# Documentation Update - Issue #10: Benchmark Data Feature

## Update Status: COMPLETE

All documentation has been successfully updated and synchronized with the benchmark data feature implementation.

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## Changed Files

### CHANGELOG.md
- **Status**: Modified (25 lines added)
- **Location**: Lines 92-115 in Unreleased/Added section
- **Changes**: Added comprehensive entry for Issue #10 benchmark data feature

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## Documentation Added

### Feature: Benchmark Data Retrieval and Analysis (Issue #10)

#### 6 Main Functions Documented

1. **get_benchmark_data()** [lines 67-115]
   - Core OHLCV data fetching via yfinance
   - Date validation for YYYY-MM-DD format
   - Error handling with descriptive messages

2. **get_spy_data()** [lines 117-136]
   - Convenience wrapper for S&P 500 benchmark
   - Identical signature to get_benchmark_data

3. **get_sector_etf_data()** [lines 138-186]
   - Sector-specific ETF data retrieval
   - Sector validation with helpful error messages
   - Support for 11 SPDR sector funds

4. **calculate_relative_strength()** [lines 188-285]
   - IBD-style weighted rate of change (ROC) formula
   - Weighted periods: 40% 63-day, 20% 126-day, 20% 189-day, 20% 252-day
   - Data alignment via inner join
   - Customizable ROC periods

5. **calculate_rolling_correlation()** [lines 287-349]
   - Time-series correlation analysis
   - Configurable rolling window (default 60 days)
   - Comprehensive validation for data alignment

6. **calculate_beta()** [lines 351-441]
   - Systematic risk measurement
   - Covariance-variance approach with optional smoothing
   - Optional rolling beta calculation (default 252 days)
   - Efficient Markdown rolling window implementation

#### Sector ETF Mappings (11 SPDR Funds)

| Sector | Symbol |
|--------|--------|
| Communication | XLC |
| Consumer Discretionary | XLY |
| Consumer Staples | XLP |
| Energy | XLE |
| Financials | XLF |
| Healthcare | XLV |
| Industrials | XLI |
| Materials | XLB |
| Real Estate | XLRE |
| Technology | XLK |
| Utilities | XLU |

#### Test Coverage

- **Unit Tests**: 28 tests in test_benchmark.py (753 lines)
  - Data fetching and validation
  - Sector validation
  - Relative strength calculations
  - Edge cases and error handling

- **Integration Tests**: 7 tests in test_benchmark_integration.py (593 lines)
  - Complete workflow scenarios
  - Cross-function integration
  - Real data behavior validation

- **Total**: 35 tests

#### Key Features Documented

- All functions return DataFrames/Series/floats on success, error strings on failure
- Comprehensive error handling with descriptive messages
- Comprehensive docstrings with examples for all public functions
- IBD-style relative strength weighting
- Data validation and alignment checks
- Efficient rolling window implementations

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## Verification Results

### File References
- [x] benchmark.py (441 lines) - Main module
- [x] test_benchmark.py (753 lines) - Unit tests
- [x] test_benchmark_integration.py (593 lines) - Integration tests

### Line Number References
- [x] get_benchmark_data: 67-115
- [x] get_spy_data: 117-136
- [x] get_sector_etf_data: 138-186
- [x] calculate_relative_strength: 188-285
- [x] calculate_rolling_correlation: 287-349
- [x] calculate_beta: 351-441
- [x] SECTOR_ETFS mapping: 48-59

### Test Counts
- [x] Unit tests: 28 tests verified
- [x] Integration tests: 7 tests verified
- [x] Total: 35 tests

### Format Compliance
- [x] Keep a Changelog format followed
- [x] Markdown links working
- [x] Consistent with surrounding entries
- [x] Proper indentation and structure

### Inline Documentation Status
- [x] Module docstring: Present
- [x] Function docstrings: Comprehensive
- [x] Section headers: Present in code
- [x] Inline comments: Organized with headers

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## Git Status

- **Modified Files**: CHANGELOG.md
- **Status**: Modified (tracked)
- **Branch**: main (ahead of upstream/main by 22 commits)

```
Modified:   CHANGELOG.md (+25 lines)
Total:      312 lines (was 287)
```

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## Summary

All documentation has been successfully updated following the Keep a Changelog format. The CHANGELOG entry provides:

- Complete feature overview
- Documentation of all 6 main functions
- 11 sector ETF mappings
- 35 total tests (28 unit + 7 integration)
- Comprehensive line number references
- Cross-linked file references
- Consistent formatting with existing entries

**Status**: Ready for commit. All documentation is synchronized with the implementation.