"""Pagination + sorting helpers."""
from sqlalchemy import func, select
from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy.orm import InstrumentedAttribute

from app.config import settings
from app.utils.errors import ValidationError


async def paginate(query, page: int, page_size: int, db: AsyncSession):
    """Apply offset/limit to `query` and return (items, meta dict).

    `page_size` is clamped to [1, MAX_PAGE_SIZE] after validation — routers
    enforce the contract bounds via Query(ge=1, le=100) before calling this.
    """
    total = await db.scalar(select(func.count()).select_from(query.subquery()))
    items = (await db.execute(query.offset((page - 1) * page_size).limit(page_size))).scalars().all()
    return items, {"total": total or 0, "page": page, "page_size": page_size}


def sort_query(stmt, model, sort: str | None, default: str = "-created_at"):
    """Apply `?sort=-created_at` (leading `-` = descending) against a model.

    Only real columns of the model are accepted — anything else is a
    VALIDATION_ERROR (400), never a raw SQL fragment.
    """
    raw = (sort or default).strip()
    descending = raw.startswith("-")
    name = raw[1:] if descending else raw
    column: InstrumentedAttribute | None = getattr(model, name, None)
    if column is None or not isinstance(column, InstrumentedAttribute):
        raise ValidationError(f"فیلد مرتب‌سازی نامعتبر است: {name}", field="sort")
    return stmt.order_by(column.desc() if descending else column.asc())


def default_pagination(page: int | None, page_size: int | None) -> tuple[int, int]:
    """Contract defaults: page=1, page_size=20 (max 100) with 400 on out-of-bounds."""
    p = page if page is not None else 1
    ps = page_size if page_size is not None else settings.DEFAULT_PAGE_SIZE
    if p < 1:
        raise ValidationError("شماره صفحه باید بزرگ‌تر از صفر باشد", field="page")
    if ps < 1:
        raise ValidationError("اندازه صفحه باید بزرگ‌تر از صفر باشد", field="page_size")
    if ps > settings.MAX_PAGE_SIZE:
        raise ValidationError(f"اندازه صفحه نمی‌تواند از {settings.MAX_PAGE_SIZE} بیشتر باشد", field="page_size")
    return p, ps