defmodule Cluster.Strategy.Kubernetes.DNS do
  @moduledoc """
  This clustering strategy works by loading all your Erlang nodes (within Pods) in the current Kubernetes
  namespace. It will fetch the addresses of all pods under a shared headless service and attempt to connect.
  It will continually monitor and update its connections every 5s.

  It assumes that all Erlang nodes were launched under a base name, are using longnames, and are unique
  based on their FQDN, rather than the base hostname. In other words, in the following
  longname, `<basename>@<ip>`, `basename` would be the value configured through
  `application_name`.

  An example configuration is below:


      config :libcluster,
        topologies: [
          k8s_example: [
            strategy: #{__MODULE__},
            config: [
              service: "myapp-headless",
              application_name: "myapp",
              polling_interval: 10_000]]]

  """
  use GenServer
  use Cluster.Strategy
  import Cluster.Logger

  alias Cluster.Strategy.State

  @default_polling_interval 5_000

  def start_link(args), do: GenServer.start_link(__MODULE__, args)

  def init([%State{meta: nil} = state]) do
    init([%State{state | :meta => MapSet.new()}])
  end

  def init([%State{} = state]) do
    {:ok, load(state), 0}
  end

  def handle_info(:timeout, state) do
    handle_info(:load, state)
  end

  def handle_info(:load, state) do
    {:noreply, load(state)}
  end

  def handle_info(_, state) do
    {:noreply, state}
  end

  defp load(%State{topology: topology, meta: meta} = state) do
    new_nodelist = MapSet.new(get_nodes(state))
    added = MapSet.difference(new_nodelist, meta)
    removed = MapSet.difference(meta, new_nodelist)

    new_nodelist =
      case Cluster.Strategy.disconnect_nodes(
             topology,
             state.disconnect,
             state.list_nodes,
             MapSet.to_list(removed)
           ) do
        :ok ->
          new_nodelist

        {:error, bad_nodes} ->
          # Add back the nodes which should have been removed, but which couldn't be for some reason
          Enum.reduce(bad_nodes, new_nodelist, fn {n, _}, acc ->
            MapSet.put(acc, n)
          end)
      end

    new_nodelist =
      case Cluster.Strategy.connect_nodes(
             topology,
             state.connect,
             state.list_nodes,
             MapSet.to_list(added)
           ) do
        :ok ->
          new_nodelist

        {:error, bad_nodes} ->
          # Remove the nodes which should have been added, but couldn't be for some reason
          Enum.reduce(bad_nodes, new_nodelist, fn {n, _}, acc ->
            MapSet.delete(acc, n)
          end)
      end

    Process.send_after(
      self(),
      :load,
      polling_interval(state)
    )

    {:noreply, %State{state | :meta => new_nodelist}}
  end

  @spec get_nodes(State.t()) :: [atom()]
  defp get_nodes(%State{topology: topology, config: config}) do
    app_name = Keyword.fetch!(config, :application_name)
    service = Keyword.fetch!(config, :service)

    cond do
      app_name != nil and service != nil ->
        headless_service = to_charlist(service)

        case :inet_res.getbyname(headless_service, :a) do
          {:ok, {:hostent, _fqdn, [], :inet, _value, addresses}} ->
            parse_response(addresses, app_name)

          {:error, reason} ->
            error(topology, "lookup against #{service} failed: #{inspect(reason)}")
            []
        end

      app_name == nil ->
        warn(
          topology,
          "kubernetes.DNS strategy is selected, but :application_name is not configured!"
        )

        []

      service == nil ->
        warn(topology, "kubernetes strategy is selected, but :service is not configured!")
        []

      :else ->
        warn(topology, "kubernetes strategy is selected, but is not configured!")
        []
    end
  end

  defp polling_interval(%State{config: config}) do
    Keyword.get(config, :polling_interval, @default_polling_interval)
  end

  defp parse_response(addresses, app_name) do
    addresses
    |> Enum.map(&:inet_parse.ntoa(&1))
    |> Enum.map(&"#{app_name}@#{&1}")
    |> Enum.map(&String.to_atom(&1))
  end
end