code: public int bitrate_instant(){
      int _link=(seekable ? current_link : 0);
      if(samptrack==0)
        return (-1);
      int ret=(int)(bittrack/samptrack*vi[_link].rate+.5);
      bittrack=0.f;
      samptrack=0.f;
      return (ret);
    }
docs: additional data to offer since last call (or at beginning of stream)