Increase code coverage.

spec/drupol/phptree/tests/TestGraphVizSpec.php

@@ -34,7 +34,7 @@ public function it_can_generate_a_graph()
 
         $tree = new ValueNode('root', 2);
 
-        $nodes = [];
+        $nodes = [0 => new ValueNode()];
         foreach (\range('A', 'F') as $letter) {
             $nodes[] = new ValueNode($letter, 2);
         }
@@ -78,4 +78,26 @@ public function it_can_create_a_graph()
             ->render($tree)
             ->shouldReturn($result . PHP_EOL);
     }
+
+    public function it_can_display()
+    {
+        $visitor = new BreadthFirstVisitor();
+        $graph = new Graph();
+        $graphviz = new GraphViz();
+
+        $this->beConstructedWith($visitor, $graph, $graphviz);
+
+        $tree = new ValueNode('root', 2);
+
+        $nodes = [];
+        foreach (\range('A', 'F') as $letter) {
+            $nodes[] = new ValueNode($letter, 2);
+        }
+
+        $tree->add(...$nodes);
+
+        $this
+            ->display($tree)
+            ->shouldReturn($this);
+    }
 }

spec/src/TestGraphViz.php

@@ -17,6 +17,6 @@ class TestGraphViz extends GraphViz
      */
     protected function hash(NodeInterface $node)
     {
-        return $node->getValue();
+        return $node->getValue() ?? parent::hash($node);
     }
 }

src/Render/GraphViz.php

@@ -73,10 +73,6 @@ public function getGraph(NodeInterface $node): Graph
             /** @var int $hash_parent */
             $hash_parent = $this->hash($parent);
 
-            if (false === $this->graph->hasVertex($hash_parent)) {
-                $this->graph->createVertex($hash_parent);
-            }
-
             $this->graph->getVertex($hash_parent)->createEdgeTo($this->graph->getVertex($hash));
         }