Programming Elixir - A Fibonacci Server

Hi, somehow, after typing code from book i managed only to make my code stuck at infinite loop. I did comparision side by side of my code to that i get from zipped examples i got with book (code from zip works ok). I can’t find where did i do mistake. Can anyone spot what did do wrong? I know i can move on, i understand what is going on, but it’s driving me nuts, beside, there is a big chance i won’t repeat that mistake if only i would know where it is :frowning:

Anyway, here’s the code:

defmodule FibSolver do

  def fib(scheduler) do
    send(scheduler, { :read, self })
    receive do
      { :fib, n, client } ->
        send(client, { :answer, n, fib_calc(n), self })
        fib(scheduler)
      { :shutdown } ->
        exit(:normal)
    end
  end

  defp fib_calc(0), do: 0
  defp fib_calc(1), do: 1
  defp fib_calc(n), do: fib_calc(n-1) + fib_calc(n-2)
end

defmodule Scheduler do
  
  def run(num_processes, module, func, to_calculate) do
    (1..num_processes)
    |> Enum.map(fn(_) -> spawn(module, func, [self]) end)
    |> schedule_processes(to_calculate, [])
  end

  defp schedule_processes(processes, queue, results) do
    receive do
      { :ready, pid } when length(queue) > 0 ->
        [ next | tail ] = queue
        send(pid, { :fib, next, self })
        schedule_processes(processes, tail, results)
      { :ready, pid } ->
        send(pid, { :shutdown })
        if length(processes) > 1 do
          schedule_processes(List.delete(processes, pid), queue, results)
        else
          Enum.sort(results, fn {n1,_}, {n2,_} -> n1 <= n2 end)
        end
      { :answer, number, result, _pid } ->
        schedule_processes(processes, queue, [{ number, result } | results])
    end
  end
end

to_process = [37, 37, 37, 37, 37, 37]

Enum.each(1..10, fn num_processes ->
  { time, result } = :timer.tc(Scheduler, :run, [num_processes, FibSolver, :fib, to_process])
  if num_processes == 1 do
    IO.puts(inspect(result))
    IO.puts("\n #  time(s)")
  end
  :io.format("~2B     ~.2f~n", [num_processes, time/1000000.0])
  end)

Looks like that should be {:ready, self}. Can’t start the circus if it’s never telling the Scheduler it’s ready. :slight_smile:

Thank you very much. Such typos are hard to spot, i normally rely on IDE to help me with them, but i have none for elixir :slight_smile:

I think the lesson for me is to also pattern match “anything” at end of receive and exit program printing the oddity I just catch, at least when I’m learning :wink:

PS. Func fact: Single core performance of my 1.6 Ghz i5 in mac book air (OSX) is better than single core performance of 3.7 Ghz AMD (Win10)

Deciding what to do when you get a message in which you are not interested is a general problem. Should I crash, or should I ignore it, or what? Unfortunately there is no general solution which is good for everyone everywhere.

Just some philosophy.

Robert

This is very true.

For completeness: When you use an OTP GenServer, it picks the ‘let it crash’ approach for you, as it will simply attempt to call a variant of handle_call, handle_cast or handle_info that does not exist. (Of course, when you don’t have, for instance, any overridden variants of handle_info, it will default to doing nothing with any info-messages it receives).

You can of course change it to the second variant by creating a fallback function variant that accepts any other message.