"""Factory function for creating OCR engine instances from configuration."""

import logging

from app.config import settings
from app.engines.base_engine import EngineUnavailableError, OcrEngine

logger = logging.getLogger(__name__)

# Valid engine identifiers
_ENGINE_REGISTRY: dict[str, str] = {
    "paddleocr": "app.engines.paddle_engine.PaddleOcrEngine",
    "tesseract": "app.engines.tesseract_engine.TesseractEngine",
}


def create_engine(engine_name: str | None = None) -> OcrEngine:
    """Instantiate an OCR engine by name (defaults to config value).

    Args:
        engine_name: Engine identifier ("paddleocr", "tesseract").
                     Falls back to ``settings.ocr_primary_engine``.

    Returns:
        Initialized OcrEngine instance.

    Raises:
        EngineUnavailableError: If the engine cannot be loaded or initialized.
    """
    name = (engine_name or settings.ocr_primary_engine).lower().strip()

    if name not in _ENGINE_REGISTRY:
        raise EngineUnavailableError(
            f"Unknown engine '{name}'. Available: {list(_ENGINE_REGISTRY.keys())}"
        )

    module_path, class_name = _ENGINE_REGISTRY[name].rsplit(".", 1)

    try:
        import importlib

        module = importlib.import_module(module_path)
        engine_cls = getattr(module, class_name)
        engine: OcrEngine = engine_cls()
        logger.info("Created OCR engine: %s", name)
        return engine
    except EngineUnavailableError:
        raise
    except Exception as exc:
        raise EngineUnavailableError(
            f"Failed to create engine '{name}': {exc}"
        ) from exc