# Chess Game - System Design

## Architecture Overview

The chess game follows a **modular, layered architecture** with clear separation of concerns. The system is designed for extensibility, testability, and maintainability.

### Architectural Style
- **Pattern**: MVC (Model-View-Controller) with Event-Driven Architecture
- **Modularity**: ES6 Modules for component isolation
- **State Management**: Centralized game state with immutable updates
- **Communication**: Event-based pub/sub for component decoupling

## System Layers

### 1. Presentation Layer
- **ChessBoardView**: Visual board rendering
- **ChessPieceView**: Piece rendering and animations
- **UIController**: User input handling and feedback
- **ThemeManager**: Visual styling and customization

### 2. Business Logic Layer
- **GameEngine**: Core game rules and state management
- **MoveValidator**: Legal move validation and check detection
- **MoveGenerator**: All possible moves calculation
- **GameController**: Game flow orchestration
- **TurnManager**: Player turn handling

### 3. Data Layer
- **BoardState**: Current board configuration
- **GameHistory**: Move history and undo/redo
- **GameConfig**: Configuration and settings
- **PersistenceManager**: Save/load game state

### 4. AI Layer (Optional)
- **AIPlayer**: Computer opponent interface
- **MoveEvaluator**: Position evaluation
- **SearchAlgorithm**: Minimax with alpha-beta pruning

## Core Principles

### Single Responsibility
Each component has one clear purpose and reason to change.

### Open/Closed Principle
Components are open for extension but closed for modification through interfaces and hooks.

### Dependency Inversion
High-level modules depend on abstractions, not concrete implementations.

### Event-Driven Communication
Components communicate through events to minimize coupling.

## System Constraints

### Performance
- Board updates: < 16ms (60 FPS)
- Move validation: < 5ms
- AI move calculation: < 2000ms (configurable)

### Browser Compatibility
- Modern browsers (ES6+ support)
- Chrome 90+, Firefox 88+, Safari 14+, Edge 90+

### Accessibility
- Keyboard navigation support
- Screen reader compatibility
- ARIA labels for all interactive elements

## Security Considerations

### Client-Side Only (Phase 1)
- No network communication
- Local storage only for persistence
- Input validation for all user actions

### Future Network Play (Phase 2)
- WebSocket communication
- Move verification on server
- Anti-cheat measures
- Rate limiting

## Scalability Strategy

### Modular Extension Points
- Plugin system for new features
- Theme customization hooks
- AI difficulty levels
- Alternative rule sets (variants)

### Performance Optimization
- Virtual DOM for efficient rendering
- Move generation caching
- Position evaluation memoization
- Web Workers for AI computation

## Deployment Architecture

### File Structure
```
chess-game/
├── index.html          # Main entry point
├── styles/
│   ├── main.css        # Core styles
│   ├── themes/         # Visual themes
│   └── responsive.css  # Mobile support
├── src/
│   ├── core/           # Business logic
│   ├── ui/             # Presentation
│   ├── ai/             # AI components
│   └── utils/          # Shared utilities
├── assets/
│   ├── pieces/         # Piece images
│   └── sounds/         # Sound effects
└── tests/              # Test suite
```

## Technology Stack

### Core Technologies
- **HTML5**: Semantic structure
- **CSS3**: Styling and animations
- **Vanilla JavaScript**: ES6+ for logic

### Optional Enhancements
- **TypeScript**: Type safety (future)
- **Web Workers**: Background AI computation
- **Service Workers**: Offline play
- **IndexedDB**: Persistent storage

## Quality Attributes

### Maintainability
- Clear code organization
- Comprehensive documentation
- Automated testing (unit + integration)

### Testability
- Pure functions for core logic
- Dependency injection
- Mock-friendly interfaces

### Usability
- Intuitive drag-and-drop
- Visual feedback for all actions
- Responsive design for all devices

### Extensibility
- Plugin architecture
- Configuration-driven behavior
- Event hooks for customization