import logging

try:
    import faiss
    _HAS_FAISS = True
except ImportError:
    logging.warning("FAISS not installed — retrieval will be disabled. Install faiss-cpu or faiss-gpu for full functionality.")
    _HAS_FAISS = False

from sentence_transformers import SentenceTransformer

# load embedding model (still works even if FAISS missing)
_model = SentenceTransformer("all-MiniLM-L6-v2")

_index = None
_docs = []

def init_retriever(docs=None):
    """
    Initialize FAISS index if FAISS is available.
    docs: list[str] to index
    """
    global _index, _docs
    if not _HAS_FAISS:
        _docs = docs or []
        return

    if docs:
        _docs = docs
        embeddings = _model.encode(docs, convert_to_numpy=True)
        d = embeddings.shape[1]
        _index = faiss.IndexFlatL2(d)
        _index.add(embeddings)

def retrieve_context(query: str, k: int = 5):
    """
    Retrieve top-k docs matching query.
    Falls back to empty list if FAISS unavailable.
    """
    if not _HAS_FAISS or _index is None or not _docs:
        return []

    q_emb = _model.encode([query], convert_to_numpy=True)
    D, I = _index.search(q_emb, k)
    return [_docs[i] for i in I[0] if i < len(_docs)]