Implement discount budget for month repository

- Provide a repository to retrieve a discount budget
  for a given month

lib/planet_express.rb

@@ -5,6 +5,7 @@ require 'planet_express/version'
 require 'planet_express/repository'
 
 require 'planet_express/discount_budget'
+require 'planet_express/discount_budget_for_month_repository'
 
 require 'planet_express/shipping_option'
 require 'planet_express/shipping_option_repository'

lib/planet_express/discount_budget_for_month_repository.rb

@@ -0,0 +1,35 @@
+module PlanetExpress
+  class DiscountBudgetForMonthRepository
+    DEFAULT_BUDGET = 1000
+    attr_reader :data
+
+    def initialize(data: {})
+      @data = data
+    end
+
+    def for_month(date)
+      return budget_for_month(date) if budget_for_month?(date)
+
+      add_budget_for_month(date)
+      budget_for_month(date)
+    end
+
+    private
+
+    def budget_for_month(date)
+      data[start_of_month(date)]
+    end
+
+    def budget_for_month?(date)
+      !budget_for_month(date).nil?
+    end
+
+    def add_budget_for_month(date)
+      data[start_of_month(date)] = DiscountBudget.new(budget: DEFAULT_BUDGET)
+    end
+
+    def start_of_month(date)
+      Date.new(date.year, date.month, 1)
+    end
+  end
+end

spec/discount_budget_for_month_spec.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+RSpec.describe PlanetExpress::DiscountBudgetForMonthRepository do
+  subject { described_class.new }
+  let(:march_date_a) { Date.new(2021, 3, 6) }
+  let(:march_date_b) { Date.new(2021, 3, 6) }
+
+  describe '#for_month' do
+
+    it 'returns the same discount budget for the same month' do
+        expect(subject.for_month(march_date_a).object_id).to(
+          eq(subject.for_month(march_date_b).object_id))
+    end
+
+    context 'not yet initialized month' do
+      it 'returns a discount budget' do
+        result = subject.for_month(march_date_a)
+
+        expect(result).to be_an_instance_of(PlanetExpress::DiscountBudget)
+        expect(result.available_budget).to eq(PlanetExpress::DiscountBudgetForMonthRepository::DEFAULT_BUDGET)
+      end
+    end
+
+    context 'other month' do
+      let(:april_date) { Date.new(2021, 4, 12) }
+      it 'returns a different discount budget' do
+
+        expect(subject.for_month(march_date_a).object_id).to_not(eq(
+          subject.for_month(april_date).object_id))
+      end
+    end
+  end
+end

spec/discount_budget_spec.rb

@@ -5,7 +5,6 @@ RSpec.describe PlanetExpress::DiscountBudget do
   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)
@@ -14,7 +13,7 @@ RSpec.describe PlanetExpress::DiscountBudget do
     end
 
     context 'with insufficient remaining budget' do
-      it 'returns the full discount' do
+      it 'returns only the remaining budget' do
         expect(subject.apply_discount(650)).to eq(500)
         expect(subject.available_budget).to eq(0)
       end