pentabonacci.ex

defmodule Pentabonacci do
  import Integer, only: [is_odd: 1]

  defp calc(0, _), do: 0
  defp calc(1, _), do: 1
  defp calc(2, _), do: 1
  defp calc(3, _), do: 2
  defp calc(4, _), do: 4

  defp calc(n, ets) do
    case :ets.lookup(ets, n) do
      [{_, cached_result}] ->
        IO.puts("got cached #{cached_result}")
        cached_result

      _otherwise ->
        IO.puts("calculating #{n}")
        result = calc(n - 1, ets) + calc(n - 2, ets) + calc(n - 3, ets) + calc(n - 4, ets)

        IO.puts("inserting #{n} -> #{result}")
        :ets.insert(ets, {n, result})

        result
    end
  end

  def count_odd(n) do
    ets = :ets.new(:qualquercoisa, [])

    0..n
    |> Enum.map(&calc(&1, ets))
    |> Enum.count(&is_odd/1)
  end
end

Pentabonacci.count_odd(1000)

versão sem ets e com :erlang.get e :erlang.pu

defmodule Pentabonacci do
  import Integer, only: [is_odd: 1]

  defp calc(0), do: 0
  defp calc(1), do: 1
  defp calc(2), do: 1
  defp calc(3), do: 2
  defp calc(4), do: 4

  defp calc(n) do
    case :erlang.get(n) do
      :undefined ->
        IO.puts("calculating #{n}")
        result = calc(n - 1) + calc(n - 2) + calc(n - 3) + calc(n - 4)

        IO.puts("inserting #{n} -> #{result}")
        :erlang.put(n, result)
        result

      cached_result ->
        IO.puts("got cached #{cached_result}")
        cached_result
    end
  end

  def count_odd(n) do
    0..n
    |> Enum.map(&calc(&1))
    |> Enum.count(&is_odd/1)
  end
end

Pentabonacci.count_odd(1000)