// See T13608. We identify the protocol after matching a link to give
// this pattern a concrete sequence to search for. If we match a
// variable-length protocol here with a pattern like "\w{3,}://", the regex
// engine may take a very long time to match against long strings.

// We also can't use "preg_replace_callback()" with "PREG_OFFSET_CAPTURE"
// because the function does not support flags until PHP 7.4.

$pattern = '(://[^\s'.PhutilRemarkupBlockStorage::MAGIC_BYTE.']+)';

// For each match, look backward from the match position to extend the
// matching sequence so it includes the protocol specification before
// the "://" in the URI, if one exists.

// Minimum required length for a valid protocol.
$min_protocol_length = 3;
// Maximum length for a valid protocol.
$max_protocol_length = 64;

// Characters which can never appear in a protocol, and stop the search.
static $nonprotocol = array(
  ' ' => true,
  "\n" => true,
  "\t" => true,
  ':' => true,
  '/' => true,
);

$offset = 0;
while (true) {
  $match = phutil_preg_match($pattern, $text, PREG_OFFSET_CAPTURE, $offset);
  if (!$match) {
    break;
  }

  $match_text = $match[0][0];
  $match_offset = $match[0][1];

  // Only search backward to the beginning of the string or the maximum
  // protocol length, whichever is shorter.
  $max_distance = min($match_offset, $max_protocol_length);

  for ($ii = 1; $ii <= $max_distance; $ii++) {
    $c = $text[$match_offset - $ii];
    if (isset($nonprotocol[$c])) {
      $ii--;
      break;
    }
  }

  $offset = $match_offset + strlen($match_text);

  // If we found a sequence of valid protocol characters between the
  // minimum and maximum lengths, try to mark it up as a hyperlink.
  if (($ii >= $min_protocol_length) && ($ii <= $max_distance)) {
    $old_offset = $match_offset - $ii;

    $old_text = substr($text, $old_offset, $ii).$match_text;
    $new_text = $this->markupHyperlinkUngreedy($old_text);

    if ($new_text !== $old_text) {
      $text = substr_replace(
        $text,
        $new_text,
        $old_offset,
        strlen($old_text));
      $offset += (strlen($new_text) - strlen($old_text));
    }
  }
}