from collections.abc import Mapping

from django.db import IntegrityError, transaction
from django.utils import timezone

from procurement.constants import EquipmentStatus, ProcurementStatus
from procurement.models import Equipment, ProcurementItem


class EquipmentCreationError(Exception):
    """Expected business-rule failure while creating equipment from an item."""


class ProcurementItemUnavailable(EquipmentCreationError):
    """The procurement item disappeared before the command acquired its lock."""


@transaction.atomic
def create_equipment_from_procurement_item(
    *,
    item_id: int,
    attributes: Mapping[str, object],
) -> Equipment:
    """Create and link equipment as one concurrency-safe transaction."""
    try:
        item = (
            ProcurementItem.objects.select_for_update(of=("self", "request"))
            # Lock the item and its request, not the nullable equipment join.
            # equipment_id below is available without fetching Equipment.
            .select_related("request")
            .get(pk=item_id)
        )
    except ProcurementItem.DoesNotExist as exc:
        raise ProcurementItemUnavailable("Позиция закупки не найдена.") from exc

    if item.request.status != ProcurementStatus.COMPLETED:
        raise EquipmentCreationError(
            "Оборудование можно создать только из завершённой заявки."
        )
    if item.equipment_id is not None:
        raise EquipmentCreationError("Оборудование для этой позиции уже создано.")

    purchase_cost = item.actual_unit_price or item.estimated_unit_price
    if purchase_cost is None:
        raise EquipmentCreationError(
            "Для создания оборудования укажите фактическую или ориентировочную "
            "цену позиции."
        )

    inventory_number = str(attributes["inventory_number"])
    if Equipment.objects.filter(inventory_number=inventory_number).exists():
        raise EquipmentCreationError(
            f'Инвентарный номер "{inventory_number}" уже используется.'
        )

    try:
        equipment = Equipment.objects.create(
            name=item.name,
            inventory_number=inventory_number,
            serial_number=attributes.get("serial_number", ""),
            category=attributes["category"],
            department=attributes["department"],
            status=EquipmentStatus.AVAILABLE,
            responsible_person=attributes.get("responsible_person"),
            location=attributes.get("location", ""),
            purchase_date=timezone.localdate(),
            warranty_until=attributes.get("warranty_until"),
            purchase_cost=purchase_cost,
            notes=item.description,
        )
    except IntegrityError as exc:
        raise EquipmentCreationError(
            "Не удалось создать оборудование: проверьте уникальность "
            "инвентарного номера."
        ) from exc

    item.equipment = equipment
    item.save(update_fields=["equipment"])
    return equipment