fib.ex

defmodule Cache do
  def init do
    {:ok, pid} = Agent.start_link(fn -> %{} end)
    pid
  end

  def set(pid, n, value) do
    pid |> Agent.get_and_update(fn state -> {state, Map.put(state, n, value)} end)
    pid
  end

  def get(pid, n) do
    pid |> Agent.get(& &1[n])
  end

  def print_state(pid) do
    pid |> Agent.get(& &1)
  end
end

defmodule Fibonacci do
  def find(n) do
    Cache.init()
    |> Cache.set(0, 0)
    |> Cache.set(1, 1)
    |> find(n)
  end

  defp find(cache, n) do
    cache
    |> Cache.get(n)
    |> value_or_find(cache, n)
  end

  defp value_or_find(_no_value = nil, cache, n) do
    value = find(cache, n - 1) + find(cache, n - 2)
    cache |> Cache.set(n, value)
    value
  end

  defp value_or_find(value, cache, n), do: value
end