LibWeb: Let Element cache its list of classes

Instead of string splitting every time you call Element::has_class(),
we now split the "class" attribute value when it changes, and cache
the individual classes as FlyStrings in Element::m_classes.

This makes has_class() significantly faster and moves the pain point
of selector matching somewhere else.

2 files changed, 15 insertions, 9 deletions

diff --git a/Libraries/LibWeb/DOM/Element.cpp b/Libraries/LibWeb/DOM/Element.cpp
index 85fbfc90dc..6b546b97c3 100644
--- a/Libraries/LibWeb/DOM/Element.cpp
+++ b/Libraries/LibWeb/DOM/Element.cpp
@@ -97,14 +97,10 @@ void Element::set_attributes(Vector<Attribute>&& attributes)
         parse_attribute(attribute.name(), attribute.value());
 }
 
-bool Element::has_class(const StringView& class_name) const
+bool Element::has_class(const FlyString& class_name) const
 {
-    auto value = attribute("class");
-    if (value.is_empty())
-        return false;
-    auto parts = value.split_view(' ');
-    for (auto& part : parts) {
-        if (part == class_name)
+    for (auto& class_ : m_classes) {
+        if (class_ == class_name)
             return true;
     }
     return false;
@@ -140,8 +136,16 @@ RefPtr<LayoutNode> Element::create_layout_node(const StyleProperties* parent_sty
     return adopt(*new LayoutInline(*this, move(style)));
 }
 
-void Element::parse_attribute(const FlyString&, const String&)
+void Element::parse_attribute(const FlyString& name, const String& value)
 {
+    if (name == "class") {
+        auto new_classes = value.split_view(' ');
+        m_classes.clear();
+        m_classes.ensure_capacity(new_classes.size());
+        for (auto& new_class : new_classes) {
+            m_classes.unchecked_append(new_class);
+        }
+    }
 }
 
 enum class StyleDifference {
diff --git a/Libraries/LibWeb/DOM/Element.h b/Libraries/LibWeb/DOM/Element.h
index 70c9d83f6b..7fbf3ab394 100644
--- a/Libraries/LibWeb/DOM/Element.h
+++ b/Libraries/LibWeb/DOM/Element.h
@@ -58,7 +58,7 @@ public:
             callback(attribute.name(), attribute.value());
     }
 
-    bool has_class(const StringView&) const;
+    bool has_class(const FlyString&) const;
 
     virtual void apply_presentational_hints(StyleProperties&) const { }
     virtual void parse_attribute(const FlyString& name, const String& value);
@@ -86,6 +86,8 @@ private:
     Vector<Attribute> m_attributes;
 
     RefPtr<StyleProperties> m_resolved_style;
+
+    Vector<FlyString> m_classes;
 };
 
 template<>