my $ntext = length $text;
my $bwt2 = join('', 
    map{substr($text, ($_ - 1), 1)} 
    sort{
      my $j = -1;
      my $diff = 0;
      while( $j++<$ntext ) {
        $diff = substr( $text, ($j+$a) % $ntext, 1 ) cmp substr( $text, ($j+$b) % $ntext, 1 );
        last if $diff;
      }
      return $diff;
    } 0..$ntext-1
);
print "$text: $bwt2\n";