from __future__ import annotations
import os
from pathlib import Path
from typing import Any
import yaml
from dotenv import load_dotenv

REQUIRED_KEYS = ["secret_key", "database.url"]
_PREFIX = "FABLEDNETMON_"


def load_config(config_path: Path | str | None = None) -> dict[str, Any]:
    """Load config from YAML file with env var overrides.

    Env var naming: FABLEDNETMON_ + uppercased dotted path, dots -> __
    Example: database.url -> FABLEDNETMON_DATABASE__URL
    List values are never overridden by env vars.
    """
    load_dotenv()

    if config_path is None:
        config_path = Path("config.yaml")
    config_path = Path(config_path)

    if config_path.exists():
        with config_path.open() as f:
            cfg: dict[str, Any] = yaml.safe_load(f) or {}
    else:
        cfg = {}

    # Apply env var overrides (scalars only)
    for key, value in os.environ.items():
        if not key.startswith(_PREFIX):
            continue
        path = key[len(_PREFIX):].lower().split("__")
        _set_nested(cfg, path, value)

    # Validate required keys
    for dotted in REQUIRED_KEYS:
        parts = dotted.split(".")
        node = cfg
        for part in parts:
            if not isinstance(node, dict) or part not in node:
                raise ValueError(
                    f"Required config key '{dotted}' is missing. "
                    f"Set it in config.yaml or via env var "
                    f"FABLEDNETMON_{dotted.upper().replace('.', '__')}"
                )
            node = node[part]

    return cfg


def _set_nested(cfg: dict, path: list[str], value: str) -> None:
    """Set a scalar value at a nested dict path. Skips if target is a list."""
    node = cfg
    for part in path[:-1]:
        node = node.setdefault(part, {})
    key = path[-1]
    existing = node.get(key)
    if isinstance(existing, list):
        return  # never override list values
    node[key] = value