"""
vector_store.py
---------------
Creates, persists, and loads a FAISS vector index.

Key behaviours
--------------
* build()  — embeds documents and stores FAISS index
* load()   — loads existing FAISS index from disk
* search() — returns top-K (Document, normalized score)
"""

import logging
from pathlib import Path
from typing import List, Tuple

from langchain.schema import Document
from langchain_community.vectorstores import FAISS

from app.config import VECTOR_DB_PATH
from components.embedder import HuggingFaceEmbedder

logger = logging.getLogger(__name__)


class VectorStore:
    """
    FAISS vector store wrapper (stable + production-safe).
    """

    def __init__(
        self,
        embedder: HuggingFaceEmbedder | None = None,
        index_path: str = VECTOR_DB_PATH,
    ) -> None:
        self.embedder = embedder or HuggingFaceEmbedder()
        self.index_path = index_path
        self._store: FAISS | None = None

    # ─────────────────────────────────────────────
    # Build index
    # ─────────────────────────────────────────────
    def build(self, documents: List[Document]) -> None:
        if not documents:
            raise ValueError("Cannot build vector store from empty documents.")

        logger.info("Building FAISS index from %d chunks …", len(documents))

        self._store = FAISS.from_documents(documents, self.embedder)
        self._persist()

        logger.info("FAISS index saved to '%s'.", self.index_path)

    # ─────────────────────────────────────────────
    # Load index
    # ─────────────────────────────────────────────
    def load(self) -> bool:
        index_file = Path(self.index_path) / "index.faiss"

        if not index_file.exists():
            logger.info("No existing FAISS index found at '%s'.", self.index_path)
            return False

        logger.info("Loading FAISS index from '%s' …", self.index_path)

        self._store = FAISS.load_local(
            self.index_path,
            self.embedder,
            allow_dangerous_deserialization=True,
        )

        logger.info("FAISS index loaded (%d vectors).", self._store.index.ntotal)
        return True

    # ─────────────────────────────────────────────
    # SEARCH (FIXED VERSION)
    # ─────────────────────────────────────────────
    def search(
        self,
        query: str,
        k: int = 4,
    ) -> List[Tuple[Document, float]]:
        """
        Returns:
            List of (Document, relevance_score 0–1)
        """
        self._require_store()

        # STEP 1: get raw FAISS distances (stable)
        results = self._store.similarity_search_with_score(query, k=k)

        processed: List[Tuple[Document, float]] = []

        for doc, distance in results:
            # Convert distance → similarity score (0–1)
            score = 1.0 / (1.0 + float(distance))
            processed.append((doc, score))

        return processed

    # ─────────────────────────────────────────────
    # Add documents
    # ─────────────────────────────────────────────
    def add_documents(self, documents: List[Document]) -> None:
        self._require_store()

        self._store.add_documents(documents)
        self._persist()

        logger.info("Added %d chunks to index.", len(documents))

    # ─────────────────────────────────────────────
    # Status
    # ─────────────────────────────────────────────
    @property
    def is_ready(self) -> bool:
        return self._store is not None

    # ─────────────────────────────────────────────
    # Internal helpers
    # ─────────────────────────────────────────────
    def _persist(self) -> None:
        Path(self.index_path).mkdir(parents=True, exist_ok=True)
        self._store.save_local(self.index_path)

    def _require_store(self) -> None:
        if self._store is None:
            raise RuntimeError(
                "Vector store not ready. Call build() or load() first."
            )