class String
  def unpack!(fmt)
    ret = unpack(fmt)
    slice!(0,ret.pack(fmt).size)
    ret
  end
end

# Use like this.
str = "\0\xaa\x01\x55\xff"
until str.empty?
  case str.unpack!('C').first
  when 0
    puts str.unpack('C')
  when 1
    puts str.unpack('n')
  end
end