rivet_utils

defmodule Rivet.Utils.Interval2 do
  @moduledoc """
  Make sure only one of a method callback is ever running.

  Interval and last run time are stored in state.  You can adjust interval
  on the fly, but it'll kick in on second iteration after this one because
  the next one is already queued.

  Contributor: Brandon Gillespie

  Usage:

  ```
  use Interval2, tick: 1_000
  # or use Interval2, ... options
  ```

  In process startup:

  ```
    interval_start(state)
  ```

  Then later:

  ```
  def interval(state) do
    # do a thing
    {:ok, state}
  end
  ```

  Supported options:

    tick: integer        # REQUIRED
    error_logger: {M, f} # default: {Logger, :error} # two-arg arity
    callback: :interval  # function name to call

  """
  require Logger

  defmacro __using__(opts) do
    quote location: :keep, bind_quoted: [opts: opts] do
      # required
      @tick Keyword.get(opts, :tick)
      @error_logger Keyword.get(opts, :error_logger, {Logger, :error})
      @method Keyword.get(opts, :callback, :interval)

      defp interval_error(s, msg, opts \\ []) do
        {log_m, log_f} = @error_logger
        apply(log_m, log_f, ["Interval: " <> msg, opts])
        {:noreply, s}
      end

      def handle_info(:interval, state) do
        start = System.system_time(:millisecond)
        with {:noreply, state} <- interval_run(state), do: interval_queue(state, start)
      end

      defp interval_run(state) do
        case apply(__MODULE__, @method, [state]) do
          {:ok, state} -> {:noreply, state}
          other -> interval_error(state, "Unexpected result from interval call", result: other)
        end
      rescue
        err ->
          interval_error(state, "Unexpected exception during interval call",
            traceback: Exception.format(:error, err, __STACKTRACE__)
          )
      end

      def interval_queue(state, start) do
        Process.send_after(
          self(),
          :interval,
          calc_interval(@tick, @tick - System.system_time(:millisecond) - start)
        )
      end

      def interval_start(state), do: interval_queue(state, 0)

      defp calc_interval(tick, diff) when diff <= 1 do
        interval_error(nil, "Process has exceeded interval, skipping to next interval")
        {:ok, tick}
      end

      defp calc_interval(tick, diff) when diff < tick / 2 do
        interval_error(nil, "Process is running slow")
        {:ok, diff}
      end

      defp calc_interval(_, diff), do: diff
    end
  end
end