... And in the darkness bind them !

gistfile1.rb

#!/usr/bin/env ruby

def is_valid_isbn13?(isbn13)
  sum = 0
  13.times { |i| sum += i.modulo(2)==0 ? isbn13[i].to_i : isbn13[i].to_i*3 }
  0 == sum.modulo(10)
end
 
def isbn13_checksum(isbn12)
  sum = 0
  12.times { |i| sum += i.modulo(2)==0 ? isbn12[i].to_i : isbn12[i].to_i*3 }
  10 - sum.modulo(10)
end


isbn_numbers = []

f = File.new('isbns', 'r')
while (line = f.gets)
    line.delete!(' ')
    line.delete!('-')
    line.chop!
    if is_valid_isbn13?(line)
        if isbn13_checksum(line)
            isbn_numbers << line
        end
    end
    puts line.size
end

puts isbn_numbers