use strict;

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() {
    return 0 unless defined $_[0];
    my @octets = split (/\./, $_[0]);
    print "\toctets array is @octets\n";
    my $valid = 1;
    my $index = 0;
    my $flagvalue = 0;

    while($index < 4) {
        $valid = ValidateOctet($octets[$index]);
        $index++;
    }
    return $valid;

    sub ValidateOctet() {
        return 0 unless defined $_[0];  # bad if no argument passed
        return 0 unless $valid;  # return bad if any previous argument is bad
        my $oc = $_[0];

        return 0 if $oc > 255;  # bad if value is above 255
        print "\tchecking octet $oc\n";
        return 1 if ($oc eq $flagvalue);  # good if equal to flag value 
            print "\t\tUnequal: oc is $oc, flagvalue is $flagvalue\n";
        return 0 if $flagvalue;  # bad if this is a second non-zero and it's not equal to 255.
        $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
        # $oc + 0 forces value to number vice string
        my $bitand = ($oc + 0) & ($oc + 1);
        return ( $bitand ? 0 : 1);
    }
}