Implement style invalidation tree walk for targeted style recalc upon class change.

Source/core/dom/shadow/SelectRuleFeatureSet.cpp

Index: Source/core/dom/shadow/SelectRuleFeatureSet.cpp
diff --git a/Source/core/dom/shadow/SelectRuleFeatureSet.cpp b/Source/core/dom/shadow/SelectRuleFeatureSet.cpp
index 02ab9488fc36a42faff5c91c7eaf4845f0efe419..25757e6bc7463c69a3e5cbe978cbf07cabbe35e9 100644
--- a/Source/core/dom/shadow/SelectRuleFeatureSet.cpp
+++ b/Source/core/dom/shadow/SelectRuleFeatureSet.cpp
@@ -33,6 +33,8 @@
 
 #include "core/css/CSSSelector.h"
 
+#include "wtf/BitVector.h"
+
 namespace WebCore {
 
 SelectRuleFeatureSet::SelectRuleFeatureSet()
@@ -83,5 +85,53 @@ void SelectRuleFeatureSet::collectFeaturesFromSelector(const CSSSelector& select
     }
 }
 
+bool SelectRuleFeatureSet::checkSelectorsForClassChange(const SpaceSplitString& changedClasses) const
+{
+    unsigned changedSize = changedClasses.size();
+    for (unsigned i = 0; i < changedSize; ++i) {
+        if (hasSelectorForClass(changedClasses[i]))
+            return true;
+    }
+    return false;
+}
+
+bool SelectRuleFeatureSet::checkSelectorsForClassChange(const SpaceSplitString& oldClasses, const SpaceSplitString& newClasses) const

[ojan, 2014/01/30 19:08:39]

It's a bummer there's so much copy-paste between this and
RuleFeatureSet::computeInvalidationSetsForClassChange. I don't have a good
suggestion for how to share the code though. :(

[chrishtr, 2014/01/30 19:25:48]

Agreed. I tried several approaches, but all turned out silly. I hope to be able
to refactor this class to call into a feature set instead, but for the moment it
was too hard.

+{
+    if (!oldClasses.size())
+        return checkSelectorsForClassChange(newClasses);
+
+    // Class vectors tend to be very short. This is faster than using a hash table.
+    BitVector remainingClassBits;
+    remainingClassBits.ensureSize(oldClasses.size());
+
+    for (unsigned i = 0; i < newClasses.size(); ++i) {
+        bool found = false;
+        for (unsigned j = 0; j < oldClasses.size(); ++j) {
+            if (newClasses[i] == oldClasses[j]) {
+                // Mark each class that is still in the newClasses so we can skip doing
+                // an n^2 search below when looking for removals. We can't break from
+                // this loop early since a class can appear more than once.
+                remainingClassBits.quickSet(j);
+                found = true;
+            }
+        }
+        // Class was added.
+        if (!found) {
+            if (hasSelectorForClass(newClasses[i]))
+                return true;
+        }
+    }
+
+    for (unsigned i = 0; i < oldClasses.size(); ++i) {
+        if (remainingClassBits.quickGet(i))
+            continue;
+
+        // Class was removed.
+        if (hasSelectorForClass(oldClasses[i]))
+            return true;
+    }
+    return false;
+}
+
 }