"""
Async document/topic/settings storage service for DocuVault.

This module replaces the legacy flat-file + filelock implementation with:
  - Async SQLAlchemy ORM for document and topic persistence (PostgreSQL)
  - MinIO SDK (via asyncio.to_thread) for binary object storage

Public function names are PRESERVED from the old flat-file implementation so
that api/documents.py and api/topics.py can be updated in Plan 05 with minimal
changes (async def + await + session parameter).

Phase 3 D-12: load_settings / save_settings / mask_api_key / settings_masked removed.
All AI config comes from DB (users.ai_provider / users.ai_model set by admin).

D-05: Storage service layer switched to PostgreSQL + MinIO.
D-06: Object key schema: {user_id}/{document_id}/{uuid4()}{ext} — human filename in DB only.
D-03: documents.user_id is None (nullable) in Phase 1 — no auth system yet.
"""
from __future__ import annotations

import sys
import uuid
from datetime import datetime, timezone
from typing import Optional

from sqlalchemy import select, delete, text, or_
from sqlalchemy import func as sql_func
from sqlalchemy.ext.asyncio import AsyncSession

from db.models import Document, DocumentTopic, Topic
from storage import get_storage_backend


# ── Lazy singleton storage backend ────────────────────────────────────────────

_storage = None


def _backend():
    """Return the lazily-instantiated StorageBackend singleton.

    Mirrors the module-level singleton behaviour of the old filelock objects so
    the MinIO client is created once per process, not once per request.
    """
    global _storage
    _storage = _storage or get_storage_backend()
    return _storage


# ── Private helpers ────────────────────────────────────────────────────────────

def _doc_to_dict(doc: Document, topic_names: list) -> dict:
    """Convert a Document ORM row + resolved topic names to the legacy dict shape."""
    return {
        "id": str(doc.id),
        "original_name": doc.filename,
        "filename": doc.filename,
        "mime_type": doc.content_type,
        "size_bytes": doc.size_bytes,
        "extracted_text": doc.extracted_text or "",
        "topics": topic_names,
        "created_at": doc.created_at.isoformat() if doc.created_at else None,
        "classified_at": doc.updated_at.isoformat() if doc.status == "classified" else None,
    }


async def _load_topic_names(session: AsyncSession, doc_id: uuid.UUID) -> list:
    """Return the list of topic names for a given document UUID."""
    q = await session.execute(
        select(Topic.name)
        .join(DocumentTopic, DocumentTopic.topic_id == Topic.id)
        .where(DocumentTopic.document_id == doc_id)
    )
    return [row[0] for row in q]


# ── Documents ─────────────────────────────────────────────────────────────────


async def save_metadata(session: AsyncSession, meta: dict) -> None:
    """Update a Document row from the legacy metadata dict shape.

    Keys consumed: id, extracted_text, topics (list[str]), classified_at.
    """
    try:
        uid = uuid.UUID(meta["id"])
    except (ValueError, KeyError):
        return

    doc = await session.get(Document, uid)
    if doc is None:
        return

    doc.extracted_text = meta.get("extracted_text", "")

    topics_list = meta.get("topics")
    if topics_list:
        await update_document_topics(session, meta["id"], topics_list)

    doc.status = "classified" if meta.get("classified_at") else "pending"
    await session.commit()


async def get_metadata(session: AsyncSession, doc_id: str) -> Optional[dict]:
    """Return the legacy metadata dict for a document, or None if not found."""
    try:
        uid = uuid.UUID(doc_id)
    except ValueError:
        return None

    doc = await session.get(Document, uid)
    if doc is None:
        return None

    topic_names = await _load_topic_names(session, uid)
    return _doc_to_dict(doc, topic_names)


async def list_metadata(
    session: AsyncSession, user_id: uuid.UUID, topic: Optional[str] = None
) -> list:
    """Return a list of metadata dicts for a specific user, optionally filtered by topic name.

    D-16: always filters by user_id — a user can only see their own documents.
    """
    stmt = select(Document).where(Document.user_id == user_id).order_by(Document.created_at.desc())
    if topic is not None:
        stmt = (
            stmt.join(DocumentTopic, DocumentTopic.document_id == Document.id)
            .join(Topic, Topic.id == DocumentTopic.topic_id)
            .where(Topic.name == topic)
        )
    result = await session.execute(stmt)
    docs = result.scalars().all()

    rows = []
    for doc in docs:
        topic_names = await _load_topic_names(session, doc.id)
        rows.append(_doc_to_dict(doc, topic_names))
    return rows


async def delete_document(session: AsyncSession, doc_id: str) -> bool:
    """Delete a document's MinIO object and its PostgreSQL row.

    Returns False if the document is not found; True on success.
    MinIO deletion failures are logged to stderr but do not prevent the DB row
    deletion (the bytes may already be gone).
    """
    try:
        uid = uuid.UUID(doc_id)
    except ValueError:
        return False

    doc = await session.get(Document, uid)
    if doc is None:
        return False

    try:
        await _backend().delete_object(doc.object_key)
    except Exception as exc:
        print(f"[storage] WARNING: MinIO delete_object failed for {doc.object_key!r}: {exc}", file=sys.stderr)

    # Atomic quota decrement (STORE-06, D-07).
    # user_id is always set post-migration (Plan 03-03+) — guard removed.
    # Use CASE WHEN instead of GREATEST() for SQLite compatibility
    # (PostgreSQL supports both; SQLite lacks the GREATEST scalar function).
    await session.execute(
        text(
            "UPDATE quotas "
            "SET used_bytes = CASE WHEN used_bytes > :delta THEN used_bytes - :delta ELSE 0 END "
            "WHERE user_id = :uid"
        ),
        {"delta": doc.size_bytes, "uid": str(doc.user_id)},
    )

    await session.delete(doc)
    await session.commit()
    return True


async def update_document_topics(
    session: AsyncSession, doc_id: str, topics: list
) -> Optional[dict]:
    """Replace all topic associations for a document.

    Auto-creates topics that don't yet exist. Returns the refreshed metadata
    dict, or None if the document is not found.
    """
    try:
        uid = uuid.UUID(doc_id)
    except ValueError:
        return None

    doc = await session.get(Document, uid)
    if doc is None:
        return None

    # Remove all existing associations
    await session.execute(
        delete(DocumentTopic).where(DocumentTopic.document_id == uid)
    )

    # Re-insert, deduplicating by name
    seen: set = set()
    for name in topics:
        if name in seen:
            continue
        seen.add(name)
        topic_dict = await create_topic(session, name)
        session.add(
            DocumentTopic(
                document_id=uid,
                topic_id=uuid.UUID(topic_dict["id"]),
            )
        )

    doc.status = "classified"
    await session.commit()
    return await get_metadata(session, doc_id)


async def remove_topic_from_all_documents(
    session: AsyncSession, topic_name: str
) -> int:
    """Delete all DocumentTopic rows for the named topic.

    Returns the number of rows deleted.
    """
    q = await session.execute(
        select(Topic).where(sql_func.lower(Topic.name) == topic_name.lower())
    )
    topic = q.scalars().first()
    if topic is None:
        return 0

    result = await session.execute(
        delete(DocumentTopic).where(DocumentTopic.topic_id == topic.id)
    )
    await session.commit()
    return result.rowcount


# ── Topics ────────────────────────────────────────────────────────────────────

async def load_topics(session: AsyncSession) -> list:
    """Return all topics ordered by name."""
    q = await session.execute(select(Topic).order_by(Topic.name))
    return [
        {"id": str(t.id), "name": t.name, "description": t.description, "color": t.color}
        for t in q.scalars()
    ]


async def load_topics_for_user(session: AsyncSession, user_id: uuid.UUID) -> list:
    """Return system topics (user_id IS NULL) + the user's own topics, ordered by name.

    D-08 + D-17 + DOC-04: layered topic namespace. System topics are visible to all
    users; per-user topics are visible only to their owner. A user's topic list is
    the union of both sets.
    """
    q = await session.execute(
        select(Topic).where(
            or_(Topic.user_id == user_id, Topic.user_id.is_(None))
        ).order_by(Topic.name)
    )
    return [
        {"id": str(t.id), "name": t.name, "description": t.description, "color": t.color}
        for t in q.scalars()
    ]


async def save_topics(session: AsyncSession, topics: list) -> None:
    """Idempotent bulk replace — delete all Topic rows then insert the list.

    # legacy: not used by current endpoints; preserved for API compatibility.
    """
    await session.execute(delete(Topic))
    for t in topics:
        session.add(
            Topic(
                id=uuid.UUID(t["id"]) if t.get("id") else uuid.uuid4(),
                name=t["name"],
                description=t.get("description", ""),
                color=t.get("color", "#6366f1"),
            )
        )
    await session.commit()


async def get_topic(session: AsyncSession, topic_id: str) -> Optional[dict]:
    """Return a topic dict by UUID string, or None if not found."""
    try:
        uid = uuid.UUID(topic_id)
    except ValueError:
        return None
    t = await session.get(Topic, uid)
    if t is None:
        return None
    return {"id": str(t.id), "name": t.name, "description": t.description, "color": t.color}


async def create_topic(
    session: AsyncSession,
    name: str,
    description: str = "",
    color: str = "#6366f1",
    user_id: Optional[uuid.UUID] = None,
) -> dict:
    """Create a topic, or return the existing one (case-insensitive, namespace-scoped dedup).

    D-08: user_id=None creates a system topic (visible to all users).
    D-08: user_id=<uuid> creates a per-user topic (visible only to that user).

    Deduplication is scoped by user_id namespace:
      - System topics (user_id=None) dedup against other system topics only
      - Per-user topics dedup within that user's namespace only
    This allows "Finance" to exist as both a system topic and a per-user topic.

    SQLite note: Uses a branching approach instead of IS NOT DISTINCT FROM
    (SQLite doesn't support that PostgreSQL construct for NULL comparison).
    """
    if user_id is None:
        q = await session.execute(
            select(Topic).where(
                sql_func.lower(Topic.name) == name.lower(),
                Topic.user_id.is_(None),
            )
        )
    else:
        q = await session.execute(
            select(Topic).where(
                sql_func.lower(Topic.name) == name.lower(),
                Topic.user_id == user_id,
            )
        )
    existing = q.scalars().first()
    if existing is not None:
        return {
            "id": str(existing.id),
            "name": existing.name,
            "description": existing.description,
            "color": existing.color,
        }

    topic = Topic(name=name, description=description, color=color, user_id=user_id)
    session.add(topic)
    await session.commit()
    return {
        "id": str(topic.id),
        "name": topic.name,
        "description": topic.description,
        "color": topic.color,
    }


async def update_topic(
    session: AsyncSession,
    topic_id: str,
    name: Optional[str] = None,
    description: Optional[str] = None,
    color: Optional[str] = None,
) -> Optional[dict]:
    """Update non-None fields on a topic. Returns updated dict or None."""
    try:
        uid = uuid.UUID(topic_id)
    except ValueError:
        return None
    t = await session.get(Topic, uid)
    if t is None:
        return None
    if name is not None:
        t.name = name
    if description is not None:
        t.description = description
    if color is not None:
        t.color = color
    await session.commit()
    return {"id": str(t.id), "name": t.name, "description": t.description, "color": t.color}


async def delete_topic(session: AsyncSession, topic_id: str) -> Optional[str]:
    """Delete a topic and cascade-remove its DocumentTopic rows.

    Returns the deleted topic name, or None if not found.
    """
    try:
        uid = uuid.UUID(topic_id)
    except ValueError:
        return None
    t = await session.get(Topic, uid)
    if t is None:
        return None
    name = t.name
    await session.delete(t)  # ondelete="CASCADE" removes DocumentTopic rows
    await session.commit()
    return name


async def topic_doc_counts(
    session: AsyncSession, user_id: Optional[uuid.UUID] = None
) -> dict:
    """Return a mapping of topic name -> document count.

    If user_id is provided, counts only documents belonging to that user.
    This ensures a user sees the count of their own documents for each topic,
    not the global count across all users.
    """
    stmt = (
        select(Topic.name, sql_func.count(DocumentTopic.document_id))
        .join(DocumentTopic, DocumentTopic.topic_id == Topic.id, isouter=True)
    )
    if user_id is not None:
        stmt = stmt.join(
            Document, Document.id == DocumentTopic.document_id, isouter=True
        ).where(
            or_(Document.user_id == user_id, Document.user_id.is_(None))
        )
    stmt = stmt.group_by(Topic.name)
    q = await session.execute(stmt)
    return {name: count for name, count in q}


# ── Public surface ─────────────────────────────────────────────────────────────

__all__ = [
    "save_metadata",
    "get_metadata",
    "list_metadata",
    "delete_document",
    "update_document_topics",
    "remove_topic_from_all_documents",
    "load_topics",
    "load_topics_for_user",
    "save_topics",
    "get_topic",
    "create_topic",
    "update_topic",
    "delete_topic",
    "topic_doc_counts",
]