Roman Numeral Ruby Quiz

roman_numerals.rb

#!/usr/bin/env ruby
MAPPINGS = {
  "I" => 1,
  "V" => 5,
  "X" => 10,
  "L" => 50,
  "C" => 100,
  "D" => 500,
  "M" => 1000
}

def to_arabic(line)
  parts = line.chomp.split("")
  return 0 if parts.empty?

  sorted_values = MAPPINGS.values.sort
  sorted_names = sorted_values.map {|v| MAPPINGS.invert[v] }

  part = parts[0]
  next_part = parts[1]

  if !next_part.nil? && sorted_names.index(part) < sorted_names.index(next_part)
    addition = MAPPINGS[next_part] - MAPPINGS[part] || 0
    pass_along = parts[2..-1]
  else
    addition = MAPPINGS[part] || 0
    pass_along = parts[1..-1]
  end

  addition + to_arabic(pass_along.join)
end

def to_roman(line)
  string = ""

  sorted_values = MAPPINGS.values.sort
  sorted_names = sorted_values.map {|v| MAPPINGS.invert[v] }

  digits = line.chomp.split("")

  digits.map {|d| d.to_i }.each_with_index do |digit, i|
    position = digits.size - i
    current_letter = MAPPINGS.invert[10 ** (position - 1)]
    next_letter = sorted_names[sorted_names.index(current_letter) + 1]
    next_next_letter = sorted_names[sorted_names.index(current_letter) + 2]

    string << case digit
      when 0..3 then current_letter * digit
      when 4 then (current_letter + next_letter)
      when 5..8 then next_letter + (current_letter * (digit - 5))
      when 9 then current_letter + next_next_letter
    end
  end

  string
end

$stdin.each_line do |line|
  if line =~ /^\d+$/
    puts to_roman(line)
  elsif line =~ Regexp.new('^[' + MAPPINGS.keys.join + ']+$')
    puts to_arabic(line)
  end
end