--- sum([bits=16])

    ʸ bits ӥåȤΥå׻ޤ
    ʲƱǤ

        sum = 0
        str.each_byte {|c| sum += c}
        sum = sum & ((1 << bits) - 1) if bits != 0

    㤨аʲΥɤ System V  sum(1) [manual page] ޥɤ
    Ʊͤޤ

      sum = 0
      while gets
        sum += $_.sum
      end
      sum %= 65536

