Done in ruby tuesday 16 for code kata, thanks Jimmy Ngu

roman_numeral_converter_spec.rb

# Refer http://literacy.kent.edu/Minigrants/Cinci/romanchart.htm
require 'rspec/given'
require 'roman_numeral_converter'

describe RomanNumeralConverter do
  
  Given(:converter) { RomanNumeralConverter.new }

  Then { converter.convert(1) == 'I' }
  Then { converter.convert(2) == 'II' }
  Then { converter.convert(3) == 'III' }
  Then {converter.convert(4) == 'IV'}
  Then {converter.convert(5) == 'V'}
  Then {converter.convert(6) == 'VI'}
  Then {converter.convert(7) == 'VII'}
  Then {converter.convert(8) == 'VIII'}
  Then {converter.convert(9) == 'IX'}
  Then {converter.convert(10) == 'X'}

  Then {converter.convert(11) == 'XI'}
  Then {converter.convert(12) == 'XII'}
  Then {converter.convert(13) == 'XIII'}
  Then {converter.convert(14) == 'XIV'}
  Then {converter.convert(15) == 'XV'}
  Then {converter.convert(16) == 'XVI'}
  Then {converter.convert(17) == 'XVII'}
  Then {converter.convert(18) == 'XVIII'}
  Then {converter.convert(19) == 'XIX'}

  Then {converter.convert(20) == 'XX'}
  Then {converter.convert(33) == 'XXXIII'}
  Then {converter.convert(48) == 'XLVIII'}
  Then {converter.convert(77) == 'LXXVII'}
  Then {converter.convert(95) == 'XCV'}

  Then {converter.convert(124) == 'CXXIV'}
  Then {converter.convert(433) == 'CDXXXIII' }
  Then {converter.convert(900) == 'CM' }
  Then {converter.convert(1500) == 'MD' }
  Then {converter.convert(1800) == 'MDCCC' }

  Then {converter.convert(4999) == 'MMMMCMXCIX'}

end

roman_numeral_ruby.rb

class RomanNumeralConverter

  def convert(n)
    case n
      when 1..3       then 'I' * n
      when 4          then 'IV'
      when 5..8       then 'V'  + convert(n - 5)
      when 9          then 'IX'
      when 10..39     then 'X'  + convert(n - 10)
      when 40..49     then 'XL' + convert(n % 10)
      when 50..89     then 'L'  + convert(n % 50)
      when 90..99     then 'XC' + convert(n % 90)
      when 100..399   then 'C'  + convert(n - 100)
      when 400..499   then 'CD' + convert(n % 400)
      when 500..899   then 'D'  + convert(n % 500)
      when 900..999   then 'CM' + convert(n % 900)
      when 1000..5000 then 'M'  + convert(n - 1000)
      else ''
    end
  end

end