from django.db.models import Count
from .models import *

@transaction.atomic
def get_suitable_pool(subnetwork_size):
    """
    Find suitable pools for a certain network size.

    First, filter for all pools that offer the requested subnetwork_size.

    Then find those pools that are not fully exhausted:

    The number of available networks in a pool is 2^(subnetwork_size-network_size.

    The number of available networks in a pool is given by the number of VPNNetworkreservations.

    """

    return VPNPool.objects.annotate(
        num_reservations=Count('vpnnetworkreservation'),
        max_reservations=2**(F('subnetwork_size')-F('network_size'))).filter(
            num_reservations__lt=F('max_reservations'),
            subnetwork_size=subnetwork_size)