"""Backend selector — reads `backends.storage` from config and returns
a singleton instance of the chosen backend."""

from __future__ import annotations

import os
from functools import lru_cache

from src.config import load_config

from .base import Storage
from .local import LocalStorage


def _resolve_backend_name() -> str:
    cfg = load_config()
    name = (cfg.section("backends").get("storage") or "local").strip().lower()
    # Auto-promotion: on Streamlit Cloud the disk is ephemeral, so silently
    # treat 'local' as 'r2' if RUNTIME=cloud is set. Same trick we use for
    # the inventory_db backend. Local dev is unaffected.
    if name == "local" and os.environ.get("RUNTIME", "").lower() == "cloud":
        return "r2"
    return name


@lru_cache(maxsize=1)
def get_storage() -> Storage:
    name = _resolve_backend_name()
    if name == "local":
        return LocalStorage()
    if name == "r2":
        from .r2 import R2Storage  # lazy: keeps boto3 out of the local hot path
        return R2Storage()
    raise ValueError(
        f"Unknown backends.storage = {name!r}. Expected 'local' or 'r2'."
    )