Awesome roman numeral algorithm in Ruby :)

roman_numerals.rb

require 'pry'

# ```
#  1  => I
# 10  => X
#  7  => VII
# ```

# There is no need to be able to convert numbers larger than about 3000. (The Romans themselves didn't tend to go any higher)

# Wikipedia says: Modern Roman numerals ... are written by expressing each digit separately starting with the left most digit and skipping any digit with a value of zero.

# To see this in practice, consider the example of 1990.

# In Roman numerals 1990 is MCMXC:

# 1000=M
# 900=CM
# 90=XC

# 2008 is written as MMVIII:

# 2000=MM
# 8=VIII

# See also: http://www.novaroma.org/via_romana/numbers.html

class Integer

  @@values = {
    1=>"I",
    4=>"IV",
    5=>"V",
    9=>"IX",
    10=>"X",
    40=>"XL",
    50=>"L",
    90=>"XC",
    100=>"C",
    400=>"CD",
    500=>"D",
    900=>"CM",
    1000=>"M"
  }

  def to_roman_descending #this one requires numeral-arabic pairs in descending order, i.e. 1000=>"M" on down
    return 0 if self == 0

    roman = ""
    integer = self
    @@values.each do |k,v|
      until integer < k
        roman << v
        integer -= k
      end
    end
    roman
  end

  def to_roman
    integer = self
    roman = ""

    while integer > 0
      if @@values[integer]
        roman += @@values[integer]
        return roman
      end

      roman += @@values[next_lower_key(integer)] # increment the roman numeral string here
      integer -= next_lower_key(integer) # decrement the arabic integer here
    end
  end

  def next_lower_key(integer)
    arabics = @@values.keys
    next_lower_index = (arabics.push(integer).sort.index(integer))-1
    arabics[next_lower_index]
  end
end

roman_numerals_spec.rb

require_relative 'roman_numerals'

RSpec.configure do |config|
  # Use color in STDOUT
  config.color_enabled = true

  # Use color not only in STDOUT but also in pagers and files
  config.tty = true

  # Use the specified formatter
  config.formatter = :progress # :progress, :html, :textmate
end

describe Integer, '#to_roman' do

  it 'converts 1' do
    1.to_roman.should eq 'I'
  end

  it 'converts 2' do
    2.to_roman.should eq 'II'
  end

  it 'converts 3' do
    3.to_roman.should eq 'III'
  end

  it 'converts 4' do
    4.to_roman.should eq 'IV'
  end

  it 'converts 5' do
    5.to_roman.should eq 'V'
  end

  it 'converts 6' do
    6.to_roman.should eq 'VI'
  end

  it 'converts 9' do
    9.to_roman.should eq 'IX'
  end


  it 'converts 27' do
    27.to_roman.should eq 'XXVII'
  end

  it 'converts 48' do
    48.to_roman.should eq 'XLVIII'
  end

  it 'converts 59' do
    59.to_roman.should eq 'LIX'
  end

  it 'converts 93' do
    93.to_roman.should eq 'XCIII'
  end

  it 'converts 141' do
    141.to_roman.should eq 'CXLI'
  end

  it 'converts 163' do
    163.to_roman.should eq 'CLXIII'
  end

  it 'converts 402' do
    402.to_roman.should eq 'CDII'
  end

  it 'converts 575' do
    575.to_roman.should eq 'DLXXV'
  end

  it 'converts 911' do
    911.to_roman.should eq 'CMXI'
  end

  it 'converts 1024' do
    1024.to_roman.should eq 'MXXIV'
  end

  it 'converts 3000' do
    3000.to_roman.should eq 'MMM'
  end

end