"""Application settings, loaded from ``backend/.env`` and the environment.

The database URL is assembled with :func:`sqlalchemy.engine.URL.create` rather
than string formatting. That matters here: the TiDB password contains ``@``,
and interpolating it into a DSN by hand silently truncates the password at the
first ``@`` (it gets parsed as the user/host separator). ``URL.create``
percent-encodes each component correctly.
"""

from functools import lru_cache
from pathlib import Path
from typing import Union

from pydantic_settings import BaseSettings, SettingsConfigDict
from sqlalchemy import URL

# backend/app/core/config.py -> backend/
BASE_DIR = Path(__file__).resolve().parents[2]


class Settings(BaseSettings):
    """Runtime configuration for the API service."""

    model_config = SettingsConfigDict(
        env_file=BASE_DIR / ".env",
        env_file_encoding="utf-8",
        extra="ignore",
    )

    app_name: str = "paper-doc API"
    api_prefix: str = "/api"
    debug: bool = True

    # The Vite dev server proxies /api to this service, so CORS is normally not
    # exercised during development; it is configured for direct-origin setups.
    cors_origins: list[str] = [
        "http://127.0.0.1:5173",
        "http://localhost:5173",
    ]

    # --- Database (TiDB) -------------------------------------------------
    db_host: str = "127.0.0.1"
    db_port: int = 4000
    db_user: str = "root"
    db_password: str = ""
    db_name: str = "paper_doc"

    # When set, this wins over the DB_* parts above and is used verbatim.
    database_url: str | None = None

    # --- Connection pool -------------------------------------------------
    db_echo: bool = False
    db_pool_pre_ping: bool = True
    db_pool_recycle: int = 3600

    @property
    def sqlalchemy_url(self) -> Union[URL, str]:
        """The SQLAlchemy connection URL.

        Returns a :class:`~sqlalchemy.engine.URL` unless ``DATABASE_URL`` was
        supplied explicitly, in which case that string is passed through.
        """
        if self.database_url:
            return self.database_url
        return URL.create(
            drivername="mysql+pymysql",
            username=self.db_user,
            password=self.db_password,
            host=self.db_host,
            port=self.db_port,
            database=self.db_name,
        )

    @property
    def safe_database_url(self) -> str:
        """The URL rendered with the password masked, for logs and diagnostics."""
        url = self.sqlalchemy_url
        if isinstance(url, str):
            return URL.make_url(url).render_as_string(hide_password=True)
        return url.render_as_string(hide_password=True)


@lru_cache
def get_settings() -> Settings:
    """Return the process-wide settings singleton."""
    return Settings()