# see https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring.rst#monitoring
defmodule Mongo.Monitor do
  @moduledoc false
  use GenServer
  use Bitwise
  require Logger
  alias Mongo.ServerDescription
  alias Mongo.Events.ServerHeartbeatStartedEvent
  alias Mongo.Events.{ServerHeartbeatStartedEvent, ServerHeartbeatFailedEvent,
                      ServerHeartbeatSucceededEvent}

  # this is not configurable because the specification says so
  # see https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring.rst#minheartbeatfrequencyms
  @min_heartbeat_frequency_ms 500

  def start_link(args, gen_server_opts \\ []) do
    GenServer.start_link(__MODULE__, args, gen_server_opts)
  end

  # We need to stop asynchronously because a Monitor can call the Topology
  # which may try to stop the same Monitor that called it. Ending in a timeout.
  # See issues #139 for some information.
  def stop(pid) do
    GenServer.cast(pid, :stop)
  end

  def force_check(pid) do
    GenServer.call(pid, :check, :infinity)
  end

  ## GenServer callbacks

  @doc false
  def init([server_description, topology_pid, heartbeat_frequency_ms, connection_opts]) do
    opts = # monitors don't authenticate and use the "admin" database
      connection_opts
      |> Keyword.put(:database, "admin")
      |> Keyword.put(:skip_auth, true)
      |> Keyword.put(:after_connect, {__MODULE__, :connected, [self(), topology_pid]})
      |> Keyword.put(:backoff_min, heartbeat_frequency_ms)
      |> Keyword.put(:backoff_max, heartbeat_frequency_ms)
      |> Keyword.put(:backoff_type, :rand)
      |> Keyword.put(:connection_type, :monitor)
      |> Keyword.put(:topology_pid, topology_pid)
      |> Keyword.put(:pool_size, 1)
      |> Keyword.put(:idle_interval, 5_000)

    {:ok, pid} = DBConnection.start_link(Mongo.MongoDBConnection, opts)
    :ok = GenServer.cast(self(), :check)
    {:ok, %{
      connection_pid: pid,
      topology_pid: topology_pid,
      server_description: server_description,
      heartbeat_frequency_ms: heartbeat_frequency_ms,
      opts: opts
    }}
  end

  @doc false
  def terminate(reason, state) do
    GenServer.stop(state.connection_pid, reason)
  end

  @doc false
  def connected(_connection, me, topology_pid) do
    GenServer.cast(topology_pid, {:connected, me})
  end

  @doc false
  def handle_cast(:check, state) do
    check(state)
  end
  def handle_cast(:stop, state) do
    exit(:normal)
    {:noreply, state}
  end

  @doc false
  def handle_call(:check, _from, state) do
    {_, state, diff} = check(state)
    {:reply, diff, state}
  end

  @doc false
  def handle_info(:timeout, state) do
    check(state)
  end

  ## Private functions

  defp check(state) do
    diff = :os.system_time(:milli_seconds) - state.server_description.last_update_time
    if diff < @min_heartbeat_frequency_ms do
      {:noreply, state, diff}
    else
      server_description = is_master(state.connection_pid, state.server_description, state.opts)

      :ok = GenServer.cast(state.topology_pid, {:server_description, server_description})
      {:noreply, %{state | server_description: server_description}, state.heartbeat_frequency_ms}
    end
  end

  # TODO: Remove this try/rescue once a new version of db_connection is released
  defp call_is_master(conn_pid, opts) do
    start_time = System.monotonic_time
    result = try do
      Mongo.direct_command(conn_pid, [isMaster: 1], opts)
    rescue
      e -> {:error, e}
    end
    finish_time = System.monotonic_time
    rtt = System.convert_time_unit(finish_time - start_time, :native, :millisecond)
    finish_time = System.convert_time_unit(finish_time, :native, :millisecond)

    {result, finish_time, rtt}
  end

  # see https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring.rst#network-error-when-calling-ismaster
  defp is_master(conn_pid, last_server_description, opts) do
    :ok = Mongo.Events.notify(%ServerHeartbeatStartedEvent{
      connection_pid: conn_pid
    })

    {result, finish_time, rtt} = call_is_master(conn_pid, opts)
    case result do
      {:ok, is_master_reply} ->
        notify_success(rtt, is_master_reply, conn_pid)
        ServerDescription.from_is_master(last_server_description, rtt, finish_time, is_master_reply)

      {:error, error} ->
        if last_server_description.type in [:unknown, :possible_primary] do
          notify_error(rtt, error, conn_pid)
          ServerDescription.from_is_master_error(last_server_description, error)
        else
          {result, finish_time, rtt} = call_is_master(conn_pid, opts)
          case result do
            {:ok, is_master_reply} ->
              notify_success(rtt, is_master_reply, conn_pid)
              ServerDescription.from_is_master(last_server_description, rtt, finish_time, is_master_reply)
            {:error, error} ->
              notify_error(rtt, error, conn_pid)
              ServerDescription.from_is_master_error(last_server_description, error)
          end
        end
    end
  end

  defp notify_error(rtt, error, conn_pid) do
    :ok = Mongo.Events.notify(%ServerHeartbeatFailedEvent{
            duration: rtt,
             failure: error,
      connection_pid: conn_pid
    })
  end

  defp notify_success(rtt, reply, conn_pid) do
    :ok = Mongo.Events.notify(%ServerHeartbeatSucceededEvent{
      duration: rtt,
      reply: reply,
      connection_pid: conn_pid
    })
  end
end