defmodule RomanNumerals do
  @doc """
  Convert the number to a roman number.
  """
  @romans %{
    1 => ["I", "X", "C", "M"],
    2 => ["II", "XX", "CC", "MM"],
    3 => ["III",  "XXX", "CCC", "MMM"],
    4 => ["IV",  "XL", "CD"],
    5 => ["V",  "L", "D"],
    6 => ["VI",  "LX", "DC"],
    7 => ["VII",  "LXX", "DCC"],
    8 => ["VIII",  "LXXX", "DCCC"],
    9 => ["IX",  "XC", "CM"]
  }

  @spec numeral(pos_integer) :: String.t()
  def numeral(number) do
    number
    |> Integer.digits()
    |> Enum.reverse()
    |> Enum.with_index(&to_roman_digit/2)
    |> Enum.reverse()
    |> Enum.join()
  end

  defp to_roman_digit(0, _index), do: ""
  defp to_roman_digit(digit, index) do
    @romans
    |> Map.fetch!(digit)
    |> Enum.at(index)
  end
end