Add perf test suite

Author: sirreal

tests/phpunit/tests/html-api/README-performance.md

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+# CSS Selector Performance Tests
+
+This directory contains performance tests for the WordPress HTML API CSS selector parsing functionality.
+
+## Test Files
+
+### wpCssSelectorPerformance.php
+Comprehensive performance tests that measure execution time for various CSS selector parsing scenarios:
+
+- **Type selectors**: `div`, `span`, `article`, etc.
+- **Class selectors**: `.class`, `.my-class`, etc.
+- **ID selectors**: `#id`, `#my-id`, etc.
+- **Attribute selectors**: `[attr]`, `[attr="value"]`, etc.
+- **Compound selectors**: `div.class#id[attr]`
+- **Complex selectors**: `div > p`, `nav ul li`
+- **Selector lists**: `div, p, span`
+- **Unicode selectors**: `.café`, `#résumé`, etc.
+- **Escaped selectors**: `.\\31 23`, `#\\2e test`
+- **Large selectors**: Complex selectors with many parts
+- **Malformed selectors**: Error handling performance
+
+### wpCssSelectorBenchmark.php
+Focused benchmark tests that measure throughput (operations per second) and memory usage:
+
+- **Throughput tests**: How many selectors can be parsed per second
+- **Memory scaling**: How memory usage scales with selector complexity
+- **Error handling**: Performance of parsing invalid selectors
+- **Real-world scenarios**: Performance with typical framework/theme selectors
+
+## Running Performance Tests
+
+### Run All Performance Tests
+```bash
+vendor/bin/phpunit tests/phpunit/tests/html-api/wpCssSelectorPerformance.php
+vendor/bin/phpunit tests/phpunit/tests/html-api/wpCssSelectorBenchmark.php
+```
+
+### Run Specific Test Groups
+```bash
+# Run only performance tests
+vendor/bin/phpunit --group performance
+
+# Run only benchmark tests
+vendor/bin/phpunit --group benchmark
+
+# Run both performance and benchmark tests
+vendor/bin/phpunit --group performance,benchmark
+```
+
+### Run Individual Tests
+```bash
+# Run a specific performance test
+vendor/bin/phpunit --filter test_compound_selector_parsing_performance tests/phpunit/tests/html-api/wpCssSelectorPerformance.php
+
+# Run a specific benchmark test
+vendor/bin/phpunit --filter test_simple_selector_parsing_throughput tests/phpunit/tests/html-api/wpCssSelectorBenchmark.php
+```
+
+## Understanding Results
+
+### Performance Test Results
+Performance tests output timing information to error_log:
+```
+Performance [Type Selector Parsing]: avg=0.05ms, min=0.03ms, max=0.12ms, iterations=1000
+```
+
+- **avg**: Average execution time in milliseconds
+- **min**: Fastest execution time
+- **max**: Slowest execution time
+- **iterations**: Number of test iterations
+
+### Benchmark Test Results
+Benchmark tests output throughput and memory information:
+```
+[BENCHMARK] Simple Selector Parsing: 67543.21 ops/sec, 0.0148ms avg, 0.00KB memory, 0.00KB peak
+```
+
+- **ops/sec**: Operations (parsings) per second
+- **avg**: Average time per operation in milliseconds
+- **memory**: Memory used during test in KB
+- **peak**: Peak memory usage in KB
+
+## Performance Expectations
+
+### Current Performance Thresholds
+- **Simple selectors**: > 50,000 ops/sec
+- **Compound selectors**: > 10,000 ops/sec
+- **Complex selectors**: > 5,000 ops/sec
+- **Selector lists**: > 3,000 ops/sec
+- **Attribute selectors**: > 8,000 ops/sec
+- **Unicode selectors**: > 15,000 ops/sec
+- **Error handling**: > 20,000 ops/sec
+
+### Memory Usage
+- Memory usage should scale roughly linearly with selector complexity
+- Error handling should not consume excessive memory
+- Memory leaks should be avoided during repeated parsing
+
+## Monitoring Performance
+
+### In CI/CD
+These tests can be integrated into CI/CD pipelines to:
+- Monitor performance regressions
+- Ensure performance standards are maintained
+- Compare performance across different PHP versions
+
+### Local Development
+Use these tests to:
+- Measure performance impact of code changes
+- Identify performance bottlenecks
+- Optimize parsing algorithms
+
+## Troubleshooting
+
+### Tests Failing Due to Performance
+If performance tests fail consistently:
+
+1. **Check system load**: High CPU usage can affect timing
+2. **Run multiple times**: Single runs may have variance
+3. **Update thresholds**: Hardware differences may require adjustment
+4. **Profile specific cases**: Use tools like Xdebug to identify bottlenecks
+
+### Adjusting Performance Thresholds
+Thresholds can be adjusted in the test files:
+- `wpCssSelectorPerformance.php`: Modify `PERFORMANCE_THRESHOLD_MS` constant
+- `wpCssSelectorBenchmark.php`: Modify assertion values in individual tests
+
+### Adding New Performance Tests
+When adding new selector functionality:
+1. Add parsing performance tests to `wpCssSelectorPerformance.php`
+2. Add throughput benchmarks to `wpCssSelectorBenchmark.php`
+3. Update this documentation with new performance expectations
+
+## Best Practices
+
+### Writing Performance Tests
+- Use realistic test data that represents actual usage
+- Include both positive and negative test cases
+- Test edge cases and error conditions
+- Consider memory usage alongside execution time
+
+### Interpreting Results
+- Look for trends over time, not just individual runs
+- Compare relative performance between different selector types
+- Consider the context of how selectors are used in real applications
+- Monitor both average and peak performance metrics
+
+### Optimization Guidelines
+Based on test results, focus optimization efforts on:
+- Most commonly used selector types
+- Cases with highest memory usage
+- Scenarios with slowest throughput
+- Error handling paths that are frequently exercised