roman_numeral.rb

module RomanNumeral

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

  def self.i_to_old_roman int
    i_to_roman int, VALUES_OF_LETTERS
  end

  def self.i_to_modern_roman int
    i_to_roman int, VALUES_OF_LETTERS.merge(MODERN_ROMAN_PATTERNS)
  end

  def self.i_to_roman int, pattern
    roman = ""
    pattern.sort_by {|k,v| v}.reverse.each do |pair|
      roman += pair[0].to_s * (int / pair[1])
      int = int % pair[1]
    end
    roman
  end

end

RomanNumeral.i_to_modern_roman 179 # => "CLXXIX"
RomanNumeral.i_to_modern_roman 2019 # => "MMXIX"