use strict;
use warnings;

open TXT, "<masks.txt" or die "Can't open masks.txt\n";

while (<TXT>) {
    chomp;
    my @words = split (/ +/);
    print "Checking mask $words[2]\n";
    my $maskvalid = ValidateMasks ($words[2]);
    print "Mask is $maskvalid\n\n";
}

sub ValidateMasks {
    my $mask = shift;
    
    return 0 unless defined $mask;
    my @octets = split /\./, $mask;

    print "\toctets array is @octets\n";

    return 0 unless 4 == @octets; # Bad if too few parts
    
    my $flagvalue = 0;

    for my $octet (@octets) {
        return 0 if $octet > 255;     # bad if value is above 255
        print "\tchecking octet $octet\n";
        next if ($octet eq $flagvalue);    # good if equal to flag value
        print "\t\tUnequal: oc is $octet, flagvalue is $flagvalue\n";
        
        # bad if this is a second non-zero and it's not equal to 255.
        return 0 if $flagvalue; 
        $flagvalue = 255;    # All further octets after this one must be 255

        # This is the first non-zero octet.
        # Should be equal to a power of 2 minus 1
        # X is a power of 2 if (X & X-1) = 0
        # $octet + 0 forces value to number vice string
        return 0 unless ($octet + 0) & ($octet + 1);
    }
    
    return 1;
}