Limit recursion depth by configurable

Fixes https://github.com/erusev/parsedown/issues/681

src/Configurables/RecursionLimiter.php

@@ -0,0 +1,51 @@
+<?php
+
+namespace Erusev\Parsedown\Configurables;
+
+use Erusev\Parsedown\Configurable;
+
+final class RecursionLimiter implements Configurable
+{
+    /** @var int */
+    private $maxDepth;
+
+    /** @var int */
+    private $currentDepth;
+
+    /**
+     * @param int $maxDepth
+     * @param int $currentDepth
+     */
+    private function __construct($maxDepth, $currentDepth)
+    {
+        $this->maxDepth = $maxDepth;
+        $this->currentDepth = $currentDepth;
+    }
+
+    /** @return self */
+    public static function initial()
+    {
+        return self::maxDepth(256);
+    }
+
+    /**
+     * @param int $maxDepth
+     * @return self
+     */
+    public static function maxDepth($maxDepth)
+    {
+        return new self($maxDepth, 0);
+    }
+
+    /** @return self */
+    public function increment()
+    {
+        return new self($this->maxDepth, $this->currentDepth + 1);
+    }
+
+    /** @return bool */
+    public function isDepthExceeded()
+    {
+        return ($this->maxDepth < $this->currentDepth);
+    }
+}

src/Parsedown.php

@@ -13,6 +13,7 @@ use Erusev\Parsedown\Components\Inlines\PlainText;
 use Erusev\Parsedown\Components\StateUpdatingBlock;
 use Erusev\Parsedown\Configurables\BlockTypes;
 use Erusev\Parsedown\Configurables\InlineTypes;
+use Erusev\Parsedown\Configurables\RecursionLimiter;
 use Erusev\Parsedown\Html\Renderable;
 use Erusev\Parsedown\Html\Renderables\Text;
 use Erusev\Parsedown\Parsing\Excerpt;
@@ -82,6 +83,14 @@ final class Parsedown
      */
     public static function blocks(Lines $Lines, State $State)
     {
+        $RecursionLimiter = $State->get(RecursionLimiter::class)->increment();
+
+        if ($RecursionLimiter->isDepthExceeded()) {
+            $State = $State->setting(new BlockTypes([], []));
+        }
+
+        $State = $State->setting($RecursionLimiter);
+
         /** @var Block[] */
         $Blocks = [];
         /** @var Block|null */
@@ -180,6 +189,14 @@ final class Parsedown
         # standardize line breaks
         $text = \str_replace(["\r\n", "\r"], "\n", $text);
 
+        $RecursionLimiter = $State->get(RecursionLimiter::class)->increment();
+
+        if ($RecursionLimiter->isDepthExceeded()) {
+            return [Plaintext::build(new Excerpt($text, 0), $State)];
+        }
+
+        $State = $State->setting($RecursionLimiter);
+
         /** @var Inline[] */
         $Inlines = [];