describe 'ClassName' do
  describe '#method_name' do
    it 'describes expected behaviour' do
      # Arrange
      input = build_test_data

      # Act
      result = subject.method_name(input)

      # Assert
      expect(result).to eq(expected)
    end
  end
end

bundle exec rspec           # RSpec
bundle exec rake test       # Minitest

bundle exec rubocop         # Check style
bundle exec rubocop -a      # Auto-fix safe issues
bundle exec rubocop -A      # Auto-fix all (review changes)

# ❌ Imperative
results = []
items.each { |i| results << transform(i) }

# ✅ Functional
results = items.map { |i| transform(i) }

valid_items = items.select(&:valid?)
invalid_items = items.reject(&:valid?)

total = items.reduce(0) { |sum, item| sum + item.value }

items
  .select(&:active?)
  .map(&:transform)
  .reduce(initial) { |acc, x| combine(acc, x) }

CONSTANTS = ['a', 'b', 'c'].freeze
hash = { key: 'value' }.freeze