sub nextI 
  {
    if (!$size)
      {
        $size = -s INDEX;
      }
    local ($a,$b,$c,$dummy);
    $cond = 0;
    seek (INDEX,$index * 8,0);

    while ($cond == 0)
      {
	&readnext;
	if ($frame == 1)
	  {
	    $cond = 1;
	  }
	else
	  {
	    $index++;
            if ($index > ($size/8-1))
              {
                $index = $size/8-1;
                &prevI;
                print "$index\n";
                exit 1;
              }
	  }
      }
  }

sub readnext
  {
    read (INDEX,$a,4);
    read (INDEX,$b,1);
    read (INDEX,$c,1);
    read (INDEX,$dummy,2);
    $offset = unpack ("L",$a);
    $frame = unpack ("C",$b);
    $file = unpack ("C",$c);
  }

sub prevI 
  {
    local ($a,$b,$c,$dummy);
    $cond = 0;
    seek (INDEX,$index * 8,0);

    while ($cond == 0)
      {
	&readnext;
	if ($frame == 1)
	  {
	    $cond = 1;
	  }
	else
	  {
	    $index--;
	    seek (INDEX,$index * 8,0);
	  }
      }
  }

1;