from django.db import transaction
from django.db.models import Count, F


from .models import *

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)


def allowed_vpn_network_reservation_size():
    """
    Find all possible sizes of subnetworks that are available.

    Select all pools with free networks.

    Get their subnetwork sizes, reduce to a set

    """

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

    return set([ pool.subnetwork_size for pool in pools ])