From 0805060e5ef5366f7e2f8454a69086b0ff451acb Mon Sep 17 00:00:00 2001
From: Sam Atkins <atkinssj@serenityos.org>
Date: Mon, 31 Jul 2023 17:46:57 +0100
Subject: [PATCH] LibWeb: Speed up CSS namespace checking

CSSStyleSheet now caches the CSSNamespaceRule for the default namespace,
which is the only one we currently care about. This saves us from
iterating over its list of rules every time we want to know what that
default namespace is.

The spec dictates that `@namespace` rules are only valid near the start
of a stylesheet, so we also take advantage of that to quit searching
for namespaces as soon as we see a non-import rule.

Also renamed `namespace_filter()` to `default_namespace()` since that's
what it actually returns.

This makes github.com/serenityos/serenity snappy again. :^)
---
 .../Libraries/LibWeb/CSS/CSSStyleSheet.cpp    | 46 +++++++++++++++----
 Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h |  6 ++-
 .../Libraries/LibWeb/CSS/StyleComputer.cpp    |  2 +-
 3 files changed, 43 insertions(+), 11 deletions(-)

diff --git a/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp b/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp
index d3a29d41f9..b2055922d9 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp
+++ b/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp
@@ -6,7 +6,6 @@
 
 #include <LibWeb/Bindings/CSSStyleSheetPrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
-#include <LibWeb/CSS/CSSNamespaceRule.h>
 #include <LibWeb/CSS/CSSStyleSheet.h>
 #include <LibWeb/CSS/Parser/Parser.h>
 #include <LibWeb/CSS/StyleComputer.h>
@@ -30,6 +29,12 @@ CSSStyleSheet::CSSStyleSheet(JS::Realm& realm, CSSRuleList& rules, MediaList& me
 
     for (auto& rule : *m_rules)
         rule->set_parent_style_sheet(this);
+
+    recalculate_namespaces();
+
+    m_rules->on_change = [this]() {
+        recalculate_namespaces();
+    };
 }
 
 JS::ThrowCompletionOr<void> CSSStyleSheet::initialize(JS::Realm& realm)
@@ -138,17 +143,40 @@ void CSSStyleSheet::set_style_sheet_list(Badge<StyleSheetList>, StyleSheetList*
     m_style_sheet_list = list;
 }
 
-Optional<StringView> CSSStyleSheet::namespace_filter() const
+Optional<StringView> CSSStyleSheet::default_namespace() const
 {
-    for (JS::NonnullGCPtr<CSSRule> rule : *m_rules) {
-        if (rule->type() == CSSRule::Type::Namespace) {
-            auto& namespace_rule = verify_cast<CSSNamespaceRule>(*rule);
-            if (!namespace_rule.namespace_uri().is_empty() && namespace_rule.prefix().is_empty())
-                return namespace_rule.namespace_uri().view();
-        }
-    }
+    if (m_default_namespace_rule)
+        return m_default_namespace_rule->namespace_uri().view();
 
     return {};
 }
 
+void CSSStyleSheet::recalculate_namespaces()
+{
+    for (JS::NonnullGCPtr<CSSRule> rule : *m_rules) {
+        // "Any @namespace rules must follow all @charset and @import rules and precede all other
+        // non-ignored at-rules and style rules in a style sheet.
+        // ...
+        // A syntactically invalid @namespace rule (whether malformed or misplaced) must be ignored."
+        // https://drafts.csswg.org/css-namespaces/#syntax
+        switch (rule->type()) {
+        case CSSRule::Type::Import:
+            continue;
+
+        case CSSRule::Type::Namespace:
+            break;
+
+        default:
+            // Any other types mean that further @namespace rules are invalid, so we can stop here.
+            return;
+        }
+
+        auto& namespace_rule = verify_cast<CSSNamespaceRule>(*rule);
+        if (!namespace_rule.namespace_uri().is_empty() && namespace_rule.prefix().is_empty())
+            m_default_namespace_rule = namespace_rule;
+
+        // FIXME: Store qualified namespace rules.
+    }
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h b/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h
index c6d031f71e..cffc45f0b8 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h
+++ b/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Function.h>
+#include <LibWeb/CSS/CSSNamespaceRule.h>
 #include <LibWeb/CSS/CSSRule.h>
 #include <LibWeb/CSS/CSSRuleList.h>
 #include <LibWeb/CSS/CSSStyleRule.h>
@@ -48,7 +49,7 @@ public:
 
     void set_style_sheet_list(Badge<StyleSheetList>, StyleSheetList*);
 
-    Optional<StringView> namespace_filter() const;
+    Optional<StringView> default_namespace() const;
 
 private:
     CSSStyleSheet(JS::Realm&, CSSRuleList&, MediaList&, Optional<AK::URL> location);
@@ -56,7 +57,10 @@ private:
     virtual JS::ThrowCompletionOr<void> initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
 
+    void recalculate_namespaces();
+
     JS::GCPtr<CSSRuleList> m_rules;
+    JS::GCPtr<CSSNamespaceRule> m_default_namespace_rule;
 
     JS::GCPtr<StyleSheetList> m_style_sheet_list;
     JS::GCPtr<CSSRule> m_owner_css_rule;
diff --git a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
index e951d292f4..221c363a2d 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
+++ b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
@@ -225,7 +225,7 @@ Vector<MatchingRule> StyleComputer::filter_namespace_rules(DOM::Element const& e
     Vector<MatchingRule> filtered_rules;
 
     for (auto const& rule : rules) {
-        auto namespace_uri = rule.sheet->namespace_filter();
+        auto namespace_uri = rule.sheet->default_namespace();
         if (namespace_uri.has_value()) {
             if (namespace_uri.value() == element.namespace_uri())
                 filtered_rules.append(rule);