defmodule SyncCallonAsync do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  def init(_) do
    {:ok, %{pending: []}}
  end

  def sync do
    GenServer.call(__MODULE__, :sync)
  end

  def handle_call(:sync, {pid, _} = from, %{pending: pend} = state) do
    mref = Process.monitor(pid)

    dispatch_req(from)
    {:noreply, %{state | pending: [{from, mref}| pend]}}
  end

  def dispatch_req(from) do
    current = self()
    spawn(fn ->
      Process.sleep(1000)
      send current, {:delayed_reply, from, :done}
    end)
  end

  def handle_info(:timeout, state) do
    {:noreply, state}
  end

  def handle_info({:delayed_reply, from, reply}, %{pending: pend} = state) do
    case List.keytake(pend, from, 0) do
      {{_from, mref}, rest} ->
        Process.demonitor(mref)
        GenServer.reply(from, reply)
        {:noreply, %{state | pending: rest}}
      false ->
        {:noreply, state}
    end
  end
end

defmodule Run do

  def measure(function) do
    function
    |> :timer.tc
    |> elem(0)
    |> Kernel./(1_000_000)
  end

  def run_for(times \\ 10) do
    SyncCallonAsync.start_link
    1..times |> Enum.each(fn (_) -> Task.async(fn -> measure(&SyncCallonAsync.sync/0) |> IO.inspect end) end)
  end
end