A simple Ruby refactoring exercise to improve a conditional-heavy method. Includes inline regression tests for both RSpec and Minitest.

README.md

Refactoring Excercise

Here’s a messy, unoptimized Ruby method. Let’s clean it up.

• How can we make this more extensible?
• How should it handle invalid inputs (negative prices, nil codes)
• How could we extract this logic into a separate class/module

discount_calculator.rb

require "bundler/inline"

gemfile do
  gem "rspec"
  gem "minitest"
end

def messy_calculate_discount(price, discount_code)
  if discount_code == 'SUMMER10'
    discount = price * 0.1
  elsif discount_code == 'WELCOME5'
    discount = price * 0.05
  elsif discount_code == 'SUPER20'
    discount = price * 0.2
  else
    discount = 0
  end
  final_price = price - discount
  if final_price < 0
    final_price = 0
  end
  return final_price
end

# ------------------------------------------
# RSpec Tests (if using rspec)
# Run: rspec discount_calculator.rb
# ------------------------------------------
if defined?(RSpec)
  RSpec.describe "Discount Calculator" do
    it 'applies SUMMER10 correctly' do
      expect(messy_calculate_discount(100, 'SUMMER10')).to eq(90)
    end

    it 'applies WELCOME5 correctly' do
      expect(messy_calculate_discount(100, 'WELCOME5')).to eq(95)
    end

    it 'applies SUPER20 correctly' do
      expect(messy_calculate_discount(100, 'SUPER20')).to eq(80)
    end

    it 'returns original price if code is unknown' do
      expect(messy_calculate_discount(100, 'UNKNOWN')).to eq(100)
    end

    it 'caps final price at 0' do
      expect(messy_calculate_discount(100, 'SUPER20')).to be >= 0
      expect(messy_calculate_discount(-100, 'SUPER20')).to eq(0)
    end
  end
end

# ------------------------------------------
# Minitest Tests (if using minitest)
# Run: ruby discount_calculator.rb
# ------------------------------------------
if __FILE__ == $0
  require 'minitest/autorun'

  class DiscountCalculatorTest < Minitest::Test
    def test_summer10
      assert_equal 90, messy_calculate_discount(100, 'SUMMER10')
    end

    def test_welcome5
      assert_equal 95, messy_calculate_discount(100, 'WELCOME5')
    end

    def test_super20
      assert_equal 80, messy_calculate_discount(100, 'SUPER20')
    end

    def test_unknown_code
      assert_equal 100, messy_calculate_discount(100, 'INVALID')
    end

    def test_negative_price_returns_zero
      assert_equal 0, messy_calculate_discount(-100, 'SUMMER10')
    end

    def test_nil_code
      assert_equal 100, messy_calculate_discount(100, nil)
    end
  end
end