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

from apps.core.errors import ApiError
from apps.core.jalali import jalali_date, jalali_time
from apps.reservations.models import Reservation, Slot


class ReservationService:
    """Booking with backend-enforced availability, double-booking prevention,
    and admin approval (which triggers a customer SMS)."""

    @transaction.atomic
    def book(self, customer, slot, title: str = "", notes: str = "") -> Reservation:
        slot = Slot.objects.select_for_update().get(pk=slot.pk)
        if slot.blocked:
            raise ApiError("SLOT_BLOCKED", "This slot is blocked.")
        if slot.date < timezone.localdate():
            raise ApiError("SLOT_PAST", "Cannot book a slot in the past.")
        if slot.booked_count >= slot.capacity:
            raise ApiError("SLOT_FULL", "This slot is fully booked.")
        slot.booked_count += 1
        slot.save(update_fields=["booked_count"])
        return Reservation.objects.create(
            customer=customer,
            slot=slot,
            title=title,
            status=Reservation.Status.PENDING,
            notes=notes,
        )

    @transaction.atomic
    def approve(self, reservation, user) -> Reservation:
        reservation = Reservation.objects.select_for_update().select_related("slot", "customer").get(pk=reservation.pk)
        if reservation.status != Reservation.Status.PENDING:
            raise ApiError("INVALID_STATUS", "Only pending reservations can be approved.")
        reservation.status = Reservation.Status.CONFIRMED
        reservation.approved_by = user
        reservation.approved_at = timezone.now()
        reservation.save(update_fields=["status", "approved_by", "approved_at"])

        from apps.notifications.services import NotificationService

        customer = reservation.customer
        NotificationService().send_event(
            "reservation.approved",
            {
                "customer_name": customer.company_name or customer.user.get_full_name() or customer.user.username,
                "reservation_title": reservation.title,
                "slot_date": jalali_date(reservation.slot.date),
                "slot_time": jalali_time(reservation.slot.start_time),
            },
            recipient=customer.user.phone or customer.user.username,
        )
        return reservation

    @transaction.atomic
    def reject(self, reservation, user) -> Reservation:
        reservation = Reservation.objects.select_for_update().select_related("slot").get(pk=reservation.pk)
        if reservation.status not in (Reservation.Status.PENDING, Reservation.Status.CONFIRMED):
            raise ApiError("INVALID_STATUS", "This reservation cannot be rejected.")
        reservation.status = Reservation.Status.REJECTED
        reservation.approved_by = user
        reservation.approved_at = timezone.now()
        reservation.save(update_fields=["status", "approved_by", "approved_at"])
        slot = reservation.slot
        slot.booked_count = max(0, slot.booked_count - 1)
        slot.save(update_fields=["booked_count"])
        return reservation

    @transaction.atomic
    def cancel(self, reservation) -> Reservation:
        reservation = (
            Reservation.objects.select_for_update()
            .select_related("slot")
            .get(pk=reservation.pk)
        )
        if reservation.status == Reservation.Status.CANCELLED:
            return reservation
        if reservation.status == Reservation.Status.WEIGHED:
            raise ApiError("RESERVATION_LOCKED", "Cannot cancel a reservation that has been weighed.")
        reservation.status = Reservation.Status.CANCELLED
        reservation.save(update_fields=["status"])
        slot = reservation.slot
        slot.booked_count = max(0, slot.booked_count - 1)
        slot.save(update_fields=["booked_count"])
        return reservation

    @transaction.atomic
    def reschedule(self, reservation, new_slot) -> Reservation:
        self.cancel(reservation)
        return self.book(reservation.customer, new_slot, title=reservation.title)