"""Optional compiled kernels (``rust/simurg-core``, plan §18 P9).

``core`` is the ``simurg_core`` extension module when it is installed, else ``None``; every caller keeps a pure
Python/NumPy path with bit-identical results. ``SIMURG_PURE_PYTHON=1`` forces the pure paths (tests, debugging).
"""

from __future__ import annotations

import os
from types import ModuleType

core: ModuleType | None
if os.environ.get("SIMURG_PURE_PYTHON", "") not in ("", "0"):
    core = None
else:
    try:
        import simurg_core as core  # type: ignore[no-redef]
    except ImportError:  # pragma: no cover - depends on the installation
        core = None


def backend() -> str:
    """``"rust <version>"`` or ``"python"`` (recorded in run manifests)."""
    return f"rust {core.__version__}" if core is not None else "python"