"""Database engine + session dependency.

SQLAlchemy 2.0 sync engine — per standards, DB/CRUD routes are sync ``def``
(FastAPI runs them in a threadpool); ``async def`` is reserved for AI + SSE
routes, which do not hold DB sessions across awaits.
"""
from __future__ import annotations

from collections.abc import Generator

from sqlalchemy import create_engine
from sqlalchemy.orm import Session, sessionmaker

from .config import get_settings

_engine = None
_SessionLocal = None


def get_engine():
    global _engine, _SessionLocal
    if _engine is None:
        settings = get_settings()
        if not settings.database_url:
            raise RuntimeError("DATABASE_URL is not configured")
        _engine = create_engine(settings.database_url, pool_pre_ping=True)
        _SessionLocal = sessionmaker(bind=_engine, expire_on_commit=False)
    return _engine


def get_db() -> Generator[Session, None, None]:
    """FastAPI dependency: one session per request, closed afterwards."""
    get_engine()
    assert _SessionLocal is not None
    db = _SessionLocal()
    try:
        yield db
    finally:
        db.close()