PRD.txt

# Product Requirements Document
## Smart Code Diffing Tool

**Version:** 1.0  
**Date:** September 2025  
**Author:** Product Team

---

## Executive Summary

This PRD outlines the requirements for a next-generation code diffing tool that performs structural and semantic comparison of source code files. Unlike traditional line-based diff tools, this solution will understand code structure at the Abstract Syntax Tree (AST) level, enabling intelligent comparison of functions, classes, and other code elements regardless of their position in files.

---

## Problem Statement

### Current Challenges

1. **Order Dependency**: Traditional diff tools (including Beyond Compare) compare files line-by-line, making them sensitive to code reorganization
2. **Noise from Formatting**: Whitespace changes, indentation, and code formatting create unnecessary diff noise
3. **Poor Refactoring Detection**: Moving or renaming functions/methods appears as deletions and additions rather than moves
4. **Limited Context Understanding**: Current tools don't understand the semantic meaning of code changes
5. **Cross-File Refactoring**: No ability to track code moved between files

### User Pain Points

- Developers spend excessive time reviewing cosmetic changes during code reviews
- Difficult to identify actual logic changes when code has been reformatted
- Merging becomes complex when team members reorganize code structure
- No way to compare code at the function/method level across different file versions

---

## Market Analysis

### Existing Solutions

Based on research, several products partially address this need:

1. **SemanticMerge (Unity/Plastic SCM)**
   - Performs structure-aware merge and diff for C#, VB.NET, Java, and C
   - Diffs code on a structural level rather than text positioning level
   - Can track methods moved between files
   - Now integrated into Unity Version Control

2. **SemanticDiff**
   - Language-aware diff for VS Code and GitHub
   - Filters out invariant changes (whitespace, optional parentheses)
   - Detects moved code and refactorings
   - Supports Python, Rust, Java, C#, TypeScript

3. **Diffsitter**
   - Tree-sitter based AST diff tool
   - Ignores formatting differences
   - Supports multiple languages through tree-sitter parsers

4. **Diff/AST (diffast)**
   - Compares ASTs node by node
   - Uses tree edit distance algorithms
   - Exports changes as XML or N-Triples

5. **Code Compare (Devart)**
   - Structure comparison mode for C#, C++, JavaScript, Java, Visual Basic, and XML
   - Integrates with Visual Studio

### Gap Analysis

While existing solutions provide AST-based comparison, they have limitations:
- Limited language support
- Poor handling of cross-file refactoring
- Lack of customizable comparison rules
- Limited integration with modern development workflows
- No unified solution that handles all major programming languages

---

## Product Vision

### Mission Statement
Build an intelligent, language-aware code diffing tool that understands code structure and semantics, enabling developers to focus on meaningful changes while ignoring cosmetic differences.

### Key Differentiators
1. **Universal Language Support**: Support for 20+ programming languages
2. **Intelligent Function Matching**: Advanced algorithms to match functions across files
3. **Customizable Rules**: User-defined rules for what constitutes a "meaningful" change
4. **IDE Integration**: Deep integration with major IDEs
5. **Performance**: Handle large codebases (100k+ files) efficiently

---

## Technical Architecture

### Core Components

#### 1. Parser Engine
- **Multi-language Parser Framework**
  - Leverage tree-sitter for broad language support
  - Custom parsers for languages not supported by tree-sitter
  - Plugin architecture for adding new language support
  
- **AST Generation**
  - Convert source code to normalized AST representation
  - Store metadata (comments, formatting) separately
  - Support incremental parsing for performance

#### 2. Semantic Analysis Engine
- **Symbol Resolution**
  - Track variable and function declarations
  - Resolve references across files
  - Build dependency graphs

- **Type Information**
  - Extract and compare type signatures
  - Understand type equivalence (e.g., `int` vs `Int32`)

#### 3. Matching Algorithm
- **Function-Level Matching**
  ```
  Algorithm: Hierarchical Signature Matching
  1. Extract function signatures (name, parameters, return type)
  2. Calculate similarity scores using:
     - Signature similarity (weighted 40%)
     - Body similarity using AST edit distance (weighted 40%)
     - Context similarity (surrounding code) (weighted 20%)
  3. Apply Hungarian algorithm for optimal matching
  4. Handle many-to-many mappings for split/merged functions
  ```

- **Cross-File Tracking**
  - Maintain a global symbol table
  - Track moves between files
  - Detect file renames and splits

#### 4. Diff Computation Engine
- **Tree Edit Distance**
  - Implement optimized Zhang-Shasha algorithm
  - Use heuristics to prune search space
  - Cache intermediate results

- **Change Classification**
  - Categorize changes: move, rename, modify, add, delete
  - Detect refactoring patterns (extract method, inline, etc.)

#### 5. Presentation Layer
- **Multiple View Modes**
  - Traditional side-by-side view
  - Unified diff view
  - Structure view (showing code organization)
  - Function-centric view

### System Architecture

```mermaid
graph TB
    subgraph Input
        F1[File Version 1]
        F2[File Version 2]
    end
    
    subgraph Parser Layer
        LP[Language Detector]
        PE[Parser Engine]
        AST1[AST Builder 1]
        AST2[AST Builder 2]
    end
    
    subgraph Analysis Layer
        SA[Semantic Analyzer]
        ST[Symbol Table]
        MA[Matching Algorithm]
    end
    
    subgraph Diff Engine
        TED[Tree Edit Distance]
        CC[Change Classifier]
        RD[Refactoring Detector]
    end
    
    subgraph Output
        VR[Visual Renderer]
        API[API Output]
        EX[Export Formats]
    end
    
    F1 --> LP
    F2 --> LP
    LP --> PE
    PE --> AST1
    PE --> AST2
    AST1 --> SA
    AST2 --> SA
    SA --> ST
    ST --> MA
    MA --> TED
    TED --> CC
    CC --> RD
    RD --> VR
    RD --> API
    RD --> EX
```

### Data Structures

```typescript
interface ASTNode {
    type: NodeType;
    id: string;
    children: ASTNode[];
    metadata: {
        line: number;
        column: number;
        originalText: string;
    };
}

interface Function {
    signature: FunctionSignature;
    body: ASTNode;
    dependencies: string[];
    hash: string;
}

interface FunctionSignature {
    name: string;
    parameters: Parameter[];
    returnType: Type;
    modifiers: string[];
}

interface MatchResult {
    similarity: number;
    mapping: Map<string, string>;
    changes: Change[];
}

interface Change {
    type: 'add' | 'delete' | 'modify' | 'move' | 'rename';
    source?: CodeElement;
    target?: CodeElement;
    details: ChangeDetail;
}
```

---

## Functional Requirements

### Core Features

#### F1: Multi-Language Support
- Support for 20+ programming languages initially
- Extensible architecture for adding new languages
- Language-specific semantic understanding

#### F2: Structural Comparison
- Compare code at function/method level
- Compare classes and interfaces
- Compare modules and namespaces
- Ignore order of declarations

#### F3: Semantic Understanding
- Identify renamed identifiers
- Detect moved code blocks
- Recognize refactoring patterns
- Understand type equivalence

#### F4: Customization
- User-defined comparison rules
- Configurable similarity thresholds
- Custom ignore patterns
- Language-specific settings

#### F5: Integration
- Command-line interface
- REST API for programmatic access
- IDE plugins (VS Code, IntelliJ, Visual Studio)
- Git integration
- CI/CD pipeline integration

### User Stories

1. **As a developer**, I want to compare two versions of my code and see only meaningful changes, so I can focus on logic changes during code review.

2. **As a code reviewer**, I want to identify functions that have been moved between files, so I can verify refactoring was done correctly.

3. **As a team lead**, I want to configure comparison rules for my team, so we have consistent code review standards.

4. **As a DevOps engineer**, I want to integrate the tool into our CI pipeline, so we can automatically analyze code changes.

---

## Non-Functional Requirements

### Performance
- Process files up to 50,000 lines in under 2 seconds
- Handle repositories with 100,000+ files
- Support incremental comparison for large changesets
- Memory usage under 2GB for typical operations

### Scalability
- Horizontal scaling for enterprise deployments
- Distributed processing for large codebases
- Caching mechanism for repeated comparisons

### Usability
- Intuitive UI with minimal learning curve
- Keyboard shortcuts for power users
- Responsive design for various screen sizes
- Accessibility compliance (WCAG 2.1 AA)

### Reliability
- 99.9% uptime for cloud service
- Graceful handling of malformed code
- Automatic recovery from parser failures
- Comprehensive error reporting

---

## Implementation Roadmap

### Phase 1: MVP (3 months)
- Core AST-based comparison engine
- Support for 5 languages (Java, Python, JavaScript, C++, C#)
- Basic function-level matching
- Command-line interface
- Simple web UI

### Phase 2: Enhanced Features (3 months)
- Cross-file refactoring detection
- Advanced matching algorithms
- 10 additional language support
- VS Code extension
- REST API

### Phase 3: Enterprise Features (3 months)
- Custom rule engine
- Git integration
- CI/CD plugins
- Performance optimizations
- Multi-user collaboration features

### Phase 4: Advanced Intelligence (6 months)
- ML-based code similarity detection
- Automatic refactoring pattern learning
- Code review assistance AI
- Advanced visualization modes

---

## Success Metrics

### Key Performance Indicators
1. **Adoption Rate**: 10,000 active users within first year
2. **Accuracy**: 95% precision in function matching
3. **Performance**: 90% of comparisons complete within 2 seconds
4. **User Satisfaction**: NPS score > 50
5. **Integration**: Support for top 5 IDEs and version control systems

### User Feedback Metrics
- Time saved in code reviews (target: 30% reduction)
- False positive rate in change detection (target: < 5%)
- User-reported bugs per release (target: < 10)

---

## Technical Decisions

### Technology Stack Recommendations

#### Backend
- **Language**: Rust or Go for performance-critical components
- **Parser Framework**: Tree-sitter for multi-language support
- **Database**: PostgreSQL for metadata, Redis for caching
- **Message Queue**: RabbitMQ for distributed processing

#### Frontend
- **Web Framework**: React with TypeScript
- **Visualization**: D3.js for tree visualization
- **Code Editor**: Monaco Editor for syntax highlighting

#### Infrastructure
- **CI/CD**: GitHub Actions or GitLab CI
- **Monitoring**: Prometheus + Grafana
- **Cloud**: AWS or GCP with auto-scaling

### Algorithm Selection
- **Tree Matching**: Modified Zhang-Shasha algorithm with heuristic optimizations
- **String Similarity**: Levenshtein distance with token-based weighting
- **Graph Matching**: Hungarian algorithm for bipartite matching

---

## Risk Analysis

### Technical Risks
1. **Parser Accuracy**: Some languages may have ambiguous grammars
   - *Mitigation*: Fallback to line-based diff for unsupported constructs

2. **Performance at Scale**: Large files may exceed processing limits
   - *Mitigation*: Implement streaming and chunking strategies

3. **Cross-Language Support**: Maintaining parsers for many languages
   - *Mitigation*: Partner with language communities, use tree-sitter

### Business Risks
1. **Market Competition**: Established tools may add similar features
   - *Mitigation*: Focus on superior UX and integration

2. **Adoption Barriers**: Developers resistant to changing tools
   - *Mitigation*: Offer free tier, excellent documentation

---

## Competitive Advantages

1. **Comprehensive Language Support**: More languages than any competitor
2. **Intelligence**: Advanced ML-based matching algorithms
3. **Performance**: Fastest processing for large codebases
4. **Customization**: Most flexible rule configuration
5. **Integration**: Deepest IDE and VCS integration

---

## Conclusion

This smart code diffing tool addresses a significant gap in the developer tooling market. By understanding code at a structural and semantic level, it will dramatically improve the code review process, reduce merge conflicts, and help teams maintain higher code quality. The proposed architecture and implementation plan provide a clear path to building a best-in-class solution that can compete with and surpass existing offerings.