GenServer implementation that wraps a function in order to allow calls to be rate limited.

ratelimit.ex

defmodule Auscrape.RateLimit do
  use GenServer
  use Timex

  def start_link(fun, interval, opts \\ []) do
    GenServer.start_link(__MODULE__, {fun, interval}, opts)
  end

  def call_fun(server, args) do
    GenServer.call(server, {:call, args}, :infinity)
  end

  def call_fun(server) do
    call_fun(server, [])
  end

  defp current_time() do
    Time.now |> Time.to_msecs |> round
  end

  def init({fun, interval}) do
    {:ok, {fun, interval, current_time, 0}}
  end

  def handle_call({:call, args}, from, {fun, interval, start_time, tick}) do
    wait_time = (start_time + tick * interval - current_time) |> max(0)
    fn ->
      receive do
      after
        wait_time ->
          GenServer.reply(from, {:ok, apply(fun, args)})
      end
    end |> spawn
    {:noreply, {fun, interval, start_time, tick + 1}}
  end
  
end