"""Async database engine, session factory, and the FastAPI session dependency.

The repository layer builds on ``get_session``; ``get_engine`` also backs the
readiness probe. Everything is lazy so importing the app never opens a
connection (important for tests and for ``--help``-style invocations).
"""

from collections.abc import AsyncIterator

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

from app.core.config import get_settings

_engine: AsyncEngine | None = None
_sessionmaker: async_sessionmaker[AsyncSession] | None = None


def get_engine() -> AsyncEngine:
    global _engine
    if _engine is None:
        _engine = create_async_engine(get_settings().database_url, pool_pre_ping=True)
    return _engine


def get_sessionmaker() -> async_sessionmaker[AsyncSession]:
    global _sessionmaker
    if _sessionmaker is None:
        _sessionmaker = async_sessionmaker(
            get_engine(), expire_on_commit=False, class_=AsyncSession
        )
    return _sessionmaker


async def get_session() -> AsyncIterator[AsyncSession]:
    """FastAPI dependency. One session per request; commits are explicit in the
    service layer."""
    async with get_sessionmaker()() as session:
        yield session