[LayoutNG] Introduce NGPaintFragmentTraversal::MoveToParent() and MoveToPrevious()

This patch inttroduces |MoveToParent()| and |MoveToPrevious()| in
|NGPaintFragmentTraversal| class for the patch[1], which uses |MoveToPrevious()|
to find text fragment in same line.

This patch also utilize |MoveToParent()| in |MoveToNext()| to share code for
improving code health.

[1] http://crrev.com/c/1104082 [LayoutNG] Adapt accessibility module to
LayoutNG;

Bug: 774229
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: If65e3015b8f4ed768325c0e13df121c0bc378580
Reviewed-on: https://chromium-review.googlesource.com/1109609Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569199}

third_party/blink/renderer/core/paint/ng/ng_paint_fragment_traversal.cc

@@ -201,33 +201,53 @@ void NGPaintFragmentTraversal::MoveToNext() {
 }
 
 void NGPaintFragmentTraversal::MoveToNextSiblingOrAncestor() {
-  if (IsAtEnd())
-    return;
-
-  while (true) {
+  while (!IsAtEnd()) {
     // Check if we have a next sibling.
     auto& stack_top = stack_.back();
     if (++stack_top.index < stack_top.parent->Children().size()) {
       current_ = stack_top.parent->Children()[stack_top.index].get();
       return;
     }
+    MoveToParent();
+  }
+}
 
-    // Check the next parent in the stack. If the stack is not empty, traverse
-    // its next sibiling.
-    stack_.pop_back();
-    if (!stack_.IsEmpty())
-      continue;
-
+void NGPaintFragmentTraversal::MoveToParent() {
+  if (IsAtEnd())
+    return;
+  DCHECK(!stack_.IsEmpty());
+  const NGPaintFragment& parent = *stack_.back().parent;
+  stack_.pop_back();
+  if (&parent == &root_) {
+    DCHECK(stack_.IsEmpty());
+    current_ = nullptr;
+    return;
+  }
+  if (stack_.IsEmpty()) {
     // We might have started with |MoveTo()|, and thus computing parent stack
-    // was deferred. Check parents until we reach the |root_|.
-    const NGPaintFragment* parent = current_->Parent();
-    DCHECK(parent);
-    if (parent == &root_) {
-      current_ = nullptr;
-      return;
-    }
-    Push(*parent);
+    // was deferred.
+    Push(parent);
+    return;
+  }
+  DCHECK_EQ(&parent,
+            stack_.back().parent->Children()[stack_.back().index].get());
+  current_ = &parent;
+}
+
+void NGPaintFragmentTraversal::MoveToPrevious() {
+  if (IsAtEnd())
+    return;
+  DCHECK(!stack_.IsEmpty());
+  auto& stack_top = stack_.back();
+  if (stack_top.index == 0) {
+    // There is no previous sibling of |current_|. We move to parent.
+    MoveToParent();
+    return;
   }
+  --stack_top.index;
+  current_ = stack_top.parent->Children()[stack_top.index].get();
+  while (!current_->Children().IsEmpty())
+    Push(*current_, current_->Children().size() - 1);
 }
 
 Vector<NGPaintFragmentWithContainerOffset>

third_party/blink/renderer/core/paint/ng/ng_paint_fragment_traversal.h

@@ -73,12 +73,24 @@ class CORE_EXPORT NGPaintFragmentTraversal {
   void MoveTo(const NGPaintFragment& fragment);
 
   // Move to the next node using the pre-order depth-first-search.
+  // Note: When |IsAtEnd()| is true, this function does nothing.
   void MoveToNext();
 
   // Move to the next sibling, or next ancestor node using the pre-order
   // depth-first-search, skipping children of the current node.
   void MoveToNextSiblingOrAncestor();
 
+  // Move to the parent of current fragment. When |current_| is a child of
+  // |root_|, this function makes |IsAtEnd()| to true.
+  // Note: When |IsAtEnd()| is true, this function does nothing.
+  void MoveToParent();
+
+  // Move to the previous node using the pre-order depth-first-search. When
+  // |current_| is the first child of |root_|, this function makes |IsAtEnd()|
+  // to true.
+  // Note: When |IsAtEnd()| is true, this function does nothing.
+  void MoveToPrevious();
+
   //
   // Following functions are static, similar to DOM traversal utilities.
   //
@@ -119,7 +131,12 @@ class CORE_EXPORT NGPaintFragmentTraversal {
   void Push(const NGPaintFragment& parent, unsigned index);
   void Push(const NGPaintFragment& fragment);
 
+  // |current_| holds a |NGPaintFragment| specified by |index|th child of
+  // |parent| of the last element of |stack_|.
   const NGPaintFragment* current_ = nullptr;
+
+  // The root of subtree where traversing is taken place. |root_| is excluded
+  // from traversal. |current_| can't |root_|.
   const NGPaintFragment& root_;
 
   // The stack of parent and its child index up to the root. Each stack entry

third_party/blink/renderer/core/paint/ng/ng_paint_fragment_traversal_test.cc

@@ -41,6 +41,16 @@ class NGPaintFragmentTraversalTest : public RenderingTest,
     return results;
   }
 
+  Vector<const NGPaintFragment*> ToReverseDepthFirstList(
+      NGPaintFragmentTraversal* traversal) const {
+    Vector<const NGPaintFragment*> results;
+    for (; *traversal; traversal->MoveToPrevious()) {
+      const NGPaintFragment& fragment = **traversal;
+      results.push_back(&fragment);
+    }
+    return results;
+  }
+
   LayoutBlockFlow* layout_block_flow_;
   NGPaintFragment* root_fragment_;
 };
@@ -83,6 +93,46 @@ TEST_F(NGPaintFragmentTraversalTest, MoveToNextWithRoot) {
                                 span->Children()[0].get(), br}));
 }
 
+TEST_F(NGPaintFragmentTraversalTest, MoveToPrevious) {
+  SetUpHtml("t", R"HTML(
+    <div id=t>
+      line0
+      <span style="background: red">red</span>
+      <br>
+      line1
+    </div>
+  )HTML");
+  NGPaintFragmentTraversal traversal(*root_fragment_);
+  NGPaintFragment* line0 = root_fragment_->Children()[0].get();
+  NGPaintFragment* line1 = root_fragment_->Children()[1].get();
+  NGPaintFragment* span = line0->Children()[1].get();
+  NGPaintFragment* br = line0->Children()[2].get();
+  traversal.MoveTo(*line1->Children()[0].get());
+  EXPECT_THAT(ToReverseDepthFirstList(&traversal),
+              ElementsAreArray({line1->Children()[0].get(), line1, br,
+                                span->Children()[0].get(), span,
+                                line0->Children()[0].get(), line0}));
+}
+
+TEST_F(NGPaintFragmentTraversalTest, MoveToPreviousWithRoot) {
+  SetUpHtml("t", R"HTML(
+    <div id=t>
+      line0
+      <span style="background: red">red</span>
+      <br>
+      line1
+    </div>
+  )HTML");
+  NGPaintFragment* line0 = root_fragment_->Children()[0].get();
+  NGPaintFragment* span = line0->Children()[1].get();
+  NGPaintFragment* br = line0->Children()[2].get();
+  NGPaintFragmentTraversal traversal(*line0);
+  traversal.MoveTo(*br);
+  EXPECT_THAT(ToReverseDepthFirstList(&traversal),
+              ElementsAreArray({br, span->Children()[0].get(), span,
+                                line0->Children()[0].get()}));
+}
+
 TEST_F(NGPaintFragmentTraversalTest, MoveTo) {
   SetUpHtml("t", R"HTML(
     <div id=t>