feature #39097 [DomCrawler] Cache discovered namespaces (simonberger, fabpot)

This PR was merged into the 5.3-dev branch.

Discussion
----------

[DomCrawler] Cache discovered namespaces

| Q             | A
| ------------- | ---
| Branch?       | 5.x
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Address #39067
| License       | MIT

Discovering namespaces is by far the most expensive task when filtering for nodes and the xpath contains prefixes.
When `Crawler::filterRelativeXPath` is called multiple times with (identical) prefixes the slowdown is huge.

This fix brings the runtime of the linked ticket down from 27 seconds to 9 seconds in my test. Compared to a pure PHP version which takes < 0.5 seconds the design of the crawler API is the limiting factor. There are still many repeated namespace queries caused by new Crawler instances. Ideas to solve this are discussed in the ticket.

Commits
-------

a8e85ecbbd Make some CS changes
4c74dead48 Cache discovered namespaces in DomCrawler

src/Symfony/Component/DomCrawler/Crawler.php

@@ -33,6 +33,11 @@ class Crawler implements \Countable, \IteratorAggregate
      */
     private $namespaces = [];
 
+    /**
+     * @var \ArrayObject A map of cached namespaces
+     */
+    private $cachedNamespaces;
+
     /**
      * @var string The base href value
      */
@@ -68,6 +73,7 @@ class Crawler implements \Countable, \IteratorAggregate
         $this->uri = $uri;
         $this->baseHref = $baseHref ?: $uri;
         $this->html5Parser = class_exists(HTML5::class) ? new HTML5(['disable_html_ns' => true]) : null;
+        $this->cachedNamespaces = new \ArrayObject();
 
         $this->add($node);
     }
@@ -99,6 +105,7 @@ class Crawler implements \Countable, \IteratorAggregate
     {
         $this->nodes = [];
         $this->document = null;
+        $this->cachedNamespaces = new \ArrayObject();
     }
 
     /**
@@ -967,12 +974,14 @@ class Crawler implements \Countable, \IteratorAggregate
      */
     private function filterRelativeXPath(string $xpath): object
     {
-        $prefixes = $this->findNamespacePrefixes($xpath);
-
         $crawler = $this->createSubCrawler(null);
+        if (null === $this->document) {
+            return $crawler;
+        }
+
+        $domxpath = $this->createDOMXPath($this->document, $this->findNamespacePrefixes($xpath));
 
         foreach ($this->nodes as $node) {
-            $domxpath = $this->createDOMXPath($node->ownerDocument, $prefixes);
             $crawler->add($domxpath->query($xpath, $node));
         }
 
@@ -1189,10 +1198,14 @@ class Crawler implements \Countable, \IteratorAggregate
             return $this->namespaces[$prefix];
         }
 
+        if (isset($this->cachedNamespaces[$prefix])) {
+            return $this->cachedNamespaces[$prefix];
+        }
+
         // ask for one namespace, otherwise we'd get a collection with an item for each node
         $namespaces = $domxpath->query(sprintf('(//namespace::*[name()="%s"])[last()]', $this->defaultNamespacePrefix === $prefix ? '' : $prefix));
 
-        return ($node = $namespaces->item(0)) ? $node->nodeValue : null;
+        return $this->cachedNamespaces[$prefix] = ($node = $namespaces->item(0)) ? $node->nodeValue : null;
     }
 
     private function findNamespacePrefixes(string $xpath): array
@@ -1217,6 +1230,7 @@ class Crawler implements \Countable, \IteratorAggregate
         $crawler->isHtml = $this->isHtml;
         $crawler->document = $this->document;
         $crawler->namespaces = $this->namespaces;
+        $crawler->cachedNamespaces = $this->cachedNamespaces;
         $crawler->html5Parser = $this->html5Parser;
 
         return $crawler;