"""Async SQLAlchemy engine and session factory.

The engine is built lazily rather than at import time. Creating it eagerly would mean that
importing anything under `app` requires the database driver to be installed and the URL to
be parseable — which makes pure-logic tests depend on infrastructure they never touch.
"""

from collections.abc import AsyncGenerator
from functools import lru_cache

from sqlalchemy.ext.asyncio import (
    AsyncEngine,
    AsyncSession,
    async_sessionmaker,
    create_async_engine,
)

from app.config import get_settings


@lru_cache
def get_engine() -> AsyncEngine:
    settings = get_settings()
    return create_async_engine(settings.database_url, echo=False, pool_pre_ping=True)


@lru_cache
def get_session_factory() -> async_sessionmaker[AsyncSession]:
    return async_sessionmaker(get_engine(), class_=AsyncSession, expire_on_commit=False)


async def get_session() -> AsyncGenerator[AsyncSession, None]:
    """FastAPI dependency yielding a session that commits on success."""
    async with get_session_factory()() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise