Shinjuku.rb #4

roman.rb

class Integer
  def to_roman
    raise if self <= 0 || self >= 4000
    
    a = ("%04d" % self).split(//).map(&:to_i).reverse
    m = { 1 => 'I', 5 => 'V', 10 => 'X', 50 => 'L', 100 => 'C', 500 => 'D', 1000 => 'M' }
    
    (0..3).inject("") do |s,i|
       i = 3 - i
       s += case a[i]
            when 0..3
              m[10**i] * a[i]
            when 4
              m[10**i] *(5-a[i]) + m[5*10**i]
            when 5..8
              m[5*10**i] + m[10**i] * (a[i]-5)
            when 9
              m[10**i] + m[10**(i+1)]
            end
    end   
  end
end