"""OCR fallback for scanned PDFs (Tesseract, rus+eng).

Triggered by the extract worker only when `extract_text` returns < 100 chars
(a strong scan signal). Rasterizes each PDF page via pymupdf and runs
pytesseract. Both libs are imported lazily so this module imports cleanly even
in images without tesseract/pymupdf installed (the bot/analyze images).

Yandex Vision is a future alternative behind the same `ocr_pdf(path)` entry.
"""

from __future__ import annotations

from pathlib import Path

from ..logging import get_logger
from .extractor import ExtractionError

log = get_logger(__name__)


class OCRError(Exception):
    """OCR failed (Tesseract unavailable, rasterization failed, no text)."""


def ocr_pdf(path: str | Path, *, lang: str = "rus+eng") -> str:
    """Rasterize and OCR a PDF → plaintext. Raises OCRError on failure."""
    p = Path(path)
    try:
        import pymupdf
        import pytesseract
        from PIL import Image
    except ImportError as exc:
        raise OCRError(f"OCR backend not installed: {exc.name}") from exc

    try:
        doc = pymupdf.open(p)
    except Exception as exc:  # pymupdf кидает разные типы
        raise OCRError(f"Не удалось открыть PDF для OCR {p.name}: {exc}") from exc

    parts: list[str] = []
    try:
        for page_index in range(doc.page_count):
            page = doc[page_index]
            pix = page.get_pixmap(dpi=300)
            img = Image.frombytes("RGB", (pix.width, pix.height), pix.samples)
            text = pytesseract.image_to_string(img, lang=lang)
            if text:
                parts.append(text)
    except pytesseract.pytesseract.TesseractError as exc:
        # Tesseract itself failed (missing language data, etc.) — surface as
        # an OCR infra failure so the retry/DLQ path can refund appropriately.
        raise OCRError(f"Tesseract engine failed for {p.name}: {exc}") from exc
    except Exception as exc:
        raise OCRError(f"OCR failed for {p.name}: {exc}") from exc
    finally:
        doc.close()

    stripped = "\n".join(parts).strip()
    if len(stripped) < 100:
        raise ExtractionError(
            f"OCR тоже дал мало текста ({len(stripped)} симв.). "
            "Файл, видимо, не содержит распознаваемого текста."
        )
    log.info("ocr_done", file=p.name, chars=len(stripped))
    return stripped