gistfile1.rb

class String
  #
  # Convert ISBN-10 to ISBN-13
  #
  #   http://www.infonet.co.jp/ueyama/ip/glossary/isbn.html
  #   http://en.wikipedia.org/wiki/Check_digit#ISBN_13
  #
  # @return [String] String as ISBN-13 format
  #
  def toISBN13
    raise TypeError.new 'must be 10 digit chars in length' unless self.ISBN10?

    base = [9,7,8] + self[0...-1].split(//).map(&:to_i)
    check_digit = 10 - base.zip([1,3]*6).inject(0) { |s, v| s += v[0] * v[1] } % 10
    check_digit = 0 if check_digit === 10
    return base.join + check_digit.to_s
  end

  def ISBN10?
    return /^\d{9}(\d|X)$/ =~ self
  end
end

# http://www.amazon.co.jp/dp/4048687158/
p "4048687158".toISBN13 # => "9784048687157"

# http://www.amazon.co.jp/dp/4873113946/
p "4873113946".toISBN13 # => "9784873113944"

# not 10 digit chars
p "12345".toISBN13 # => raise TypeError