Use a Zipper to process MakeWellFormed, removing quadratic behavior.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

configdoc/usage.xml

@@ -503,7 +503,7 @@
  </directive>
  <directive id="Core.EscapeInvalidTags">
   <file name="HTMLPurifier/Strategy/MakeWellFormed.php">
-   <line>66</line>
+   <line>72</line>
   </file>
   <file name="HTMLPurifier/Strategy/RemoveForeignElements.php">
    <line>26</line>

library/HTMLPurifier/Injector.php

@@ -35,19 +35,16 @@ abstract class HTMLPurifier_Injector
     protected $currentNesting;
 
     /**
-     * Reference to InputTokens variable in Context. This is an array
+     * Reference to current token.
-     * list of the input tokens that are being processed.
+     * @type HTMLPurifier_Token
-     * @type array
      */
-    protected $inputTokens;
+    protected $currentToken;
 
     /**
-     * Reference to InputIndex variable in Context. This is an integer
+     * Reference to InputZipper variable in Context.
-     * array index for $this->inputTokens that indicates what token
+     * @type HTMLPurifier_Zipper
-     * is currently being processed.
-     * @type int
      */
-    protected $inputIndex;
+    protected $inputZipper;
 
     /**
      * Array of elements and attributes this injector creates and therefore
@@ -58,33 +55,33 @@ abstract class HTMLPurifier_Injector
     public $needed = array();
 
     /**
-     * Index of inputTokens to rewind to.
+     * Number of elements to rewind backwards (relative).
      * @type bool|int
      */
-    protected $rewind = false;
+    protected $rewindOffset = false;
 
     /**
      * Rewind to a spot to re-perform processing. This is useful if you
      * deleted a node, and now need to see if this change affected any
      * earlier nodes. Rewinding does not affect other injectors, and can
      * result in infinite loops if not used carefully.
-     * @param bool|int $index
+     * @param bool|int $offset
      * @warning HTML Purifier will prevent you from fast-forwarding with this
      *          function.
      */
-    public function rewind($index)
+    public function rewindOffset($offset)
     {
-        $this->rewind = $index;
+        $this->rewindOffset = $offset;
     }
 
     /**
-     * Retrieves rewind, and then unsets it.
+     * Retrieves rewind offset, and then unsets it.
      * @return bool|int
      */
-    public function getRewind()
+    public function getRewindOffset()
     {
-        $r = $this->rewind;
+        $r = $this->rewindOffset;
-        $this->rewind = false;
+        $this->rewindOffset = false;
         return $r;
     }
 
@@ -108,8 +105,8 @@ abstract class HTMLPurifier_Injector
             return $result;
         }
         $this->currentNesting =& $context->get('CurrentNesting');
-        $this->inputTokens    =& $context->get('InputTokens');
+        $this->currentToken   =& $context->get('CurrentToken');
-        $this->inputIndex     =& $context->get('InputIndex');
+        $this->inputZipper    =& $context->get('InputZipper');
         return false;
     }
 
@@ -183,14 +180,14 @@ abstract class HTMLPurifier_Injector
     protected function forward(&$i, &$current)
     {
         if ($i === null) {
-            $i = $this->inputIndex + 1;
+            $i = count($this->inputZipper->back) - 1;
         } else {
-            $i++;
+            $i--;
         }
-        if (!isset($this->inputTokens[$i])) {
+        if ($i < 0) {
             return false;
         }
-        $current = $this->inputTokens[$i];
+        $current = $this->inputZipper->back[$i];
         return true;
     }
 
@@ -237,35 +234,17 @@ abstract class HTMLPurifier_Injector
     protected function backward(&$i, &$current)
     {
         if ($i === null) {
-            $i = $this->inputIndex - 1;
+            $i = count($this->inputZipper->front) - 1;
         } else {
             $i--;
         }
         if ($i < 0) {
             return false;
         }
-        $current = $this->inputTokens[$i];
+        $current = $this->inputZipper->front[$i];
         return true;
     }
 
-    /**
-     * Initializes the iterator at the current position. Use in a do {} while;
-     * loop to force the _forward and _backward functions to start at the
-     * current location.
-     * @warning Please prevent previous references from interfering with this
-     *          functions by setting $i = null beforehand!
-     * @param int $i Current integer index variable for inputTokens
-     * @param HTMLPurifier_Token $current Current token variable.
-     *          Do NOT use $token, as that variable is also a reference
-     */
-    protected function current(&$i, &$current)
-    {
-        if ($i === null) {
-            $i = $this->inputIndex;
-        }
-        $current = $this->inputTokens[$i];
-    }
-
     /**
      * Handler that is called when a text token is processed
      */

library/HTMLPurifier/Injector/AutoParagraph.php

@@ -307,13 +307,13 @@ class HTMLPurifier_Injector_AutoParagraph extends HTMLPurifier_Injector
      */
     private function _pLookAhead()
     {
-        $this->current($i, $current);
+        if ($this->currentToken instanceof HTMLPurifier_Token_Start) {
-        if ($current instanceof HTMLPurifier_Token_Start) {
             $nesting = 1;
         } else {
             $nesting = 0;
         }
         $ok = false;
+        $i = null;
         while ($this->forwardUntilEndToken($i, $current, $nesting)) {
             $result = $this->_checkNeedsP($current);
             if ($result !== null) {

library/HTMLPurifier/Injector/RemoveEmpty.php

@@ -57,8 +57,9 @@ class HTMLPurifier_Injector_RemoveEmpty extends HTMLPurifier_Injector
             return;
         }
         $next = false;
-        for ($i = $this->inputIndex + 1, $c = count($this->inputTokens); $i < $c; $i++) {
+        $deleted = 1; // the current tag
-            $next = $this->inputTokens[$i];
+        for ($i = count($this->inputZipper->back) - 1; $i >= 0; $i--, $deleted++) {
+            $next = $this->inputZipper->back[$i];
             if ($next instanceof HTMLPurifier_Token_Text) {
                 if ($next->is_whitespace) {
                     continue;
@@ -82,16 +83,16 @@ class HTMLPurifier_Injector_RemoveEmpty extends HTMLPurifier_Injector
             if (isset($token->attr['id']) || isset($token->attr['name'])) {
                 return;
             }
-            $token = $i - $this->inputIndex + 1;
+            $token = $deleted + 1;
-            for ($b = $this->inputIndex - 1; $b > 0; $b--) {
+            for ($b = 0, $c = count($this->inputZipper->front); $b < $c; $b++) {
-                $prev = $this->inputTokens[$b];
+                $prev = $this->inputZipper->front[$b];
                 if ($prev instanceof HTMLPurifier_Token_Text && $prev->is_whitespace) {
                     continue;
                 }
                 break;
             }
             // This is safe because we removed the token that triggered this.
-            $this->rewind($b - 1);
+            $this->rewindOffset($b+$deleted);
             return;
         }
     }

library/HTMLPurifier/Strategy/MakeWellFormed.php

@@ -21,10 +21,16 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
     protected $tokens;
 
     /**
-     * Current index in $tokens.
+     * Current token.
-     * @type int
+     * @type HTMLPurifier_Token
      */
-    protected $t;
+    protected $token;
+
+    /**
+     * Zipper managing the true state.
+     * @type HTMLPurifier_Zipper
+     */
+    protected $zipper;
 
     /**
      * Current nesting of elements.
@@ -67,23 +73,25 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
         // used for autoclose early abortion
         $global_parent_allowed_elements = $definition->info_parent_def->child->getAllowedElements($config);
         $e = $context->get('ErrorCollector', true);
-        $t = false; // token index
         $i = false; // injector index
-        $token = false; // the current token
+        list($zipper, $token) = HTMLPurifier_Zipper::fromArray($tokens);
+        if ($token === NULL) {
+            return array();
+        }
         $reprocess = false; // whether or not to reprocess the same token
         $stack = array();
 
         // member variables
         $this->stack =& $stack;
-        $this->t =& $t;
         $this->tokens =& $tokens;
+        $this->token =& $token;
+        $this->zipper =& $zipper;
         $this->config = $config;
         $this->context = $context;
 
         // context variables
         $context->register('CurrentNesting', $stack);
-        $context->register('InputIndex', $t);
+        $context->register('InputZipper', $zipper);
-        $context->register('InputTokens', $tokens);
         $context->register('CurrentToken', $token);
 
         // -- begin INJECTOR --
@@ -142,32 +150,28 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
         //      punt ($reprocess = true; continue;) and it does that for us.
 
         // isset is in loop because $tokens size changes during loop exec
-        for ($t = 0;
+        for (;;
-             $t == 0 || isset($tokens[$t - 1]);
+             // only increment if we don't need to reprocess
-            // only increment if we don't need to reprocess
+             $reprocess ? $reprocess = false : $token = $zipper->next($token)) {
-             $reprocess ? $reprocess = false : $t++) {
 
             // check for a rewind
-            if (is_int($i) && $i >= 0) {
+            if (is_int($i)) {
                 // possibility: disable rewinding if the current token has a
                 // rewind set on it already. This would offer protection from
                 // infinite loop, but might hinder some advanced rewinding.
-                $rewind_to = $this->injectors[$i]->getRewind();
+                $rewind_offset = $this->injectors[$i]->getRewindOffset();
-                if (is_int($rewind_to) && $rewind_to < $t) {
+                if (is_int($rewind_offset)) {
-                    if ($rewind_to < 0) {
+                    for ($j = 0; $j < $rewind_offset; $j++) {
-                        $rewind_to = 0;
+                        if (empty($zipper->front)) break;
-                    }
+                        $token = $zipper->prev($token);
-                    while ($t > $rewind_to) {
-                        $t--;
-                        $prev = $tokens[$t];
                         // indicate that other injectors should not process this token,
                         // but we need to reprocess it
-                        unset($prev->skip[$i]);
+                        unset($token->skip[$i]);
-                        $prev->rewind = $i;
+                        $token->rewind = $i;
-                        if ($prev instanceof HTMLPurifier_Token_Start) {
+                        if ($token instanceof HTMLPurifier_Token_Start) {
                             array_pop($this->stack);
-                        } elseif ($prev instanceof HTMLPurifier_Token_End) {
+                        } elseif ($token instanceof HTMLPurifier_Token_End) {
-                            $this->stack[] = $prev->start;
+                            $this->stack[] = $token->start;
                         }
                     }
                 }
@@ -175,7 +179,7 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
             }
 
             // handle case of document end
-            if (!isset($tokens[$t])) {
+            if ($token === NULL) {
                 // kill processing if stack is empty
                 if (empty($this->stack)) {
                     break;
@@ -191,16 +195,14 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
                 }
 
                 // append, don't splice, since this is the end
-                $tokens[] = new HTMLPurifier_Token_End($top_nesting->name);
+                $token = new HTMLPurifier_Token_End($top_nesting->name);
 
                 // punt!
                 $reprocess = true;
                 continue;
             }
 
-            $token = $tokens[$t];
+            //echo '<br>'; printZipper($zipper, $token);//printTokens($this->stack);
-
-            //echo '<br>'; printTokens($tokens, $t); printTokens($this->stack);
             //flush();
 
             // quick-check: if it's not a tag, no need to process
@@ -213,8 +215,10 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
                         if ($token->rewind !== null && $token->rewind !== $i) {
                             continue;
                         }
-                        $injector->handleText($token);
+                        // XXX fuckup
-                        $this->processToken($token, $i);
+                        $r = $token;
+                        $injector->handleText($r);
+                        $token = $this->processToken($r, $i);
                         $reprocess = true;
                         break;
                     }
@@ -243,9 +247,11 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
                 $ok = true;
             } elseif ($type && $type !== 'empty' && $token instanceof HTMLPurifier_Token_Empty) {
                 // claims to be empty but really is a start tag
-                $this->swap(new HTMLPurifier_Token_End($token->name));
+                // NB: this assignment is required
-                $this->insertBefore(
+                $old_token = $token;
-                    new HTMLPurifier_Token_Start($token->name, $token->attr, $token->line, $token->col, $token->armor)
+                $token = new HTMLPurifier_Token_End($token->name);
+                $token = $this->insertBefore(
+                    new HTMLPurifier_Token_Start($old_token->name, $old_token->attr, $old_token->line, $old_token->col, $old_token->armor)
                 );
                 // punt (since we had to modify the input stream in a non-trivial way)
                 $reprocess = true;
@@ -293,7 +299,7 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
                         $elements = $wrapdef->child->getAllowedElements($config);
                         if (isset($elements[$token->name]) && isset($parent_elements[$wrapname])) {
                             $newtoken = new HTMLPurifier_Token_Start($wrapname);
-                            $this->insertBefore($newtoken);
+                            $token = $this->insertBefore($newtoken);
                             $reprocess = true;
                             continue;
                         }
@@ -330,15 +336,6 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
                             // errors need to be updated
                             $new_token = new HTMLPurifier_Token_End($parent->name);
                             $new_token->start = $parent;
-                            if ($carryover) {
-                                $element = clone $parent;
-                                // [TagClosedAuto]
-                                $element->armor['MakeWellFormed_TagClosedError'] = true;
-                                $element->carryover = true;
-                                $this->processToken(array($new_token, $token, $element));
-                            } else {
-                                $this->insertBefore($new_token);
-                            }
                             // [TagClosedSuppress]
                             if ($e && !isset($parent->armor['MakeWellFormed_TagClosedError'])) {
                                 if (!$carryover) {
@@ -347,8 +344,17 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
                                     $e->send(E_NOTICE, 'Strategy_MakeWellFormed: Tag carryover', $parent);
                                 }
                             }
+                            if ($carryover) {
+                                $element = clone $parent;
+                                // [TagClosedAuto]
+                                $element->armor['MakeWellFormed_TagClosedError'] = true;
+                                $element->carryover = true;
+                                $token = $this->processToken(array($new_token, $token, $element));
+                            } else {
+                                $token = $this->insertBefore($new_token);
+                            }
                         } else {
-                            $this->remove();
+                            $token = $this->remove();
                         }
                         $reprocess = true;
                         continue;
@@ -366,14 +372,14 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
                     if ($token->rewind !== null && $token->rewind !== $i) {
                         continue;
                     }
-                    $injector->handleElement($token);
+                    $r = $token;
-                    $this->processToken($token, $i);
+                    $injector->handleElement($r);
+                    $token = $this->processToken($r, $i);
                     $reprocess = true;
                     break;
                 }
                 if (!$reprocess) {
                     // ah, nothing interesting happened; do normal processing
-                    $this->swap($token);
                     if ($token instanceof HTMLPurifier_Token_Start) {
                         $this->stack[] = $token;
                     } elseif ($token instanceof HTMLPurifier_Token_End) {
@@ -396,16 +402,12 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
                     if ($e) {
                         $e->send(E_WARNING, 'Strategy_MakeWellFormed: Unnecessary end tag to text');
                     }
-                    $this->swap(
+                    $token = new HTMLPurifier_Token_Text($generator->generateFromToken($token));
-                        new HTMLPurifier_Token_Text(
-                            $generator->generateFromToken($token)
-                        )
-                    );
                 } else {
-                    $this->remove();
                     if ($e) {
                         $e->send(E_WARNING, 'Strategy_MakeWellFormed: Unnecessary end tag removed');
                     }
+                    $token = $this->remove();
                 }
                 $reprocess = true;
                 continue;
@@ -425,8 +427,9 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
                     if ($token->rewind !== null && $token->rewind !== $i) {
                         continue;
                     }
-                    $injector->handleEnd($token);
+                    $r = $token;
-                    $this->processToken($token, $i);
+                    $injector->handleEnd($r);
+                    $token = $this->processToken($r, $i);
                     $this->stack[] = $current_parent;
                     $reprocess = true;
                     break;
@@ -454,19 +457,15 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
             // we didn't find the tag, so remove
             if ($skipped_tags === false) {
                 if ($escape_invalid_tags) {
-                    $this->swap(
-                        new HTMLPurifier_Token_Text(
-                            $generator->generateFromToken($token)
-                        )
-                    );
                     if ($e) {
                         $e->send(E_WARNING, 'Strategy_MakeWellFormed: Stray end tag to text');
                     }
+                    $token = new HTMLPurifier_Token_Text($generator->generateFromToken($token));
                 } else {
-                    $this->remove();
                     if ($e) {
                         $e->send(E_WARNING, 'Strategy_MakeWellFormed: Stray end tag removed');
                     }
+                    $token = $this->remove();
                 }
                 $reprocess = true;
                 continue;
@@ -499,18 +498,17 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
                     $replace[] = $element;
                 }
             }
-            $this->processToken($replace);
+            $token = $this->processToken($replace);
             $reprocess = true;
             continue;
         }
 
-        $context->destroy('CurrentNesting');
-        $context->destroy('InputTokens');
-        $context->destroy('InputIndex');
         $context->destroy('CurrentToken');
+        $context->destroy('CurrentNesting');
+        $context->destroy('InputZipper');
 
-        unset($this->injectors, $this->stack, $this->tokens, $this->t);
+        unset($this->injectors, $this->stack, $this->tokens);
-        return $tokens;
+        return $zipper->toArray($token);
     }
 
     /**
@@ -560,7 +558,7 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
         // array(number nodes to delete, new node 1, new node 2, ...)
 
         $delete = array_shift($token);
-        $old = array_splice($this->tokens, $this->t, $delete, $token);
+        list($old, $r) = $this->zipper->splice($this->token, $delete, $token);
 
         if ($injector > -1) {
             // determine appropriate skips
@@ -571,6 +569,8 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
             }
         }
 
+        return $r;
+
     }
 
     /**
@@ -580,7 +580,9 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
      */
     private function insertBefore($token)
     {
-        array_splice($this->tokens, $this->t, 0, array($token));
+        // NB not $this->zipper->insertBefore(), due to positioning
+        // differences
+        return $this->zipper->splice($this->token, 0, array($token))[1];
     }
 
     /**
@@ -589,17 +591,7 @@ class HTMLPurifier_Strategy_MakeWellFormed extends HTMLPurifier_Strategy
      */
     private function remove()
     {
-        array_splice($this->tokens, $this->t, 1);
+        return $this->zipper->delete();
-    }
-
-    /**
-     * Swap current token with new token. Cursor points to new token (no
-     * change).  You must reprocess after this.
-     * @param HTMLPurifier_Token $token
-     */
-    private function swap($token)
-    {
-        $this->tokens[$this->t] = $token;
     }
 }
 

library/HTMLPurifier/Zipper.php

@@ -20,7 +20,7 @@
 
 class HTMLPurifier_Zipper
 {
-    private $front, $back;
+    public $front, $back;
 
     public function __construct($front, $back) {
         $this->front = $front;
@@ -95,6 +95,14 @@ class HTMLPurifier_Zipper
         return empty($this->back) ? NULL : array_pop($this->back);
     }
 
+    /**
+     * Returns true if we are at the end of the list.
+     * @return bool
+     */
+    public function done() {
+        return empty($this->back);
+    }
+
     /**
      * Insert element before hole.
      * @param Element to insert
@@ -115,14 +123,16 @@ class HTMLPurifier_Zipper
      * Splice in multiple elements at hole.  Functional specification
      * in terms of array_splice:
      *
-     *      $r1 = array_splice($arr, $i, $delete, $replacement);
+     *      $arr1 = $arr;
+     *      $old1 = array_splice($arr1, $i, $delete, $replacement);
      *
      *      list($z, $t) = HTMLPurifier_Zipper::fromArray($arr);
      *      $t = $z->advance($t, $i);
-     *      $t = $z->splice($t, $delete, $replacement);
+     *      list($old2, $t) = $z->splice($t, $delete, $replacement);
-     *      $r2 = $z->toArray($t);
+     *      $arr2 = $z->toArray($t);
      *
-     *      assert($r1 === $r2);
+     *      assert($old1 === $old2);
+     *      assert($arr1 === $arr2);
      *
      * NB: the absolute index location after this operation is
      * *unchanged!*
@@ -131,8 +141,10 @@ class HTMLPurifier_Zipper
      */
     public function splice($t, $delete, $replacement) {
         // delete
+        $old = array();
         $r = $t;
         for ($i = $delete; $i > 0; $i--) {
+            $old[] = $r;
             $r = $this->delete();
         }
         // insert
@@ -140,6 +152,6 @@ class HTMLPurifier_Zipper
             $this->insertAfter($r);
             $r = $replacement[$i];
         }
-        return $r;
+        return array($old, $r);
     }
 }

tests/HTMLPurifier/Strategy/MakeWellFormed/EndRewindInjector.php

@@ -24,7 +24,7 @@ class HTMLPurifier_Strategy_MakeWellFormed_EndRewindInjector extends HTMLPurifie
         ) {
             $token = false;
             $prev->_InjectorTest_EndRewindInjector_delete = true;
-            $this->rewind($i);
+            $this->rewindOffset(1);
         }
     }
 }

tests/HTMLPurifier/Strategy/MakeWellFormed_InjectorTest.php

@@ -27,7 +27,7 @@ class HTMLPurifier_Strategy_MakeWellFormed_InjectorTest extends HTMLPurifier_Str
         $i->start->skip = array(0 => true, 1 => true);
         $mock->expectAt(1, 'handleEnd', array($i));
         $mock->expectCallCount('handleEnd', 2);
-        $mock->setReturnValue('getRewind', false);
+        $mock->setReturnValue('getRewindOffset', false);
         $this->config->set('AutoFormat.AutoParagraph', false);
         $this->config->set('AutoFormat.Linkify',       false);
         $this->config->set('AutoFormat.Custom', array($mock));

tests/HTMLPurifier/ZipperTest.php

@@ -16,7 +16,8 @@ class HTMLPurifier_ZipperTest extends HTMLPurifier_Harness
         $z->insertBefore(4);
         $z->insertAfter(5);
         $this->assertIdentical($z->toArray($t), array(0,1,4,3,5));
-        $t = $z->splice($t, 2, array(6,7));
+        list($old, $t) = $z->splice($t, 2, array(6,7));
+        $this->assertIdentical($old, array(3,5));
         $this->assertIdentical($t, 6);
         $this->assertIdentical($z->toArray($t), array(0,1,4,6,7));
     }

tests/common.php

@@ -182,10 +182,34 @@ function printTokens($tokens, $index = null)
     $string = '<pre>';
     $generator = new HTMLPurifier_Generator(HTMLPurifier_Config::createDefault(), new HTMLPurifier_Context);
     foreach ($tokens as $i => $token) {
-        if ($index === $i) $string .= '[<strong>';
+        $string .= printToken($generator, $token, $i, $index == $i);
-        $string .= "<sup>$i</sup>";
+    }
-        $string .= $generator->escape($generator->generateFromToken($token));
+    $string .= '</pre>';
-        if ($index === $i) $string .= '</strong>]';
+    echo $string;
+}
+
+function printToken($generator, $token, $i, $isCursor)
+{
+    $string = "";
+    if ($isCursor) $string .= '[<strong>';
+    $string .= "<sup>$i</sup>";
+    $string .= $generator->escape($generator->generateFromToken($token));
+    if ($isCursor) $string .= '</strong>]';
+    return $string;
+}
+
+function printZipper($zipper, $token)
+{
+    $string = '<pre>';
+    $generator = new HTMLPurifier_Generator(HTMLPurifier_Config::createDefault(), new HTMLPurifier_Context);
+    foreach ($zipper->front as $i => $t) {
+        $string .= printToken($generator, $t, $i, false);
+    }
+    if ($token !== NULL) {
+        $string .= printToken($generator, $token, "", true);
+    }
+    for ($i = count($zipper->back)-1; $i >= 0; $i--) {
+        $string .= printToken($generator, $zipper->back[$i], $i, false);
     }
     $string .= '</pre>';
     echo $string;