"""Admin-side user management: list, get, update, soft-delete, assign groups.

Self-service updates (locale, password, display_name) live in
`services.auth` — this module is for admin operations on other users.
"""

from __future__ import annotations

import uuid
from dataclasses import dataclass
from datetime import datetime, timezone
from typing import Iterable

from sqlalchemy import func, or_, select

from app.db.session import session_scope
from app.models.auth import Group, User, UserGroup
from app.services.bootstrap import ADMIN_GROUP_NAME


class UserNotFound(Exception):
    pass


class LastAdminProtected(Exception):
    """Refusing to strip admin from the last active admin."""


class SystemGroupProtected(Exception):
    """Refusing to delete or rename a built-in system group."""


@dataclass(frozen=True)
class UserView:
    id: uuid.UUID
    email: str
    display_name: str | None
    locale: str
    is_active: bool
    deleted_at: datetime | None
    created_at: datetime
    updated_at: datetime
    groups: list[tuple[uuid.UUID, str]]


def _to_view(u: User) -> UserView:
    return UserView(
        id=u.id,
        email=u.email,
        display_name=u.display_name,
        locale=u.locale,
        is_active=u.is_active,
        deleted_at=u.deleted_at,
        created_at=u.created_at,
        updated_at=u.updated_at,
        groups=[(g.id, g.name) for g in u.groups if g.deleted_at is None],
    )


def list_users(
    *,
    q: str | None = None,
    is_active: bool | None = None,
    include_deleted: bool = False,
    limit: int = 50,
    offset: int = 0,
) -> tuple[list[UserView], int]:
    """Return (rows, total_count) with case-insensitive search on email + display_name."""
    with session_scope() as s:
        stmt = select(User)
        count_stmt = select(func.count()).select_from(User)
        if not include_deleted:
            stmt = stmt.where(User.deleted_at.is_(None))
            count_stmt = count_stmt.where(User.deleted_at.is_(None))
        if is_active is not None:
            stmt = stmt.where(User.is_active.is_(is_active))
            count_stmt = count_stmt.where(User.is_active.is_(is_active))
        if q:
            like = f"%{q.lower()}%"
            stmt = stmt.where(
                or_(func.lower(User.email).like(like), func.lower(User.display_name).like(like))
            )
            count_stmt = count_stmt.where(
                or_(func.lower(User.email).like(like), func.lower(User.display_name).like(like))
            )
        stmt = stmt.order_by(User.email.asc()).limit(limit).offset(offset)
        rows = s.scalars(stmt).all()
        total = int(s.scalar(count_stmt) or 0)
        views = [_to_view(u) for u in rows]
        return views, total


def get_user(user_id: uuid.UUID, *, include_deleted: bool = False) -> UserView:
    with session_scope() as s:
        u = s.get(User, user_id)
        if u is None or (u.deleted_at is not None and not include_deleted):
            raise UserNotFound()
        return _to_view(u)


def update_user(
    user_id: uuid.UUID,
    *,
    display_name: str | None | object = ...,
    locale: str | None = None,
    is_active: bool | None = None,
) -> UserView:
    """Partial update. Pass display_name=None to clear; omit to leave unchanged."""
    with session_scope() as s:
        u = s.get(User, user_id)
        if u is None or u.deleted_at is not None:
            raise UserNotFound()
        if display_name is not ...:
            if display_name in (None, ""):
                u.display_name = None
            else:
                u.display_name = display_name.strip() or None
        if locale is not None:
            u.locale = locale
        if is_active is not None:
            # If deactivating the last active admin, refuse.
            if not is_active and _is_last_active_admin(s, u):
                raise LastAdminProtected("cannot deactivate the last active admin")
            u.is_active = is_active
        return _to_view(u)


def soft_delete_user(user_id: uuid.UUID) -> None:
    with session_scope() as s:
        u = s.get(User, user_id)
        if u is None or u.deleted_at is not None:
            raise UserNotFound()
        if _is_last_active_admin(s, u):
            raise LastAdminProtected("cannot delete the last active admin")
        u.deleted_at = datetime.now(tz=timezone.utc)
        u.is_active = False


def set_user_groups(user_id: uuid.UUID, group_ids: Iterable[uuid.UUID]) -> UserView:
    """Replace the user's group memberships with the given set."""
    desired = set(group_ids)
    with session_scope() as s:
        u = s.get(User, user_id)
        if u is None or u.deleted_at is not None:
            raise UserNotFound()

        # Resolve admin group id once.
        admin_group_id = s.scalar(
            select(Group.id).where(Group.name == ADMIN_GROUP_NAME, Group.is_system.is_(True))
        )
        is_currently_admin = admin_group_id in {g.id for g in u.groups}
        will_be_admin = admin_group_id in desired
        if is_currently_admin and not will_be_admin and _is_last_active_admin(s, u):
            raise LastAdminProtected("cannot remove admin from the last active admin")

        # Refuse silently for unknown groups: validate first.
        if desired:
            known = set(
                s.scalars(
                    select(Group.id).where(Group.id.in_(desired), Group.deleted_at.is_(None))
                ).all()
            )
            unknown = desired - known
            if unknown:
                raise ValueError(f"unknown groups: {sorted(map(str, unknown))}")

        current = {g.id for g in u.groups}
        to_add = desired - current
        to_remove = current - desired

        for gid in to_remove:
            row = s.get(UserGroup, (u.id, gid))
            if row is not None:
                s.delete(row)
        for gid in to_add:
            s.add(UserGroup(user_id=u.id, group_id=gid))

        s.flush()
        s.refresh(u)
        return _to_view(u)


def _is_last_active_admin(s, user: User) -> bool:
    """True when `user` is currently in the admin system group and removing/blocking
    them would leave the platform with zero active admins."""
    admin_group_id = s.scalar(
        select(Group.id).where(Group.name == ADMIN_GROUP_NAME, Group.is_system.is_(True))
    )
    if admin_group_id is None:
        return False
    if admin_group_id not in {g.id for g in user.groups}:
        return False
    other_admins = s.scalar(
        select(func.count())
        .select_from(User)
        .join(UserGroup, UserGroup.user_id == User.id)
        .where(
            UserGroup.group_id == admin_group_id,
            User.id != user.id,
            User.deleted_at.is_(None),
            User.is_active.is_(True),
        )
    )
    return int(other_admins or 0) == 0