RomanNumerals

roman_numerals.rb

module RomanNumerals
  extend self

  CARDINALS =
    {    1 => %w|I V X|,
        10 => %w|X L C|,
       100 => %w|C D M|,
      1000 => 'M'
    }

  def to_roman(num)
    return '' unless (1...4000).include? num

    ("%04d" % num).each_char.with_index.inject('') do |ret, obj|
      char, i = *obj
      ret << convert(char.to_i, 10 ** (3 - i))
    end
  end

  private
  def convert(num, base)
    min, mid, max = CARDINALS[base]

    case remainder = num % 10
    when 1..3; min * remainder
    when 4;    "#{min}#{mid}"
    when 5;    mid
    when 6..8; "#{mid}#{min * (remainder - 5)}"
    when 9;    "#{min}#{max}"
    else ''
    end
  end
end

roman_numerals_spec.rb

require './roman_numerals'
describe RomanNumerals do
  subject { RomanNumerals.to_roman(number) }

  shared_examples_for 'change to roman' do |number, roman|
    let(:number) { number }
    it { should == roman }
  end

  it_behaves_like 'change to roman', -1, ''
  it_behaves_like 'change to roman', 0, ''

  it_behaves_like 'change to roman', 1, 'I'
  it_behaves_like 'change to roman', 2, 'II'
  it_behaves_like 'change to roman', 3, 'III'
  it_behaves_like 'change to roman', 4, 'IV'
  it_behaves_like 'change to roman', 5, 'V'

  it_behaves_like 'change to roman', 6, 'VI'
  it_behaves_like 'change to roman', 7, 'VII'
  it_behaves_like 'change to roman', 8, 'VIII'
  it_behaves_like 'change to roman', 9, 'IX'
  it_behaves_like 'change to roman', 10, 'X'

  it_behaves_like 'change to roman', 49, 'XLIX'
  it_behaves_like 'change to roman', 50, 'L'
  it_behaves_like 'change to roman', 51, 'LI'

  it_behaves_like 'change to roman', 99, 'XCIX'
  it_behaves_like 'change to roman', 100, 'C'
  it_behaves_like 'change to roman', 101, 'CI'

  it_behaves_like 'change to roman', 499, 'CDXCIX'
  it_behaves_like 'change to roman', 500, 'D'
  it_behaves_like 'change to roman', 501, 'DI'

  it_behaves_like 'change to roman', 999, 'CMXCIX'
  it_behaves_like 'change to roman', 1000, 'M'
  it_behaves_like 'change to roman', 1001, 'MI'

  it_behaves_like 'change to roman', 1999, 'MCMXCIX'
  it_behaves_like 'change to roman', 2000, 'MM'
  it_behaves_like 'change to roman', 2001, 'MMI'

  it_behaves_like 'change to roman', 3999, 'MMMCMXCIX'
  it_behaves_like 'change to roman', 4000, ''
end