function phutil_utf8_strlen_compare($string, $operator, $value) {
  // To compute the length of a UTF8 string, we currently must vectorize it
  // with "phutil_utf8v()". This operation has complexity "O(N)" on the
  // length of the string, so it is relatively expensive if the string
  // is long.

  // If we know which comparison we're performing, we can sometimes perform
  // the comparison in "O(1)" without vectorizing the string, since extreme
  // cases are trivially true or false, as we can easily find an upper and
  // lower bound for the character length of string from the byte length.

  // A byte string of length "N" has a maximum UTF8 character length of "N"
  // (if each byte is a one-byte character, like regular ASCII text).

  // A byte string of length "N" has a minimum UTF8 character length of
  // "ceil(N / 6)".

  $max_length = strlen($string);
  $min_length = (int)ceil($max_length / 6);

  switch ($operator) {
    case '<':
      if ($max_length < $value) {
        return true;
      }
      if ($min_length >= $value) {
        return false;
      }
      break;
    default:
      throw new Exception(
        pht(
          'Unknown string comparison operator "%s"!',
          $operator));
  }

  $actual_length = phutil_utf8_strlen($string);

  // ...