"""Budget validation helpers for finance-aware print dispatch."""

import calendar
from datetime import datetime, timezone
from typing import TYPE_CHECKING
from zoneinfo import ZoneInfo, ZoneInfoNotFoundError

from fastapi import HTTPException
from sqlalchemy import case, func, select
from sqlalchemy.ext.asyncio import AsyncSession

from backend.app.models.finance import BudgetReservation, CostCenter, CostCenterMember, WalletTransaction
from backend.app.models.print_queue import PrintQueueItem
from backend.app.models.settings import Settings
from backend.app.models.user import User

if TYPE_CHECKING:
    from backend.app.core.auth import ApiKeyActor


async def is_billing_enabled(db: AsyncSession) -> bool:
    # Consider any 'billing_enabled' setting with a true-ish value as enabling billing.
    result = await db.execute(
        select(func.count())
        .select_from(Settings)
        .where(Settings.key == "billing_enabled", func.lower(func.coalesce(Settings.value, "")) == "true")
    )
    count = int(result.scalar_one() or 0)
    return count > 0


async def is_printer_kill_switch_enabled(db: AsyncSession) -> bool:
    """Return True when billing and the printer kill-switch are both enabled."""

    result = await db.execute(
        select(Settings.key, Settings.value).where(Settings.key.in_(("billing_enabled", "printer_kill_switch_enabled")))
    )
    values = {key: (value or "").strip().lower() for key, value in result.all()}
    return values.get("billing_enabled") == "true" and values.get("printer_kill_switch_enabled") == "true"


async def _get_budget_window_start_utc(db: AsyncSession) -> datetime:
    result = await db.execute(
        select(Settings).where(Settings.key.in_(["finance_budget_reset_day", "finance_budget_reset_timezone"]))
    )
    values = {setting.key: setting.value for setting in result.scalars().all()}

    desired_day = 1
    try:
        parsed = int(values.get("finance_budget_reset_day") or 1)
        if 1 <= parsed <= 31:
            desired_day = parsed
    except (TypeError, ValueError):
        pass

    timezone_name = values.get("finance_budget_reset_timezone") or "UTC"
    try:
        tz = ZoneInfo(timezone_name)
    except ZoneInfoNotFoundError:
        tz = ZoneInfo("UTC")

    now = datetime.now(tz)
    current_month_reset_day = min(desired_day, calendar.monthrange(now.year, now.month)[1])
    if now.day < current_month_reset_day:
        month = now.month - 1
        year = now.year
        if month == 0:
            month = 12
            year -= 1
    else:
        month = now.month
        year = now.year

    reset_day = min(desired_day, calendar.monthrange(year, month)[1])
    return datetime(year, month, reset_day, tzinfo=tz).astimezone(timezone.utc)


async def _cost_center_spend(db: AsyncSession, cost_center_id: int, *, monthly: bool) -> float:
    spend_expr = case((WalletTransaction.amount < 0, -WalletTransaction.amount), else_=0.0)
    conditions = [
        WalletTransaction.cost_center_id == cost_center_id,
        WalletTransaction.cost_center_id.is_not(None),
        WalletTransaction.is_voided.is_(False),
    ]
    if monthly:
        conditions.append(WalletTransaction.created_at >= await _get_budget_window_start_utc(db))

    result = await db.execute(select(func.coalesce(func.sum(spend_expr), 0.0)).where(*conditions))
    return float(result.scalar() or 0.0)


async def get_cost_center_reserved_map(
    db: AsyncSession,
    cost_center_ids: list[int],
    *,
    exclude_queue_item_id: int | None = None,
    exclude_reservation_source_type: str | None = None,
    exclude_reservation_source_id: int | None = None,
) -> dict[int, float]:
    """Return active holds plus unreserved open queue estimates per cost center.

    Queue items that already have an active ``print_queue`` reservation are
    excluded from the queue sum because the reservation is their replacement,
    not an additional hold.
    """

    if not cost_center_ids:
        return {}

    active_queue_reservation = (
        select(BudgetReservation.id)
        .where(
            BudgetReservation.status == "active",
            BudgetReservation.source_type == "print_queue",
            BudgetReservation.source_id == PrintQueueItem.id,
        )
        .exists()
    )
    queue_conditions = [
        PrintQueueItem.cost_center_id.in_(cost_center_ids),
        PrintQueueItem.status.in_(("pending", "printing")),
        ~active_queue_reservation,
    ]
    if exclude_queue_item_id is not None:
        queue_conditions.append(PrintQueueItem.id != exclude_queue_item_id)

    queue_rows = await db.execute(
        select(PrintQueueItem.cost_center_id, func.coalesce(func.sum(PrintQueueItem.estimated_cost), 0.0))
        .where(*queue_conditions)
        .group_by(PrintQueueItem.cost_center_id)
    )
    reserved_map = {int(center_id): float(value) for center_id, value in queue_rows.all() if center_id is not None}

    reservation_conditions = [
        BudgetReservation.cost_center_id.in_(cost_center_ids),
        BudgetReservation.status == "active",
    ]
    if exclude_reservation_source_type is not None and exclude_reservation_source_id is not None:
        reservation_conditions.append(
            ~(
                (BudgetReservation.source_type == exclude_reservation_source_type)
                & (BudgetReservation.source_id == exclude_reservation_source_id)
            )
        )
    reservation_rows = await db.execute(
        select(BudgetReservation.cost_center_id, func.coalesce(func.sum(BudgetReservation.amount), 0.0))
        .where(*reservation_conditions)
        .group_by(BudgetReservation.cost_center_id)
    )
    for center_id, value in reservation_rows.all():
        if center_id is not None:
            reserved_map[int(center_id)] = reserved_map.get(int(center_id), 0.0) + float(value or 0.0)
    return reserved_map


async def validate_print_budget(
    db: AsyncSession,
    *,
    cost_center_id: int | None,
    estimated_cost: float | None,
    current_user: "User | ApiKeyActor | None",
    quantity: int = 1,
    exclude_queue_item_id: int | None = None,
    exclude_reservation_source_type: str | None = None,
    exclude_reservation_source_id: int | None = None,
) -> None:
    """Validate that a print can be assigned to a cost center budget."""
    if not await is_billing_enabled(db):
        return

    if cost_center_id is None:
        raise HTTPException(status_code=400, detail="Cost center is required when billing is enabled")

    if estimated_cost is None or estimated_cost <= 0:
        raise HTTPException(status_code=400, detail="Estimated cost is required for cost center prints")

    center = await db.scalar(select(CostCenter).where(CostCenter.id == cost_center_id).with_for_update())
    if not center:
        raise HTTPException(status_code=404, detail="Cost center not found")
    if not center.is_active:
        raise HTTPException(status_code=400, detail="Cost center is inactive")

    if current_user is not None and not current_user.is_admin:
        if center.is_private:
            if center.owner_user_id != current_user.id:
                raise HTTPException(status_code=403, detail="You cannot print with this private cost center")
        else:
            member = await db.scalar(
                select(CostCenterMember).where(
                    CostCenterMember.cost_center_id == cost_center_id,
                    CostCenterMember.user_id == current_user.id,
                )
            )
            if not member or not member.can_print:
                raise HTTPException(status_code=403, detail="You cannot print with this cost center")

    budget_limit = center.monthly_budget if center.monthly_budget is not None else center.total_budget
    if budget_limit is None:
        return

    used = await _cost_center_spend(db, cost_center_id, monthly=center.monthly_budget is not None)
    reserved_map = await get_cost_center_reserved_map(
        db,
        [cost_center_id],
        exclude_queue_item_id=exclude_queue_item_id,
        exclude_reservation_source_type=exclude_reservation_source_type,
        exclude_reservation_source_id=exclude_reservation_source_id,
    )
    reserved = reserved_map.get(cost_center_id, 0.0)
    requested = estimated_cost * max(1, quantity)
    available = float(budget_limit) - used - reserved
    if requested > available:
        raise HTTPException(
            status_code=400,
            detail=f"Estimated print cost exceeds available cost center budget ({requested:.2f} > {available:.2f})",
        )


async def create_budget_reservation(
    db: AsyncSession,
    *,
    cost_center_id: int | None,
    estimated_cost: float | None,
    current_user: User | None,
    source_type: str,
    source_id: int | None,
    print_archive_id: int | None = None,
    exclude_queue_item_id: int | None = None,
) -> BudgetReservation | None:
    if not await is_billing_enabled(db):
        return None

    if cost_center_id is None:
        raise HTTPException(status_code=400, detail="Cost center is required when billing is enabled")

    await validate_print_budget(
        db,
        cost_center_id=cost_center_id,
        estimated_cost=estimated_cost,
        current_user=current_user,
        exclude_queue_item_id=exclude_queue_item_id,
        exclude_reservation_source_type=source_type,
        exclude_reservation_source_id=source_id,
    )

    existing = None
    if source_id is not None:
        existing = await db.scalar(
            select(BudgetReservation).where(
                BudgetReservation.status == "active",
                BudgetReservation.source_type == source_type,
                BudgetReservation.source_id == source_id,
            )
        )
    if existing is not None:
        existing.cost_center_id = cost_center_id
        existing.amount = float(estimated_cost or 0.0)
        if print_archive_id is not None:
            existing.print_archive_id = print_archive_id
        await db.flush()
        return existing

    reservation = BudgetReservation(
        cost_center_id=cost_center_id,
        amount=float(estimated_cost or 0.0),
        status="active",
        source_type=source_type,
        source_id=source_id,
        print_archive_id=print_archive_id,
    )
    db.add(reservation)
    await db.flush()
    return reservation


async def release_budget_reservation(
    db: AsyncSession,
    *,
    source_type: str | None = None,
    source_id: int | None = None,
    print_archive_id: int | None = None,
    status: str = "released",
) -> int:
    conditions = [BudgetReservation.status == "active"]
    if print_archive_id is not None:
        conditions.append(BudgetReservation.print_archive_id == print_archive_id)
    else:
        conditions.extend(
            [
                BudgetReservation.source_type == source_type,
                BudgetReservation.source_id == source_id,
            ]
        )

    result = await db.execute(select(BudgetReservation).where(*conditions))
    reservations = result.scalars().all()
    for reservation in reservations:
        reservation.status = status
        reservation.released_at = datetime.now(timezone.utc)
    if reservations:
        await db.flush()
    return len(reservations)