"""Engine registry: the single relevance engine, configured from Django settings.

There's one engine - `OpenAICompatEngine` against `ENGINE_BASE_URL` - because every
backend we support speaks the OpenAI `/v1` API, so no per-backend "kind" routing
is needed (the endpoint + the spec are enough). The Protocol + this thin registry
remain as the seam: the watch-action config's free-string `engine.kind` is
validated against `kinds()` (only the default), and a future divergent backend
could be registered here without touching the call sites or the config schema.
"""

from django.conf import settings

from .engines import Engine, OpenAICompatEngine

# The only registered kind; an empty `engine.kind` in config resolves to it.
DEFAULT_KIND = OpenAICompatEngine.kind


def _build() -> Engine:
    return OpenAICompatEngine(
        base_url=settings.ENGINE_BASE_URL,
        default_model=settings.ENGINE_MODEL,
        api_key=settings.ENGINE_API_KEY,
    )


# Built eagerly at import (the constructor only stores config, no network), so
# there's no lazy check-then-set to race under a threaded server. register()
# swaps it for tests.
_engine: Engine = _build()


def get(kind: str = "") -> Engine:
    """The configured engine. Empty kind (or the default kind) resolves to it;
    any other kind raises KeyError so the config policy maps a bad pin to a clean
    400 at the write boundary rather than a mid-poll failure."""
    if kind and kind != DEFAULT_KIND:
        raise KeyError(kind)
    return _engine


def kinds() -> list[str]:
    """Recognized engine kinds (just the default). Used by config validation to
    reject a bad `engine.kind` at create time rather than mid-poll."""
    return [DEFAULT_KIND]


def register(engine: Engine) -> None:
    """Override the engine (test-only: used to stub the LLM)."""
    global _engine
    _engine = engine