Make HTMLCollection / NodeList backward traversal consistent with forward one

Source/core/html/HTMLCollection.cpp

Index: Source/core/html/HTMLCollection.cpp
diff --git a/Source/core/html/HTMLCollection.cpp b/Source/core/html/HTMLCollection.cpp
index 8a81afed942ae52b576c675e400d6847f908be28..b66fa9d26d5ea5003523c4402deb4ef231e56878 100644
--- a/Source/core/html/HTMLCollection.cpp
+++ b/Source/core/html/HTMLCollection.cpp
@@ -276,11 +276,6 @@ template <> inline bool isMatchingElement(const HTMLTagCollection& collection, c
     return collection.elementMatches(element);
 }
 
-Element* HTMLCollection::itemBefore(const Element* previous) const
-{
-    return LiveNodeListBase::itemBefore(*this, previous);
-}
-
 Element* HTMLCollection::virtualItemAfter(Element*) const
 {
     ASSERT_NOT_REACHED();
@@ -314,6 +309,14 @@ inline Element* firstMatchingChildElement(const HTMLCollection& nodeList)
     return element;
 }
 
+inline Element* lastMatchingChildElement(const HTMLCollection& nodeList)
+{
+    Element* element = ElementTraversal::lastChild(nodeList.rootNode());
+    while (element && !isMatchingElement(nodeList, *element))
+        element = ElementTraversal::previousSibling(*element);
+    return element;
+}
+
 inline Element* nextMatchingChildElement(const HTMLCollection& nodeList, Element& current)
 {
     Element* next = &current;
@@ -323,6 +326,15 @@ inline Element* nextMatchingChildElement(const HTMLCollection& nodeList, Element
     return next;
 }
 
+inline Element* previousMatchingChildElement(const HTMLCollection& nodeList, Element& current)
+{
+    Element* previous = &current;
+    do {
+        previous = ElementTraversal::previousSibling(*previous);
+    } while (previous && !isMatchingElement(nodeList, *previous));
+    return previous;
+}
+
 Element* HTMLCollection::traverseToFirstElement() const
 {
     switch (type()) {
@@ -339,6 +351,14 @@ Element* HTMLCollection::traverseToFirstElement() const
     }
 }
 
+Element* HTMLCollection::traverseToLastElement() const
+{
+    ASSERT(canTraverseBackward());
+    if (shouldOnlyIncludeDirectChildren())
+        return lastMatchingChildElement(*this);
+    return lastMatchingElement(*this);
+}
+
 inline Element* HTMLCollection::traverseNextElement(Element& previous) const
 {
     if (overridesItemAfter())
@@ -377,6 +397,21 @@ Element* HTMLCollection::traverseForwardToOffset(unsigned offset, Element& curre
     }
 }
 
+Element* HTMLCollection::traverseBackwardToOffset(unsigned offset, Element& currentElement, unsigned& currentOffset) const
+{
+    ASSERT(currentOffset > offset);
+    ASSERT(canTraverseBackward());
+    if (shouldOnlyIncludeDirectChildren()) {

[Inactive, 2014/04/09 23:08:58]

Now we do this check once and then traverse to the right offset.
Previously, since there was no method to traverse backward to an offset (only
itemBefore()), we were doing this check for *every* item on the path to the
offset. The check was in LiveNodeListBase::itemBefore() and CollectionIndexCache
was calling it in a for loop until it reached the right offset.

+        Element* previous = &currentElement;
+        while ((previous = previousMatchingChildElement(*this, *previous))) {
+            if (--currentOffset == offset)
+                return previous;
+        }
+        return 0;
+    }
+    return traverseMatchingElementsBackwardToOffset(*this, offset, currentElement, currentOffset);
+}
+
 Element* HTMLCollection::namedItem(const AtomicString& name) const
 {
     // http://msdn.microsoft.com/workshop/author/dhtml/reference/methods/nameditem.asp