defmodule FLAMEK8sBackend do
  @behaviour FLAME.Backend

  require Logger

  defstruct token_path: "/var/run/secrets/kubernetes.io/serviceaccount",
            env: %{},
            base_pod: nil,
            parent_ref: nil,
            runner_node_basename: nil,
            runner_pod_ip: nil,
            runner_pod_name: nil,
            runner_node_name: nil,
            boot_timeout: nil,
            container_name: nil,
            remote_terminator_pid: nil,
            log: false,
            conn: nil

  @valid_opts ~w(token_path container_name terminator_sup log)a
  @required_config ~w()a

  @impl true
  def init(opts) do
    :global_group.monitor_nodes(true)
    conf = Application.get_env(:flame, __MODULE__) || []
    [node_base | _ip] = node() |> to_string() |> String.split("@")

    default = %FLAMEK8sBackend{
      boot_timeout: 30_000,
      runner_node_basename: node_base
    }

    provided_opts =
      conf
      |> Keyword.merge(opts)
      |> Keyword.validate!(@valid_opts)

    state = struct(default, provided_opts)

    for key <- @required_config do
      unless Map.get(state, key) do
        raise ArgumentError, "missing :#{key} config for #{inspect(__MODULE__)}"
      end
    end

    parent_ref = make_ref()

    encoded_parent =
      parent_ref
      |> FLAME.Parent.new(self(), __MODULE__)
      |> FLAME.Parent.encode()

    new_env =
      Map.merge(
        %{PHX_SERVER: "false", DRAGONFLY_PARENT: encoded_parent},
        state.env
      )

    {:ok, conn} = K8s.Conn.from_service_account(state.token_path, insecure_skip_tls_verify: true)

    {:ok, base_pod} =
      K8s.Client.get("v1", "pod",
        namespace: System.get_env("POD_NAMESPACE"),
        name: System.get_env("POD_NAME")
      )
      |> K8s.Client.put_conn(conn)
      |> K8s.Client.run()

    new_state =
      struct(state, conn: conn, base_pod: base_pod, env: new_env, parent_ref: parent_ref)

    {:ok, new_state}
  end

  @impl true
  def remote_spawn_monitor(%FLAMEK8sBackend{} = state, term) do
    case term do
      func when is_function(func, 0) ->
        {pid, ref} = Node.spawn_monitor(state.runner_node_name, func)
        {:ok, {pid, ref}}

      {mod, fun, args} when is_atom(mod) and is_atom(fun) and is_list(args) ->
        {pid, ref} = Node.spawn_monitor(state.runner_node_name, mod, fun, args)
        {:ok, {pid, ref}}

      other ->
        raise ArgumentError,
              "expected a null arity function or {mod, func, args}. Got: #{inspect(other)}"
    end
  end

  @impl true
  def system_shutdown do
    System.stop()
  end

  def with_elapsed_ms(func) when is_function(func, 0) do
    {micro, result} = :timer.tc(func)
    {result, div(micro, 1000)}
  end

  @impl true
  def remote_boot(%FLAMEK8sBackend{parent_ref: parent_ref} = state) do
    log(state, "Remote Boot")

    {new_state, req_connect_time} =
      with_elapsed_ms(fn ->
        {:ok, created_pod} =
          state
          |> create_runner_pod()
          |> K8s.Client.create()
          |> K8s.Client.put_conn(state.conn)
          |> K8s.Client.run()

        log(state, "Pod Created")

        wait_result =
          created_pod
          |> K8s.Client.get()
          |> K8s.Client.put_conn(state.conn)
          |> K8s.Client.wait_until(
            find: ["status", "podIP"],
            eval: &(not is_nil(&1)),
            timeout: Integer.floor_div(state.boot_timeout, 1000)
          )

        case wait_result do
          {:ok, pod} ->
            log(state, "Pod Scheduled. IP: #{pod["status"]["podIP"]}")

            struct!(state,
              runner_pod_ip: pod["status"]["podIP"],
              runner_pod_name: pod["metadata"]["name"]
            )

          {:error, _} ->
            Logger.error("failed to schedule runner pod within #{state.boot_timeout}ms")
            exit(:timeout)
        end
      end)

    remaining_connect_window = state.boot_timeout - req_connect_time
    log(state, "Waiting for Remote UP. Remaining: #{remaining_connect_window}")

    remote_terminator_pid =
      receive do
        {^parent_ref, {:remote_up, remote_terminator_pid}} ->
          log(state, "Remote is Up!")
          remote_terminator_pid
      after
        remaining_connect_window ->
          Logger.error("failed to connect to runner pod within #{state.boot_timeout}ms")
          exit(:timeout)
      end

    new_state =
      struct!(new_state,
        remote_terminator_pid: remote_terminator_pid,
        runner_node_name: :"#{state.runner_node_basename}@#{new_state.runner_pod_ip}"
      )

    {:ok, remote_terminator_pid, new_state}
  end

  @impl true
  def handle_info({:nodedown, down_node}, state) do
    if down_node == state.runner_node_name do
      log(state, "Runner #{state.runner_node_name} Down")
      {:stop, {:shutdown, :noconnection}, state}
    else
      log(state, "Other Runner #{down_node} Down")
      {:noreply, state}
    end
  end

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

  def handle_info({ref, {:remote_shutdown, :idle}}, %{parent_ref: ref} = state) do
    namespace = System.get_env("POD_NAMESPACE")
    runner_pod_name = state.runner_pod_name
    log(state, "Deleting Pod #{namespace}/#{runner_pod_name}")

    K8s.Client.delete("v1", "Pod",
      namespace: namespace,
      name: runner_pod_name
    )
    |> K8s.Client.put_conn(state.conn)
    |> K8s.Client.run()

    {:noreply, state}
  end

  def handle_info(msg, state) do
    log(state, "Missed message: #{inspect(msg)}")
    {:noreply, state}
  end

  defp create_runner_pod(state) do
    %{base_pod: base_pod, env: env} = state

    pod_name_sliced = base_pod |> get_in(~w(metadata name)) |> String.slice(0..40)
    runner_pod_name = pod_name_sliced <> rand_id(20)

    container_access =
      case state.container_name do
        nil -> Access.at(0)
        name -> K8s.Resource.NamedList.access(name)
      end

    base_container = get_in(base_pod, ["spec", "containers", container_access])

    %{
      "apiVersion" => "v1",
      "kind" => "Pod",
      "metadata" => %{
        "namespace" => get_in(base_pod, ~w(metadata namespace)),
        "name" => runner_pod_name,
        "labels" => %{
          "excluster" => get_in(base_pod, ~w(metadata labels excluster))
        }
      },
      "spec" => %{
        "restartPolicy" => "Never",
        "containers" => [
          %{
            "image" => base_container["image"],
            "name" => runner_pod_name,
            "resources" => base_container["resources"],
            "env" => encode_k8s_env(env) ++ base_container["env"]
          }
        ]
      }
    }
  end

  defp rand_id(len) do
    len
    |> :crypto.strong_rand_bytes()
    |> Base.encode16(padding: false, case: :lower)
    |> binary_part(0, len)
  end

  defp encode_k8s_env(env_map) do
    for {name, value} <- env_map, do: %{"name" => name, "value" => value}
  end

  defp log(%FLAMEK8sBackend{log: false}, _), do: :ok

  defp log(%FLAMEK8sBackend{log: level}, msg, metadata \\ []) do
    Logger.log(level, msg, metadata)
  end
end