Convert arabic to roman numerals in Ruby

roman_numerals.rb

class Fixnum

  # Constant variable with 'unique' roman numerals, plus some special cases (e.g. "IV, IX, XC, CM...")
  ROMANS = {  1000 => "M",
              900 => "CM",
              500 => "D",
              400 => "CD",
              100 => "C",
              90 => "XC",
              50 => "L",
              40 => "XL",
              10 => "X",
              9 => "IX",
              5 => "V",
              4 => "IV",
              1 => "I" }

  def to_roman
    # If the number's already on the hash return the value, otherwise decompose it into numbers that are in it.
    ROMANS[self] || break_composite_roman(self)
  end

  private

  def break_composite_roman(num)
    # We start with an empty string and divide by the largest roman numeral possible in the hash
    # Then we keep breaking down the remainder until it is 0
    ROMANS.reduce("") do | memo, (arabic, roman) |
      quotient, num = num.divmod(arabic)
      # We multiply the quotient by the key's value (e.g. "I" * 3 = "III") and push it to the end of the string
      memo << roman * quotient
    end
  end

end

roman_numerals_test.rb

#!/usr/bin/env ruby
gem 'minitest', '>= 5.0.0'
require 'minitest/autorun'
require_relative 'roman_numerals'

class RomanNumeralsTest < Minitest::Test
  def test_1
    assert_equal 'I', 1.to_roman
  end

  def test_2
    assert_equal 'II', 2.to_roman
  end

  def test_3
    assert_equal 'III', 3.to_roman
  end

  def test_4
    assert_equal 'IV', 4.to_roman
  end

  def test_5
    assert_equal 'V', 5.to_roman
  end

  def test_6
    assert_equal 'VI', 6.to_roman
  end

  def test_9
    assert_equal 'IX', 9.to_roman
  end

  def test_27
    assert_equal 'XXVII', 27.to_roman
  end

  def test_48
    assert_equal 'XLVIII', 48.to_roman
  end

  def test_59
    assert_equal 'LIX', 59.to_roman
  end

  def test_93
    assert_equal 'XCIII', 93.to_roman
  end

  def test_141
    assert_equal 'CXLI', 141.to_roman
  end

  def test_163
    assert_equal 'CLXIII', 163.to_roman
  end

  def test_402
    assert_equal 'CDII', 402.to_roman
  end

  def test_575
    assert_equal 'DLXXV', 575.to_roman
  end

  def test_911
    assert_equal 'CMXI', 911.to_roman
  end

  def test_1024
    assert_equal 'MXXIV', 1024.to_roman
  end

  def test_3000
    assert_equal 'MMM', 3000.to_roman
  end

  def test_99
    assert_equal 'XCIX', 99.to_roman
  end

  def test_2015
    assert_equal 'MMXV', 2015.to_roman
  end

  def test_3999
    assert_equal 'MMMCMXCIX', 3999.to_roman
  end

end