feat: introduce helper NodeTraverser for recursive operations on nodes

src/Mapper/Tree/NodeTraverser.php

@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace CuyZ\Valinor\Mapper\Tree;
+
+/**
+ * @template T
+ */
+final class NodeTraverser
+{
+    /** @var callable(Node): T */
+    private $callback;
+
+    /**
+     * @param callable(Node): T $callback
+     */
+    public function __construct(callable $callback)
+    {
+        $this->callback = $callback;
+    }
+
+    /**
+     * @return iterable<T>
+     */
+    public function traverse(Node $node): iterable
+    {
+        return $this->recurse($node);
+    }
+
+    /**
+     * @return iterable<T>
+     */
+    private function recurse(Node $node): iterable
+    {
+        yield ($this->callback)($node);
+
+        foreach ($node->children() as $child) {
+            yield from $this->recurse($child);
+        }
+    }
+}

tests/Unit/Mapper/Tree/NodeTraverserTest.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace CuyZ\Valinor\Tests\Unit\Mapper\Tree;
+
+use CuyZ\Valinor\Mapper\Tree\Node;
+use CuyZ\Valinor\Mapper\Tree\NodeTraverser;
+use CuyZ\Valinor\Tests\Fake\Mapper\FakeNode;
+use PHPUnit\Framework\TestCase;
+
+final class NodeTraverserTest extends TestCase
+{
+    public function test_nodes_are_visited(): void
+    {
+        $node = FakeNode::branch([
+            'foo' => [],
+            'bar' => [],
+        ]);
+
+        $visited = [...(new NodeTraverser(
+            fn (Node $node) => $node
+        ))->traverse($node)];
+
+        self::assertContains($node, $visited);
+        self::assertContains($node->children()['foo'], $visited);
+        self::assertContains($node->children()['bar'], $visited);
+    }
+}