# encoding: UTF-8

CHARS = ('A'..'Z').to_a

def number_to_char(number)
  raise ArgumentError if number < 1

  result = ''
  while number > 0
    number, remainder = (number - 1).divmod(26)
    result.prepend(CHARS[remainder])
  end
  result
end

def char_to_number(char)
  raise ArgumentError unless char =~ /^[A-Z]*$/
  char.each_char.inject(0) { |sum, c| sum * 26 + CHARS.index(c) + 1 }
end

input = 'XFD'

number = char_to_number(input)
char = number_to_char(number)

puts "Input: #{input} to Number: #{number} to Char: #{char}"