defmodule IP.Address.ULA do
  alias IP.Address
  alias IP.Prefix.EUI64
  use Bitwise

  @moduledoc false

  @doc """
  Generates an IPv6 Unique Local Address
  """
  @spec generate(binary, non_neg_integer, true | false) :: {:ok, Address.ipv6()} | {:error, term}
  def generate(mac, subnet_id, locally_assigned)
      when is_binary(mac) and is_integer(subnet_id) and subnet_id >= 0 and subnet_id <= 0xFFFF and
             is_boolean(locally_assigned) do
    with %DateTime{} = now <- DateTime.utc_now(),
         {:ok, ntp_time} <- ntp_time(now),
         {:ok, eui} <- EUI64.eui_portion(mac),
         {:ok, digest} <- generate_digest(ntp_time, eui),
         {:ok, global_id} <- last_40_bits_of_digest(digest) do
      generate_address(locally_assigned, subnet_id, global_id)
    end
  end

  defp ntp_time(%DateTime{} = time) do
    seconds = DateTime.to_unix(time)
    {msec, _} = time.microsecond
    {:ok, ((seconds + 0x83AA7E80) <<< 32) + msec}
  end

  defp generate_digest(ntp_time, eui) do
    with key <- <<ntp_time::unsigned-integer-size(64), eui::unsigned-integer-size(64)>>,
         digest <- :crypto.hash(:sha, key),
         digest <- :binary.decode_unsigned(digest) do
      {:ok, digest}
    end
  end

  defp last_40_bits_of_digest(digest) do
    {:ok, digest &&& 0xFFFFFFFFFF}
  end

  defp generate_address(locally_assigned, subnet_id, global_id) do
    address =
      (0xFC <<< 120) + (local_assignment_bit(locally_assigned) <<< 120) + (global_id <<< 80) +
        ((subnet_id &&& 0xFFFF) <<< 64)

    {:ok, address}
  end

  defp local_assignment_bit(true), do: 1
  defp local_assignment_bit(false), do: 0
end