roman1.ex

defmodule RomanNumeral do
  @mapping [
    {1000, 'M'},
    {900,  'CM'},
    {500,  'D'},
    {400,  'CD'},
    {100,  'C'},
    {90,   'XC'},
    {50,   'L'},
    {40,   'XL'},
    {10,   'X'},
    {9,    'IX'},
    {5,    'V'},
    {4,    'IV'},
    {1,    'I'},
  ]
 
  def to_arabic(roman_numeral) do
    to_arabic(String.to_char_list(roman_numeral), @mapping)
  end
  
  defp to_arabic([], _mapping), do: 0
  
  defp to_arabic([ h1, h2 | rest ], mapping = [{arabic_value, [h1, h2]} | _]) do
      arabic_value + to_arabic(rest, mapping)
  end
  
  defp to_arabic([ h1 | rest ], mapping = [{arabic_value, [h1]} | _]) do
      arabic_value + to_arabic(rest, mapping)
  end

  defp to_arabic(roman,  [ _ | other_mappings]) do
      to_arabic(roman, other_mappings)
  end
  
end