script to convert isbn13 to isbn10

isbn13to10.rb

#!/usr/bin/env ruby                                                                                       

# returns isbn10 if the given string represents a isbn13                                                  
# returns the given string otherwise                                                                      
def isbn13to10(line)
  isbn13 = line.tr_s('- ', '')
  return line unless isbn13.match(/\d{13}/)

  isbn10 = isbn13[3,9]
  checksum = 0
  isbn10.split('').each_with_index do |c, i|
    checksum += c.to_i * (10 - i)
  end
  checksum = 11 - (checksum % 11)
  checksum = 'X' if checksum == 10
  checksum = '0' if checksum == 11
  isbn10 + checksum.to_s
end

content = File.read(ARGV[0])
puts *content.split("\n").map { |line| isbn13to10(line) }