"""Database engine + session.

Reads DATABASE_URL from the environment (provisioned Postgres). The engine is
created with a short connect timeout and a probe: if Postgres is unreachable at
startup, we fall back to a local SQLite file so the app still runs (Preview,
offline dev). The rest of the app never chooses a database — it imports
``SessionLocal`` and ``engine`` from here.
"""

from __future__ import annotations

import os
from collections.abc import Generator

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


class Base(DeclarativeBase):
    pass


def _normalize(url: str) -> str:
    # SQLAlchemy + psycopg3 wants the postgresql+psycopg driver.
    if url.startswith("postgresql://"):
        return url.replace("postgresql://", "postgresql+psycopg://", 1)
    if url.startswith("postgres://"):
        return url.replace("postgres://", "postgresql+psycopg://", 1)
    return url


def _make_engine():
    url = os.getenv("DATABASE_URL", "").strip()
    if url:
        pg_url = _normalize(url)
        try:
            eng = create_engine(
                pg_url,
                pool_pre_ping=True,
                connect_args={"connect_timeout": 5},
            )
            with eng.connect() as conn:
                conn.execute(text("SELECT 1"))
            print("[db] using Postgres")
            return eng
        except Exception as exc:  # noqa: BLE001
            print(f"[db] Postgres unreachable ({exc}); falling back to SQLite")

    sqlite_path = os.path.join(os.path.dirname(__file__), "..", "app_fallback.db")
    eng = create_engine(
        f"sqlite:///{os.path.abspath(sqlite_path)}",
        connect_args={"check_same_thread": False},
    )
    print("[db] using SQLite fallback")
    return eng


engine = _make_engine()
SessionLocal = sessionmaker(bind=engine, autoflush=False, autocommit=False)


def get_db() -> Generator[Session, None, None]:
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()