#!/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_first_last [options] xyzfile(s)

write the first and last frame of a xyz file

Options:
		-f   (to select the first frame only)
		-l   (to select the last frame only)

							by Nick 12 Sept 2016
EOF
    return;
}

$select_first=1;
$select_last=1;


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 "-f"){ # first
        $select_last=0;
        next;
    }
    if($p eq "-l"){ # last 
        $select_first=0;
        next;
    }
    if($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
}

$actualframes=0;
while ($filename=shift(@inputfiles)){
    open(INPUT, $filename) || die "Cannot open $filename\n";

    while(<INPUT>){
	s/^\s*//;	# remove leading spaces
	@li = split('\s+');

	if($#li==0){
	    $actualframes++;
	    $natms = $li[0];
	    $commentline=<INPUT>;
	    @lines=();
	    for($i=0;$i<$natms;$i++){
		$_=<INPUT>;
		push @lines,$_; # store everything
	    }
	    if($select_first==1 && $actualframes==1){
		print $natms."\n";
		print $commentline;
		for($i=0;$i<=$#lines;$i++){
		    print $lines[$i];
		}
	    }
	}
    }
    close(INPUT);
}
if($select_last==1){
    print $natms."\n";
    print $commentline;
    for($i=0;$i<=$#lines;$i++){
	print $lines[$i];
    }
}