class BookOutline

class BookOutline {
    
    /**
     * The book manager.
     *
     * @var \Drupal\book\BookManagerInterface
     */
    protected $bookManager;
    
    /**
     * Constructs a new BookOutline.
     *
     * @param \Drupal\book\BookManagerInterface $book_manager
     *   The book manager.
     */
    public function __construct(BookManagerInterface $book_manager) {
        $this->bookManager = $book_manager;
    }
    
    /**
     * Fetches the book link for the previous page of the book.
     *
     * @param array $book_link
     *   A fully loaded book link that is part of the book hierarchy.
     *
     * @return array
     *   A fully loaded book link for the page before the one represented in
     *   $book_link.
     */
    public function prevLink(array $book_link) {
        // If the parent is zero, we are at the start of a book.
        if ($book_link['pid'] == 0) {
            return NULL;
        }
        $flat = $this->bookManager
            ->bookTreeGetFlat($book_link);
        reset($flat);
        $curr = NULL;
        do {
            $prev = $curr;
            $key = key($flat);
            $curr = current($flat);
            next($flat);
        } while ($key && $key != $book_link['nid']);
        if ($key == $book_link['nid']) {
            // The previous page in the book may be a child of the previous visible link.
            if ($prev['depth'] == $book_link['depth']) {
                // The subtree will have only one link at the top level - get its data.
                $tree = $this->bookManager
                    ->bookSubtreeData($prev);
                $data = array_shift($tree);
                // The link of interest is the last child - iterate to find the deepest one.
                while ($data['below']) {
                    $data = end($data['below']);
                }
                $this->bookManager
                    ->bookLinkTranslate($data['link']);
                return $data['link'];
            }
            else {
                $this->bookManager
                    ->bookLinkTranslate($prev);
                return $prev;
            }
        }
    }
    
    /**
     * Fetches the book link for the next page of the book.
     *
     * @param array $book_link
     *   A fully loaded book link that is part of the book hierarchy.
     *
     * @return array
     *   A fully loaded book link for the page after the one represented in
     *   $book_link.
     */
    public function nextLink(array $book_link) {
        $flat = $this->bookManager
            ->bookTreeGetFlat($book_link);
        reset($flat);
        do {
            $key = key($flat);
            next($flat);
        } while ($key && $key != $book_link['nid']);
        if ($key == $book_link['nid']) {
            $next = current($flat);
            if ($next) {
                $this->bookManager
                    ->bookLinkTranslate($next);
            }
            return $next;
        }
    }
    
    /**
     * Formats the book links for the child pages of the current page.
     *
     * @param array $book_link
     *   A fully loaded book link that is part of the book hierarchy.
     *
     * @return array
     *   HTML for the links to the child pages of the current page.
     */
    public function childrenLinks(array $book_link) {
        $flat = $this->bookManager
            ->bookTreeGetFlat($book_link);
        $children = [];
        if ($book_link['has_children']) {
            // Walk through the array until we find the current page.
            do {
                $link = array_shift($flat);
            } while ($link && $link['nid'] != $book_link['nid']);
            // Continue though the array and collect the links whose parent is this page.
            while (($link = array_shift($flat)) && $link['pid'] == $book_link['nid']) {
                $data['link'] = $link;
                $data['below'] = '';
                $children[] = $data;
            }
        }
        if ($children) {
            return $this->bookManager
                ->bookTreeOutput($children);
        }
        return '';
    }

}