Add an Auto Balanced Node.

spec/drupol/phptree/Node/ABNodeSpec.php

@@ -0,0 +1,69 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace spec\drupol\phptree\Node;
+
+use drupol\phptree\Node\ABNode;
+
+class ABNodeSpec extends NodeObjectBehavior
+{
+    public function it_balance_a_tree()
+    {
+        $this->beConstructedWith(1);
+
+        $data = \range(0, 40);
+
+        $nodes = [];
+        foreach ($data as $key => $value) {
+            $nodes[$key] = new ABNode(2);
+        }
+
+        $this
+            ->add(...$nodes);
+
+        $this->offsetGet(0)->count()->shouldReturn(40);
+        $this
+            ->offsetGet(0)
+            ->offsetGet(0)
+            ->count()
+            ->shouldReturn(19);
+        $this
+            ->offsetGet(0)
+            ->offsetGet(1)
+            ->count()
+            ->shouldReturn(19);
+
+        $this
+            ->add(new ABNode(2));
+
+        $this
+            ->offsetGet(0)
+            ->offsetGet(0)
+            ->count()
+            ->shouldReturn(19);
+        $this
+            ->offsetGet(0)
+            ->offsetGet(1)
+            ->count()
+            ->shouldReturn(20);
+
+        $this
+            ->add(new ABNode(2));
+
+        $this
+            ->offsetGet(0)
+            ->offsetGet(0)
+            ->count()
+            ->shouldReturn(20);
+        $this
+            ->offsetGet(0)
+            ->offsetGet(1)
+            ->count()
+            ->shouldReturn(20);
+    }
+    public function it_is_initializable()
+    {
+        $this->shouldHaveType(ABNode::class);
+    }
+}

src/Node/ABNode.php

@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace drupol\phptree\Node;
+
+/**
+ * Class ABNode.
+ *
+ * An auto-balanced node.
+ */
+class ABNode extends NaryNode
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function add(NodeInterface ...$nodes): NodeInterface
+    {
+        foreach ($nodes as $node) {
+            if (0 === $this->count()) {
+                parent::add($node);
+
+                continue;
+            }
+
+            $count = [];
+
+            foreach ($this->children() as $child) {
+                $count[$child->count()] = $child;
+            }
+
+            $keys = \array_keys($count);
+            $keys[] = 0;
+
+            if (\min($keys) === \max($keys)) {
+                parent::add($node);
+
+                continue;
+            }
+
+            \ksort($count);
+
+            $child = \array_shift($count);
+            $child->add($node);
+        }
+
+        return $this;
+    }
+}