class Data

abstract class Data implements \IteratorAggregate, \Countable {
    
    /**
     * Various representations of JSON:API objects.
     *
     * @var \Drupal\jsonapi\JsonApiResource\ResourceIdentifierInterface[]
     */
    protected $data;
    
    /**
     * The number of resources permitted in this collection.
     *
     * @var int
     */
    protected $cardinality;
    
    /**
     * Holds a boolean indicating if there is a next page.
     *
     * @var bool
     */
    protected $hasNextPage;
    
    /**
     * Holds the total count of entities.
     *
     * @var int
     */
    protected $count;
    
    /**
     * Instantiates a Data object.
     *
     * @param \Drupal\jsonapi\JsonApiResource\ResourceIdentifierInterface[] $data
     *   The resources or resource identifiers for the collection.
     * @param int $cardinality
     *   The number of resources that this collection may contain. Related
     *   resource collections may handle both to-one or to-many relationships. A
     *   to-one relationship should have a cardinality of 1. Use -1 for unlimited
     *   cardinality.
     */
    public function __construct(array $data, $cardinality = -1) {
        assert(Inspector::assertAllObjects($data, ResourceIdentifierInterface::class));
        assert($cardinality >= -1 && $cardinality !== 0, 'Cardinality must be -1 for unlimited cardinality or a positive integer.');
        assert($cardinality === -1 || count($data) <= $cardinality, 'If cardinality is not unlimited, the number of given resources must not exceed the cardinality of the collection.');
        $this->data = array_values($data);
        $this->cardinality = $cardinality;
    }
    
    /**
     * Returns an iterator for entities.
     *
     * @return \ArrayIterator
     *   An \ArrayIterator instance
     */
    public function getIterator() : \ArrayIterator {
        return new \ArrayIterator($this->data);
    }
    
    /**
     * Returns the number of entities.
     *
     * @return int
     *   The number of parameters
     */
    public function count() : int {
        return count($this->data);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getTotalCount() {
        return $this->count;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setTotalCount($count) {
        $this->count = $count;
    }
    
    /**
     * Returns the collection as an array.
     *
     * @return \Drupal\Core\Entity\EntityInterface[]
     *   The array of entities.
     */
    public function toArray() {
        return $this->data;
    }
    
    /**
     * Checks if there is a next page in the collection.
     *
     * @return bool
     *   TRUE if the collection has a next page.
     */
    public function hasNextPage() {
        return (bool) $this->hasNextPage;
    }
    
    /**
     * Sets the has next page flag.
     *
     * Once the collection query has been executed and we build the entity
     * collection, we now if there will be a next page with extra entities.
     *
     * @param bool $has_next_page
     *   TRUE if the collection has a next page.
     */
    public function setHasNextPage($has_next_page) {
        $this->hasNextPage = (bool) $has_next_page;
    }
    
    /**
     * Gets the cardinality of this collection.
     *
     * @return int
     *   The cardinality of the resource collection. -1 for unlimited cardinality.
     */
    public function getCardinality() {
        return $this->cardinality;
    }
    
    /**
     * Returns a new Data object containing the entities of $this and $other.
     *
     * @param \Drupal\jsonapi\JsonApiResource\Data $a
     *   A Data object to be merged.
     * @param \Drupal\jsonapi\JsonApiResource\Data $b
     *   A Data object to be merged.
     *
     * @return static
     *   A new merged Data object.
     */
    public static function merge(Data $a, Data $b) {
        return new static(array_merge($a->toArray(), $b->toArray()));
    }
    
    /**
     * Returns a new, deduplicated Data object.
     *
     * @param \Drupal\jsonapi\JsonApiResource\Data $collection
     *   The Data object to deduplicate.
     *
     * @return static
     *   A new merged Data object.
     */
    public static function deduplicate(Data $collection) {
        $deduplicated = [];
        foreach ($collection as $resource) {
            $dedupe_key = $resource->getTypeName() . ':' . $resource->getId();
            if ($resource instanceof EntityAccessDeniedHttpException && ($error = $resource->getError()) && !is_null($error['relationship_field'])) {
                $dedupe_key .= ':' . $error['relationship_field'];
            }
            $deduplicated[$dedupe_key] = $resource;
        }
        return new static(array_values($deduplicated));
    }

}