"""Backend registry for garment detectors."""

import logging
from typing import Type

from ._types import DetectorBackend

logger = logging.getLogger(__name__)

_BACKENDS: dict[str, Type[DetectorBackend]] = {}
_INSTANCES: dict[str, DetectorBackend] = {}


def register(name: str):
    """Decorator to register a detector backend class."""

    def decorator(cls: Type[DetectorBackend]):
        _BACKENDS[name] = cls
        return cls

    return decorator


def get_backend(name: str) -> DetectorBackend:
    """Get a backend instance by name (lazy-instantiated singleton)."""
    if name not in _BACKENDS:
        available = list_available()
        raise ValueError(
            f"Unknown detector backend: '{name}'. "
            f"Available: {available}"
        )

    if name not in _INSTANCES:
        _INSTANCES[name] = _BACKENDS[name]()

    return _INSTANCES[name]


def list_registered() -> list[str]:
    """List all registered backend names (even if deps are missing)."""
    return list(_BACKENDS.keys())


def list_available() -> list[tuple[str, str]]:
    """List backends whose dependencies are installed.

    Returns (name, description) tuples for UI display.
    """
    available = []
    for name, cls in _BACKENDS.items():
        try:
            instance = get_backend(name)
            if instance.is_available():
                available.append((instance.description, name))
        except Exception as e:
            logger.debug("Backend '%s' not available: %s", name, e)
    return available