Page Comparison

/**
 * Implements hook_page_attachments_alter().
 */
function example_page_attachments_alter(array &$attachments) {
  if (isset($attachments['#attached']['html_head_link'])) {
    $node = \Drupal::routeMatch()->getParameter('node');
    $host = \Drupal::request()->getSchemeAndHttpHost();
    if ($node instanceof NodeInterface) {
      $siblings = \Drupal::service('example.context_helpers')->getSiblings($node);
      if (!empty($siblings)) {
        $paths = \Drupal::service('example.context_helpers')->getAliases($siblings);
        if (!empty($paths)) {
          $links = [];
          $props = [];
          foreach ($paths as $i => $path) {
            $props[0]['rel'] = 'alternate';
            $props[0]['hreflang'] = $path['lang'];
            $props[0]['href'] = $host . $path['link'];
            $props[1] = TRUE;
            $links[] = $props;
          }
        }
      }
    }
    foreach ($attachments['#attached']['html_head_link'] as $key => $link) {
      if (isset($link[0]['rel']) && !empty($link[0]['rel']) && $link[0]['rel'] == 'alternate' && isset($link[0]['href'])) {
        $href = $link[0]['href'];
        $href = str_replace('https://', '', $href);
        $href = str_replace('http://', '', $href);
        $href_arr = explode('/', $href);
        if (isset($href_arr[2]) && $href_arr[2] !== 'node') {
          $attachments['#attached']['html_head_link'][$key][0]['hreflang'] = $href_arr[1] . '-' . $href_arr[2];
        }
      }
    }
    
    // Setting default one.
    $attachments['#attached']['html_head_link'][] = [['rel' => 'alternate', 'hreflang' => 'x-default', 'href' => 'https://example.com']];
  }
}

  /**
   * Helper function to get siblings of the node if sibling relationship exists.
   *
   * @param Node $node
   *    node interface.
   *
   * @return array $siblings
   *    array of node siblings ids.
   */
  public function getSiblings(Node $node) {
    $siblings = [];
    if (isset($node->field_master_node)) {
      $master = $node->field_master_node->target_id;
      if (isset($master)) {
        $siblings = $this->getChildNodes($master);
      }
    }
    return $siblings;
  }
  /**
   * Helper function to get node aliases and language / region prefix by nid.
   *
   * @param array $nids
   *    Node ids.
   *
   * @return array $aliases
   *    Array of aliases.
   */
  public function getAliases($nids) {
    $languages = \Drupal::languageManager()->getLanguages();
    if (!is_array($nids)) {
      $nids = [$nids];
    }
    $aliases = [];
    $i = 0;
    foreach ($nids as $nid) {
      foreach ($languages as $lng_code => $lang) {
        $alias = \Drupal::service('path.alias_manager')->getAliasByPath('/node/' . $nid, $lng_code);
        if (strpos($alias, '/node/') === FALSE) {
          $al_arr = explode('/', $alias);
          $alias = '/' . $lng_code . $alias;
          $aliases[$i]['link'] = $alias;
  
          $aliases[$i]['lang'] = $al_arr[1] . '-' . $lng_code;
          $i++;
        }
      }
    }
    return $aliases;
  }