Example of a refactored discount_calculator.rb logic into a reusable class using a hash lookup for cleaner, more scalable code, with safeguards and test coverage in both RSpec and Minitest.

discount_calculator_solution.rb

require "bundler/inline"

gemfile do
  gem "rspec"
  gem "minitest"
end

class DiscountCalculator
  DISCOUNTS = {
    'SUMMER10' => 0.10,
    'WELCOME5' => 0.05,
    'SUPER20'  => 0.20
  }.freeze

  def self.calculate(price, code)
    return 0 unless price.is_a?(Numeric) && price >= 0

    discount_rate = DISCOUNTS[code] || 0
    discounted_price = price - (price * discount_rate)
    [discounted_price, 0].max
  end
end

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

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

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

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

      it 'caps final price at 0' do
        expect(described_class.calculate(100, 'SUPER20')).to be >= 0
        expect(described_class.calculate(-100, 'SUPER20')).to eq(0)
      end
    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, DiscountCalculator.calculate(100, 'SUMMER10')
    end

    def test_welcome5
      assert_equal 95, DiscountCalculator.calculate(100, 'WELCOME5')
    end

    def test_super20
      assert_equal 80, DiscountCalculator.calculate(100, 'SUPER20')
    end

    def test_unknown_code
      assert_equal 100, DiscountCalculator.calculate(100, 'INVALID')
    end

    def test_negative_price_returns_zero
      assert_equal 0, DiscountCalculator.calculate(-100, 'SUMMER10')
    end

    def test_nil_code
      assert_equal 100, DiscountCalculator.calculate(100, nil)
    end
  end
end