"""
Configuration definition.

Converts per-run (environment) variables and config files into the
"python world" using `pydantic`.

Pydantic models might have convenience methods attached.
"""

from __future__ import annotations

import functools
import json
from datetime import datetime, timedelta
from enum import Enum
from pathlib import Path
from secrets import token_urlsafe

from jose import JWTError, jwt
from jose.constants import ALGORITHMS
from passlib.context import CryptContext
from pydantic import BaseModel, BaseSettings, Field, validator
from sqlalchemy import create_engine
from sqlalchemy.engine import Engine


class Settings(BaseSettings):
    """
    Per-run settings
    """

    production_mode: bool = False
    data_dir: Path = Path("./tmp")
    openapi_url: str = "/openapi.json"
    docs_url: str | None = "/docs"
    redoc_url: str | None = "/redoc"

    @staticmethod
    @functools.lru_cache
    def get() -> Settings:
        return Settings()

    @property
    def config_file(self) -> Path:
        return self.data_dir.joinpath("config.json")


class DBType(Enum):
    """
    Supported database types
    """

    sqlite = "sqlite"
    mysql = "mysql"


class DBConfig(BaseModel):
    """
    Database connection configuration
    """

    type: DBType = DBType.sqlite
    user: str | None = None
    password: str | None = None
    host: str | None = None
    database: str | None = Settings.get().data_dir.joinpath("vpn.db")

    mysql_driver: str = "pymysql"
    mysql_args: list[str] = ["charset=utf8mb4"]

    @property
    async def db_engine(self) -> Engine:
        """
        Construct an SQLAlchemy engine
        """

        if self.type is DBType.sqlite:
            # SQLite backend
            return create_engine(
                f"sqlite:///{self.database}",
                connect_args={"check_same_thread": False},
            )

        elif self.type is DBType.mysql:
            # MySQL backend
            if self.mysql_args:
                args_str = "?" + "&".join(self.mysql_args)
            else:
                args_str = ""

            return create_engine(
                f"mysql+{self.mysql_driver}://"
                f"{self.user}:{self.password}@{self.host}"
                f"/{self.database}{args_str}",
                pool_recycle=3600,
            )


class JWTConfig(BaseModel):
    """
    Configuration for JSON Web Tokens
    """

    secret: str | None = None
    hash_algorithm: str = ALGORITHMS.HS256
    expiry_minutes: int = 30

    @validator("secret")
    @classmethod
    def ensure_secret(cls, value: str | None) -> str:
        """
        Generate a per-run secret if `None` was loaded from the config file
        """

        if value is None:
            return token_urlsafe(128)

        return value

    async def create_token(
        self,
        username: str,
        expiry_minutes: int | None = None,
    ) -> str:
        """
        Build and sign a JSON Web Token
        """

        if expiry_minutes is None:
            expiry_minutes = self.expiry_minutes

        return jwt.encode(
            {
                "sub": username,
                "exp": datetime.utcnow() + timedelta(minutes=expiry_minutes),
            },
            self.secret,
            algorithm=self.hash_algorithm,
        )

    async def decode_token(
        self,
        token: str,
    ) -> str | None:
        """
        Verify a JSON Web Token, then extract the username
        """

        # decode JWT token
        try:
            payload = jwt.decode(
                token,
                self.secret,
                algorithms=[self.hash_algorithm],
            )

        except JWTError:
            return None

        # check expiry
        expiry = payload.get("exp")
        if expiry is None:
            return None

        if datetime.fromtimestamp(expiry) < datetime.utcnow():
            return None

        # get username
        username = payload.get("sub")
        if username is None:
            return None

        return username


class CryptoConfig(BaseModel):
    """
    Configuration for hash algorithms
    """

    schemes: list[str] = ["bcrypt"]

    @property
    async def crypt_context(self) -> CryptContext:
        return CryptContext(
            schemes=self.schemes,
            deprecated="auto",
        )


class Config(BaseModel):
    """
    Configuration for `kiwi-vpn-api`
    """

    db: DBConfig = Field(default_factory=DBConfig)
    jwt: JWTConfig = Field(default_factory=JWTConfig)
    crypto: CryptoConfig = Field(default_factory=CryptoConfig)

    @staticmethod
    async def load() -> Config | None:
        """
        Load configuration from config file
        """

        try:
            with open(Settings.get().config_file, "r") as config_file:
                return Config.parse_obj(json.load(config_file))

        except FileNotFoundError:
            return None

    async def save(self) -> None:
        """
        Save configuration to config file
        """

        with open(Settings.get().config_file, "w") as config_file:
            config_file.write(self.json(indent=2))