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CLAUDE.md

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+# PyGenRay
+
+Python package for simulated 2D ray-based acoustic propagation modeling in underwater environments, designed for acoustic tomography applications.
+
+## Environment
+
+Use `uv` for all environment and package management — never `pip` or `python` directly.
+
+```bash
+uv sync --group dev   # install with dev dependencies
+uv run pytest tests/  # run tests
+```
+
+## Testing
+
+```bash
+uv run pytest tests/              # full suite
+uv run pytest tests/ --cov=pygenray  # with coverage
+uv run pytest --regenerate-physics tests/  # regenerate physics regression fixtures
+```
+
+## Architecture
+
+Source lives in `src/pygenray/`. Key modules:
+
+- `environment.py` — `OceanEnvironment2D`, sound speed profiles (e.g. `munk_ssp()`)
+- `launch_rays.py` — `shoot_rays()` (fan, parallel), `shoot_ray()` (single)
+- `ray_objects.py` — `Ray` and `RayFan` data classes with plotting methods
+- `eigenrays.py` — `find_eigenrays()` using regula falsi root-finding
+- `integration_processes.py` — core ray ODEs (`derivsrd()`), Numba JIT-compiled
+
+## Coordinate Convention
+
+There is a sign flip between the user-facing API and internal ODE integration:
+
+- **User-facing:** positive `z` = downward (ocean depth), positive launch angle = toward surface
+- **Internal ODE:** negative `z` convention
+
+Respect this distinction when modifying ray integration or `Ray`/`RayFan` attribute handling.
+
+## Performance
+
+`derivsrd()` in `integration_processes.py` is JIT-compiled with Numba (`fastmath=True`). Avoid introducing pure-Python loops in hot paths. `shoot_rays()` uses multiprocessing — be careful with shared state.