.s_shipment?
+    end
+
+    def discount_amount(shipment)
+      desired_amount = price(shipment) - cheapest_available_shipping_price(shipment)
+      discount_budget(shipment).maximum_possible_discount(desired_amount)
+    end
+  end
+end

spec/s_shipment_discount_rule_spec.rb

@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+RSpec.describe PlanetExpress::SShipmentDiscountRule do
+  let(:budget_repo) { PlanetExpress::DiscountBudgetForMonthRepository.new }
+  let(:shipping_option_repo) { PlanetExpress::ShippingOptionRepository.new }
+  let(:shipment_repo) { PlanetExpress::ShipmentRepository.new(data: []) }
+
+  subject do
+    described_class.new(
+      discount_budget_for_month_repository: budget_repo,
+      shipping_option_repository: shipping_option_repo,
+      shipment_repository: shipment_repo)
+  end
+
+  context 's_shipment' do
+
+    context 'more expensive than cheapest option' do
+      let(:shipping_option) do
+        shipping_option_repo.find_by_provider_and_package_size('MR', 'S')
+      end
+
+      let(:shipment) do
+        PlanetExpress::Shipment.new(Date.today,
+                                    shipping_option)
+      end
+
+      it 'does applies the discount' do
+        expect do
+          subject.apply_to(shipment)
+        end.to(change { shipment.discounts.length }.by(1))
+
+        expect(shipment.discounts.last.amount).to eq(50)
+      end
+
+      it 'changes the discount budget' do
+        expect do
+          subject.apply_to(shipment)
+        end.to(change { budget_repo.for_month(shipment.month).available_budget }.by(-50))
+      end
+    end
+
+    context 'equals cheapest options' do
+      let(:shipping_option) do
+        shipping_option_repo.find_by_provider_and_package_size('LP', 'S')
+      end
+
+      let(:shipment) do
+        PlanetExpress::Shipment.new(Date.today,
+                                    shipping_option)
+      end
+
+      it 'does not apply the discount' do
+        expect do
+          subject.apply_to(shipment)
+        end.to_not(change { shipment.discounts })
+      end
+    end
+  end
+
+  context 'non s_shipment' do
+    let(:shipping_option) do
+      shipping_option_repo.find_by_provider_and_package_size('LP', 'M')
+    end
+
+    let(:shipment) do
+      PlanetExpress::Shipment.new(Date.today,
+                                  shipping_option)
+    end
+
+    it 'does not apply the discount' do
+      expect do
+        subject.apply_to(shipment)
+      end.to_not(change { shipment.discounts })
+    end
+  end
+end