Implement a discount budget

A discount budget to represent the available budget for
discounts

lib/planet_express.rb

@@ -4,6 +4,8 @@ require 'planet_express/version'
 
 require 'planet_express/repository'
 
+require 'planet_express/discount_budget'
+
 require 'planet_express/shipping_option'
 require 'planet_express/shipping_option_repository'
 

lib/planet_express/discount_budget.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+module PlanetExpress
+  class DiscountBudget
+    attr_reader :budget
+
+    def initialize(budget:)
+      @budget = budget
+      @applied_discounts = []
+    end
+
+    def apply_discount(discount)
+      possible_discount = maximum_possible_discount(discount)
+      applied_discounts.push(possible_discount)
+      possible_discount
+    end
+
+    def available_budget
+      budget - spent_budget
+    end
+
+    private
+
+    attr_reader :applied_discounts
+
+    def spent_budget
+      applied_discounts.sum
+    end
+
+    def maximum_possible_discount(discount)
+      [available_budget, discount].min
+    end
+  end
+end

spec/discount_budget_spec.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+RSpec.describe PlanetExpress::DiscountBudget do
+  let(:budget) { 500 }
+  subject { described_class.new(budget: budget) }
+
+  describe '#apply_discount' do
+
+    context 'with sufficient remaining budget' do
+      it 'returns the full discount' do
+        expect(subject.apply_discount(120)).to eq(120)
+        expect(subject.available_budget).to eq(380)
+      end
+    end
+
+    context 'with insufficient remaining budget' do
+      it 'returns the full discount' do
+        expect(subject.apply_discount(650)).to eq(500)
+        expect(subject.available_budget).to eq(0)
+      end
+    end
+  end
+end