"""Load and merge TOML configuration files into AppConfig."""

from __future__ import annotations

import os
import tomllib
from pathlib import Path
from typing import Any

from app.core.app_config_models import AppConfig

_CONFIG_DIR_ENV = "CONFIG_DIR"


def default_config_dir() -> Path:
    override = (os.environ.get(_CONFIG_DIR_ENV) or "").strip()
    if override:
        return Path(override).expanduser().resolve()
    # api/app/core/app_config_loader.py -> api/config
    return Path(__file__).resolve().parents[2] / "config"


def _deep_merge(base: dict[str, Any], overlay: dict[str, Any]) -> dict[str, Any]:
    merged = dict(base)
    for key, value in overlay.items():
        if key in merged and isinstance(merged[key], dict) and isinstance(value, dict):
            merged[key] = _deep_merge(merged[key], value)
        else:
            merged[key] = value
    return merged


def _read_toml(path: Path) -> dict[str, Any]:
    with path.open("rb") as handle:
        return tomllib.load(handle)


def load_app_config(app_environment: str, *, config_dir: Path | None = None) -> AppConfig:
    root = config_dir or default_config_dir()
    default_path = root / "default.toml"
    if not default_path.is_file():
        raise FileNotFoundError(f"Missing default config: {default_path}")

    merged = _read_toml(default_path)
    env = (app_environment or "development").strip().lower()
    overlay_path = root / f"{env}.toml"
    if overlay_path.is_file():
        merged = _deep_merge(merged, _read_toml(overlay_path))

    return AppConfig.model_validate(merged)