sub word_to_indicator {
        my($n, $w) = @_;
        my($p, $d) = 0;
        reverse map { 
                $p += $d = chr(ord("A") + $_) eq substr($w, $p, 1) ? 1 : 0;
                $d;
        } 0 .. $n - 1;
}

if (1) {
        # here, N = 5, K = 2.
        my @combination = qw{AB AC AD AE BC BD BE CD CE DE};
        for my $combination (@combination) {
                my @indicator = word_to_indicator(5, $combination);
                my($N, $K, $rank, $C) = perm_decode(@indicator);
                print "$combination : @indicator : $N $K $C $rank\n";
        }
}