fib.exs

#START:solver1
defmodule FibSolver do

  def fib(scheduler) do
    send scheduler, { :ready, self }
    receive do
      { :fib, n, client } ->
        send client, { :answer, n, fib_calc(n), self }
        fib(scheduler)
      { :shutdown } ->
        exit(:normal)
    end
  end
#END:solver1

#START:solver2
  # very inefficient, deliberately
  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
#END:solver2

#START:scheduler
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
#END:scheduler

#START:main
to_process = List.duplicate(37, 20)

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
#END:main