Determine subnet sizes within a VNET

Terraform
1

I need to programmatically determine the next available subnet address space within a given VNET CIDR range, when passed an object containing multiple subnets which could be of varying sizes.

The input we have at the moment is a list of objects which specify a list of virtual networks, as an example:

  virtual_networks = [
    {
      location = "westeurope"
      size     = 24
      subnets = [
        {
          type = "web"
          size = 27
        },
        {
          type = "app"
          size = 26
        },
        {
          type = "db"
          size = 26
        },
        {
          type = "private"
          size = 27
        }
      ]
    }
  ]

The CIDR range for the VNET is obtained using a provider for Azure IPAM, based on the specified size of the VNET. This ensures that the CIDR range is available within our environment.

This is then passed to a local which determines the details for each subnet which we use to create the subnets:

  subnets = var.virtual_network_enabled ? flatten([
    for k, v in var.virtual_networks : [
      for i, subnet in v.subnets : {
        vnet_location  = v.location
        type           = subnet.type
        size           = subnet.size
        address_prefix = cidrsubnet(azureipam_reservation.virtual_network[v.location].cidr, subnet.size - v.size, i)
      }
    ]
  ]) : []

The logic, using cidrsubnet, works fine if all subnets are the same size. This falls over when we pass different subnet sizes as per the example above.

Is there a better way to determine these subnet sizes? Have I missed a function that is going to do this for me?

Thanks in advance!