nextcloud-mcp-server/nextcloud_mcp_server/search/bm25_hybrid.py

"""BM25 hybrid search algorithm using Qdrant native RRF fusion."""

import logging
from typing import Any

from qdrant_client import models
from qdrant_client.models import FieldCondition, Filter, MatchValue

from nextcloud_mcp_server.config import get_settings
from nextcloud_mcp_server.embedding import get_bm25_service, get_embedding_service
from nextcloud_mcp_server.observability.metrics import record_qdrant_operation
from nextcloud_mcp_server.search.algorithms import SearchAlgorithm, SearchResult
from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client

logger = logging.getLogger(__name__)


class BM25HybridSearchAlgorithm(SearchAlgorithm):
    """
    Hybrid search combining dense semantic vectors with BM25 sparse vectors.

    Uses Qdrant's native Reciprocal Rank Fusion (RRF) to automatically merge
    results from both dense (semantic) and sparse (BM25 keyword) searches.
    This provides the best of both worlds: semantic understanding for conceptual
    queries and precise keyword matching for specific terms, acronyms, and codes.

    The fusion happens efficiently in the database using the prefetch mechanism,
    eliminating the need for application-layer result merging.
    """

    def __init__(self, score_threshold: float = 0.0):
        """
        Initialize BM25 hybrid search algorithm.

        Args:
            score_threshold: Minimum RRF score (0-1, default: 0.0 to allow RRF scoring)
                           Note: RRF produces normalized scores, so threshold is typically lower
        """
        self.score_threshold = score_threshold

    @property
    def name(self) -> str:
        return "bm25_hybrid"

    @property
    def requires_vector_db(self) -> bool:
        return True

    async def search(
        self,
        query: str,
        user_id: str,
        limit: int = 10,
        doc_type: str | None = None,
        **kwargs: Any,
    ) -> list[SearchResult]:
        """
        Execute hybrid search using dense + sparse vectors with native RRF fusion.

        Returns unverified results from Qdrant. Access verification should be
        performed separately at the final output stage using verify_search_results().

        Args:
            query: Natural language or keyword search query
            user_id: User ID for filtering
            limit: Maximum results to return
            doc_type: Optional document type filter
            **kwargs: Additional parameters (score_threshold override)

        Returns:
            List of unverified SearchResult objects ranked by RRF fusion score

        Raises:
            McpError: If vector sync is not enabled or search fails
        """
        settings = get_settings()
        score_threshold = kwargs.get("score_threshold", self.score_threshold)

        logger.info(
            f"BM25 hybrid search: query='{query}', user={user_id}, "
            f"limit={limit}, score_threshold={score_threshold}, doc_type={doc_type}"
        )

        # Generate dense embedding for semantic search
        embedding_service = get_embedding_service()
        dense_embedding = await embedding_service.embed(query)
        logger.debug(f"Generated dense embedding (dimension={len(dense_embedding)})")

        # Generate sparse embedding for BM25 keyword search
        bm25_service = get_bm25_service()
        sparse_embedding = bm25_service.encode(query)
        logger.debug(
            f"Generated sparse embedding "
            f"({len(sparse_embedding['indices'])} non-zero terms)"
        )

        # Build Qdrant filter
        filter_conditions = [
            FieldCondition(
                key="user_id",
                match=MatchValue(value=user_id),
            )
        ]

        # Add doc_type filter if specified
        if doc_type:
            filter_conditions.append(
                FieldCondition(
                    key="doc_type",
                    match=MatchValue(value=doc_type),
                )
            )

        query_filter = Filter(must=filter_conditions)

        # Execute hybrid search with Qdrant native RRF fusion
        qdrant_client = await get_qdrant_client()
        try:
            # Use prefetch to run both dense and sparse searches
            # Qdrant will automatically merge results using RRF
            search_response = await qdrant_client.query_points(
                collection_name=settings.get_collection_name(),
                prefetch=[
                    # Dense semantic search
                    models.Prefetch(
                        query=dense_embedding,
                        using="dense",
                        limit=limit * 2,  # Get extra for deduplication
                        filter=query_filter,
                    ),
                    # Sparse BM25 search
                    models.Prefetch(
                        query=models.SparseVector(
                            indices=sparse_embedding["indices"],
                            values=sparse_embedding["values"],
                        ),
                        using="sparse",
                        limit=limit * 2,  # Get extra for deduplication
                        filter=query_filter,
                    ),
                ],
                # RRF fusion query (no additional query needed, just fusion)
                query=models.FusionQuery(fusion=models.Fusion.RRF),
                limit=limit * 2,  # Get extra for deduplication
                score_threshold=score_threshold,
                with_payload=True,
                with_vectors=False,  # Don't return vectors to save bandwidth
            )
            record_qdrant_operation("search", "success")
        except Exception:
            record_qdrant_operation("search", "error")
            raise

        logger.info(
            f"Qdrant RRF fusion returned {len(search_response.points)} results "
            f"(before deduplication)"
        )

        if search_response.points:
            # Log top 3 RRF scores to help with threshold tuning
            top_scores = [p.score for p in search_response.points[:3]]
            logger.debug(f"Top 3 RRF fusion scores: {top_scores}")

        # Deduplicate by (doc_id, doc_type) - multiple chunks per document
        seen_docs = set()
        results = []

        for result in search_response.points:
            doc_id = int(result.payload["doc_id"])
            doc_type = result.payload.get("doc_type", "note")
            doc_key = (doc_id, doc_type)

            # Skip if we've already seen this document
            if doc_key in seen_docs:
                continue

            seen_docs.add(doc_key)

            # Return unverified results (verification happens at output stage)
            results.append(
                SearchResult(
                    id=doc_id,
                    doc_type=doc_type,
                    title=result.payload.get("title", "Untitled"),
                    excerpt=result.payload.get("excerpt", ""),
                    score=result.score,  # RRF fusion score
                    metadata={
                        "chunk_index": result.payload.get("chunk_index"),
                        "total_chunks": result.payload.get("total_chunks"),
                        "search_method": "bm25_hybrid_rrf",
                    },
                )
            )

            if len(results) >= limit:
                break

        logger.info(f"Returning {len(results)} unverified results after deduplication")
        if results:
            result_details = [
                f"{r.doc_type}_{r.id} (score={r.score:.3f}, title='{r.title}')"
                for r in results[:5]  # Show top 5
            ]
            logger.debug(f"Top results: {', '.join(result_details)}")

        return results