Refactored tree flattening

Author: lazychaser

README.markdown

@@ -417,6 +417,16 @@ This will output something like this:
 - Another root
 ```
 
+##### Building flat tree
+
+Also, you can build a flat tree: a list of nodes where child nodes are immediately
+after parent node. This is helpful when you get nodes with custom order
+(i.e. alphabetically) and don't want to use recursion to iterate over your nodes.
+
+```php
+$nodes = Category::get()->toFlatTree();
+```
+
 ##### Getting a subtree
 
 Sometimes you don't need whole tree to be loaded and just some subtree of specific node.

src/Collection.php

@@ -77,7 +77,7 @@ public function toTree($root = false)
      *
      * @return int
      */
-    protected function getRootNodeId($root)
+    protected function getRootNodeId($root = false)
     {
         if (NestedSet::isNode($root)) {
             return $root->getKey();
@@ -106,44 +106,38 @@ protected function getRootNodeId($root)
      * Build a list of nodes that retain the order that they were pulled from
      * the database.
      *
-     * @return Collection|static
+     * @param bool $root
+     *
+     * @return static
      */
-    public function toFlattenedTree()
+    public function toFlatTree($root = false)
     {
-        $tree = $this->toTree();
+        $result = new static;
 
-        return $tree->flattenTree();
-    }
+        if ($this->isEmpty()) return $result;
 
-    /**
-     * Flatten a tree into a non recursive array
-     */
-    public function flattenTree()
-    {
-        $items = [];
-
-        foreach ($this->items as $node) {
-            $items = array_merge($items, $this->flattenNode($node));
-        }
+        $groupedNodes = $this->groupBy($this->first()->getParentIdName());
 
-        return new static($items);
+        return $result->flattenTree($groupedNodes, $this->getRootNodeId($root));
     }
 
     /**
-     * Flatten a single node
+     * Flatten a tree into a non recursive array.
+     *
+     * @param Collection $groupedNodes
+     * @param mixed $parentId
      *
-     * @param $node
-     * @return array
+     * @return $this
      */
-    protected function flattenNode($node)
+    protected function flattenTree(self $groupedNodes, $parentId)
     {
-        $items = [];
-        $items[] = $node;
+        foreach ($groupedNodes->get($parentId, []) as $node) {
+            $this->push($node);
 
-        foreach ($node->children as $childNode) {
-            $items = array_merge($items, $this->flattenNode($childNode));
+            $this->flattenTree($groupedNodes, $node->getKey());
         }
 
-        return $items;
+        return $this;
     }
+
 }

tests/NodeTest.php

@@ -806,6 +806,16 @@ public function testRebuildFailsWithInvalidPK()
     {
         Category::rebuildTree([ [ 'id' => 24 ] ]);
     }
+
+    public function testFlatTree()
+    {
+        $node = $this->findCategory('mobile');
+        $tree = $node->descendants()->orderBy('name')->get()->toFlatTree();
+
+        $this->assertCount(5, $tree);
+        $this->assertEquals('samsung', $tree[2]->name);
+        $this->assertEquals('galaxy', $tree[3]->name);
+    }
 }
 
 function all($items)