Add a new level parameter to the BreadthFirst visitor.

Author: drupol

spec/drupol/phptree/Visitor/BreadthFirstVisitorSpec.php

@@ -45,4 +45,39 @@ public function it_can_traverse_a_tree()
             ->traverse($tree)
             ->shouldYield(new \ArrayIterator($rootAndNodes));
     }
+
+    public function it_can_traverse_a_tree_at_a_specific_level_only()
+    {
+        $tree = new Node();
+
+        $nodes = [];
+        $nodes1 = [];
+        $nodes2 = [];
+        foreach (\range('1', '5') as $lowercaseValue) {
+            $node1 = new Node();
+
+            foreach (\range('A', 'E') as $uppercaseValue) {
+                $node2 = new Node();
+
+                $node1->add($node2);
+
+                $nodes2[] = $node2;
+            }
+
+            $nodes1[] = $node1;
+            $nodes[] = $node1;
+        }
+
+        $tree->add(...$nodes);
+
+        $rootAndNodes = \array_merge($nodes2);
+
+        $this
+            ->traverse($tree, 2)
+            ->shouldYield(new \ArrayIterator($rootAndNodes));
+
+        $this
+            ->traverse($tree, 0)
+            ->shouldYield(new \ArrayIterator([$tree]));
+    }
 }

src/Visitor/BreadthFirstVisitor.php

@@ -14,18 +14,22 @@ class BreadthFirstVisitor extends AbstractVisitor
     /**
      * {@inheritdoc}
      */
-    public function traverse(NodeInterface $node): \Traversable
+    public function traverse(NodeInterface $node, int $level = null): \Traversable
     {
         $queue = new \SplQueue();
         $queue->enqueue($node);
 
-        yield $node;
+        if (null === $level || $level === $node->depth()) {
+            yield $node;
+        }
 
         while ($queue->count() > 0) {
             foreach ($queue->dequeue()->children() as $child) {
                 $queue->enqueue($child);
 
-                yield $child;
+                if (null === $level || $level === $child->depth()) {
+                    yield $child;
+                }
             }
         }
     }