"""
vector_store.py
───────────────
Handles text chunking, embedding, and FAISS vector store creation/querying.

Responsibilities:
  - Split raw Documents into overlapping chunks
  - Embed chunks using a local HuggingFace sentence-transformer
  - Build and expose a FAISS index for similarity search
  - Provide a clean retrieve() function used by the RAG pipeline
"""

import logging

from langchain_core.documents import Document
from langchain_text_splitters import RecursiveCharacterTextSplitter
from langchain_community.embeddings import HuggingFaceEmbeddings
from langchain_community.vectorstores import FAISS

from config import cfg

logger = logging.getLogger(__name__)


# ── Public API ────────────────────────────────────────────────────────────────

def build_index(documents: list[Document]) -> FAISS:
    """
    Chunk → embed → index the supplied documents.

    Parameters
    ----------
    documents : list[Document]
        Raw documents returned by data_loader.load_documents().

    Returns
    -------
    FAISS
        A ready-to-query FAISS vector store.
    """
    chunks = _chunk_documents(documents)
    embeddings = _load_embeddings()
    index = _create_faiss_index(chunks, embeddings)
    return index


def retrieve(index: FAISS, query: str, k: int | None = None) -> list[Document]:
    """
    Retrieve the top-k most relevant chunks for a given query.

    Parameters
    ----------
    index : FAISS
        The FAISS vector store built by build_index().
    query : str
        The user's natural-language question.
    k : int, optional
        Number of results to return. Defaults to cfg.top_k.

    Returns
    -------
    list[Document]
        Retrieved chunks, most relevant first.
    """
    k = k or cfg.top_k
    results = index.similarity_search(query, k=k)
    logger.debug("Retrieved %d chunks for query: '%s'", len(results), query[:80])
    return results


# ── Internal helpers ──────────────────────────────────────────────────────────

def _chunk_documents(documents: list[Document]) -> list[Document]:
    """Split documents into smaller overlapping chunks."""
    splitter = RecursiveCharacterTextSplitter(
        chunk_size=cfg.chunk_size,
        chunk_overlap=cfg.chunk_overlap,
        separators=["\n\n", "\n", ". ", " ", ""],
    )
    chunks = splitter.split_documents(documents)
    logger.info(
        "Chunking: %d raw docs → %d chunks (size=%d, overlap=%d)",
        len(documents), len(chunks), cfg.chunk_size, cfg.chunk_overlap,
    )
    return chunks


def _load_embeddings() -> HuggingFaceEmbeddings:
    """Load the local sentence-transformer embedding model (cached after first call)."""
    logger.info("Loading embedding model: %s", cfg.embed_model)
    return HuggingFaceEmbeddings(
        model_name=cfg.embed_model,
        model_kwargs={"device": "cpu"},
        encode_kwargs={"normalize_embeddings": True},
    )


def _create_faiss_index(chunks: list[Document], embeddings: HuggingFaceEmbeddings) -> FAISS:
    """Embed all chunks and build the FAISS index."""
    logger.info("Building FAISS index over %d chunks …", len(chunks))
    index = FAISS.from_documents(chunks, embeddings)
    logger.info("FAISS index built ✓  (vectors: %d)", index.index.ntotal)
    return index