my $do_shift = 0;
sub get_elem {
  my $id = shift;
  if (++$do_shift == 30) {
    shift_mru();
    $do_shift = 0;
  }
  if (exists $count{$id}) {
    $count{$id} |= 0x80000000;
    return $cache{$id};
  }
  else{
    return;
  }
}

sub shift_mru {
  while (my ($id, $count) = each %count) {
    $count{$id} >>= 1;
  }
}