Engineering Standards

Foundational principles for high-quality software. Use this skill when:

• Reviewing code for quality issues
• Planning architectural changes
• Identifying refactoring opportunities
• Evaluating technical debt

SOLID Principles

Single Responsibility (SRP)

A class/module should have one reason to change.

Detection patterns:

• Class with 5+ public methods doing unrelated things
• Method longer than 50 lines
• Class name contains "And" or "Manager" with mixed concerns
• File imports from 10+ unrelated modules

Refactoring:

• Extract class for each responsibility
• Split into focused modules
• Use composition over inheritance

Open/Closed (OCP)

Open for extension, closed for modification.

Detection patterns:

• Switch/case on type with frequent additions
• if/else chains checking instance types
• Modifying existing code to add new features

Refactoring:

• Strategy pattern for varying behaviors
• Plugin architecture for extensions
• Dependency injection for configurability

Liskov Substitution (LSP)

Subtypes must be substitutable for their base types.

Detection patterns:

• Override that throws "not implemented"
• Subclass that ignores parent behavior
• Type checks before calling inherited methods

Refactoring:

• Favor composition over inheritance
• Extract interface for true polymorphism
• Use abstract base with required overrides

Interface Segregation (ISP)

Clients shouldn't depend on methods they don't use.

Detection patterns:

• Interface with 10+ methods
• Implementing classes that stub methods as no-ops
• "Fat" interfaces with unrelated method groups

Refactoring:

• Split into role-specific interfaces
• Use mixins/traits for optional behaviors
• Compose multiple focused interfaces

Dependency Inversion (DIP)

Depend on abstractions, not concretions.

Detection patterns:

• Direct instantiation of dependencies (new ConcreteClass())
• Hard-coded database/API connections
• Test files creating production instances

Refactoring:

• Constructor injection
• Factory pattern for complex creation
• Interface-based dependencies

DRY (Don't Repeat Yourself)

Detection patterns:

• Copy-pasted code blocks (3+ occurrences)
• Similar functions with minor variations
• Duplicated validation logic
• Repeated configuration values

Refactoring:

• Extract shared function/class
• Parameterize variations
• Create configuration constants
• Use template method pattern

Exceptions (acceptable duplication):

• Test code clarity (explicit over DRY)
• Cross-boundary isolation (microservices)
• Performance-critical paths

YAGNI (You Aren't Gonna Need It)

Detection patterns:

• Unused parameters "for future use"
• Abstract classes with single implementation
• Configuration options never used
• Speculative generalization

Guidance:

• Build for current requirements
• Refactor when needs emerge
• Delete dead code immediately
• Prefer simple over flexible

KISS (Keep It Simple, Stupid)

Detection patterns:

• Cyclomatic complexity > 10
• Nested callbacks/promises 4+ deep
• Generic solutions for specific problems
• Framework overkill for simple tasks

Refactoring:

• Flatten control flow
• Extract named functions
• Use early returns
• Choose boring technology

Code Smells Quick Reference

When to Apply

Always apply:

• SRP, DRY for production code
• KISS for all code

Apply with judgment:

• OCP when extension points are clear
• ISP when interfaces grow beyond 5 methods
• DIP at module boundaries

Defer:

• YAGNI violations until pattern emerges 3+ times

See [references/refactoring-catalog.md](references/refactoring-catalog.md) for detailed refactoring techniques.