Refactor Skill

This skill provides detailed guidance for the REFACTOR phase of the RED-GREEN-REFACTOR cycle in Test-Driven Development.

When to use this skill:

• After tests pass (GREEN phase complete)
• When code duplication is identified
• Before implementing new features that may duplicate existing patterns
• During code reviews to improve code quality

Core Principle: Refactor-First Mindset

Before writing new code, always ask:

1. Does similar functionality already exist?
2. Can I refactor existing code to handle both cases?
3. What patterns or abstractions are already in use?
4. Would this be better as a modification to existing code?

Refactoring Workflow

Phase 1: Search for Existing Patterns

CRITICAL: Always search before implementing new functionality

Use code search tools to find:

• Similar function names or patterns
• Duplicate logic or algorithms
• Existing abstractions that could be extended
• Related utilities or helpers

Search strategies:

• Grep for similar function/method names
• Search for similar algorithm patterns
• Look for related types or interfaces
• Check for existing error handling patterns
• Search for similar data transformations

Phase 2: Analyze Duplication

When similar code is found, evaluate:

Questions to ask:

• Is this duplication intentional or accidental?
• Do these implementations handle the same concept?
• Can they be unified without increasing complexity?
• Is there a common abstraction that emerges?

Red flags for duplication:

• Copy-pasted code with minor variations
• Similar logic in different locations
• Repeated error handling patterns
• Multiple implementations of same algorithm
• Similar data validation or transformation logic

Phase 3: Choose Refactoring Strategy

Based on the duplication analysis, choose appropriate strategy:

1. Extract Function/Method

• When: Same code block appears multiple times
• Action: Extract to shared function
• Benefit: Single source of truth

2. Parameterize Function

• When: Similar functions with minor variations
• Action: Add parameters to handle variations
• Benefit: Reduces function count

3. Extract Common Interface/Trait

• When: Multiple types share similar behavior
• Action: Define shared interface
• Benefit: Polymorphic usage

4. Introduce Abstraction Layer

• When: Complex logic duplicated across features
• Action: Create abstraction that handles complexity
• Benefit: Simpler calling code

5. Use Existing Abstraction

• When: Functionality fits existing abstraction
• Action: Extend or modify existing abstraction
• Benefit: Consistency with codebase patterns

6. Keep Duplication

• When: Concepts are fundamentally different despite similar code
• When: Unification would increase coupling inappropriately
• When: Code will diverge in future
• Action: Document why duplication is intentional
• Benefit: Avoids premature abstraction

Phase 4: Refactor Safely

Safety rules for refactoring:

1. All tests must be GREEN before starting

   • Never refactor with failing tests
   • Tests are your safety net

2. Make small, incremental changes

   • One refactoring at a time
   • Run tests after each change
   • Commit frequently

3. Run ALL tests after each change

   • Ensures no regression
   • Catches unintended side effects
   • Maintains confidence

4. If tests fail, revert immediately

   • Don't try to fix forward
   • Revert and try different approach
   • Smaller steps if needed

5. Keep refactoring separate from feature work

   • Don't mix refactoring and new features in same commit
   • Refactoring commits should not change behavior
   • Feature commits add new behavior

Phase 5: Verify and Document

After refactoring:

Verification checklist:

• All tests still pass
• No new compiler warnings
• Code is more readable
• Duplication is reduced or eliminated
• Performance is not degraded
• Documentation is updated

Documentation:

• Update function/module documentation
• Add comments explaining complex refactorings
• Document design decisions (especially if keeping duplication)
• Update architecture diagrams if needed

Refactoring Anti-Patterns

❌ Anti-Pattern 1: Premature Abstraction

Problem: Creating abstractions before understanding the pattern Solution: Follow "rule of three" - wait until duplication appears 3 times

❌ Anti-Pattern 2: Over-Engineering

Problem: Creating overly complex abstractions for simple duplication Solution: Keep refactoring simple and focused

❌ Anti-Pattern 3: Breaking Encapsulation

Problem: Exposing internals to reduce duplication Solution: Sometimes duplication is better than bad coupling

❌ Anti-Pattern 4: Big Bang Refactoring

Problem: Changing too much at once Solution: Small, incremental changes with test verification

❌ Anti-Pattern 5: Refactoring Without Tests

Problem: Changing code without safety net Solution: Write tests first if they don't exist

When NOT to Refactor

Skip refactoring when:

• Tests are not passing (fix tests first)
• Code will be deleted soon
• Time pressure is extreme (but add TODO)
• Concepts are fundamentally different despite similar code
• System is stable and working (if it ain't broke...)
• Refactoring would introduce inappropriate coupling

Integration with TDD Workflow

Refactoring is Step 4 in the RED-GREEN-REFACTOR cycle:

1. RED: Write test that fails
2. GREEN: Write minimal code to pass
3. VERIFY: Run all tests
4. REFACTOR: Improve code quality ← (this skill)
5. VERIFY: Run all tests again
6. REPEAT: Next test

When implementing new features:

1. Search for existing similar patterns (use this skill)
2. Decide: refactor existing vs. create new
3. If refactoring: do it before adding new feature
4. Then proceed with TDD cycle for new feature

File Organization

For guidance on splitting large files into modules: See FILE-ORGANIZATION.md for guidelines on when and how to split files.

Practical Checklist

Before writing new code:

• Search codebase for similar functionality
• Review existing abstractions in the area
• Check if existing code can be extended
• Consider refactoring existing code first
• Document decision (refactor vs. new code)

During refactoring:

• All tests passing before starting
• Make one small change at a time
• Run all tests after each change
• Commit working changes frequently
• Revert if tests fail

After refactoring:

• All tests still passing
• Code is more maintainable
• Duplication reduced or eliminated
• Documentation updated
• No performance regression