Solution 10: Roman Numeral to Integer

Given a string containing a Roman numeral between I and MMMCMXCIX, return an integer of the value it represents.

def roman_to_int(s)
  numeral = s
  int = 0

  while (numeral.length > 0)
    (number, numeral) = get_int_from_string(numeral)
    int += number
  end

  int
end

def get_int_from_string(numeral)
  numerals = ['I', 'V', 'X', 'L', 'C', 'D', 'M']
  examine = numeral.slice(0,2)
  take_two = false
  first = numerals.index examine[0]
  second = numerals.index examine[1]
  if (!second.nil? && first < second)
    if examine[0] == 'C' && examine[1] == 'M'
      number = 900
    elsif examine[0] == 'C' && examine[1] == 'D'
      number = 400
    elsif examine[0] == 'X' && examine[1] == 'C'
      number = 90
    elsif examine[0] == 'X' && examine[1] == 'L'
      number = 40
    elsif examine[0] == 'I' && examine[1] == 'X'
      number = 9
    elsif examine[0] == 'I' && examine[1] == 'V'
      number = 4
    end
  take_two = true
  elsif examine[0] == 'M'
    number = 1000
  elsif examine[0] == 'D'
    number = 500
  elsif examine[0] == 'C'
    number = 100
  elsif examine[0] == 'L'
    number = 50
  elsif examine[0] == 'X'
    number = 10
  elsif examine[0] == 'V'
    number = 5
  elsif examine[0] == 'I'
    number = 1
  else
    number = 0
  end

  string = take_two ? numeral.slice(2..) : numeral.slice(1..)

  [number, string]
end