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diff --git a/dom/base/ChildIterator.h b/dom/base/ChildIterator.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef ChildIterator_h
+#define ChildIterator_h
+
+#include "nsIContent.h"
+
+/**
+ * Iterates over the children on a node. If a child is an insertion point,
+ * iterates over the children inserted there instead, or the default content
+ * if no children are inserted there.
+ *
+ * The FlattenedChildIterator expands any anonymous content bound from an XBL
+ * binding's <xbl:content> element.
+ */
+
+#include <stdint.h>
+#include "nsAutoPtr.h"
+
+class nsIContent;
+
+namespace mozilla {
+namespace dom {
+
+// This class iterates normal DOM child nodes of a given DOM node with
+// <xbl:children> nodes replaced by the elements that have been filtered into that
+// insertion point. Any bindings on the given element are ignored for purposes
+// of determining which insertion point children are filtered into. The iterator
+// can be initialized to start at the end by providing false for aStartAtBeginning
+// in order to start iterating in reverse from the last child.
+class ExplicitChildIterator
+{
+public:
+  explicit ExplicitChildIterator(const nsIContent* aParent,
+                                 bool aStartAtBeginning = true)
+    : mParent(aParent),
+      mChild(nullptr),
+      mDefaultChild(nullptr),
+      mIndexInInserted(0),
+      mIsFirst(aStartAtBeginning)
+  {
+  }
+
+  ExplicitChildIterator(const ExplicitChildIterator& aOther)
+    : mParent(aOther.mParent), mChild(aOther.mChild),
+      mDefaultChild(aOther.mDefaultChild),
+      mShadowIterator(aOther.mShadowIterator ?
+                      new ExplicitChildIterator(*aOther.mShadowIterator) :
+                      nullptr),
+      mIndexInInserted(aOther.mIndexInInserted), mIsFirst(aOther.mIsFirst) {}
+
+  ExplicitChildIterator(ExplicitChildIterator&& aOther)
+    : mParent(aOther.mParent), mChild(aOther.mChild),
+      mDefaultChild(aOther.mDefaultChild),
+      mShadowIterator(Move(aOther.mShadowIterator)),
+      mIndexInInserted(aOther.mIndexInInserted), mIsFirst(aOther.mIsFirst) {}
+
+  nsIContent* GetNextChild();
+
+  // Looks for aChildToFind respecting insertion points until aChildToFind is
+  // found.  This version can take shortcuts that the two-argument version
+  // can't, so can be faster (and in fact can be O(1) instead of O(N) in many
+  // cases).
+  bool Seek(nsIContent* aChildToFind);
+
+  // Looks for aChildToFind respecting insertion points until aChildToFind is found.
+  // or aBound is found. If aBound is nullptr then the seek is unbounded. Returns
+  // whether aChildToFind was found as an explicit child prior to encountering
+  // aBound.
+  bool Seek(nsIContent* aChildToFind, nsIContent* aBound)
+  {
+    // It would be nice to assert that we find aChildToFind, but bz thinks that
+    // we might not find aChildToFind when called from ContentInserted
+    // if first-letter frames are about.
+
+    // We can't easily take shortcuts here because we'd have to have a way to
+    // compare aChildToFind to aBound.
+    nsIContent* child;
+    do {
+      child = GetNextChild();
+    } while (child && child != aChildToFind && child != aBound);
+
+    return child == aChildToFind;
+  }
+
+  // Returns the current target of this iterator (which might be an explicit
+  // child of the node, fallback content of an insertion point or
+  // a node distributed to an insertion point.
+  nsIContent* Get() const;
+
+  // The inverse of GetNextChild. Properly steps in and out of insertion
+  // points.
+  nsIContent* GetPreviousChild();
+
+protected:
+  // The parent of the children being iterated. For the FlattenedChildIterator,
+  // if there is a binding attached to the original parent, mParent points to
+  // the <xbl:content> element for the binding.
+  const nsIContent* mParent;
+
+  // The current child. When we encounter an insertion point,
+  // mChild remains as the insertion point whose content we're iterating (and
+  // our state is controled by mDefaultChild or mIndexInInserted depending on
+  // whether the insertion point expands to its default content or not).
+  nsIContent* mChild;
+
+  // If non-null, this points to the current default content for the current
+  // insertion point that we're iterating (i.e. mChild, which must be an
+  // nsXBLChildrenElement or HTMLContentElement). Once this transitions back
+  // to null, we continue iterating at mChild's next sibling.
+  nsIContent* mDefaultChild;
+
+  // If non-null, this points to an iterator of the explicit children of
+  // the ShadowRoot projected by the current shadow element that we're
+  // iterating.
+  nsAutoPtr<ExplicitChildIterator> mShadowIterator;
+
+  // If not zero, we're iterating inserted children for an insertion point. This
+  // is an index into mChild's inserted children array (mChild must be an
+  // nsXBLChildrenElement). The index is one past the "current" child (as
+  // opposed to mChild which represents the "current" child).
+  uint32_t mIndexInInserted;
+
+  // A flag to let us know that we haven't started iterating yet.
+  bool mIsFirst;
+};
+
+// Iterates over the flattened children of a node, which accounts for anonymous
+// children and nodes moved by insertion points. If a node has anonymous
+// children, those are iterated over.  The iterator can be initialized to start
+// at the end by providing false for aStartAtBeginning in order to start
+// iterating in reverse from the last child.
+class FlattenedChildIterator : public ExplicitChildIterator
+{
+public:
+  explicit FlattenedChildIterator(const nsIContent* aParent,
+                                  bool aStartAtBeginning = true)
+    : ExplicitChildIterator(aParent, aStartAtBeginning), mXBLInvolved(false)
+  {
+    Init(false);
+  }
+
+  FlattenedChildIterator(FlattenedChildIterator&& aOther)
+    : ExplicitChildIterator(Move(aOther)), mXBLInvolved(aOther.mXBLInvolved) {}
+
+  FlattenedChildIterator(const FlattenedChildIterator& aOther)
+    : ExplicitChildIterator(aOther), mXBLInvolved(aOther.mXBLInvolved) {}
+
+  bool XBLInvolved() { return mXBLInvolved; }
+
+protected:
+  /**
+   * This constructor is a hack to help AllChildrenIterator which sometimes
+   * doesn't want to consider XBL.
+   */
+  FlattenedChildIterator(const nsIContent* aParent, uint32_t aFlags,
+                         bool aStartAtBeginning = true)
+    : ExplicitChildIterator(aParent, aStartAtBeginning), mXBLInvolved(false)
+  {
+    bool ignoreXBL = aFlags & nsIContent::eAllButXBL;
+    Init(ignoreXBL);
+  }
+
+  void Init(bool aIgnoreXBL);
+
+  // For certain optimizations, nsCSSFrameConstructor needs to know if the
+  // child list of the element that we're iterating matches its .childNodes.
+  bool mXBLInvolved;
+};
+
+/**
+ * AllChildrenIterator traverses the children of an element including before /
+ * after content and optionally XBL children.  The iterator can be initialized
+ * to start at the end by providing false for aStartAtBeginning in order to
+ * start iterating in reverse from the last child.
+ *
+ * Note: it assumes that no mutation of the DOM or frame tree takes place during
+ * iteration, and will break horribly if that is not true.
+ */
+class AllChildrenIterator : private FlattenedChildIterator
+{
+public:
+  AllChildrenIterator(const nsIContent* aNode, uint32_t aFlags,
+                      bool aStartAtBeginning = true) :
+    FlattenedChildIterator(aNode, aFlags, aStartAtBeginning),
+    mOriginalContent(aNode), mAnonKidsIdx(aStartAtBeginning ? UINT32_MAX : 0),
+    mFlags(aFlags), mPhase(aStartAtBeginning ? eAtBegin : eAtEnd) { }
+
+  AllChildrenIterator(AllChildrenIterator&& aOther)
+    : FlattenedChildIterator(Move(aOther)),
+      mOriginalContent(aOther.mOriginalContent),
+      mAnonKids(Move(aOther.mAnonKids)), mAnonKidsIdx(aOther.mAnonKidsIdx),
+      mFlags(aOther.mFlags), mPhase(aOther.mPhase)
+#ifdef DEBUG
+      , mMutationGuard(aOther.mMutationGuard)
+#endif
+      {}
+
+#ifdef DEBUG
+  ~AllChildrenIterator() { MOZ_ASSERT(!mMutationGuard.Mutated(0)); }
+#endif
+
+  // Returns the current target the iterator is at, or null if the iterator
+  // doesn't point to any child node (either eAtBegin or eAtEnd phase).
+  nsIContent* Get() const;
+
+  // Seeks the given node in children of a parent element, starting from
+  // the current iterator's position, and sets the iterator at the given child
+  // node if it was found.
+  bool Seek(nsIContent* aChildToFind);
+
+  nsIContent* GetNextChild();
+  nsIContent* GetPreviousChild();
+  const nsIContent* Parent() const { return mOriginalContent; }
+
+  enum IteratorPhase
+  {
+    eAtBegin,
+    eAtBeforeKid,
+    eAtExplicitKids,
+    eAtAnonKids,
+    eAtAfterKid,
+    eAtEnd
+  };
+  IteratorPhase Phase() const { return mPhase; }
+
+private:
+  // Helpers.
+  void AppendNativeAnonymousChildren();
+  void AppendNativeAnonymousChildrenFromFrame(nsIFrame* aFrame);
+
+  const nsIContent* mOriginalContent;
+
+  // mAnonKids is an array of native anonymous children, mAnonKidsIdx is index
+  // in the array. If mAnonKidsIdx < mAnonKids.Length() and mPhase is
+  // eAtAnonKids then the iterator points at a child at mAnonKidsIdx index. If
+  // mAnonKidsIdx == mAnonKids.Length() then the iterator is somewhere after
+  // the last native anon child. If mAnonKidsIdx == UINT32_MAX then the iterator
+  // is somewhere before the first native anon child.
+  nsTArray<nsIContent*> mAnonKids;
+  uint32_t mAnonKidsIdx;
+
+  uint32_t mFlags;
+  IteratorPhase mPhase;
+#ifdef DEBUG
+  // XXX we should really assert there are no frame tree changes as well, but
+  // there's no easy way to do that.
+  nsMutationGuard mMutationGuard;
+#endif
+};
+
+/**
+ * StyleChildrenIterator traverses the children of the element from the
+ * perspective of the style system, particularly the children we need to traverse
+ * during restyle. This is identical to AllChildrenIterator with eAllChildren,
+ * _except_ that we detect and skip any native anonymous children that are used
+ * to implement pseudo-elements (since the style system needs to cascade those
+ * using different algorithms).
+ *
+ * Note: it assumes that no mutation of the DOM or frame tree takes place during
+ * iteration, and will break horribly if that is not true.
+ */
+class StyleChildrenIterator : private AllChildrenIterator {
+public:
+  explicit StyleChildrenIterator(const nsIContent* aContent)
+    : AllChildrenIterator(aContent, nsIContent::eAllChildren)
+  {
+    MOZ_COUNT_CTOR(StyleChildrenIterator);
+  }
+  ~StyleChildrenIterator() { MOZ_COUNT_DTOR(StyleChildrenIterator); }
+
+  nsIContent* GetNextChild();
+
+  // Returns true if we cannot find all the children we need to style by
+  // traversing the siblings of the first child.
+  static bool IsNeeded(const Element* aParent);
+};
+
+} // namespace dom
+} // namespace mozilla
+
+#endif