class Link

final class Link implements CacheableDependencyInterface {
    use CacheableDependencyTrait;
    
    /**
     * The link URI.
     *
     * @var \Drupal\Core\Url
     */
    protected $uri;
    
    /**
     * The URI, as a string.
     *
     * @var string
     */
    protected $href;
    
    /**
     * The link relation type.
     *
     * @var string
     */
    protected $rel;
    
    /**
     * The link target attributes.
     *
     * @var string[]
     *   An associative array where the keys are the attribute keys and values are
     *   either string or an array of strings.
     */
    protected $attributes;
    
    /**
     * JSON:API Link constructor.
     *
     * @param \Drupal\Core\Cache\CacheableMetadata $cacheability
     *   Any cacheability metadata associated with the link. For example, a
     *   'call-to-action' link might reference a registration resource if an event
     *   has vacancies or a wait-list resource otherwise. Therefore, the link's
     *   cacheability might be depend on a certain entity's values other than the
     *   entity on which the link will appear.
     * @param \Drupal\Core\Url $url
     *   The Url object for the link.
     * @param string $link_relation_type
     *   An array of registered or extension RFC8288 link relation types.
     * @param array $target_attributes
     *   An associative array of target attributes for the link.
     *
     * @see https://tools.ietf.org/html/rfc8288#section-2.1
     */
    public function __construct(CacheableMetadata $cacheability, Url $url, string $link_relation_type, array $target_attributes = []) {
        assert(Inspector::assertAllStrings(array_keys($target_attributes)));
        assert(Inspector::assertAll(function ($target_attribute_value) {
            return is_string($target_attribute_value) || is_array($target_attribute_value);
        }, array_values($target_attributes)));
        $generated_url = $url->setAbsolute()
            ->toString(TRUE);
        $this->href = $generated_url->getGeneratedUrl();
        $this->uri = $url;
        $this->rel = $link_relation_type;
        $this->attributes = $target_attributes;
        $this->setCacheability($cacheability->addCacheableDependency($generated_url));
    }
    
    /**
     * Gets the link's URI.
     *
     * @return \Drupal\Core\Url
     *   The link's URI as a Url object.
     */
    public function getUri() {
        return $this->uri;
    }
    
    /**
     * Gets the link's URI as a string.
     *
     * @return string
     *   The link's URI as a string.
     */
    public function getHref() {
        return $this->href;
    }
    
    /**
     * Gets the link's relation type.
     *
     * @return string
     *   The link's relation type.
     */
    public function getLinkRelationType() {
        return $this->rel;
    }
    
    /**
     * Gets the link's target attributes.
     *
     * @return string[]
     *   The link's target attributes.
     */
    public function getTargetAttributes() {
        return $this->attributes;
    }
    
    /**
     * Compares two links.
     *
     * @param \Drupal\jsonapi\JsonApiResource\Link $a
     *   The first link.
     * @param \Drupal\jsonapi\JsonApiResource\Link $b
     *   The second link.
     *
     * @return int
     *   0 if the links can be considered identical, an integer greater than or
     *   less than 0 otherwise.
     */
    public static function compare(Link $a, Link $b) {
        // Any string concatenation would work, but a Link header-like format makes
        // it clear what is being compared.
        $a_string = sprintf('<%s>;rel="%s"', $a->getHref(), $a->rel);
        $b_string = sprintf('<%s>;rel="%s"', $b->getHref(), $b->rel);
        $cmp = strcmp($a_string, $b_string);
        // If the `href` or `rel` of the links are not equivalent, it's not
        // necessary to compare target attributes.
        if ($cmp === 0) {
            return (int) (!empty(DiffArray::diffAssocRecursive($a->getTargetAttributes(), $b->getTargetAttributes())));
        }
        return $cmp;
    }
    
    /**
     * Merges two equivalent links into one link with the merged cacheability.
     *
     * The links must share the same URI, link relation type and attributes.
     *
     * @param \Drupal\jsonapi\JsonApiResource\Link $a
     *   The first link.
     * @param \Drupal\jsonapi\JsonApiResource\Link $b
     *   The second link.
     *
     * @return static
     *   A new JSON:API Link object with the cacheability of both links merged.
     */
    public static function merge(Link $a, Link $b) {
        assert(static::compare($a, $b) === 0, 'Only equivalent links can be merged.');
        $merged_cacheability = (new CacheableMetadata())->addCacheableDependency($a)
            ->addCacheableDependency($b);
        return new static($merged_cacheability, $a->getUri(), $a->getLinkRelationType(), $a->getTargetAttributes());
    }

}