Spatzerny roman to int and int to roman

roman.rb

class Integer
  INT_TO_ROMAN = {
      1000 => 'M',
      500  => 'D',
      100  => 'C',
      50   => 'L',
      10   => 'X',
      5    => 'V',
      1    => 'I',
  }
  
  public
  def to_roman
    str = ""

    mod = roman_digit(str, self, 1000, 100)
    mod = roman_digit(str, mod, 500, 100)
    mod = roman_digit(str, mod, 100, 10)
    mod = roman_digit(str, mod, 50, 10)
    mod = roman_digit(str, mod, 10, 1)
    mod = roman_digit(str, mod, 5, 1)
    roman_digit(str, mod, 1)

    str
  end

  protected
  def roman_digit(str, n, value, sub_value = nil)
    div, mod = n.divmod value
    str << INT_TO_ROMAN[value] * div

    # Add MC or IX or whatever.
    if sub_value 
      div, mod = mod.divmod(value - sub_value) 
      str << "#{INT_TO_ROMAN[sub_value]}#{INT_TO_ROMAN[value]}" * div
    end

    mod
  end
end

class String
  # Invert the conversion above
  ROMAN_TO_INT = {}
  Integer::INT_TO_ROMAN.each_pair {|value, letter| ROMAN_TO_INT[letter] = value }

  def to_i(base = 10)
    if base == :roman
      total = 0
      
      strip.each_char.with_index do |letter, i|
        value = ROMAN_TO_INT[letter.upcase] || raise("Character '#{letter}' not valid in a Roman numeral!")
        next_value = ROMAN_TO_INT[self[i + 1]] || 0
        total += ((next_value > value) ? -value : +value)      
      end
      
      total
    else
      super(base)
    end
  end
end
  
puts 1989.to_roman # => MCMLXXXIX 
puts 2004.to_roman # => MMIV
puts 2012.to_roman # => MMXII
puts (1..20).map(&:to_roman)

puts "III".to_i(:roman) # => 3
puts "XCIX".to_i(:roman) # => 99
puts "IIJ".to_i(:roman) # => error J not a roman numeral