class ResourceIdentifier

class ResourceIdentifier implements ResourceIdentifierInterface {
    const ARITY_KEY = 'arity';
    
    /**
     * The JSON:API resource type name.
     *
     * @var string
     */
    protected $resourceTypeName;
    
    /**
     * The JSON:API resource type.
     *
     * @var \Drupal\jsonapi\ResourceType\ResourceType
     */
    protected $resourceType;
    
    /**
     * The resource ID.
     *
     * @var string
     */
    protected $id;
    
    /**
     * The relationship's metadata.
     *
     * @var array
     */
    protected $meta;
    
    /**
     * ResourceIdentifier constructor.
     *
     * @param \Drupal\jsonapi\ResourceType\ResourceType|string $resource_type
     *   The JSON:API resource type or a JSON:API resource type name.
     * @param string $id
     *   The resource ID.
     * @param array $meta
     *   Any metadata for the ResourceIdentifier.
     */
    public function __construct($resource_type, $id, array $meta = []) {
        assert(is_string($resource_type) || $resource_type instanceof ResourceType);
        assert(!isset($meta[static::ARITY_KEY]) || is_int($meta[static::ARITY_KEY]) && $meta[static::ARITY_KEY] >= 0);
        $this->resourceTypeName = is_string($resource_type) ? $resource_type : $resource_type->getTypeName();
        $this->id = $id;
        $this->meta = $meta;
        if (!is_string($resource_type)) {
            $this->resourceType = $resource_type;
        }
    }
    
    /**
     * Gets the ResourceIdentifier's JSON:API resource type name.
     *
     * @return string
     *   The JSON:API resource type name.
     */
    public function getTypeName() {
        return $this->resourceTypeName;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getResourceType() {
        if (!isset($this->resourceType)) {
            
            /** @var \Drupal\jsonapi\ResourceType\ResourceTypeRepositoryInterface $resource_type_repository */
            $resource_type_repository = \Drupal::service('jsonapi.resource_type.repository');
            $this->resourceType = $resource_type_repository->getByTypeName($this->getTypeName());
        }
        return $this->resourceType;
    }
    
    /**
     * Gets the ResourceIdentifier's ID.
     *
     * @return string
     *   The ID.
     */
    public function getId() {
        return $this->id;
    }
    
    /**
     * Whether this ResourceIdentifier has an arity.
     *
     * @return int
     *   TRUE if the ResourceIdentifier has an arity, FALSE otherwise.
     */
    public function hasArity() {
        return isset($this->meta[static::ARITY_KEY]);
    }
    
    /**
     * Gets the ResourceIdentifier's arity.
     *
     * One must check self::hasArity() before calling this method.
     *
     * @return int
     *   The arity.
     */
    public function getArity() {
        assert($this->hasArity());
        return $this->meta[static::ARITY_KEY];
    }
    
    /**
     * Returns a copy of the given ResourceIdentifier with the given arity.
     *
     * @param int $arity
     *   The new arity; must be a non-negative integer.
     *
     * @return static
     *   A newly created ResourceIdentifier with the given arity, otherwise
     *   the same.
     */
    public function withArity($arity) {
        return new static($this->getResourceType(), $this->getId(), [
            static::ARITY_KEY => $arity,
        ] + $this->getMeta());
    }
    
    /**
     * Gets the resource identifier objects metadata.
     *
     * @return array
     *   The metadata.
     */
    public function getMeta() {
        return $this->meta;
    }
    
    /**
     * Determines if two ResourceIdentifiers are the same.
     *
     * This method does not consider parallel relationships with different arity
     * values to be duplicates. For that, use the isParallel() method.
     *
     * @param \Drupal\jsonapi\JsonApiResource\ResourceIdentifier $a
     *   The first ResourceIdentifier object.
     * @param \Drupal\jsonapi\JsonApiResource\ResourceIdentifier $b
     *   The second ResourceIdentifier object.
     *
     * @return bool
     *   TRUE if both relationships reference the same resource and do not have
     *   two distinct arity's, FALSE otherwise.
     *
     *   For example, if $a and $b both reference the same resource identifier,
     *   they can only be distinct if they *both* have an arity and those values
     *   are not the same. If $a or $b does not have an arity, they will be
     *   considered duplicates.
     */
    public static function isDuplicate(ResourceIdentifier $a, ResourceIdentifier $b) {
        return static::compare($a, $b) === 0;
    }
    
    /**
     * Determines if two ResourceIdentifiers identify the same resource object.
     *
     * This method does not consider arity.
     *
     * @param \Drupal\jsonapi\JsonApiResource\ResourceIdentifier $a
     *   The first ResourceIdentifier object.
     * @param \Drupal\jsonapi\JsonApiResource\ResourceIdentifier $b
     *   The second ResourceIdentifier object.
     *
     * @return bool
     *   TRUE if both relationships reference the same resource, even when they
     *   have differing arity values, FALSE otherwise.
     */
    public static function isParallel(ResourceIdentifier $a, ResourceIdentifier $b) {
        return static::compare($a->withArity(0), $b->withArity(0)) === 0;
    }
    
    /**
     * Compares ResourceIdentifier objects.
     *
     * @param \Drupal\jsonapi\JsonApiResource\ResourceIdentifier $a
     *   The first ResourceIdentifier object.
     * @param \Drupal\jsonapi\JsonApiResource\ResourceIdentifier $b
     *   The second ResourceIdentifier object.
     *
     * @return int
     *   Returns 0 if $a and $b are duplicate ResourceIdentifiers. If $a and $b
     *   identify the same resource but have distinct arity values, then the
     *   return value will be arity $a minus arity $b. -1 otherwise.
     */
    public static function compare(ResourceIdentifier $a, ResourceIdentifier $b) {
        $result = strcmp(sprintf('%s:%s', $a->getTypeName(), $a->getId()), sprintf('%s:%s', $b->getTypeName(), $b->getId()));
        // If type and ID do not match, return their ordering.
        if ($result !== 0) {
            return $result;
        }
        // If both $a and $b have an arity, then return the order by arity.
        // Otherwise, they are considered equal.
        return $a->hasArity() && $b->hasArity() ? $a->getArity() - $b->getArity() : 0;
    }
    
    /**
     * Deduplicates an array of ResourceIdentifier objects.
     *
     * @param \Drupal\jsonapi\JsonApiResource\ResourceIdentifier[] $resource_identifiers
     *   The list of ResourceIdentifiers to deduplicate.
     *
     * @return \Drupal\jsonapi\JsonApiResource\ResourceIdentifier[]
     *   A deduplicated array of ResourceIdentifier objects.
     *
     * @see self::isDuplicate()
     */
    public static function deduplicate(array $resource_identifiers) {
        return array_reduce(array_slice($resource_identifiers, 1), function ($deduplicated, $current) {
            assert($current instanceof static);
            return array_merge($deduplicated, array_reduce($deduplicated, function ($duplicate, $previous) use ($current) {
                return $duplicate ?: static::isDuplicate($previous, $current);
            }, FALSE) ? [] : [
                $current,
            ]);
        }, array_slice($resource_identifiers, 0, 1));
    }
    
    /**
     * Determines if an array of ResourceIdentifier objects is duplicate free.
     *
     * @param \Drupal\jsonapi\JsonApiResource\ResourceIdentifier[] $resource_identifiers
     *   The list of ResourceIdentifiers to assess.
     *
     * @return bool
     *   Whether all the given resource identifiers are unique.
     */
    public static function areResourceIdentifiersUnique(array $resource_identifiers) {
        return count($resource_identifiers) === count(static::deduplicate($resource_identifiers));
    }
    
    /**
     * Creates a ResourceIdentifier object.
     *
     * @param \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem $item
     *   The entity reference field item from which to create the relationship.
     * @param int $arity
     *   (optional) The arity of the relationship.
     *
     * @return self
     *   A new ResourceIdentifier object.
     */
    public static function toResourceIdentifier(EntityReferenceItem $item, $arity = NULL) {
        $property_name = static::getDataReferencePropertyName($item);
        $target = $item->get($property_name)
            ->getValue();
        if ($target === NULL) {
            return static::getVirtualOrMissingResourceIdentifier($item);
        }
        assert($target instanceof EntityInterface);
        
        /** @var \Drupal\jsonapi\ResourceType\ResourceTypeRepositoryInterface $resource_type_repository */
        $resource_type_repository = \Drupal::service('jsonapi.resource_type.repository');
        $resource_type = $resource_type_repository->get($target->getEntityTypeId(), $target->bundle());
        // Remove unwanted properties from the meta value, usually 'entity'
        // and 'target_id'.
        $properties = TypedDataInternalPropertiesHelper::getNonInternalProperties($item);
        $main_property_name = $item->getDataDefinition()
            ->getMainPropertyName();
        $meta = array_diff_key($properties, array_flip([
            $property_name,
            $main_property_name,
        ]));
        if (!is_null($arity)) {
            $meta[static::ARITY_KEY] = $arity;
        }
        $meta["drupal_internal__{$main_property_name}"] = $properties[$main_property_name];
        return new static($resource_type, $target->uuid(), $meta);
    }
    
    /**
     * Creates an array of ResourceIdentifier objects.
     *
     * @param \Drupal\Core\Field\EntityReferenceFieldItemListInterface $items
     *   The entity reference field items from which to create the relationship
     *   array.
     *
     * @return self[]
     *   An array of new ResourceIdentifier objects with appropriate arity values.
     */
    public static function toResourceIdentifiers(EntityReferenceFieldItemListInterface $items) {
        $relationships = [];
        foreach ($items->filterEmptyItems() as $item) {
            // Create a ResourceIdentifier from the field item. This will make it
            // comparable with all previous field items. Here, it is assumed that the
            // resource identifier is unique so it has no arity. If a parallel
            // relationship is encountered, it will be assigned later.
            $relationship = static::toResourceIdentifier($item);
            if ($relationship->getResourceType()
                ->isInternal()) {
                continue;
            }
            // Now, iterate over the previously seen resource identifiers in reverse
            // order. Reverse order is important so that when a parallel relationship
            // is encountered, it will have the highest arity value so the current
            // relationship's arity value can simply be incremented by one.
            
            /** @var \Drupal\jsonapi\JsonApiResource\ResourceIdentifier $existing */
            foreach (array_reverse($relationships, TRUE) as $index => $existing) {
                $is_parallel = static::isParallel($existing, $relationship);
                if ($is_parallel) {
                    // A parallel relationship has been found. If the previous
                    // relationship does not have an arity, it must now be assigned an
                    // arity of 0.
                    if (!$existing->hasArity()) {
                        $relationships[$index] = $existing->withArity(0);
                    }
                    // Since the new ResourceIdentifier is parallel, it must have an arity
                    // assigned to it that is the arity of the last parallel
                    // relationship's arity + 1.
                    $relationship = $relationship->withArity($relationships[$index]->getArity() + 1);
                    break;
                }
            }
            // Finally, append the relationship to the list of ResourceIdentifiers.
            $relationships[] = $relationship;
        }
        return $relationships;
    }
    
    /**
     * Creates an array of ResourceIdentifier objects with arity on every value.
     *
     * @param \Drupal\Core\Field\EntityReferenceFieldItemListInterface $items
     *   The entity reference field items from which to create the relationship
     *   array.
     *
     * @return self[]
     *   An array of new ResourceIdentifier objects with appropriate arity values.
     *   Unlike self::toResourceIdentifiers(), this method does not omit arity
     *   when an identifier is not parallel to any other identifier.
     */
    public static function toResourceIdentifiersWithArityRequired(EntityReferenceFieldItemListInterface $items) {
        return array_map(function (ResourceIdentifier $identifier) {
            return $identifier->hasArity() ? $identifier : $identifier->withArity(0);
        }, static::toResourceIdentifiers($items));
    }
    
    /**
     * Creates a ResourceIdentifier object.
     *
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity from which to create the resource identifier.
     *
     * @return self
     *   A new ResourceIdentifier object.
     */
    public static function fromEntity(EntityInterface $entity) {
        
        /** @var \Drupal\jsonapi\ResourceType\ResourceTypeRepositoryInterface $resource_type_repository */
        $resource_type_repository = \Drupal::service('jsonapi.resource_type.repository');
        $resource_type = $resource_type_repository->get($entity->getEntityTypeId(), $entity->bundle());
        return new static($resource_type, $entity->uuid());
    }
    
    /**
     * Helper method to determine which field item property contains an entity.
     *
     * @param \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem $item
     *   The entity reference item for which to determine the entity property
     *   name.
     *
     * @return string
     *   The property name which has an entity as its value.
     */
    protected static function getDataReferencePropertyName(EntityReferenceItem $item) {
        foreach ($item->getDataDefinition()
            ->getPropertyDefinitions() as $property_name => $property_definition) {
            if ($property_definition instanceof DataReferenceDefinitionInterface) {
                return $property_name;
            }
        }
    }
    
    /**
     * Creates a ResourceIdentifier for a NULL or FALSE entity reference item.
     *
     * @param \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem $item
     *   The entity reference field item.
     *
     * @return self
     *   A new ResourceIdentifier object.
     */
    protected static function getVirtualOrMissingResourceIdentifier(EntityReferenceItem $item) {
        $resource_type_repository = \Drupal::service('jsonapi.resource_type.repository');
        $property_name = static::getDataReferencePropertyName($item);
        $value = $item->get($property_name)
            ->getValue();
        assert($value === NULL);
        $field = $item->getParent();
        assert($field instanceof EntityReferenceFieldItemListInterface);
        $host_entity = $field->getEntity();
        assert($host_entity instanceof EntityInterface);
        $resource_type = $resource_type_repository->get($host_entity->getEntityTypeId(), $host_entity->bundle());
        assert($resource_type instanceof ResourceType);
        $relatable_resource_types = $resource_type->getRelatableResourceTypesByField($resource_type->getPublicName($field->getName()));
        assert(!empty($relatable_resource_types));
        $get_metadata = function ($type) {
            return [
                'links' => [
                    'help' => [
                        'href' => "https://www.drupal.org/docs/8/modules/json-api/core-concepts#{$type}",
                        'meta' => [
                            'about' => "Usage and meaning of the '{$type}' resource identifier.",
                        ],
                    ],
                ],
            ];
        };
        $resource_type = reset($relatable_resource_types);
        // A non-empty entity reference field that refers to a non-existent entity
        // is not a data integrity problem. For example, Term entities' "parent"
        // entity reference field uses target_id zero to refer to the non-existent
        // "<root>" term. And references to entities that no longer exist are not
        // cleaned up by Drupal; hence we map it to a "missing" resource.
        if ($field->getFieldDefinition()
            ->getSetting('target_type') === 'taxonomy_term' && $item->get('target_id')
            ->getCastedValue() === 0) {
            if (count($relatable_resource_types) !== 1) {
                throw new \RuntimeException('Relationships to virtual resources are possible only if a single resource type is relatable.');
            }
            return new static($resource_type, 'virtual', $get_metadata('virtual'));
        }
        else {
            // In case of a dangling reference, it is impossible to determine which
            // resource type it used to reference, because that requires knowing the
            // referenced bundle, which Drupal does not store.
            // If we can reliably determine the resource type of the dangling
            // reference, use it; otherwise conjure a fake resource type out of thin
            // air, one that indicates we don't know the bundle.
            $resource_type = count($relatable_resource_types) > 1 ? new ResourceType('?', '?', '') : reset($relatable_resource_types);
            return new static($resource_type, 'missing', $get_metadata('missing'));
        }
    }

}