sub common {
  my ($a, $b) = @_;
  my $min = 0;
  my $max = length $a < length $b ? length $a : length $b;
  
  while ($min < $max) {
    my $h = ($max - $min + 1) >> 1;
    my $c = substr($a, $min, $h);
    my $d = substr($b, $min, $h);
    if ($c eq $d) {
      $min += $h;
    }
    else {
      $max = $min + $h - 1;
    }
  }

  return (substr($a, 0, $min),
          substr($a, $min),
          substr($b, $min));
}