RomanNumerals.ex

defmodule RomanNumerals do

  @vals %{
    "I" => 1,
    "V" => 5,
    "X" => 10,
    "L" => 50,
    "C" => 100,
    "D" => 500,
    "M" => 1000
  }

  def to_arabic(roman_numeral) do
    convert(String.graphemes(roman_numeral))
  end

  def convert([]) do
    0
  end

  def convert([h1, h2|tail]) do
    num1 = val(h1)
    num2 = val(h2)

    num1 =
      cond do
        num2 > num1 ->
          num1 * -1
        true ->
          num1
      end
    num1 + num2 + convert(tail)
  end

  def convert([h1|tail]) do
    val(h1) + convert(tail)
  end

  defp val(letter) do
    @vals[letter]
  end

end