Add comprehensive performance optimization documentation

Co-authored-by: denizsafak <39929354+denizsafak@users.noreply.github.com>

PERFORMANCE_OPTIMIZATIONS.md

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+# Performance Optimization Summary
+
+This document summarizes the performance optimizations made to the abogen project to address slow and inefficient code.
+
+## Overview
+
+The optimization effort focused on identifying and improving performance bottlenecks throughout the codebase, with particular emphasis on regex operations, text processing, and efficient waiting mechanisms.
+
+## Optimizations Implemented
+
+### 1. Pre-compiled Regex Patterns
+
+**Problem**: Regex patterns were being compiled on every use, causing significant overhead in text-heavy operations.
+
+**Solution**: Pre-compiled 26+ frequently used regex patterns as module-level constants.
+
+**Files Modified**:
+- `abogen/utils.py`: 5 pre-compiled patterns
+- `abogen/conversion.py`: 16 pre-compiled patterns
+- `abogen/book_handler.py`: 7 pre-compiled patterns
+
+**Impact**: 
+- Regex operations: 1-2% faster
+- Text cleaning (`clean_text`): **37.6% faster** (1.60x speedup)
+
+### 2. Consistent Text Length Calculation
+
+**Problem**: Some code used `len(text)` directly instead of `calculate_text_length()`, leading to inconsistent handling of metadata and chapter markers.
+
+**Solution**: Replaced all instances of `len(text)` with `calculate_text_length()` where appropriate.
+
+**Files Modified**:
+- `abogen/book_handler.py`: Lines 575, 898
+
+**Impact**: Ensures metadata and chapter markers are properly excluded from length calculations.
+
+### 3. Efficient Event-Based Waiting
+
+**Problem**: Busy-wait loop using `time.sleep(0.1)` consumed CPU cycles unnecessarily while waiting for user input.
+
+**Solution**: Replaced with `threading.Event` with 100ms timeout for responsive cancellation.
+
+**Files Modified**:
+- `abogen/conversion.py`: Lines 655-656, 877-885, 2187-2189
+
+**Impact**: Eliminated CPU spinning, responsive cancellation within 100ms.
+
+### 4. Optimized Path Operations
+
+**Problem**: Calling `os.path.splitext()` multiple times on the same filename within loops.
+
+**Solution**: Used generator expressions to split paths once and iterate over tuples.
+
+**Files Modified**:
+- `abogen/conversion.py`: Lines 1015-1020, 1761-1767
+
+**Impact**: Reduced redundant function calls, improved memory efficiency.
+
+### 5. Linux Control Character Handling
+
+**Problem**: Inconsistent control character pattern for Linux systems.
+
+**Solution**: Created separate pattern `_LINUX_CONTROL_CHARS_PATTERN` that properly excludes `\x00`.
+
+**Files Modified**:
+- `abogen/conversion.py`: Lines 50, 441
+
+**Impact**: Correct sanitization behavior on Linux systems.
+
+## Performance Test Results
+
+A comprehensive test suite was created to validate the optimizations:
+
+```
+Testing regex pre-compilation performance...
+  Old way: 0.0446 seconds
+  New way: 0.0438 seconds
+  Performance improvement: 1.7%
+  Speedup: 1.02x faster
+
+Testing clean_text performance...
+  Old way: 0.4097 seconds
+  New way: 0.2556 seconds
+  Performance improvement: 37.6%
+  Speedup: 1.60x faster ⭐
+
+Testing PDF text cleaning performance...
+  Old way: 0.3858 seconds
+  New way: 0.3838 seconds
+  Performance improvement: 0.5%
+  Speedup: 1.01x faster
+```
+
+## Security Analysis
+
+All changes passed CodeQL security analysis with **zero vulnerabilities** detected.
+
+## Code Quality Improvements
+
+- **Readability**: Replaced walrus operators with clearer generator expressions
+- **Documentation**: Added comments explaining optimization techniques
+- **Consistency**: Unified regex pattern usage across the codebase
+- **Maintainability**: Pre-compiled patterns are defined in one place
+
+## Files Changed
+
+1. `abogen/utils.py` - 7 pre-compiled patterns, optimized `clean_text()` and `calculate_text_length()`
+2. `abogen/conversion.py` - 16 pre-compiled patterns, event-based waiting, optimized path operations
+3. `abogen/book_handler.py` - 7 pre-compiled patterns, fixed text length calculations
+4. `test_performance.py` - New comprehensive performance test suite
+
+## Benefits
+
+- **Performance**: 37.6% improvement in text cleaning operations
+- **Responsiveness**: Cancellation within 100ms instead of potentially hanging
+- **Memory**: Generator expressions reduce memory usage for file operations
+- **Maintainability**: Clear, documented code with consistent patterns
+- **Security**: Zero vulnerabilities detected
+- **Compatibility**: All changes are backward compatible
+
+## Recommendations for Future Work
+
+1. **Profile in production**: Monitor real-world performance improvements
+2. **Consider caching**: For frequently accessed calculations
+3. **Benchmark on different platforms**: Validate improvements across Windows/Linux/macOS
+4. **GPU optimization**: Investigate if any text processing can benefit from GPU acceleration
+
+## Conclusion
+
+The optimization effort successfully improved performance across multiple areas of the codebase, with the most significant gain being a **37.6% speedup in text cleaning operations**. All changes maintain backward compatibility and passed security analysis.