class LinkCollectionNormalizer

class LinkCollectionNormalizer extends NormalizerBase {
  
  /**
   * The normalizer $context key name for the key of an individual link.
   *
   * @var string
   */
  const LINK_KEY = 'jsonapi_links_object_link_key';
  
  /**
   * The normalizer $context key name for the context object of the link.
   *
   * @var string
   */
  const LINK_CONTEXT = 'jsonapi_links_object_context';
  
  /**
   * {@inheritdoc}
   */
  protected $supportedInterfaceOrClass = LinkCollection::class;
  
  /**
   * A random string to use when hashing links.
   *
   * This string is unique per instance of a link collection, but always the
   * same within it. This means that link key hashes will be non-deterministic
   * for outside observers, but two links within the same collection will always
   * have the same hash value.
   *
   * This is not used for cryptographic purposes.
   *
   * @var string
   */
  protected $hashSalt;
  
  /**
   * {@inheritdoc}
   */
  public function normalize($object, $format = NULL, array $context = []) {
    assert($object instanceof LinkCollection);
    $normalized = [];
    /* @var \Drupal\jsonapi\JsonApiResource\Link $link */
    foreach ($object as $key => $links) {
      $is_multiple = count($links) > 1;
      foreach ($links as $link) {
        $link_key = $is_multiple ? sprintf('%s--%s', $key, $this->hashByHref($link)) : $key;
        $attributes = $link->getTargetAttributes();
        $normalization = array_merge([
          'href' => $link->getHref(),
        ], !empty($attributes) ? [
          'meta' => $attributes,
        ] : []);
        $normalized[$link_key] = new CacheableNormalization($link, $normalization);
      }
    }
    return CacheableNormalization::aggregate($normalized);
  }
  
  /**
   * Hashes a link by its href.
   *
   * @param \Drupal\jsonapi\JsonApiResource\Link $link
   *   A link to be hashed.
   *
   * @return string
   *   A 7 character alphanumeric hash.
   */
  protected function hashByHref(Link $link) {
    if (!$this->hashSalt) {
      $this->hashSalt = Crypt::randomBytesBase64();
    }
    return substr(str_replace([
      '-',
      '_',
    ], '', Crypt::hashBase64($this->hashSalt . $link->getHref())), 0, 7);
  }

}