Add comprehensive GitHub Copilot instructions for hexhamming development

.github/copilot-instructions.md

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+# hexhamming - Fast Hamming Distance Calculation
+
+hexhamming is a Python C extension module that provides blazingly fast bitwise Hamming distance calculation for hexadecimal strings and byte arrays. It uses vectorized algorithms (SSE4.1, AVX2, ARM NEON) for optimal performance.
+
+Always reference these instructions first and fallback to search or bash commands only when you encounter unexpected information that does not match the info here.
+
+## Working Effectively
+
+### Essential Setup Commands
+Run these commands in sequence to set up the development environment:
+
+```bash
+# Install development dependencies
+pip3 install -r requirements-dev.txt
+# Takes ~30 seconds - includes pytest, black, pytest-benchmark
+
+# Build and install the package (RECOMMENDED METHOD)
+pip3 install .
+# Takes 2-3 minutes. NEVER CANCEL - C++ compilation requires time.
+# Set timeout to 5+ minutes for build commands.
+```
+
+### Alternative Build Methods
+If the recommended method fails due to network issues:
+
+```bash
+# Legacy build method (always works offline)
+python3 setup.py install --user
+# Takes 2-3 minutes. NEVER CANCEL. Shows deprecation warnings but works reliably.
+```
+
+### Running Tests
+```bash
+# Run the complete test suite with benchmarks  
+python3 -m pytest -vls .
+# Takes ~27 seconds. NEVER CANCEL - includes performance benchmarks.
+# Set timeout to 2+ minutes for test commands.
+
+# Quick test run (no benchmarks)
+python3 -m pytest test/ -k "not bench"
+# Takes ~5 seconds for functional tests only.
+```
+
+### Code Quality and CI Requirements
+```bash
+# CRITICAL: Check code formatting before committing
+black --check .
+# Returns exit code 1 if formatting needed
+
+# Fix code formatting (if needed)
+black .
+# Reformats Python files to match project standards
+
+# Validate package manifest
+python3 -m pip install check-manifest
+python3 -m check_manifest
+# Verifies all files are properly included in package
+```
+
+## Validation Scenarios
+
+### ALWAYS Test These After Making Changes
+After any code modifications, run these validation scenarios:
+
+```bash
+# 1. Basic string functionality test
+python3 -c "
+from hexhamming import hamming_distance_string
+result = hamming_distance_string('deadbeef', '00000000')
+assert result == 24, f'Expected 24, got {result}'
+print('✓ String hamming distance: PASS')
+"
+
+# 2. Bytes functionality test  
+python3 -c "
+from hexhamming import hamming_distance_bytes
+result = hamming_distance_bytes(b'\xde\xad\xbe\xef', b'\x00\x00\x00\x00')
+assert result == 24, f'Expected 24, got {result}'
+print('✓ Bytes hamming distance: PASS')
+"
+
+# 3. Within distance check test
+python3 -c "
+from hexhamming import check_hexstrings_within_dist
+result1 = check_hexstrings_within_dist('ffff', 'fffe', 2)
+result2 = check_hexstrings_within_dist('ffff', '0000', 2)
+assert result1 == True, f'Expected True, got {result1}'
+assert result2 == False, f'Expected False, got {result2}'
+print('✓ Within distance check: PASS')
+"
+
+# 4. Algorithm switching test
+python3 -c "
+from hexhamming import set_algo, hamming_distance_string
+set_algo('classic')
+result = hamming_distance_string('abc', 'def')
+assert isinstance(result, int), f'Algorithm switch failed'
+print('✓ Algorithm switching: PASS')
+"
+```
+
+### Build Validation Scenarios
+Test that your changes don't break the build:
+
+```bash
+# Clean build test (removes any cached artifacts)
+rm -rf build/ dist/ *.egg-info/ test/__pycache__/ .pytest_cache/
+pip3 install .
+# Takes 2-3 minutes. NEVER CANCEL.
+
+# Verify installation worked
+python3 -c "import hexhamming; print('✓ Import successful')"
+```
+
+## Common Build Issues and Solutions
+
+### Network Connectivity Problems
+If `pip install` or `python -m build` fail with network errors:
+- Use the legacy method: `python3 setup.py install --user`
+- This method works completely offline after initial dependency installation
+
+### Build Dependencies Missing
+If you get compiler errors:
+```bash
+# Install build essentials (Ubuntu/Debian)
+sudo apt-get update && sudo apt-get install build-essential python3-dev
+
+# On other systems, ensure you have a C++ compiler and Python headers
+```
+
+### Test Failures
+If tests fail after your changes:
+- Focus only on failures related to your changes
+- Ignore unrelated benchmark timing variations
+- Always run the validation scenarios above to verify core functionality
+
+## Performance Expectations
+
+### CRITICAL Timing Information - NEVER CANCEL
+- **Development setup**: 30 seconds to 2 minutes
+- **Package build**: 2-3 minutes (C++ compilation takes time)
+- **Full test suite**: ~27 seconds (includes performance benchmarks)
+- **Code formatting**: 1-2 seconds
+- **Manifest check**: 5-10 seconds
+
+**NEVER CANCEL builds or long-running commands.** C++ compilation and performance benchmarks require time to complete. Always set timeouts of 5+ minutes for builds and 2+ minutes for tests.
+
+## Repository Structure
+
+### Key Files and Directories
+```
+hexhamming/
+├── README.rst              # Main documentation and usage examples
+├── setup.py                # Build configuration with C++ extension
+├── requirements-dev.txt    # Development dependencies (pytest, black, etc.)
+├── hexhamming/            # C++ source code directory
+│   ├── python_hexhamming.cc    # Main C++ implementation
+│   ├── python_hexhamming.h     # Header with vectorized algorithms
+│   └── _version.h              # Version information
+├── test/
+│   └── test_hexhamming.py      # Comprehensive test suite with benchmarks
+├── .github/workflows/
+│   └── pythonpackage.yml      # CI/CD pipeline using cibuildwheel
+└── MANIFEST.in             # Package file inclusion rules
+```
+
+### Important Code Locations
+- **Algorithm implementations**: `hexhamming/python_hexhamming.h` (lines 150-630)
+- **Python bindings**: `hexhamming/python_hexhamming.cc`
+- **Performance tests**: `test/test_hexhamming.py` (lines 100+)
+- **Build configuration**: `setup.py` (platform-specific optimizations)
+
+## Development Workflow
+
+### Making Changes
+1. **ALWAYS** run the setup commands first
+2. Make your code changes
+3. **IMMEDIATELY** test with validation scenarios
+4. Run `black --check .` and fix formatting if needed
+5. Run full test suite: `python3 -m pytest -vls .`
+6. Run `check-manifest` to verify package integrity
+
+### Before Committing
+```bash
+# Required checks that CI will run
+black --check .           # Code formatting
+python3 -m check_manifest # Package manifest
+python3 -m pytest -vls .  # Full test suite
+```
+
+### CI/CD Information
+The project uses GitHub Actions with cibuildwheel for cross-platform wheel building:
+- Builds wheels for Linux (manylinux, musllinux), macOS, Windows
+- Tests on Python 3.6-3.10
+- **Build time in CI**: 15-45 minutes depending on platform
+- **Network dependencies**: Requires PyPI access for dependencies
+
+If CI fails on build steps, it's often due to:
+1. Code formatting issues (run `black .`)
+2. Package manifest issues (run `check-manifest`)
+3. Test failures (run validation scenarios locally)
+
+## Troubleshooting
+
+### "Module not found" errors
+Ensure you've installed the package: `pip3 install .`
+
+### Compilation errors
+Check that you have build tools: `sudo apt-get install build-essential python3-dev`
+
+### Test failures
+Run validation scenarios individually to isolate issues.
+
+### Network timeouts
+Use legacy build method: `python3 setup.py install --user`
+
+### Formatting issues
+Run `black .` to fix all formatting issues automatically.