"""Pluggable OCR.

The engine is chosen by the OCR_ENGINE env var:

    tesseract  (default)  -- pytesseract + Pillow + the system `tesseract` binary
    none                  -- OCR disabled; images are marked 'skipped'

Swapping in another engine later (e.g. a Claude-vision call) is just another
branch in `run_ocr` behind the same signature. `run_ocr` raises OCRUnavailable
when the configured engine's dependencies are missing (the worker then marks the
row 'skipped' rather than 'failed'), and lets real OCR errors propagate.
"""

from __future__ import annotations

import io
import os


class OCRUnavailable(RuntimeError):
    """The configured OCR engine can't run (missing binary / library)."""


def engine() -> str:
    return os.environ.get("OCR_ENGINE", "tesseract").lower()


def enabled() -> bool:
    return engine() != "none"


def run_ocr(data: bytes, mime: str) -> str:
    """Return extracted text for an image. Raises OCRUnavailable if the engine
    is missing, or any other exception on a genuine OCR failure."""
    eng = engine()
    if eng == "none":
        raise OCRUnavailable("OCR_ENGINE=none")
    if eng == "tesseract":
        return _tesseract(data)
    raise OCRUnavailable(f"unknown OCR_ENGINE={eng!r}")


def _tesseract(data: bytes) -> str:
    try:
        import pytesseract
        from PIL import Image
    except ImportError as e:
        raise OCRUnavailable(f"pytesseract/Pillow not installed: {e}") from e
    try:
        img = Image.open(io.BytesIO(data))
    except Exception as e:  # not a decodable image
        raise OCRUnavailable(f"not a decodable image: {e}") from e
    try:
        return pytesseract.image_to_string(img)
    except pytesseract.TesseractNotFoundError as e:
        raise OCRUnavailable("tesseract binary not found on PATH") from e