isbn_amazon_redirector.rb

#! /usr/bin/ruby -Ku
# To rewrite URL, the following may be written in .htaccess:
#
#   RewriteEngine On
#   RewriteBase ^/path/to
#   RewriteRule ^([\dX]+) /path/to/?$1 [L,R]

def isbn10_check_digit(common)
  d = common.split(//)
  c = (11 - (2..10).inject(0) {|a, x| a + x*d[10-x].to_i} % 11) % 11
  return c == 10 ? 'X' : c.to_s
end

def isbn13_check_digit(common)
  d = [9, 7, 8] + common.split(//)
  c = (10 - d.zip([1,3]*6).inject(0) {|a, x| a + x[0].to_i*x[1]} % 10) % 10
  return c.to_s
end

def check_isbn10(isbn10)
  d = isbn10.split(//)
  c = isbn10_check_digit(d[0,9].join)
  return c == d[-1]
end

def check_isbn13(isbn13)
  d = isbn13.split(//)
  c = isbn13_check_digit(d[3,9].join)
  return c == d[-1]
end

def isbn10_to_13(isbn10)
  if isbn10 =~ /\A(\d{9})[\dX]\Z/
    common = $1
    if check_isbn10(isbn10)
      return '978' + common + isbn13_check_digit(common)
    end
  end
  return nil
end

def isbn13_to_10(isbn13)
  if isbn13 =~ /\A978(\d{9})\d\Z/
    common = $1
    if check_isbn13(isbn13)
      return common + isbn10_check_digit(common)
    end
  end
  return nil
end

isbn = ENV['QUERY_STRING']
if isbn.nil?
  print "Location: http://www.amazon.co.jp/\r\n"
  print "\r\n"
  exit
end

isbn10 = nil
isbn13 = nil
if isbn =~ /\A(\d{9})([\dX])\Z/
  if check_isbn10(isbn)
    isbn10 = isbn
    isbn13 = isbn10_to_13(isbn10)
  end
elsif isbn =~ /\A978(\d{9})(\d)\Z/
  if check_isbn13(isbn)
    isbn13 = isbn
    isbn10 = isbn13_to_10(isbn13)
  end
end

#$stderr.puts "#{isbn.inspect}, #{isbn10.inspect}, #{isbn13.inspect}"
if isbn10
  location = "http://www.amazon.co.jp/dp/#{isbn10}/mrkn-22"
else
  location = "http://www.amazon.co.jp/"
end
print "Location: #{location}\r\n"
print "X-ISBN10: #{isbn10}\r\n"
print "X-ISBN13: #{isbn13}\r\n"
print "\r\n"