#!/usr/bin/perl
use Getopt::Long;

if (($#ARGV + 1) < 5){die <<EOS;}
*** $0
*** Generate ITAB file to process interferograms with either single or sequential reference SLC ***
*** Copyright 2015, Gamma Remote Sensing, v1.9 11-Apr-2015 clw ******

usage: $0 <itab> <max> <step> <stride> <offset>
  itab    (output) ITAB file with 4 columns, text format
            1: ref. SLC index from SLC_tab
            2: second SLC index of the interferometric pair
            3: ITAB file line counter
            4: Active flag (0: not included  1: included in time-series analysis)
  max     number of SLCs in the SLC_tab
  step    difference in SLC index between the reference and second SLC for each interferogram (default: 1)
  stride  increment of the index of the starting SLC, if stride is 0, then reference will not change, and the index of the second SLC will increase by step
  start    index of the first SLC used in generating the itab,  (start >= 1)

  -r ref  index of the reference SLC when stride is 0, default is the start index
  -a      do not generate itab records where the ref. SLC and second SLC have the same index
 
  NOTE: itab records refer to line numbers of the entries in the SLC_tab
    
EOS

$itab = $ARGV[0];
$max = $ARGV[1];
$step = $ARGV[2];
$stride = $ARGV[3];
$start = $ARGV[4];
$refx = $start;
$aflg = 0;

GetOptions ('r=i' => \$refx, 'a' => \$aflg);
($refx >= 1 && $refx <= $max) or  die "\nERROR $0: reference index must be between 1 and $max: $refx\n";

open(ITAB, ">$itab") or die "ERROR $0: cannot create itab file: $itab\n\n";
print "ITAB file: $ARGV[0]\n"; 

$step =~ /\d+$/ && $step > 0 or die "ERROR $0: invalid step: $step\n"; 
$max =~ /\d+$/ && $max >= $step or die "ERROR $0: invalid maximum SLC number: $max\n";
$stride =~ /\d+$/ && $stride >= 0 or die "ERROR $0: invalid stride: $stride\n";
$start >= 1 or die "ERROR $0: starting SLC index must be >= 1\n";

$ic = 1;
if ($stride == 0) {
  print "reference SLC index: $refx\n";
  for ($n=0;  $n <= $max - $start; $n += $step) {
    next if ($aflg and $refx eq $start + $n);
    printf "%4d %4d %4d %4d\n",  $refx, $start + $n, $ic, 1;
    printf ITAB "%4d %4d %4d %4d\n",  $refx, $start + $n, $ic, 1;
    $ic++;
  }
}
else {
  for ($n= 0; $n <= ($max - $start - 1); $n += $stride) {
    if (($n + $start + $step) <= $max) {
      next if ($aflg and  $n + $start eq $n + $step + $start);
      printf "%4d %4d %4d %4d\n", $n + $start, $n + $step + $start, $ic, 1;
      printf ITAB "%4d %4d %4d %4d\n", $n + $start, $n + $start + $step, $ic, 1;
      $ic++;
    }
  }
}  
exit 0;