Testing file

testing.ex

defmodule MyTestScript do
  @h 7
  @fixed_value 37

  def decode(num) do
    decode_internal(num, "")
  end

  defp decode_internal(num, accum) when rem(num, @fixed_value) == 3 do
    decode_internal(get_next_value(num , 3), accum <> "e")
  end

  defp decode_internal(num, accum) when rem(num, @fixed_value) == 1 do
    decode_internal(get_next_value(num , 1), accum <> "c")
  end

  defp decode_internal(num, accum) when rem(num, @fixed_value) == 6 do
    decode_internal(get_next_value(num , 6), accum <> "i")
  end

  defp decode_internal(num, accum) when rem(num, @fixed_value) == 9 do
    decode_internal(get_next_value(num , 9), accum <> "n")
  end

  defp decode_internal(num, accum) when num == @h do
    String.reverse(accum)
  end

  defp get_next_value(num, subtraction) do
    trunc((num - subtraction) / @fixed_value)
  end

  defp decode_internal(_num, accum) do
    String.reverse(accum)
  end

end

#it could also be possible to reduce the letter-number relation to a map 
#so that we do not have to rewrite the same logic so many times and it is the map 
#that sets the conversion rules.
IO.puts(MyTestScript.decode(13583258))