pattern-detector

What this skill does

# Pattern Detector Skill

Detect design patterns and anti-patterns in code with recommendations.

## Instructions

You are a design pattern expert. When invoked:

1. **Identify Design Patterns**: Recognize common patterns in use:
   - Creational: Singleton, Factory, Builder, Prototype
   - Structural: Adapter, Decorator, Facade, Proxy, Composite
   - Behavioral: Observer, Strategy, Command, State, Iterator
   - Architectural: MVC, MVVM, Repository, Service Layer

2. **Detect Anti-Patterns**: Find problematic patterns:
   - God Object (too many responsibilities)
   - Spaghetti Code (complex, tangled logic)
   - Lava Flow (obsolete code kept around)
   - Golden Hammer (overusing one solution)
   - Cargo Cult (copying without understanding)
   - Premature Optimization
   - Magic Numbers and Strings

3. **Analyze Implementation**:
   - Is pattern implemented correctly?
   - Is pattern appropriate for use case?
   - Are there simpler alternatives?
   - Does it follow best practices?

4. **Provide Recommendations**:
   - Suggest better patterns when appropriate
   - Show how to fix anti-patterns
   - Explain trade-offs
   - Give refactoring guidance

## Common Design Patterns

### Singleton Pattern
```javascript
// ✓ Good implementation
class DatabaseConnection {
  static #instance = null;

  constructor() {
    if (DatabaseConnection.#instance) {
      throw new Error('Use getInstance()');
    }
    this.connection = null;
  }

  static getInstance() {
    if (!DatabaseConnection.#instance) {
      DatabaseConnection.#instance = new DatabaseConnection();
    }
    return DatabaseConnection.#instance;
  }
}

// Usage
const db = DatabaseConnection.getInstance();
```

### Factory Pattern
```javascript
// ✓ Good implementation
class PaymentFactory {
  static createPayment(type, amount) {
    switch(type) {
      case 'credit':
        return new CreditCardPayment(amount);
      case 'paypal':
        return new PayPalPayment(amount);
      case 'crypto':
        return new CryptoPayment(amount);
      default:
        throw new Error(`Unknown payment type: ${type}`);
    }
  }
}

// Usage
const payment = PaymentFactory.createPayment('credit', 100);
```

### Observer Pattern
```javascript
// ✓ Good implementation
class EventEmitter {
  constructor() {
    this.events = {};
  }

  on(event, listener) {
    if (!this.events[event]) {
      this.events[event] = [];
    }
    this.events[event].push(listener);
  }

  emit(event, data) {
    if (this.events[event]) {
      this.events[event].forEach(listener => listener(data));
    }
  }

  off(event, listener) {
    if (this.events[event]) {
      this.events[event] = this.events[event].filter(l => l !== listener);
    }
  }
}
```

### Strategy Pattern
```javascript
// ✓ Good implementation
class PriceCalculator {
  constructor(strategy) {
    this.strategy = strategy;
  }

  calculate(price) {
    return this.strategy.calculate(price);
  }
}

class RegularPrice {
  calculate(price) { return price; }
}

class MemberPrice {
  calculate(price) { return price * 0.9; }
}

class VIPPrice {
  calculate(price) { return price * 0.8; }
}

// Usage
const calculator = new PriceCalculator(new MemberPrice());
const finalPrice = calculator.calculate(100); // 90
```

## Common Anti-Patterns

### God Object
```javascript
// ❌ Bad - God Object
class Application {
  // Handles everything: auth, database, UI, business logic, etc.
  login(user) { }
  logout() { }
  saveData(data) { }
  loadData() { }
  renderUI() { }
  processPayment() { }
  sendEmail() { }
  // ... 50 more methods
}

// ✓ Good - Single Responsibility
class AuthService {
  login(user) { }
  logout() { }
}

class DataService {
  save(data) { }
  load() { }
}

class PaymentService {
  process(payment) { }
}
```

### Spaghetti Code
```javascript
// ❌ Bad - Spaghetti Code
function processOrder(order) {
  if (order.items.length > 0) {
    let total = 0;
    for (let i = 0; i < order.items.length; i++) {
      if (order.items[i].discount) {
        if (order.user.membership === 'gold') {
          total += order.items[i].price * 0.8 * 0.9;
        } else if (order.user.membership === 'silver') {
          total += order.items[i].price * 0.9 * 0.95;
        } else {
          total += order.items[i].price * 0.9;
        }
      } else {
        total += order.items[i].price;
      }
    }
    if (order.shippingMethod === 'express') {
      total += 20;
    } else if (order.shippingMethod === 'standard') {
      total += 10;
    }
    return total;
  }
  return 0;
}

// ✓ Good - Clean, separated concerns
function processOrder(order) {
  if (!order.items.length) return 0;

  const itemsTotal = calculateItemsTotal(order.items, order.user);
  const shipping = calculateShipping(order.shippingMethod);
  return itemsTotal + shipping;
}

function calculateItemsTotal(items, user) {
  return items.reduce((total, item) => {
    const price = applyDiscount(item.price, item.discount);
    return total + applyMembershipDiscount(price, user.membership);
  }, 0);
}

function calculateShipping(method) {
  const rates = { express: 20, standard: 10 };
  return rates[method] || 0;
}
```

### Magic Numbers
```javascript
// ❌ Bad - Magic numbers
function canVote(age) {
  return age >= 18;
}

function isPremium(spent) {
  return spent > 1000;
}

// ✓ Good - Named constants
const VOTING_AGE = 18;
const PREMIUM_THRESHOLD = 1000;

function canVote(age) {
  return age >= VOTING_AGE;
}

function isPremium(spent) {
  return spent > PREMIUM_THRESHOLD;
}
```

### Cargo Cult Programming
```javascript
// ❌ Bad - Copied pattern without understanding
class SimpleDataStore {
  // Unnecessary complexity for simple use case
  private strategy: StorageStrategy;
  private observer: Observer;
  private factory: DataFactory;
  private singleton: Singleton;

  // Just needs simple key-value storage!
}

// ✓ Good - KISS (Keep It Simple)
class SimpleDataStore {
  private data = new Map();

  set(key, value) {
    this.data.set(key, value);
  }

  get(key) {
    return this.data.get(key);
  }
}
```

## Usage Examples

```
@pattern-detector
@pattern-detector src/
@pattern-detector --show-patterns
@pattern-detector --anti-patterns-only
@pattern-detector UserService.js
```

## Report Format

```markdown
# Pattern Detection Report

## Summary
- Files analyzed: 34
- Design patterns found: 12
- Anti-patterns detected: 8
- Recommendations: 15

---

## Design Patterns Detected

### ✓ Singleton Pattern (2 instances)

**Location**: src/config/Database.js:12
**Pattern**: Singleton
**Implementation**: Good
**Purpose**: Ensure single database connection
**Notes**: Correctly implemented with private constructor

**Location**: src/services/Logger.js:8
**Pattern**: Singleton
**Implementation**: Problematic
**Issue**: Not thread-safe, uses simple boolean flag
**Recommendation**: Use the proper instance check or consider dependency injection

---

### ✓ Factory Pattern (3 instances)

**Location**: src/payments/PaymentFactory.js:23
**Pattern**: Factory Method
**Implementation**: Good
**Purpose**: Create different payment processor instances
**Notes**: Clean implementation, easily extensible

---

### ✓ Observer Pattern (4 instances)

**Location**: src/events/EventBus.js:15
**Pattern**: Observer (Pub/Sub)
**Implementation**: Good
**Purpose**: Decoupled event handling
**Notes**: Well-implemented event system

**Location**: src/state/Store.js:45
**Pattern**: Observer
**Implementation**: Good
**Purpose**: State management with subscriptions
**Notes**: Similar to Redux pattern

---

### ✓ Strategy Pattern (1 instance)

**Location**: src/sorting/Sorter.js:34
**Pattern**: Strategy
**Implementation**: Good
**Purpose**: Pluggable sorting algorithms
**Notes**: Good use of strategy for runtime algorithm selection

---

### ⚠️ Decorator Pattern (2 instances)

**Location**: src/api/middleware.js:67
**Pattern**: Decorator (Middleware)
**Implementation**: Acceptable
**Purpose**: Request/response modification
**Notes**: Could be simplified, consider