Add iterator object to iterate efficiently over an Element's attributes

Source/core/dom/ElementData.h

Index: Source/core/dom/ElementData.h
diff --git a/Source/core/dom/ElementData.h b/Source/core/dom/ElementData.h
index e209f7fb1b8d76c4ed606df990128f10ce8e1580..99f2115f0f4afdccfa5e3b36fd7ba1d95613cafd 100644
--- a/Source/core/dom/ElementData.h
+++ b/Source/core/dom/ElementData.h
@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
+ * Copyright (C) 2014 Apple Inc. All rights reserved.

[esprehn, 2014/05/28 20:03:54]

Do we really need to add Apple copyrights?

[Inactive, 2014/05/29 15:14:01]

I don't know what the exact rules are but it seems (to me) like the right thing
to do since I am porting over once of their patches.

FYI, I have done this so far when porting patches over to Blink.

[Inactive, 2014/05/29 15:17:59]

Also, this CL seems to hint that I am not the only one doing this:
https://codereview.chromium.org/305653007/

  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -42,6 +43,44 @@ class ShareableElementData;
 class StylePropertySet;
 class UniqueElementData;
 
+class AttributeConstIterator {
+public:
+    AttributeConstIterator(const Attribute* array, unsigned index)
+        : m_array(array)
+        , m_index(index)
+    { }
+
+    const Attribute* operator*() const { return &m_array[m_index]; }
+    const Attribute* operator->() const { return &m_array[m_index]; }
+    AttributeConstIterator& operator++() { ++m_index; return *this; }
+
+    bool operator==(const AttributeConstIterator& other) const { return m_index == other.m_index; }
+    bool operator!=(const AttributeConstIterator& other) const { return !(*this == other); }
+
+    unsigned index() const { return m_index; }
+
+private:
+    const Attribute* m_array;
+    unsigned m_index;
+};
+
+class AttributeIteratorAccessor {

[esprehn, 2014/05/28 20:03:54]

This class seems unnecessary.

[Inactive, 2014/05/29 15:14:01]

The main reason for this class (besides avoiding multiple calls to isUnique() as
explained below), is that this will let us use range iterators when we
eventually start using C++11 (http://www.stroustrup.com/C++11FAQ.html#for):
for (const Attribute& attribute : attributesIterator())

For the range-for statement to work, the object needs to have begin() and end()
methods defines (i.e. not attributesBegin() / attributesEnd()). Defining those
on ElementData may be a bit confusing. Defining those on Element would
definitely be confusing as it would not be obvious the iterators are for the
Element's attributes.

+public:
+    AttributeIteratorAccessor(const Attribute* array, unsigned size)
+        : m_array(array)
+        , m_size(size)
+    { }
+
+    AttributeConstIterator begin() const { return AttributeConstIterator(m_array, 0); }
+    AttributeConstIterator end() const { return AttributeConstIterator(m_array, m_size); }
+
+    unsigned size() const { return m_size; }
+
+private:
+    const Attribute* m_array;
+    unsigned m_size;
+};
+
 // ElementData represents very common, but not necessarily unique to an element,
 // data such as attributes, inline style, and parsed class names and ids.
 class ElementData : public RefCounted<ElementData> {
@@ -66,6 +105,8 @@ public:
     size_t length() const;
     bool isEmpty() const { return !length(); }
 
+    AttributeIteratorAccessor attributesIterator() const;

[esprehn, 2014/05/28 20:03:54]

Can we just add attributesBegin() and attributesEnd() instead? Why do we need
this class?

[Inactive, 2014/05/29 15:14:01]

If we do this then we will call ElementData::isUnique() twice, once in
attributesBegin() and once in attributesEnd(), right? Instead of now calling it
only once when calling attributesData().

[Inactive, 2014/05/30 13:16:03]

Note that we had issues before with callers not caching attributeCount() before
the loop and thus calling ElementData::isUnique() unnecessarily for every loop
iteration. The proposal at the time was to introduce iterators to avoid this.
However, if attributesEnd() hides a call to ElementData::isUnique(), then the
same type of error is also possible if the caller fails to cache the end
iterator before the loop.

+
     const Attribute& attributeItem(unsigned index) const;
     const Attribute* getAttributeItem(const QualifiedName&) const;
     size_t getAttributeItemIndex(const QualifiedName&, bool shouldIgnoreCase = false) const;
@@ -202,13 +243,11 @@ inline const Attribute* ElementData::attributeBase() const
 
 inline size_t ElementData::getAttributeItemIndex(const QualifiedName& name, bool shouldIgnoreCase) const
 {
-    const Attribute* begin = attributeBase();
-    // Cache length for performance as ElementData::length() contains a conditional branch.
-    unsigned length = this->length();
-    for (unsigned i = 0; i < length; ++i) {
-        const Attribute& attribute = begin[i];
-        if (attribute.name().matchesPossiblyIgnoringCase(name, shouldIgnoreCase))
-            return i;
+    AttributeIteratorAccessor attributes = attributesIterator();
+    AttributeConstIterator end = attributes.end();
+    for (AttributeConstIterator it = attributes.begin(); it != end; ++it) {
+        if (it->name().matchesPossiblyIgnoringCase(name, shouldIgnoreCase))
+            return it.index();
     }
     return kNotFound;
 }
@@ -217,19 +256,17 @@ inline size_t ElementData::getAttributeItemIndex(const QualifiedName& name, bool
 // can tune the behavior (hasAttribute is case sensitive whereas getAttribute is not).
 inline size_t ElementData::getAttributeItemIndex(const AtomicString& name, bool shouldIgnoreAttributeCase) const
 {
-    // Cache length for performance as ElementData::length() contains a conditional branch.
-    unsigned length = this->length();
     bool doSlowCheck = shouldIgnoreAttributeCase;
 
     // Optimize for the case where the attribute exists and its name exactly matches.
-    const Attribute* begin = attributeBase();
-    for (unsigned i = 0; i < length; ++i) {
-        const Attribute& attribute = begin[i];
+    AttributeIteratorAccessor attributes = attributesIterator();
+    AttributeConstIterator end = attributes.end();
+    for (AttributeConstIterator it = attributes.begin(); it != end; ++it) {
         // FIXME: Why check the prefix? Namespaces should be all that matter.
         // Most attributes (all of HTML and CSS) have no namespace.
-        if (!attribute.name().hasPrefix()) {
-            if (name == attribute.localName())
-                return i;
+        if (!it->name().hasPrefix()) {
+            if (name == it->localName())
+                return it.index();
         } else {
             doSlowCheck = true;
         }
@@ -240,14 +277,22 @@ inline size_t ElementData::getAttributeItemIndex(const AtomicString& name, bool
     return kNotFound;
 }
 
+inline AttributeIteratorAccessor ElementData::attributesIterator() const
+{
+    if (isUnique()) {
+        const Vector<Attribute, 4>& attributeVector = static_cast<const UniqueElementData*>(this)->m_attributeVector;
+        return AttributeIteratorAccessor(attributeVector.data(), attributeVector.size());
+    }
+    return AttributeIteratorAccessor(static_cast<const ShareableElementData*>(this)->m_attributeArray, m_arraySize);
+}
+
 inline const Attribute* ElementData::getAttributeItem(const QualifiedName& name) const
 {
-    const Attribute* begin = attributeBase();
-    unsigned length = this->length();
-    for (unsigned i = 0; i < length; ++i) {
-        const Attribute& attribute = begin[i];
-        if (attribute.name().matches(name))
-            return &attribute;
+    AttributeIteratorAccessor attributes = attributesIterator();
+    AttributeConstIterator end = attributes.end();
+    for (AttributeConstIterator it = attributes.begin(); it != end; ++it) {
+        if (it->name().matches(name))
+            return *it;
     }
     return 0;
 }