numerales romanos a decimal (en crystal)

romanos.cr

class String
  def from_roman
  	ans = -1
  	if self.length == 0
  		ans = 0
	elsif self[0..0] == "M"
		ans = 1000 + self[1..self.length].from_roman
	elsif self[0..1] == "CM"
		ans = 900 + self[2..self.length].from_roman
	elsif self[0..0] == "D"
		ans = 500 + self[1..self.length].from_roman
	elsif self[0..1] == "CD"
		ans = 400 + self[2..self.length].from_roman
	elsif self[0..0] == "C"
		ans = 100 + self[1..self.length].from_roman
	elsif self[0..1] == "XC"
		ans = 90 + self[2..self.length].from_roman
	elsif self[0..0] == "L"
		ans = 50 + self[1..self.length].from_roman
	elsif self[0..1] == "XL"
		ans = 40 + self[2..self.length].from_roman
	elsif self[0..0] == "X"
		ans = 10 + self[1..self.length].from_roman
	elsif self[0..1] == "IX"
		ans = 9 + self[2..self.length].from_roman
	elsif self[0..0] == "V"
		ans = 5 + self[1..self.length].from_roman
	elsif self[0..1] == "IV"
		ans = 4 + self[2..self.length].from_roman
	elsif self[0..0] == "I"
		ans = 1 + self[1..self.length].from_roman
  	end
  	ans
  end
end