#!/usr/local/bin/perl -w
$plotymax = 1E-10;
$plotymin = 1E-05; 
$temp = $$;
$GNU = "/usr/local/gnu/bin/gnuplot";
$gps2_log = 'gps.log';

$start_day   =  01;
$start_month = 'Mar';
$end_day     = 11;
$end_month   = 'Apr';

%months = qw{Jan 01 Feb 02 Mar 03 Apr 04 May 05 Jun 06
	     Jul 07 Aug 08 Sep 09 Oct 10 Nov 11 Dec 12};  

$sdate_cursor = [$months{$start_month},$start_day,0,0,0];
$edate_cursor = [$months{$end_month},$end_day,23,59,59];
$date_cursor  = [0,0,0,0,0] unless(defined($date_cursor));

open(CLOCK,"$gps2_log") or die "$gps2_log not available: $!\n";
open(GPSmk4,">>/tmp/gpsmk4.dat");

while($line = <CLOCK>) {
  next if $line =~ /last/;
  ($month,$day,$time) = ( split(/\s+/,$line) )[0..2];
  ($hour,$min,$sec) = split(':',$time);

  $month = sprintf "%02d", $months{$month};
  $day   = sprintf "%02d", $day;
  
  $data3 = ( split(/\s+/,$line) )[6];
	 
  last if (compare_dates([$month,$day,$hour,$min,$sec], $edate_cursor) > 0);
  if (compare_dates([$month,$day,$hour,$min,$sec], $sdate_cursor) > 0)
    {
      if( $data3 > $plotymax ) { $plotymax = $data3; }
      if( $data3 < $plotymin ) { $plotymin = $data3; }
      print GPSmk4 "$month-$day $time $data3\n";
    }
  # update the date cursor
  $date_cursor = [$month,$day,$hour,$min,$sec];
  
} #end of while

close(CLOCK);

print "$plotymin $plotymax\n";

@files = `ls -1 /tmp/*gps*.dat`;
chomp(@files);

$plotline = join('', 'plot', join "\,", map { " \"$_\" using 1:3" } @files);

######### Make GIF #########

open(GNUPLOT,">/tmp/$temp-gps.plt");

print GNUPLOT <<EOF;
set terminal gif
set output "/tmp/$temp-gps.gif"
set parametric
set zero 1e-20
set xdata time
set timefmt "%m-%d %H:%M:%S"
set yrange [$plotymin:$plotymax]
set format x "%d/%m\\n%H:%M"
set xrange ["$months{$start_month}-$start_day 00:00:00":"$months{$end_month}-$end_day 23:59:59"]
set grid
$plotline
exit
EOF

close(GNUPLOT);

chdir("/tmp");
`$GNU "$temp-gps.plt"`;

#--------------------

sub compare_dates {
  
  my($a, $b) = @_;
  foreach (0..4)
    {
      if    ($$a[$_]>$$b[$_]) { return  1; }
      elsif ($$a[$_]<$$b[$_]) { return -1; }
    }
  return 0;
}

#--------------------


#####################

GPS.LOG sample input
Jan  1 00:29:59 pkdesk 51909 48450 1.716667E-07
Jan  1 00:34:59 pkdesk 51909 48750 1.750000E-07
Jan  1 00:39:59 pkdesk 51909 49050 1.733333E-07
Jan  1 00:44:59 pkdesk 51909 49350 1.550000E-07
Jan  1 00:49:59 pkdesk 51909 49650 1.796667E-07
Jan  1 00:54:59 pkdesk 51909 49950 1.630000E-07
Jan  1 00:59:59 pkdesk 51909 50250 1.730000E-07
Jan  1 01:04:59 pkdesk 51909 50550 1.956667E-07
Jan  1 01:09:59 pkdesk 51909 50850 1.836667E-07
Jan  1 01:14:59 pkdesk 51909 51150 1.720000E-07
Jan  1 01:19:59 pkdesk 51909 51450 1.740000E-07


############

Sample GPSMK4.DAT output

03-01 00:04:59 8.430000E-07
03-01 00:09:59 8.433333E-07
03-01 00:14:59 8.390000E-07
03-01 00:19:59 8.463333E-07
03-01 00:24:59 8.716667E-07
03-01 00:29:59 9.013333E-07
03-01 00:34:59 8.640000E-07
03-01 00:44:59 8.336667E-07
03-01 00:49:59 8.370000E-07
03-01 00:54:59 8.383333E-07
03-01 00:59:59 8.340000E-07
03-01 01:04:59 8.306667E-07
03-01 01:09:59 8.296667E-07
03-01 01:14:59 8.416667E-07