import os
import shutil  # <-- Added this missing import
from llama_index.core import (
    VectorStoreIndex,
    SimpleDirectoryReader,
    StorageContext,
    load_index_from_storage,
    Settings
)
from llama_index.embeddings.huggingface import HuggingFaceEmbedding
from llama_index.core.llms import MockLLM
from llama_index.readers.file import PyMuPDFReader

# Set cache directory for Hugging Face models
os.environ["HF_HOME"] = "/tmp/hf"

# Configure Global Settings
# Using BAAI bge-small-zh-v1.5 for efficient Chinese/English embedding
Settings.embed_model = HuggingFaceEmbedding(model_name="BAAI/bge-small-zh-v1.5")
# Use a MockLLM since we are doing retrieval only; saves memory and avoids API calls
Settings.llm = MockLLM()

PERSIST_DIR = "./storage"
DATA_DIR = "./data"

def initialize_index():
    """Load existing index from disk or create a new one if data exists."""
    if os.path.exists(os.path.join(PERSIST_DIR, "docstore.json")):
        storage_context = StorageContext.from_defaults(persist_dir=PERSIST_DIR)
        index = load_index_from_storage(storage_context)
    else:
        os.makedirs(DATA_DIR, exist_ok=True)
        loader = SimpleDirectoryReader(
            DATA_DIR, 
            recursive=True, 
            required_exts=[".pdf"],
            file_extractor={".pdf": PyMuPDFReader()}
        )
        if os.listdir(DATA_DIR):
            documents = loader.load_data()
            index = VectorStoreIndex.from_documents(documents)
            index.storage_context.persist(persist_dir=PERSIST_DIR)
        else:
            index = VectorStoreIndex.from_documents([])
    return index

# Global variable to cache the engine
_cached_engine = None

def get_retriever():
    """Initialize and return a query engine optimized for retrieval only."""
    global _cached_engine
    if _cached_engine is None:
        index = initialize_index()
        _cached_engine = index.as_query_engine(
            similarity_top_k=8,
            response_mode="no_text"  # Returns only source nodes, no LLM synthesis
        )
    return _cached_engine

def add_document(file_path: str):
    """Move file to data directory and rebuild the index."""
    global _cached_engine
    os.makedirs(DATA_DIR, exist_ok=True)
    
    # Move the uploaded file to the persistent data folder
    dest = os.path.join(DATA_DIR, os.path.basename(file_path))
    
    # Ensure the destination doesn't exist or handle it (shutil.move requires this)
    if os.path.exists(dest):
        os.remove(dest)
        
    shutil.move(file_path, dest)
    
    # Reload all documents and rebuild index
    documents = SimpleDirectoryReader(DATA_DIR).load_data()
    index = VectorStoreIndex.from_documents(documents)
    index.storage_context.persist(persist_dir=PERSIST_DIR)
    
    # Reset engine to force reload on next query
    _cached_engine = None
    return len(documents)