#!/usr/bin/perl
use warnings;
use strict;

# Idea for improvement: 
# take maximum of serial from file and from all authoritative nameservers,
# then increment by one.

for my $f (@ARGV) {
    rename $f, "$f.old" || die "cannot rename $f to $f.old: $!";
    open (my $in, '<', "$f.old") or die "cannot open $f.old: $!";
    open (my $out, '>', "$f") or die "cannot open $f: $!";
    while (<$in>) {
	if (/(.*\bSOA\b.*?)(\d+)( \d+ \d+ \d+ \d+)/) {
	    my $serial = $2;
	    $serial++;
	    print $out "$1$serial$3\n";
	} else {
	    print $out $_;
	}
    }
    close($out) or die "cannot close $out";
}