class VersionNegotiator

class VersionNegotiator {
    
    /**
     * The separator between the version negotiator name and the version argument.
     *
     * @var string
     */
    const SEPARATOR = ':';
    
    /**
     * An array of named version negotiators.
     *
     * @var \Drupal\jsonapi\Revisions\VersionNegotiatorInterface[]
     */
    protected $negotiators = [];
    
    /**
     * Adds a version negotiator.
     *
     * @param \Drupal\jsonapi\Revisions\VersionNegotiatorInterface $version_negotiator
     *   The version negotiator.
     * @param string $negotiator_name
     *   The name of the negotiation strategy used by the version negotiator.
     */
    public function addVersionNegotiator(VersionNegotiatorInterface $version_negotiator, $negotiator_name) {
        assert(strpos(get_class($version_negotiator), 'Drupal\\jsonapi\\') === 0, 'Version negotiators are not a public API.');
        $this->negotiators[$negotiator_name] = $version_negotiator;
    }
    
    /**
     * Gets a negotiated entity revision.
     *
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity.
     * @param string $resource_version_identifier
     *   A value used to derive a revision for the given entity.
     *
     * @return \Drupal\Core\Entity\EntityInterface
     *   The loaded revision.
     *
     * @throws \Drupal\Core\Http\Exception\CacheableNotFoundHttpException
     *   When the revision does not exist.
     * @throws \Drupal\Core\Http\Exception\CacheableBadRequestHttpException
     *   When the revision ID cannot be negotiated.
     */
    public function getRevision(EntityInterface $entity, $resource_version_identifier) {
        try {
            list($version_negotiator_name, $version_argument) = explode(VersionNegotiator::SEPARATOR, $resource_version_identifier, 2);
            if (!isset($this->negotiators[$version_negotiator_name])) {
                static::throwBadRequestHttpException($resource_version_identifier);
            }
            return $this->negotiators[$version_negotiator_name]
                ->getRevision($entity, $version_argument);
        } catch (VersionNotFoundException $exception) {
            static::throwNotFoundHttpException($entity, $resource_version_identifier);
        } catch (InvalidVersionIdentifierException $exception) {
            static::throwBadRequestHttpException($resource_version_identifier);
        }
    }
    
    /**
     * Throws a cacheable error exception.
     *
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity for which a revision was requested.
     * @param string $resource_version_identifier
     *   The user input for the revision negotiation.
     *
     * @throws \Drupal\Core\Http\Exception\CacheableNotFoundHttpException
     */
    protected static function throwNotFoundHttpException(EntityInterface $entity, $resource_version_identifier) {
        $cacheability = CacheableMetadata::createFromObject($entity)->addCacheContexts([
            'url.path',
            'url.query_args:' . ResourceVersionRouteEnhancer::RESOURCE_VERSION_QUERY_PARAMETER,
        ]);
        $reason = sprintf('The requested version, identified by `%s`, could not be found.', $resource_version_identifier);
        throw new CacheableNotFoundHttpException($cacheability, $reason);
    }
    
    /**
     * Throws a cacheable error exception.
     *
     * @param string $resource_version_identifier
     *   The user input for the revision negotiation.
     *
     * @throws \Drupal\Core\Http\Exception\CacheableBadRequestHttpException
     */
    protected static function throwBadRequestHttpException($resource_version_identifier) {
        $cacheability = (new CacheableMetadata())->addCacheContexts([
            'url.query_args:' . ResourceVersionRouteEnhancer::RESOURCE_VERSION_QUERY_PARAMETER,
        ]);
        $message = sprintf('An invalid resource version identifier, `%s`, was provided.', $resource_version_identifier);
        throw new CacheableBadRequestHttpException($cacheability, $message);
    }

}