trait TreeEncodeTrait in Extensible BBCode 8.3

trait TreeEncodeTrait {

  /**
   * Concatenate the top-level text of the tree.
   *
   * The text elements are concatenated, inserting placeholders for each tag
   * element contained in the children.
   *
   * @param array $children
   *   A sequence of elements representing the children of a single element.
   *
   * @return string[]
   *   An array with two values:
   *   - A unique delimiter token.
   *   - A concatenated string, in which the i-th tag element is replaced with
   *     a placeholder string "{token:i}", and text elements are left in place.
   */
  protected static function encodeTree(array $children) : array {
    $output = [];
    foreach ($children as $i => $child) {
      if ($child instanceof TextElement) {
        $output[] = $child
          ->getText();
      }
      else {
        $output[] = $i;
      }
    }
    $text = implode('', $output);
    $token = 100000;
    while (strpos($text, (string) $token) !== FALSE) {
      $token++;
    }
    foreach ($output as $i => $item) {
      if (is_int($item)) {
        $output[$i] = "{tag:{$token}:{$item}}";
      }
    }
    return [
      $token,
      implode('', $output),
    ];
  }

  /**
   * Decode a part of the encoded tree.
   *
   * @param string $cell
   *   The text (or part of the text) of the encoded tree.
   * @param array $children
   *   The children which were previously encoded.
   * @param string $token
   *   The token used as a placeholder.
   *
   * @return \Drupal\xbbcode\Parser\Tree\TagElement
   *   A pseudo-tag element (empty name) containing the part of the tree
   *   represented by $cell.
   */
  protected static function decodeTree($cell, array $children, $token) : TagElement {
    $items = preg_split("/{tag:{$token}:(\\d+)}/", $cell, NULL, PREG_SPLIT_DELIM_CAPTURE);
    $tree = new TagElement('', '', '');
    foreach ($items as $i => $item) {
      if ($item !== '') {
        $tree
          ->append($i % 2 ? $children[$item] : new TextElement($item));
      }
    }
    return $tree;
  }

}