defmodule Cluster.EcsStrategy do
  @moduledoc """
  Clustering!!
  """

  use GenServer
  use Cluster.Strategy

  alias Cluster.Strategy.State

  @default_polling_interval 5_000

  @impl true
  def start_link(args), do: GenServer.start_link(__MODULE__, args)

  @impl true
  def init([%State{meta: nil} = state]) do
    {:ok, _pid} = Cluster.EcsClusterInfo.start_link(state.config)
    init([%State{state | :meta => MapSet.new()}])
  end

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

  @impl true
  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))
    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(new_nodelist)
           ) 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)
    )

    %State{state | meta: new_nodelist}
  end

  @spec get_nodes(State.t()) :: [atom()]
  defp get_nodes(%State{topology: _topology, config: _config}) do
    Cluster.EcsClusterInfo.get_nodes()
    |> Map.keys()
  end

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