Rename discount budget methods and expose maximum possible discount

- Rename #apply_discount to #register_applied_discount to make it
  more clear that it only registers discounts and does not apply them
- Expose maximum possible discount to make it possible to get get the possible
  discount without registering it in the budget

lib/planet_express/discount_budget.rb

@@ -9,12 +9,16 @@ module PlanetExpress
       @applied_discounts = []
     end
 
-    def apply_discount(discount)
-      possible_discount = maximum_possible_discount(discount)
+    def register_applied_discount(discount_amount)
+      possible_discount = maximum_possible_discount(discount_amount)
       applied_discounts.push(possible_discount)
       possible_discount
     end
 
+    def maximum_possible_discount(discount_amount)
+      [available_budget, discount_amount].min
+    end
+
     def available_budget
       budget - spent_budget
     end
@@ -30,9 +34,5 @@ module PlanetExpress
     def spent_budget
       applied_discounts.sum
     end
-
-    def maximum_possible_discount(discount)
-      [available_budget, discount].min
-    end
   end
 end

spec/discount_budget_spec.rb

@@ -4,10 +4,10 @@ RSpec.describe PlanetExpress::DiscountBudget do
   let(:budget) { 500 }
   subject { described_class.new(budget: budget) }
 
-  describe '#apply_discount' do
+  describe '#register_applied_discount' do
     context 'with sufficient remaining budget' do
       it 'returns the full discount' do
-        expect(subject.apply_discount(120)).to eq(120)
+        expect(subject.register_applied_discount(120)).to eq(120)
         expect(subject.available_budget).to eq(380)
         expect(subject.remaining_budget?).to eq(true)
       end
@@ -15,10 +15,24 @@ RSpec.describe PlanetExpress::DiscountBudget do
 
     context 'with insufficient remaining budget' do
       it 'returns only the remaining budget' do
-        expect(subject.apply_discount(650)).to eq(500)
+        expect(subject.register_applied_discount(650)).to eq(500)
         expect(subject.available_budget).to eq(0)
         expect(subject.remaining_budget?).to eq(false)
       end
     end
   end
+
+  describe '#maximum_possible_discount' do
+    context 'with sufficient budget' do
+      it 'returns the full discount amount' do
+        expect(subject.maximum_possible_discount(300)).to eq(300)
+      end
+    end
+
+    context 'with insufficient budget' do
+      it 'returns the maximum possible amount' do
+        expect(subject.maximum_possible_discount(600)).to eq(500)
+      end
+    end
+  end
 end