+<?php namespace BookStack\Entities\Tools;
+
+use BookStack\Entities\Models\BookChild;
+use BookStack\Entities\Models\Entity;
+use Illuminate\Support\Collection;
+
+/**
+ * Finds the next or previous content of a book element (page or chapter).
+ */
+class NextPreviousContentLocator
+{
+ protected $relativeBookItem;
+ protected $flatTree;
+ protected $currentIndex = null;
+
+ /**
+ * NextPreviousContentLocator constructor.
+ */
+ public function __construct(BookChild $relativeBookItem, Collection $bookTree)
+ {
+ $this->relativeBookItem = $relativeBookItem;
+ $this->flatTree = $this->treeToFlatOrderedCollection($bookTree);
+ $this->currentIndex = $this->getCurrentIndex();
+ }
+
+ /**
+ * Get the next logical entity within the book hierarchy.
+ */
+ public function getNext(): ?Entity
+ {
+ return $this->flatTree->get($this->currentIndex + 1);
+ }
+
+ /**
+ * Get the next logical entity within the book hierarchy.
+ */
+ public function getPrevious(): ?Entity
+ {
+ return $this->flatTree->get($this->currentIndex - 1);
+ }
+
+ /**
+ * Get the index of the current relative item.
+ */
+ protected function getCurrentIndex(): ?int
+ {
+ $index = $this->flatTree->search(function (Entity $entity) {
+ return get_class($entity) === get_class($this->relativeBookItem)
+ && $entity->id === $this->relativeBookItem->id;
+ });
+ return $index === false ? null : $index;
+ }
+
+ /**
+ * Convert a book tree collection to a flattened version
+ * where all items follow the expected order of user flow.
+ */
+ protected function treeToFlatOrderedCollection(Collection $bookTree): Collection
+ {
+ $flatOrdered = collect();
+ /** @var Entity $item */
+ foreach ($bookTree->all() as $item) {
+ $flatOrdered->push($item);
+ $childPages = $item->visible_pages ?? [];
+ $flatOrdered = $flatOrdered->concat($childPages);
+ }
+ return $flatOrdered;
+ }
+}
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