hdb => sub {
  my $input = shift;
  my $length = length $$input;
  my $i = 0;
  while( $i < $length ) {                    # redundant, but to be on the safe side
      $i++;                                  # length of next candidate
      my $possible = substr $$input, 0, $i;  # next candidate
      my $j = index $$input, $possible, $i;  # try to find it to the right
      if( $j > 0 ) {                         # if found, skip ahead
          $possible = substr $$input, 0, $j; # the candidate has to be this long at least
          $i = $j;                           # store its length
      }
      # check if we have a solution already
      if( substr( $$input, $i ) eq substr($$input, 0, $length - $i) ) {
          return $possible;                  # success !
      }
  }
  # will never reach here
  return '';
},