"""
SQLAlchemy 引擎：使用环境变量 ``database_url`` / ``DATABASE_URL``（与项目根目录 ``.env`` 一致）。
"""

from __future__ import annotations

import logging
import os
from typing import Optional

from sqlalchemy import create_engine
from sqlalchemy.engine import Engine

logger = logging.getLogger(__name__)

_engine: Engine | None = None


def _database_url_from_env() -> str:
    for key in ("database_url", "DATABASE_URL"):
        v = os.getenv(key)
        if v is not None and str(v).strip():
            return str(v).strip()
    return ""


def get_engine(*, url: Optional[str] = None, reset: bool = False) -> Engine:
    """
    返回默认业务库引擎（单例）。未配置 ``database_url`` 时抛出 ``ValueError``。

    :param url: 若传入，则忽略单例并为此 URL 新建引擎（便于测试）。
    :param reset: 为 True 时丢弃已缓存的单例，下次再按环境变量创建。
    """
    global _engine
    if reset:
        _engine = None
    if url is not None:
        return create_engine(url, pool_pre_ping=True)
    if _engine is not None:
        return _engine
    u = _database_url_from_env()
    if not u:
        raise ValueError("database_url 未配置")
    _engine = create_engine(u, pool_pre_ping=True)
    logger.info("SQLAlchemy engine initialized from database_url")
    return _engine