class NodeTraverser

final class NodeTraverser {
    private $env;
    private $visitors = [];
    
    /**
     * @param NodeVisitorInterface[] $visitors
     */
    public function __construct(Environment $env, array $visitors = []) {
        $this->env = $env;
        foreach ($visitors as $visitor) {
            $this->addVisitor($visitor);
        }
    }
    public function addVisitor(NodeVisitorInterface $visitor) : void {
        $this->visitors[$visitor->getPriority()][] = $visitor;
    }
    
    /**
     * Traverses a node and calls the registered visitors.
     */
    public function traverse(Node $node) : Node {
        ksort($this->visitors);
        foreach ($this->visitors as $visitors) {
            foreach ($visitors as $visitor) {
                $node = $this->traverseForVisitor($visitor, $node);
            }
        }
        return $node;
    }
    private function traverseForVisitor(NodeVisitorInterface $visitor, Node $node) : ?Node {
        $node = $visitor->enterNode($node, $this->env);
        foreach ($node as $k => $n) {
            if (null !== ($m = $this->traverseForVisitor($visitor, $n))) {
                if ($m !== $n) {
                    $node->setNode($k, $m);
                }
            }
            else {
                $node->removeNode($k);
            }
        }
        return $visitor->leaveNode($node, $this->env);
    }

}