app/services/summarizer.py

"""Author RAG Chatbot SaaS — Book Summarizer.

Generates a concise summary for each book using Facebook's BART model.
This runs ONCE after embedding — result stored on the Book record as ai_summary.
RULE: Summarizer runs async after embedding completes — never blocks ingestion.
"""

import structlog

logger = structlog.get_logger(__name__)

_summarizer_pipeline = None


async def get_summarizer():
    """Lazily load and cache the BART summarization pipeline.

    Returns:
        HuggingFace pipeline for summarization.
    """
    global _summarizer_pipeline
    if _summarizer_pipeline is None:
        from transformers import pipeline
        logger.info("Loading BART summarizer model (first load may take a moment)...")
        _summarizer_pipeline = pipeline(
            "summarization",
            model="facebook/bart-large-cnn",
            device=-1,  # CPU (-1), use 0 for GPU
        )
        logger.info("BART summarizer loaded successfully")
    return _summarizer_pipeline


async def summarize_book(text: str, max_length: int = 300) -> str:
    """Generate a concise summary of a book's content using BART.

    Uses the first 3000 characters as representative input (BART has input limits).
    Falls back gracefully if model fails.

    Args:
        text: Full extracted book text.
        max_length: Maximum summary length in tokens.

    Returns:
        Summary string (or empty string on failure).
    """
    if not text.strip():
        return ""

    # BART works best with ~1024 tokens input — use beginning of book
    input_text = text[:4000].strip()

    try:
        summarizer = await get_summarizer()
        result = summarizer(
            input_text,
            max_length=max_length,
            min_length=60,
            do_sample=False,
            truncation=True,
        )
        summary = result[0]["summary_text"].strip()
        logger.info("Book summary generated", length=len(summary))
        return summary
    except Exception as e:
        logger.error("BART summarization failed", error=str(e))
        return _extract_first_paragraph(text)


def _extract_first_paragraph(text: str) -> str:
    """Fallback: extract the first meaningful paragraph as a summary.

    Args:
        text: Full document text.

    Returns:
        First non-empty paragraph, truncated to 500 chars.
    """
    for paragraph in text.split("\n\n"):
        stripped = paragraph.strip()
        if len(stripped) > 100:
            return stripped[:500] + ("..." if len(stripped) > 500 else "")
    return text[:300].strip()