import secrets

from sqlalchemy import select, func
from sqlalchemy.orm import selectinload
from sqlalchemy.ext.asyncio import AsyncSession

from app.core.auth import hash_password
from app.db.models import ClassMembership, Class_, User


def generate_invite_code(length: int = 8) -> str:
    chars = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789"
    return "".join(secrets.choice(chars) for _ in range(length))


async def ensure_invite_code(db: AsyncSession, class_id: int) -> str:
    result = await db.execute(select(Class_).where(Class_.id == class_id))
    class_ = result.scalar_one_or_none()
    if class_ is None:
        return ""
    if not class_.invite_code:
        class_.invite_code = generate_invite_code()
        await db.commit()
        await db.refresh(class_)
    return class_.invite_code


async def regenerate_invite_code(db: AsyncSession, class_id: int) -> str:
    result = await db.execute(select(Class_).where(Class_.id == class_id))
    class_ = result.scalar_one_or_none()
    if class_ is None:
        return ""
    class_.invite_code = generate_invite_code()
    await db.commit()
    await db.refresh(class_)
    return class_.invite_code


async def import_members(
    db: AsyncSession, class_id: int, entries: list[dict]
) -> int:
    incoming_entries: list[tuple[str, str]] = []
    for entry in entries:
        sid = entry.get("student_id", "").strip()
        name = entry.get("name", "").strip()
        if sid and name:
            incoming_entries.append((sid, name))

    if not incoming_entries:
        return 0

    student_ids = {sid for sid, _ in incoming_entries}
    result = await db.execute(
        select(User)
        .options(
            selectinload(User.memberships),
            selectinload(User.memberships).selectinload(ClassMembership.class_),
        )
        .where(User.student_id.in_(student_ids))
    )
    existing_users = {user.student_id: user for user in result.scalars().unique().all() if user.student_id}

    count = 0
    seen_in_batch: set[str] = set()
    for sid, name in incoming_entries:
        if sid in seen_in_batch:
            continue
        seen_in_batch.add(sid)

        user = existing_users.get(sid)
        if user is not None:
            if user.get_membership(class_id) is not None:
                continue
            if user.status == "inactive":
                user.name = name
            db.add(
                ClassMembership(
                    user_id=user.id,
                    class_id=class_id,
                    membership_role="student",
                )
            )
            count += 1
            continue

        placeholder_email = f"inactive+{class_id}.{sid}@member.local"
        new_user = User(
            email=placeholder_email,
            # Keep inactive imports compatible with older schemas where password_hash is NOT NULL.
            password_hash=hash_password(secrets.token_urlsafe(24)),
            name=name,
            student_id=sid,
            role="student",
            status="inactive",
        )
        db.add(new_user)
        await db.flush()
        db.add(
            ClassMembership(
                user_id=new_user.id,
                class_id=class_id,
                membership_role="student",
            )
        )
        existing_users[sid] = new_user
        count += 1

    await db.commit()
    return count


async def get_inactive_members(
    db: AsyncSession, class_id: int, page: int = 1, page_size: int = 50
) -> tuple[list[User], int]:
    query = (
        select(User)
        .options(
            selectinload(User.memberships),
            selectinload(User.memberships).selectinload(ClassMembership.class_),
        )
        .join(ClassMembership)
        .where(
            ClassMembership.class_id == class_id,
            User.status == "inactive",
        )
    )
    count_query = (
        select(func.count(User.id))
        .join(ClassMembership)
        .where(
            ClassMembership.class_id == class_id,
            User.status == "inactive",
        )
    )

    total_result = await db.execute(count_query)
    total = total_result.scalar() or 0

    result = await db.execute(
        query.order_by(User.student_id)
        .offset((page - 1) * page_size)
        .limit(page_size)
    )
    return list(result.scalars().unique().all()), total


async def validate_registration(
    db: AsyncSession, invite_code: str, student_id: str
) -> tuple[User, int] | None:
    class_result = await db.execute(
        select(Class_).where(Class_.invite_code == invite_code)
    )
    class_ = class_result.scalar_one_or_none()
    if class_ is None:
        return None
    user_result = await db.execute(
        select(User)
        .options(
            selectinload(User.memberships),
            selectinload(User.memberships).selectinload(ClassMembership.class_),
        )
        .join(ClassMembership)
        .where(
            ClassMembership.class_id == class_.id,
            User.student_id == student_id,
            User.status == "inactive",
        )
    )
    user = user_result.scalar_one_or_none()
    if user is None:
        return None
    user.set_active_membership(class_.id)
    return user, class_.id


async def delete_inactive_member(db: AsyncSession, class_id: int, user_id: int) -> bool:
    result = await db.execute(
        select(User)
        .options(selectinload(User.memberships))
        .where(User.id == user_id)
    )
    user = result.scalar_one_or_none()
    if user is None or user.status != "inactive":
        return False
    membership = user.get_membership(class_id)
    if membership is None:
        return False
    other_memberships = [item for item in user.memberships if item.class_id != class_id]
    await db.delete(membership)
    await db.flush()
    if not other_memberships:
        await db.delete(user)
    await db.commit()
    return True


async def clear_inactive_members(db: AsyncSession, class_id: int) -> int:
    result = await db.execute(
        select(User.id)
        .join(ClassMembership)
        .where(
            ClassMembership.class_id == class_id,
            User.status == "inactive",
        )
    )
    user_ids = list(result.scalars().all())
    removed = 0
    for user_id in user_ids:
        removed += 1 if await delete_inactive_member(db, class_id, user_id) else 0
    return removed