"""Read / write the device config file at ~/assistant/state/config.json."""
import json
import os
from dataclasses import asdict, dataclass
from pathlib import Path


class ConfigMissingError(RuntimeError):
    """Raised when config.json is missing or malformed."""


@dataclass(frozen=True)
class Config:
    device_id: str
    device_token: str
    backend_ws: str


def config_path() -> Path:
    return Path(os.environ["HOME"]) / "assistant" / "state" / "config.json"


def load_config() -> Config:
    path = config_path()
    if not path.exists():
        raise ConfigMissingError(f"{path} not found; run `python -m client.pair` first")
    try:
        raw = json.loads(path.read_text())
    except json.JSONDecodeError as exc:
        raise ConfigMissingError(f"{path} is not valid JSON: {exc}") from exc
    try:
        return Config(
            device_id=str(raw["device_id"]),
            device_token=str(raw["device_token"]),
            backend_ws=str(raw["backend_ws"]),
        )
    except KeyError as exc:
        raise ConfigMissingError(f"{path} missing field: {exc}") from exc


def save_config(cfg: Config) -> None:
    path = config_path()
    path.parent.mkdir(parents=True, exist_ok=True)
    # Write to a temp file then atomically rename, so a partial write never
    # leaves a half-baked config on disk.
    tmp = path.with_suffix(".json.tmp")
    tmp.write_text(json.dumps(asdict(cfg), indent=2) + "\n")
    os.chmod(tmp, 0o600)
    os.replace(tmp, path)