"""Groups API router.

Endpoints:
- POST   /api/groups                              -- create (creator becomes owner)
- GET    /api/groups                               -- list groups (paginated), optional mine=true
- GET    /api/groups/{id}                          -- group page (name/description/members)
- PATCH  /api/groups/{id}                          -- update name/description (owner/admin)
- DELETE /api/groups/{id}                          -- delete (owner only)
- POST   /api/groups/{id}/join                     -- join (idempotent)
- POST   /api/groups/{id}/leave                    -- leave (owner must transfer first)
- POST   /api/groups/{id}/members/{user_id}/role   -- set role admin/member (owner only)
- POST   /api/groups/{id}/transfer                 -- transfer ownership (owner only)

See app/models/groups.py for the schema and delete-behavior rationale
(docs/planning/social.md US-5). This module owns create/join/leave, role
authorization, and the group page aggregate.
"""

from datetime import datetime
from typing import Annotated

from fastapi import APIRouter, Depends, HTTPException, Query, status
from pydantic import BaseModel, Field, field_validator
from sqlalchemy import func, select
from sqlalchemy.ext.asyncio import AsyncSession

from app.database import get_db
from app.dependencies import CurrentUser
from app.models.groups import Group, GroupMember, GroupRole
from app.models.user import User

router = APIRouter()

DbDep = Annotated[AsyncSession, Depends(get_db)]

_NAME_MAX_LEN = 100
_DESCRIPTION_MAX_LEN = 2000


# ──────────────────────────── Pydantic schemas ────────────────────────────────


class GroupCreate(BaseModel):
    name: str = Field(..., min_length=1, max_length=_NAME_MAX_LEN)
    description: str | None = Field(default=None, max_length=_DESCRIPTION_MAX_LEN)

    @field_validator("name")
    @classmethod
    def strip_name(cls, v: str) -> str:
        v = v.strip()
        if not v:
            raise ValueError("name must not be blank")
        return v

    @field_validator("description")
    @classmethod
    def strip_description(cls, v: str | None) -> str | None:
        if v is not None:
            v = v.strip() or None
        return v


class GroupUpdate(BaseModel):
    name: str | None = Field(default=None, min_length=1, max_length=_NAME_MAX_LEN)
    description: str | None = Field(default=None, max_length=_DESCRIPTION_MAX_LEN)

    @field_validator("name")
    @classmethod
    def strip_name(cls, v: str | None) -> str | None:
        if v is not None:
            v = v.strip()
            if not v:
                raise ValueError("name must not be blank")
        return v

    @field_validator("description")
    @classmethod
    def strip_description(cls, v: str | None) -> str | None:
        if v is not None:
            v = v.strip() or None
        return v


class GroupSummary(BaseModel):
    id: int
    name: str
    description: str | None
    owner_id: int | None
    member_count: int
    created_at: datetime


class GroupListResponse(BaseModel):
    groups: list[GroupSummary]
    total: int


class GroupMemberOut(BaseModel):
    """Public-safe member entry -- no email or other private User fields."""

    user_id: int
    username: str
    role: GroupRole
    joined_at: datetime


class GroupDetail(BaseModel):
    id: int
    name: str
    description: str | None
    owner_id: int | None
    member_count: int
    members: list[GroupMemberOut]
    created_at: datetime


class RoleUpdateRequest(BaseModel):
    role: GroupRole

    @field_validator("role")
    @classmethod
    def only_admin_or_member(cls, v: GroupRole) -> GroupRole:
        if v == GroupRole.OWNER:
            raise ValueError(
                "cannot set role to owner directly -- use POST /transfer"
            )
        return v


class TransferRequest(BaseModel):
    new_owner_id: int


class MembershipOut(BaseModel):
    group_id: int
    user_id: int
    role: GroupRole
    joined_at: datetime


# ──────────────────────────────── Helpers ─────────────────────────────────────


async def _get_group_or_404(db: AsyncSession, group_id: int) -> Group:
    group = await db.get(Group, group_id)
    if group is None:
        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Group not found")
    return group


async def _get_membership(db: AsyncSession, group_id: int, user_id: int) -> GroupMember | None:
    result = await db.execute(
        select(GroupMember).where(
            GroupMember.group_id == group_id, GroupMember.user_id == user_id
        )
    )
    return result.scalar_one_or_none()


async def _member_count(db: AsyncSession, group_id: int) -> int:
    result = await db.execute(
        select(func.count()).select_from(GroupMember).where(GroupMember.group_id == group_id)
    )
    return int(result.scalar_one())


def _require_role(membership: GroupMember | None, *allowed: GroupRole) -> None:
    if membership is None or membership.role not in allowed:
        raise HTTPException(
            status_code=status.HTTP_403_FORBIDDEN,
            detail="You do not have permission to perform this action",
        )


# ──────────────────────────────── Endpoints ───────────────────────────────────


@router.post("", response_model=GroupDetail, status_code=status.HTTP_201_CREATED)
@router.post("/", response_model=GroupDetail, status_code=status.HTTP_201_CREATED, include_in_schema=False)
async def create_group(
    body: GroupCreate,
    current_user: CurrentUser,
    db: DbDep,
):
    """Create a group. The creator becomes its owner."""
    group = Group(name=body.name, description=body.description, owner_id=current_user.id)
    db.add(group)
    await db.flush()

    db.add(GroupMember(group_id=group.id, user_id=current_user.id, role=GroupRole.OWNER))
    await db.commit()
    await db.refresh(group)

    return GroupDetail(
        id=group.id,
        name=group.name,
        description=group.description,
        owner_id=group.owner_id,
        member_count=1,
        members=[
            GroupMemberOut(
                user_id=current_user.id,
                username=current_user.username,
                role=GroupRole.OWNER,
                joined_at=group.created_at,
            )
        ],
        created_at=group.created_at,
    )


@router.get("", response_model=GroupListResponse)
@router.get("/", response_model=GroupListResponse, include_in_schema=False)
async def list_groups(
    current_user: CurrentUser,
    db: DbDep,
    skip: int = Query(0, ge=0),
    limit: int = Query(50, ge=1, le=100),
    mine: bool = Query(False, description="If true, only groups the caller is a member of"),
):
    """List groups, paginated. With ``mine=true``, restrict to the caller's own groups."""
    base_query = select(Group)
    count_query = select(func.count()).select_from(Group)

    if mine:
        base_query = base_query.join(
            GroupMember, GroupMember.group_id == Group.id
        ).where(GroupMember.user_id == current_user.id)
        count_query = count_query.join(
            GroupMember, GroupMember.group_id == Group.id
        ).where(GroupMember.user_id == current_user.id)

    total = (await db.execute(count_query)).scalar_one()

    result = await db.execute(
        base_query.order_by(Group.created_at.desc()).offset(skip).limit(limit)
    )
    groups = result.scalars().all()

    summaries = []
    for group in groups:
        summaries.append(
            GroupSummary(
                id=group.id,
                name=group.name,
                description=group.description,
                owner_id=group.owner_id,
                member_count=await _member_count(db, group.id),
                created_at=group.created_at,
            )
        )

    return GroupListResponse(groups=summaries, total=total)


@router.get("/{group_id}", response_model=GroupDetail)
async def get_group(
    group_id: int,
    current_user: CurrentUser,
    db: DbDep,
):
    """Group page: name, description, member count, and member list with roles."""
    group = await _get_group_or_404(db, group_id)

    result = await db.execute(
        select(GroupMember, User.username)
        .join(User, User.id == GroupMember.user_id)
        .where(GroupMember.group_id == group_id)
        .order_by(GroupMember.joined_at.asc())
    )
    rows = result.all()

    members = [
        GroupMemberOut(
            user_id=gm.user_id,
            username=username,
            role=gm.role,
            joined_at=gm.joined_at,
        )
        for gm, username in rows
    ]

    return GroupDetail(
        id=group.id,
        name=group.name,
        description=group.description,
        owner_id=group.owner_id,
        member_count=len(members),
        members=members,
        created_at=group.created_at,
    )


@router.patch("/{group_id}", response_model=GroupDetail)
async def update_group(
    group_id: int,
    body: GroupUpdate,
    current_user: CurrentUser,
    db: DbDep,
):
    """Update name/description. Owner/admin only."""
    group = await _get_group_or_404(db, group_id)
    membership = await _get_membership(db, group_id, current_user.id)
    _require_role(membership, GroupRole.OWNER, GroupRole.ADMIN)

    if body.name is not None:
        group.name = body.name
    if body.description is not None:
        group.description = body.description if body.description else None

    await db.commit()
    await db.refresh(group)

    return await get_group(group_id, current_user, db)


@router.delete("/{group_id}", status_code=status.HTTP_204_NO_CONTENT)
async def delete_group(
    group_id: int,
    current_user: CurrentUser,
    db: DbDep,
):
    """Delete a group. Owner only."""
    group = await _get_group_or_404(db, group_id)
    if group.owner_id != current_user.id:
        raise HTTPException(
            status_code=status.HTTP_403_FORBIDDEN,
            detail="Only the owner can delete this group",
        )

    await db.delete(group)
    await db.commit()
    return None


@router.post("/{group_id}/join", response_model=MembershipOut)
async def join_group(
    group_id: int,
    current_user: CurrentUser,
    db: DbDep,
):
    """Join a group. Idempotent -- joining an already-joined group is a no-op."""
    await _get_group_or_404(db, group_id)

    existing = await _get_membership(db, group_id, current_user.id)
    if existing is not None:
        return MembershipOut(
            group_id=group_id,
            user_id=current_user.id,
            role=existing.role,
            joined_at=existing.joined_at,
        )

    membership = GroupMember(group_id=group_id, user_id=current_user.id, role=GroupRole.MEMBER)
    db.add(membership)
    await db.commit()
    await db.refresh(membership)

    return MembershipOut(
        group_id=group_id,
        user_id=current_user.id,
        role=membership.role,
        joined_at=membership.joined_at,
    )


@router.post("/{group_id}/leave", response_model=dict)
async def leave_group(
    group_id: int,
    current_user: CurrentUser,
    db: DbDep,
):
    """Leave a group. The owner cannot leave without transferring ownership first."""
    await _get_group_or_404(db, group_id)
    membership = await _get_membership(db, group_id, current_user.id)
    if membership is None:
        raise HTTPException(
            status_code=status.HTTP_404_NOT_FOUND,
            detail="You are not a member of this group",
        )

    if membership.role == GroupRole.OWNER:
        raise HTTPException(
            status_code=status.HTTP_409_CONFLICT,
            detail="Transfer ownership before leaving this group",
        )

    await db.delete(membership)
    await db.commit()
    return {"status": "ok"}


@router.post("/{group_id}/members/{user_id}/role", response_model=MembershipOut)
async def set_member_role(
    group_id: int,
    user_id: int,
    body: RoleUpdateRequest,
    current_user: CurrentUser,
    db: DbDep,
):
    """Set a member's role (admin/member). Owner only."""
    group = await _get_group_or_404(db, group_id)
    if group.owner_id != current_user.id:
        raise HTTPException(
            status_code=status.HTTP_403_FORBIDDEN,
            detail="Only the owner can change member roles",
        )

    target = await _get_membership(db, group_id, user_id)
    if target is None:
        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Member not found")

    if target.role == GroupRole.OWNER:
        raise HTTPException(
            status_code=status.HTTP_400_BAD_REQUEST,
            detail="Use POST /transfer to change ownership",
        )

    target.role = body.role
    await db.commit()
    await db.refresh(target)

    return MembershipOut(
        group_id=group_id, user_id=user_id, role=target.role, joined_at=target.joined_at
    )


@router.post("/{group_id}/transfer", response_model=GroupDetail)
async def transfer_ownership(
    group_id: int,
    body: TransferRequest,
    current_user: CurrentUser,
    db: DbDep,
):
    """Transfer group ownership to another existing member. Owner only."""
    group = await _get_group_or_404(db, group_id)
    if group.owner_id != current_user.id:
        raise HTTPException(
            status_code=status.HTTP_403_FORBIDDEN,
            detail="Only the owner can transfer ownership",
        )

    if body.new_owner_id == current_user.id:
        raise HTTPException(
            status_code=status.HTTP_400_BAD_REQUEST,
            detail="You already own this group",
        )

    new_owner_membership = await _get_membership(db, group_id, body.new_owner_id)
    if new_owner_membership is None:
        raise HTTPException(
            status_code=status.HTTP_404_NOT_FOUND,
            detail="New owner must already be a member of this group",
        )

    current_owner_membership = await _get_membership(db, group_id, current_user.id)

    new_owner_membership.role = GroupRole.OWNER
    if current_owner_membership is not None:
        current_owner_membership.role = GroupRole.ADMIN
    group.owner_id = body.new_owner_id

    await db.commit()

    return await get_group(group_id, current_user, db)