class BookBreadcrumbBuilder

class BookBreadcrumbBuilder implements BreadcrumbBuilderInterface {
    use StringTranslationTrait;
    
    /**
     * The node storage.
     *
     * @var \Drupal\Core\Entity\EntityStorageInterface
     */
    protected $nodeStorage;
    
    /**
     * The current user account.
     *
     * @var \Drupal\Core\Session\AccountInterface
     */
    protected $account;
    
    /**
     * The entity repository service.
     *
     * @var \Drupal\Core\Entity\EntityRepositoryInterface
     */
    protected $entityRepository;
    
    /**
     * The language manager service.
     *
     * @var \Drupal\Core\Language\LanguageManagerInterface|null
     */
    protected $languageManager;
    
    /**
     * Constructs the BookBreadcrumbBuilder.
     *
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager service.
     * @param \Drupal\Core\Session\AccountInterface $account
     *   The current user account.
     * @param \Drupal\Core\Entity\EntityRepositoryInterface $entity_repository
     *   The entity repository service.
     * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
     *   The language manager service.
     */
    public function __construct(EntityTypeManagerInterface $entity_type_manager, AccountInterface $account, EntityRepositoryInterface $entity_repository, LanguageManagerInterface $language_manager) {
        $this->nodeStorage = $entity_type_manager->getStorage('node');
        $this->account = $account;
        $this->entityRepository = $entity_repository;
        $this->languageManager = $language_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public function applies(RouteMatchInterface $route_match) {
        $node = $route_match->getParameter('node');
        return $node instanceof NodeInterface && !empty($node->book);
    }
    
    /**
     * {@inheritdoc}
     */
    public function build(RouteMatchInterface $route_match) {
        $book_nids = [];
        $breadcrumb = new Breadcrumb();
        $links = [
            Link::createFromRoute($this->t('Home'), '<front>', [], [
                'language' => $this->languageManager
                    ->getCurrentLanguage(LanguageInterface::TYPE_CONTENT),
            ]),
        ];
        $breadcrumb->addCacheContexts([
            'languages:' . LanguageInterface::TYPE_CONTENT,
        ]);
        $book = $route_match->getParameter('node')->book;
        $depth = 1;
        // We skip the current node.
        while (!empty($book['p' . ($depth + 1)])) {
            $book_nids[] = $book['p' . $depth];
            $depth++;
        }
        
        /** @var \Drupal\node\NodeInterface[] $parent_books */
        $parent_books = $this->nodeStorage
            ->loadMultiple($book_nids);
        $parent_books = array_map([
            $this->entityRepository,
            'getTranslationFromContext',
        ], $parent_books);
        if (count($parent_books) > 0) {
            $depth = 1;
            while (!empty($book['p' . ($depth + 1)])) {
                if (!empty($parent_books[$book['p' . $depth]]) && ($parent_book = $parent_books[$book['p' . $depth]])) {
                    $access = $parent_book->access('view', $this->account, TRUE);
                    $breadcrumb->addCacheableDependency($access);
                    if ($access->isAllowed()) {
                        $breadcrumb->addCacheableDependency($parent_book);
                        $links[] = $parent_book->toLink();
                    }
                }
                $depth++;
            }
        }
        $breadcrumb->setLinks($links);
        $breadcrumb->addCacheContexts([
            'route.book_navigation',
        ]);
        return $breadcrumb;
    }

}