#!/usr/bin/perl
# the above is the standard linux/unix path for perl: change if your computer is nonstandard

sub print_help{
    print STDERR <<EOF;

Syntax:  xyz_skip [options] xyzfile

write one out of n (default: $skip) frames of a xyz file

Options: 
	-s skipnum (skip number)
	-b frame (begin at this frame)
	-e frame (end at this frame)
							by Nick 31 Jan 2018
EOF
    return;
}

$skip=5;
$beg=0;
$end=9.e99;

while($p=shift(@ARGV)){
    if($p=~/^[^\-]/){ # all command line options start with "-", otherwise it a filename
        if(-e $p){
            push(@inputfiles,$p);
        }else{
            print STDERR "ERROR: file $p does not exist\n";
        }
        next;
    }
    if($p eq "-s"){ # skip times
	$skip=shift(@ARGV)+0;
	$skip>0 or die "Invalid number of skipped frames $skip\n";
	next;
    }
    if($p eq "-b"){ # beginning
	$beg=shift(@ARGV)+0;
	$beg>=0 or die "Invalid beginning frame frame $beg\n";
	next;
    }
    if($p eq "-e"){ # end
	$end=shift(@ARGV)+0;
	$end>=0 or die "Invalid end frame frame $end\n";
	next;
    }
    if($p eq "-h"){ # help
	print_help;
	die "\n";
    }
    print_help;
    die "invalid option $p\n";
}

if($#inputfiles<0){
    push(@inputfiles,"-");      #       adds stdio for lack of arguments                     
}

while ($filename=shift(@inputfiles)){
    open(INPUT, $filename) || die "Cannot open $filename\n";
    $count=0;	#so that the included frames are multiples of $skip
    while(<INPUT>){
	s/^\s*//;	# remove leading spaces
	@li = split('\s+');
	if($#li==0 && $li[0]+0==$li[0] & $li[0]+0>0){
	    $count++;
	}
	$c=$count-$beg;
	if($c>=0 && $c%$skip ==0 && $count<=$end){
	    print;
	}
    }
    
    close(INPUT);
}