defmodule FactDigitSum do
  defp fact(1, acc), do: acc
  defp fact(n, acc), do: fact(n - 1, n * acc)
  def fact(n), do: fact(n - 1, n)

  defp digit_sum(0, acc), do: acc
  defp digit_sum(n, acc), do: digit_sum(div(n, 10), rem(n, 10) + acc)
  def digit_sum(n), do: digit_sum(n, 0)
end


FactDigitSum.fact(100)
|> FactDigitSum.digit_sum()
|> IO.puts()