fib_with_agents.exs

defmodule FibAgent do 
  def start_link do
    cache = Enum.into([{0, 0}, {1, 1}], HashDict.new)
    Agent.start_link(fn -> cache end)
  end
 
  def fib(pid, n) when n >= 0 do
    Agent.get_and_update(pid, &do_fib(&1, n))
  end
 
  defp do_fib(cache, n) do
    case Dict.size(cache) do
      size when size > n ->
        {cache[n], cache}
      size ->
        do_fib(cache, cache[size-2], cache[size-1], size, n)
    end
  end

  defp do_fib(cache, prev, cur, max, max) do
    next = prev+cur
    {next, Dict.put(cache, max, next)}
  end

  defp do_fib(cache, prev, cur, n, max) do
    next = prev + cur
    do_fib(Dict.put(cache, n, next), cur, next, n + 1, max)
  end

end

{:ok, agent} = FibAgent.start_link()
IO.puts FibAgent.fib(agent, 20)
IO.puts FibAgent.fib(agent, 20)