def validate_isbn(isbn)
  return true if isbn.nil?
  digits = isbn.gsub(/[^0-9X]/, '').split(//).map { |d| d == "X" ? 10 : d.to_i }
  if digits.length == 10
    # ISBN-10
    partial = total = 0
    digits.each do |d|
      partial += d
      total   += partial
    end
    return total.modulo(11) == 0
  elsif digits.length == 13
    # ISBN-13
    sum = 0
    digits[0..11].each_with_index do |d, i|
      sum += d * (i.odd? ? 3 : 1)
    end
    sum = sum % 10
    return digits[12] == (10 - sum)
  else
    return false
  end
end