LibWeb: Cache name->element mappings in HTMLCollection

This makes https://wpt.fyi/ load today instead of tomorrow, although
there's a lot of room for improvement still.

Userland/Libraries/LibWeb/Bindings/PlatformObject.cpp

@@ -47,9 +47,7 @@ JS::ThrowCompletionOr<bool> PlatformObject::is_named_property_exposed_on_object(
 
     // 1. If P is not a supported property name of O, then return false.
     // NOTE: This is in it's own variable to enforce the type.
-    auto supported_property_names = this->supported_property_names();
-    auto property_key_string = MUST(String::from_byte_string(property_key.to_string()));
-    if (!supported_property_names.contains_slow(property_key_string))
+    if (!is_supported_property_name(MUST(String::from_byte_string(property_key.to_string()))))
         return false;
 
     // 2. If O has an own property named P, then return false.
@@ -500,6 +498,11 @@ Vector<FlyString> PlatformObject::supported_property_names() const
     return {};
 }
 
+bool PlatformObject::is_supported_property_name(FlyString const& name) const
+{
+    return supported_property_names().contains_slow(name);
+}
+
 bool PlatformObject::is_supported_property_index(u32) const
 {
     return false;

Userland/Libraries/LibWeb/Bindings/PlatformObject.h

@@ -75,6 +75,7 @@ protected:
     virtual WebIDL::ExceptionOr<JS::Value> item_value(size_t index) const;
     virtual WebIDL::ExceptionOr<JS::Value> named_item_value(FlyString const& name) const;
     virtual Vector<FlyString> supported_property_names() const;
+    virtual bool is_supported_property_name(FlyString const&) const;
     virtual bool is_supported_property_index(u32) const;
 
     // NOTE: These will crash if you make has_named_property_setter return true but do not override these methods.

Userland/Libraries/LibWeb/DOM/HTMLCollection.cpp

@@ -48,6 +48,30 @@ void HTMLCollection::visit_edges(Cell::Visitor& visitor)
     Base::visit_edges(visitor);
     visitor.visit(m_root);
     visitor.visit(m_cached_elements);
+    if (m_cached_name_to_element_mappings)
+        visitor.visit(*m_cached_name_to_element_mappings);
+}
+
+void HTMLCollection::update_name_to_element_mappings_if_needed() const
+{
+    update_cache_if_needed();
+    if (m_cached_name_to_element_mappings)
+        return;
+    m_cached_name_to_element_mappings = make<HashMap<FlyString, JS::NonnullGCPtr<Element>>>();
+    for (auto const& element : m_cached_elements) {
+        // 1. If element has an ID which is not in result, append element’s ID to result.
+        if (auto const& id = element->id(); id.has_value()) {
+            if (!id.value().is_empty() && !m_cached_name_to_element_mappings->contains(id.value()))
+                m_cached_name_to_element_mappings->set(id.value(), element);
+        }
+
+        // 2. If element is in the HTML namespace and has a name attribute whose value is neither the empty string nor is in result, append element’s name attribute value to result.
+        if (element->namespace_uri() == Namespace::HTML && element->name().has_value()) {
+            auto element_name = element->name().value();
+            if (!element_name.is_empty() && !m_cached_name_to_element_mappings->contains(element_name))
+                m_cached_name_to_element_mappings->set(move(element_name), element);
+        }
+    }
 }
 
 void HTMLCollection::update_cache_if_needed() const
@@ -57,6 +81,7 @@ void HTMLCollection::update_cache_if_needed() const
         return;
 
     m_cached_elements.clear();
+    m_cached_name_to_element_mappings = nullptr;
     if (m_scope == Scope::Descendants) {
         m_root->for_each_in_subtree_of_type<Element>([&](auto& element) {
             if (m_filter(element))
@@ -107,23 +132,19 @@ Element* HTMLCollection::named_item(FlyString const& key) const
     if (key.is_empty())
         return nullptr;
 
-    update_cache_if_needed();
-
-    // 2. Return the first element in the collection for which at least one of the following is true:
-    for (auto const& element : m_cached_elements) {
-        // - it has an ID which is key;
-        if (element->id() == key)
-            return element;
-
-        // - it is in the HTML namespace and has a name attribute whose value is key;
-        if (element->namespace_uri() == Namespace::HTML && element->name() == key)
-            return element;
-    }
-
-    // or null if there is no such element.
+    update_name_to_element_mappings_if_needed();
+    if (auto it = m_cached_name_to_element_mappings->get(key); it.has_value())
+        return it.value();
     return nullptr;
 }
 
+// https://dom.spec.whatwg.org/#ref-for-dfn-supported-property-names
+bool HTMLCollection::is_supported_property_name(FlyString const& name) const
+{
+    update_name_to_element_mappings_if_needed();
+    return m_cached_name_to_element_mappings->contains(name);
+}
+
 // https://dom.spec.whatwg.org/#ref-for-dfn-supported-property-names
 Vector<FlyString> HTMLCollection::supported_property_names() const
 {
@@ -131,20 +152,9 @@ Vector<FlyString> HTMLCollection::supported_property_names() const
     Vector<FlyString> result;
 
     // 2. For each element represented by the collection, in tree order:
-    update_cache_if_needed();
-    for (auto const& element : m_cached_elements) {
-        // 1. If element has an ID which is not in result, append element’s ID to result.
-        if (auto const& id = element->id(); id.has_value()) {
-            if (!id.value().is_empty() && !result.contains_slow(id.value()))
-                result.append(id.value());
-        }
-
-        // 2. If element is in the HTML namespace and has a name attribute whose value is neither the empty string nor is in result, append element’s name attribute value to result.
-        if (element->namespace_uri() == Namespace::HTML && element->name().has_value()) {
-            auto name = element->name().value();
-            if (!name.is_empty() && !result.contains_slow(name))
-                result.append(move(name));
-        }
+    update_name_to_element_mappings_if_needed();
+    for (auto const& it : *m_cached_name_to_element_mappings) {
+        result.append(it.key);
     }
 
     // 3. Return result.

Userland/Libraries/LibWeb/DOM/HTMLCollection.h

@@ -43,6 +43,7 @@ public:
     virtual WebIDL::ExceptionOr<JS::Value> item_value(size_t index) const override;
     virtual WebIDL::ExceptionOr<JS::Value> named_item_value(FlyString const& name) const override;
     virtual Vector<FlyString> supported_property_names() const override;
+    virtual bool is_supported_property_name(FlyString const&) const override;
     virtual bool is_supported_property_index(u32) const override;
 
 protected:
@@ -57,9 +58,11 @@ private:
     virtual void visit_edges(Cell::Visitor&) override;
 
     void update_cache_if_needed() const;
+    void update_name_to_element_mappings_if_needed() const;
 
     mutable u64 m_cached_dom_tree_version { 0 };
     mutable Vector<JS::NonnullGCPtr<Element>> m_cached_elements;
+    mutable OwnPtr<HashMap<FlyString, JS::NonnullGCPtr<Element>>> m_cached_name_to_element_mappings;
 
     JS::NonnullGCPtr<ParentNode> m_root;
     Function<bool(Element const&)> m_filter;