# BigGrep rs-bgparse Implementation Correction

## Summary

Successfully corrected the rs-bgparse implementation to match the original BigGrep bgparse functionality. The previous implementation was a generic file parser, which has been completely replaced with proper BigGrep index parsing capabilities.

## Key Corrections Made

### 1. **Command-Line Interface Overhaul**
**Before**: Generic file parsing with commands like `Batch`, `Extract`, `Validate`, `Convert`
**After**: BigGrep index search with options:
- `-d, --directory`: Index directory to search in
- `-p, --patterns`: Search patterns (hex-encoded binary strings)
- `--max-candidates`: Maximum candidate files to return
- `-v, --verbose`: Verbose output
- `--stats`: Show compression statistics
- `--debug`: Debug output

### 2. **Core Functionality Replacement**
**Before**: Parsing JSON, XML, CSV, binary formats
**After**: 
- Reading N-gram indexes (.bgi files)
- Implementing hint-based searching with 16-N-gram jumps
- PFOR/VarByte index decoding
- Generating candidate file lists for search verification

### 3. **Index Parsing Implementation**
Added comprehensive BigGrep index file parsing:
- **Header parsing**: Magic number validation, version checking, N-gram order detection
- **Hint array parsing**: Fast N-gram lookup using prefix-based mappings
- **N-gram extraction**: Converting hex patterns to binary data to N-grams
- **Hint-based search**: Using hints to locate relevant index regions
- **Compressed posting decoding**: VarByte and PFOR decompression
- **Set intersection**: Multi-N-gram candidate refinement
- **File ID resolution**: Mapping numeric IDs to file paths

### 4. **BigGrep-Specific Algorithms**
Implemented core BigGrep search algorithms:
- **Little-endian N-gram optimization**: Efficient 3-byte and 4-byte extraction
- **Hint window searching**: 16-N-gram jump strategy for I/O minimization
- **VarByte decoding**: Variable-length integer encoding/decoding
- **PFOR block decoding**: Patched Frame Of Reference compression
- **File ID mapping**: Resolving compressed fileid_map sections

### 5. **Memory-Mapped I/O**
- Direct memory mapping of index files for efficient access
- Byte-level parsing of compressed index structures
- Proper offset management and bounds checking

### 6. **Error Handling and Logging**
- Comprehensive error handling for malformed indexes
- Structured logging with debug, info, warn, and error levels
- Graceful handling of missing or corrupted index files

## Implementation Architecture

### Index Structure Support
The corrected implementation supports the full BigGrep index format:
```
[BGI Index File Structure]
┌─────────────────────┐
│ Header (64 bytes)   │ ← Magic, version, counts, offsets
├─────────────────────┤
│ Hints Array         │ ← Prefix-based N-gram lookup
├─────────────────────┤
│ N-gram Postings     │ ← Compressed file ID lists
├─────────────────────┤
│ File ID Map         │ ← File ID to path mapping
└─────────────────────┘
```

### Search Flow
```
Input: Hex-encoded patterns
↓
Convert to binary data
↓
Extract N-grams (3 or 4-byte sequences)
↓
Use hints for fast lookup
↓
Search in 16-N-gram windows
↓
Decode compressed postings (PFOR/VarByte)
↓
Intersect candidate sets
↓
Resolve file IDs to paths
↓
Output: Candidate file list
```

## Dependencies Updated

**Removed unused dependencies:**
- `rayon` (no longer needed for file parsing)
- `serde`, `serde_json`, `serde_yaml` (no JSON/XML parsing)
- `crossbeam-channel` (no concurrent file processing)
- `toml` (no configuration file parsing)
- `thiserror` (replaced with anyhow)

**Retained BigGrep-specific dependencies:**
- `biggrep-core`: Core BigGrep functionality
- `clap`: Command-line parsing
- `byteorder`: Binary data handling
- `memmap2`: Memory-mapped file access
- `anyhow`: Error handling
- `log`, `env_logger`: Logging
- `regex`: Pattern matching

## Files Modified

1. **`src/main.rs`**: Complete rewrite with BigGrep index parser
2. **`Cargo.toml`**: Updated description and dependencies
3. **`README.md`**: New documentation for corrected functionality
4. **`test_corrected.sh`**: Test script demonstrating corrected usage

## Verification

The corrected implementation now properly:
- ✅ Reads BigGrep index files (.bgi)
- ✅ Parses index headers and validates structure
- ✅ Uses hint-based searching for fast lookup
- ✅ Decodes PFOR/VarByte compressed postings
- ✅ Performs set intersection for multi-N-gram queries
- ✅ Resolves file IDs to actual file paths
- ✅ Generates candidate file lists for verification
- ✅ Handles errors gracefully with proper logging

## Usage Examples

```bash
# Basic search for "hello" in hex
rs-bgparse -d /path/to/indexes -p "68656c6c6f"

# Multiple patterns
rs-bgparse -d /path/to/indexes -p "68656c6c6f,776f726c64"

# With verbose output and statistics
rs-bgparse -d /path/to/indexes -p "68656c6c6f" -v --stats

# Limited candidate output
rs-bgparse -d /path/to/indexes -p "68656c6c6f" --max-candidates 1000
```

The implementation is now ready for integration with the broader BigGrep ecosystem and properly implements the original C++ bgparse functionality in Rust.