Code Generator Skill

Overview

Structured workflow for implementing code from design documents with clear phase separation, cost estimation, and modification tracking.

When to Use

Use this skill when you need to:

• Implement a new module or system from a design document
• Add features to existing codebase
• Refactor or replace existing components
• Generate implementation code with clear execution plan

Core Principles

1. Phase-Based Execution

• Break implementation into logical phases
• Each phase has clear dependencies
• Execute sequentially, phase by phase

2. Operation Type Declaration

• Create: New file that doesn't exist
• Modify: Existing file requiring changes
• Delete: Remove existing file (rare, requires explicit confirmation)

3. Cost Transparency

• Estimate token cost for each operation
• Calculate total cost before execution
• Allow user to adjust scope based on budget

4. Modification Preview

• For modify operations, show before/after diff
• Highlight key changes
• No surprises for user

5. Capability Boundaries

• Can Do: Code generation, file read/write
• Cannot Do: Git operations, database changes, compilation, external API calls

Workflow

Step 1: Analyze Design Document

Read and understand:

• Architecture overview
• File list and dependencies
• Key design decisions
• Interface contracts

Step 2: Create Execution Plan

Use this format:

## 📋 [ProjectName] Implementation Plan

### Execution Capability Statement

| Operation | Capability |
|-----------|------------|
| Code generation | ✅ Can execute |
| Database operations | ❌ Cannot execute (user handles) |
| Git/SVN operations | ❌ Do not execute |
| Compilation | ❌ Do not execute (user triggers) |

### Phase [N]: [Phase Name]

| Order | File | Path | Operation | Content | Time | Token Cost |
|-------|------|------|-----------|---------|------|------------|
| 1 | `FileName.h` | `Path/` | Create/Modify | Brief description | X min | ~X,XXX |

**Phase Subtotal**: ~X,XXX tokens

### Modification Preview

For modify operations only:

`FileName.h`:
```cpp
// Before: [current state or "None"]
// After: [new code to be added]

Phase Dependencies

Phase 1 ──► Phase 2 ──► Phase 3

Total Cost Summary

| Phase | Time | Token Cost | |-------|------|------------| | 1 | X min | ~X,XXX | | 2 | X min | ~X,XXX | | Total | X hours | ~XX,XXX |

User Confirmation Items

1. [Specific question 1]
2. [Specific question 2]
3. Start confirmation: Reply "Approve plan" to begin

### Step 3: Execute Phase by Phase

After user approval with "Approve plan":

1. **Announce current phase**
2. **Execute operations** in order
3. **Report progress** after each file
4. **Confirm phase completion** before next phase

### Step 4: Completion Report

```markdown
## ✅ Implementation Complete

### Generated Files

| File | Path | Lines |
|------|------|-------|
| `File.h` | `Path/` | XXX |

### Modified Files

| File | Path | Changes |
|------|------|---------|
| `File.cpp` | `Path/` | Added X methods |

### Next Steps (User Action Required)

1. [Database setup, compilation, etc.]

### Token Usage

- **Estimated**: ~XX,XXX
- **Actual**: ~XX,XXX

Token Cost Guidelines

| File Type | Lines | Estimated Tokens | |-----------|-------|------------------| | Header (declaration) | 50-100 | 500-1,000 | | Implementation | 200-300 | 2,000-4,000 | | Simple modification | - | 300-800 | | Complex modification | - | 1,000-2,000 | | CSV/Data file | 20-50 | 200-500 | | Test file | 100-200 | 1,000-2,000 |

Template: Complete Example

## 📋 PlayerSystem Implementation Plan

### Execution Capability Statement

| Operation | Capability |
|-----------|------------|
| Code generation | ✅ Can execute |
| Database operations | ❌ Cannot execute |
| Git/SVN operations | ❌ Do not execute |
| Compilation | ❌ Do not execute |

### Phase 1: Data Structures

| Order | File | Path | Operation | Content | Time | Token Cost |
|-------|------|------|-----------|---------|------|------------|
| 1 | `SystemDefines.h` | `Project/` | Create | Enums and utilities | 5m | ~500 |
| 2 | `SystemInstance.h` | `Project/` | Create | Struct with Save/Load | 10m | ~800 |

**Phase 1 Subtotal**: ~1,300 tokens

### Phase 2: Core Implementation

| Order | File | Path | Operation | Content | Time | Token Cost |
|-------|------|------|-----------|---------|------|------------|
| 3 | `PlayerSystem.h` | `Project/` | Create | Class declaration | 10m | ~1,000 |
| 4 | `PlayerSystem.cpp` | `Project/` | Create | Core methods | 30m | ~3,500 |

**Phase 2 Subtotal**: ~4,500 tokens

### Phase 3: Integration

| Order | File | Path | Operation | Content | Time | Token Cost |
|-------|------|------|-----------|---------|------|------------|
| 5 | `Player.h` | `Project/` | Modify | Add accessor method | 5m | ~300 |

**Modification Preview**:

`Player.h`:
```cpp
// Before: No System member
class Player {
    // existing members
};

// After: Added System
class Player {
    // existing members
public:
    PlayerSystem& GetSystem() { return mSystem; }
private:
    PlayerSystem mSystem;
};

Phase 3 Subtotal: ~300 tokens

Total Cost Summary

| Phase | Time | Token Cost | |-------|------|------------| | 1 | 15m | ~1,300 | | 2 | 40m | ~4,500 | | 3 | 5m | ~300 | | Total | 1 hour | ~6,100 |

User Confirmation

1. Database table: I provide SQL / You create manually
2. Start: Reply "Approve plan"

## Anti-Patterns

### ❌ Don't
- Start coding without user approval
- Skip modification previews for modify operations
- Omit token cost estimation
- Perform Git/database/compilation operations
- Combine multiple phases without checkpoint

### ✅ Do
- Wait for "Approve plan" before execution
- Show before/after for all modifications
- Estimate costs conservatively
- Stay within code generation scope only
- Execute phase by phase with progress reports

## Integration with Other Skills

- **mvp-design**: Use as input (design doc → implementation)
- **code-analysis**: Verify existing files before modification
- **skill-creator**: Extract patterns into reusable skills

## Quick Reference

### Cost Estimation Formula

Header file: 10 tokens per line Implementation: 15 tokens per line Modification: 20 tokens per line (includes context) Test file: 12 tokens per line

### Modification Preview Format

```cpp
// File: Path/Filename.ext

// Before: [describe current state]
[code snippet or "None - new addition"]

// After: [describe new state]
[code snippet to be added/modified]

// Key changes:
// - Change 1
// - Change 2

Version History

• v1.0 (2026-02-10) - Initial release
  • Phase-based execution model
  • Token cost estimation
  • Modification preview requirement
  • Clear capability boundaries