while(){
    if($cnt % $nr_samples == 0){
      $lower_bound = $lower_bound+$nr_samples;
      $upper_bound = $upper_bound+$nr_samples;
    }
    $cnt++;
    last if $upper_bound > ($length-1);
    foreach my $station(@stations){
      open OUT, ">$span.$station.alpha.sac.data";
      print OUT "2 $nr_samples\n";
      for my $seeking($lower_bound..$upper_bound){
        my $eval_file_2 = $suffix ? sprintf"%s%s_%s_%s",$prefix,$suffix,$station,$span : sprintf"%s_%s_%s",$prefix,$station,$span;
        open(FILE, "< $eval_file_2") or die "Can't open $eval_file_2 for reading: $!\n";
        open(INDEX, "+>$eval_file_2.idx") or die "Can't open $eval_file_2.idx for read/write: $!\n";
        build_index(*FILE, *INDEX);
        my $line = line_with_index(*FILE, *INDEX, $seeking);
        close FILE;
        close INDEX;
        chomp $line;
        my($time,$value)=split(/\s+/,$line);
        printf OUT "%.3f %.10f\n",$time,$value;
      }
      close OUT;
    }
  }

##</code><code>##


sub build_index {
    my $data_file   = shift;
    my $index_file  = shift;
    my $offset      = 0;

    while (<$data_file>) {
        print $index_file pack("N", $offset);
        $offset = tell($data_file);
    }
}


sub line_with_index {
    my $data_file   = shift;
    my $index_file  = shift;
    my $line_number = shift;

    my $size;               # size of an index entry
    my $i_offset;           # offset into the index of the entry
    my $entry;              # index entry
    my $d_offset;           # offset into the data file

    $size = length(pack("N", 0));
    $i_offset = $size * ($line_number-1);
    seek($index_file, $i_offset, 0) or return;
    read($index_file, $entry, $size);
    $d_offset = unpack("N", $entry);
    seek($data_file, $d_offset, 0);
    return scalar(<$data_file>);
}