import pickle
import logging
import os
import shutil
from datetime import datetime
from concurrent.futures import ThreadPoolExecutor, as_completed
from pathlib import Path
from typing import List, Optional, Iterable
from langchain.schema import Document
from langchain_community.vectorstores import FAISS

from .config import get_embedding_model, VECTOR_STORE_DIR, CHUNKS_PATH, NEW_DATA, PROCESSED_DATA
from .text_processors import markdown_splitter, recursive_splitter
from . import data_loaders

logger = logging.getLogger(__name__)

MAX_WORKERS = max(2, min(8, (os.cpu_count() or 4)))


def load_vector_store() -> Optional[FAISS]:
    """Load existing vector store with proper error handling.
    Only attempt to load if required FAISS files are present.
    """
    try:
        store_dir = Path(VECTOR_STORE_DIR)
        index_file = store_dir / "index.faiss"
        meta_file = store_dir / "index.pkl"  # created by LangChain FAISS.save_local

        # If directory exists but files are missing, do not attempt load
        if not (index_file.exists() and meta_file.exists()):
            logger.info("Vector store not initialized yet; index files not found. Skipping load.")
            return None

        vector_store = FAISS.load_local(
            str(VECTOR_STORE_DIR),
            get_embedding_model(),
            allow_dangerous_deserialization=True,
        )
        logger.info("Successfully loaded existing vector store")
        return vector_store
    except Exception as e:
        logger.error(f"Failed to load vector store: {e}")
        return None


def load_chunks() -> Optional[List[Document]]:
    """Load pre-processed document chunks from cache with error handling."""
    try:
        if Path(CHUNKS_PATH).exists():
            with open(CHUNKS_PATH, 'rb') as f:
                chunks = pickle.load(f)
            logger.info(f"Successfully loaded {len(chunks)} chunks from cache")
            return chunks
        else:
            logger.info("No cached chunks found")
            return None
    except Exception as e:
        logger.error(f"Failed to load chunks: {e}")
        return None


def save_chunks(chunks: List[Document]) -> bool:
    """Save processed document chunks to cache file.
    
    Args:
        chunks: List of document chunks to save
        
    Returns:
        True if successful, False otherwise
    """
    try:
        # Ensure directory exists
        Path(CHUNKS_PATH).parent.mkdir(parents=True, exist_ok=True)
        
        with open(CHUNKS_PATH, 'wb') as f:
            pickle.dump(chunks, f)
        logger.info(f"Successfully saved {len(chunks)} chunks to {CHUNKS_PATH}")
        return True
    except Exception as e:
        logger.error(f"Failed to save chunks: {e}")
        return False


# ============================================================================
# DOCUMENT PROCESSING UTILITIES
# ============================================================================

def _iter_files(root: Path) -> Iterable[Path]:
    """Yield PDF and Markdown files under the given root directory recursively.
    
    Args:
        root: Root directory to search
        
    Yields:
        Path objects for PDF and Markdown files
    """
    if not root.exists():
        return []
    for p in root.rglob('*'):
        if p.is_file() and p.suffix.lower() in {'.pdf', '.md'}:
            yield p


def create_documents() -> List[Document]:
    """Load documents from NEW_DATA directory.
    
    Returns:
        List of loaded documents
        
    Note:
        Use create_documents_and_files() if you need both documents and file paths.
    """
    docs, _ = create_documents_and_files()
    return docs


def _load_documents_for_file(file_path: Path) -> List[Document]:
    """Load documents from a single file (PDF or Markdown).
    
    Args:
        file_path: Path to the file to load
        
    Returns:
        List of documents loaded from the file
    """
    try:
        if file_path.suffix.lower() == '.pdf':
            return data_loaders.load_pdf_documents(file_path)
        return data_loaders.load_markdown_documents(file_path)
    except Exception as e:
        logger.error(f"Failed to load {file_path}: {e}")
        return []


def create_documents_and_files() -> tuple[List[Document], List[Path]]:
    """Load documents from NEW_DATA directory and return both documents and file paths.

    Returns:
        Tuple of (documents, file_paths) where:
        - documents: List of loaded Document objects
        - file_paths: List of Path objects for files that were loaded
    """
    documents: List[Document] = []
    files = list(_iter_files(NEW_DATA))
    if not files:
        logger.info(f"No new files found under {NEW_DATA}")
        return documents, []

    worker_count = min(MAX_WORKERS, len(files)) or 1
    with ThreadPoolExecutor(max_workers=worker_count) as executor:
        futures = {executor.submit(_load_documents_for_file, file_path): file_path for file_path in files}
        for future in as_completed(futures):
            documents.extend(future.result())
    logger.info(f"Loaded {len(documents)} documents from {NEW_DATA}")
    return documents, files


def _segment_document(doc: Document) -> List[Document]:
    """Segment a document using markdown headers if applicable.
    
    Args:
        doc: Document to segment
        
    Returns:
        List of segmented documents (or original if not markdown)
    """
    source_name = str(doc.metadata.get("source", "")).lower()
    if source_name.endswith('.md'):
        try:
            md_sections = markdown_splitter.split_text(doc.page_content)
            return [Document(page_content=section.page_content, metadata={**doc.metadata, **section.metadata}) for section in md_sections]
        except Exception:
            return [doc]
    return [doc]


def _split_chunk(doc: Document) -> List[Document]:
    """Split a document into smaller chunks using recursive splitter.
    
    Args:
        doc: Document to split
        
    Returns:
        List of document chunks
    """
    try:
        return recursive_splitter.split_documents([doc])
    except Exception as exc:
        logger.error(f"Failed to split document {doc.metadata.get('source', 'unknown')}: {exc}")
        return []


def split_documents(documents: List[Document]) -> List[Document]:
    """Split documents into smaller chunks for vector store indexing.
    
    Process:
    1. Segment markdown files by headers (if applicable)
    2. Split all documents into uniform chunks using recursive splitter
    
    Args:
        documents: List of documents to split
        
    Returns:
        List of document chunks ready for indexing
    """
    if not documents:
        return []

    # First pass: optional markdown header segmentation for .md sources
    worker_count = min(MAX_WORKERS, len(documents)) or 1
    with ThreadPoolExecutor(max_workers=worker_count) as executor:
        segmented_lists = list(executor.map(_segment_document, documents))
    segmented: List[Document] = [seg for sublist in segmented_lists for seg in sublist]

    if not segmented:
        return []

    # Second pass: split into uniform chunks
    split_worker_count = min(MAX_WORKERS, len(segmented)) or 1
    with ThreadPoolExecutor(max_workers=split_worker_count) as executor:
        chunk_lists = list(executor.map(_split_chunk, segmented))

    chunks = [chunk for chunk_list in chunk_lists for chunk in chunk_list]
    logger.info(f"Split {len(segmented)} documents into {len(chunks)} chunks")
    return chunks


def create_vector_store(chunks: List[Document]) -> FAISS:
    """Create a new FAISS vector store from document chunks and persist it.
    
    Args:
        chunks: List of document chunks to index
        
    Returns:
        Created FAISS vector store
        
    Raises:
        ValueError: If chunks list is empty
    """
    if not chunks:
        raise ValueError("Cannot create vector store from empty chunks")
    vector_store = FAISS.from_documents(chunks, get_embedding_model())
    vector_store.save_local(str(VECTOR_STORE_DIR))
    logger.info("Vector store created and saved")
    return vector_store


def update_vector_store_with_chunks(chunks: List[Document]) -> FAISS:
    """Update vector store with new chunks or create if doesn't exist.
    
    Args:
        chunks: List of new document chunks to add
        
    Returns:
        Updated or newly created FAISS vector store
    """
    if not chunks:
        existing = load_vector_store()
        if existing:
            return existing

    store = load_vector_store()
    if store is None:
        store = create_vector_store(chunks)
    else:
        # Add to existing store and persist
        store.add_documents(chunks)
        store.save_local(str(VECTOR_STORE_DIR))
        logger.info(f"Added {len(chunks)} new chunks to existing vector store")
    return store


def _move_to_processed(paths: List[Path]) -> None:
    """Move processed files to processed_data folder maintaining directory structure.
    
    Args:
        paths: List of file paths to move
    """
    timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
    
    for p in paths:
        try:
            if p.exists() and p.is_file():
                # Calculate relative path from NEW_DATA
                try:
                    rel_path = p.relative_to(NEW_DATA)
                except ValueError:
                    # File is not under NEW_DATA, skip it
                    logger.warning(f"File {p} is not under NEW_DATA directory, skipping")
                    continue
                
                # Create destination path in PROCESSED_DATA with same structure
                dest_dir = PROCESSED_DATA / rel_path.parent
                dest_dir.mkdir(parents=True, exist_ok=True)
                
                # Add timestamp to filename to avoid overwriting
                dest_file = dest_dir / f"{p.stem}_{timestamp}{p.suffix}"
                
                # Move the file
                shutil.move(str(p), str(dest_file))
                logger.info(f"📦 Moved processed file: {p.name} -> {dest_file.relative_to(PROCESSED_DATA)}")
        except Exception as e:
            logger.error(f"❌ Failed to move {p}: {e}")


def _cleanup_empty_dirs(root: Path) -> None:
    """Remove empty directories under root directory (best-effort).
    
    Args:
        root: Root directory to clean up
    """
    try:
        # Walk bottom-up to remove empty directories
        dirs = [d for d in root.rglob('*') if d.is_dir()]
        for dirpath in sorted(dirs, key=lambda x: len(str(x)), reverse=True):
            try:
                if not any(dirpath.iterdir()):
                    dirpath.rmdir()
                    logger.info(f"Removed empty directory: {dirpath}")
            except Exception:
                pass
    except Exception:
        pass


def process_new_data_and_update_vector_store() -> Optional[FAISS]:
    """Process new documents and update the vector store.
    
    Workflow:
    1. Load documents from NEW_DATA directory
    2. Split documents into chunks
    3. Update chunks cache and vector store
    4. Delete processed files and clean up empty directories
    
    Returns:
        Updated FAISS vector store, or None if processing failed
    """
    try:
        docs, files = create_documents_and_files()
        if not docs:
            logger.info("No new documents to process.")
            return load_vector_store()

        chunks = split_documents(docs)

        # Save/merge chunks first (durability)
        existing_chunks = load_chunks() or []
        merged_chunks = existing_chunks + chunks

        with ThreadPoolExecutor(max_workers=2) as executor:
            save_future = executor.submit(save_chunks, merged_chunks)
            store_future = executor.submit(update_vector_store_with_chunks, chunks)
            save_success = save_future.result()
            store = store_future.result()

        if not save_success:
            logger.warning("Chunk persistence reported failure; vector store was updated but cache may be stale.")

        # If we reached here, store update succeeded; move processed source files
        _move_to_processed(files)
        _cleanup_empty_dirs(NEW_DATA)

        logger.info(
            f"✅ Processed {len(docs)} new documents into {len(chunks)} chunks, updated vector store, and moved files to processed_data."
        )
        return store
    except Exception as e:
        logger.error(f"Failed processing new data: {e}")
        return None