"""Which propagator answers, and one instance of it per process.

``get_backend()`` with no argument returns the deployment's configured default;
with a name it returns that one specifically. Both matter. The default is what
the request path should use, and the explicit name is what a caller needing
physics the default cannot do -- space weather, drag fitting, decay -- must pass.

That second case is not hypothetical. The default is ``sgp4``, which raises on
any of those; a view that forgot to name ``"orekit"`` and swallowed the error
rendered an empty chart for weeks without anything logging a complaint. If your
call needs the sidecar, say so.
"""

from __future__ import annotations

from django.conf import settings

from odm import PropagatorBackend

from .backends.orekit import OrekitBackend
from .backends.sgp4 import Sgp4Backend

BACKENDS: dict[str, type[PropagatorBackend]] = {
    "sgp4": Sgp4Backend,
    "orekit": OrekitBackend,
}

#: One instance per name. Backends are stateless apart from an HTTP client, so
#: sharing them saves a connection pool rather than risking shared state.
_INSTANCES: dict[str, PropagatorBackend] = {}

DEFAULT_BACKEND = "sgp4"


def register(name: str, backend: type[PropagatorBackend]) -> None:
    """Add a backend under ``name``. For a service with its own propagator."""
    BACKENDS[name] = backend
    _INSTANCES.pop(name, None)


def get_backend(name: str | None = None) -> PropagatorBackend:
    key = name or getattr(settings, "ORBITAL_PROPAGATOR_BACKEND", DEFAULT_BACKEND)
    try:
        cls = BACKENDS[key]
    except KeyError as exc:
        raise ValueError(
            f"unknown propagation backend {key!r}; "
            f"expected one of {', '.join(sorted(BACKENDS))}"
        ) from exc
    if key not in _INSTANCES:
        _INSTANCES[key] = cls()
    return _INSTANCES[key]