Eliminate string conversions and concatenation in MaxTextOffset

MaxTextOffset was computing the total text string for a given node in
UTF-16 just to obtain its length. Taking the length of std::string is
a constant-time operation, so eliminating this effort reduces the scale
factor from number of characters to number of nodes. On a simple test
page this led to a 24% speedup in MaxTextOffset which is used by a
variety of TextPattern methods.

Bug: 970297
Change-Id: Ie40e8032b72a45510a2a9ee6bc0e4c4cf5564627
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730094
Commit-Queue: Kevin Babbitt <kbabbitt@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#683359}

ui/accessibility/ax_node_position.cc

@@ -43,6 +43,30 @@ base::string16 AXNodePosition::GetText() const {
   return text;
 }
 
+int AXNodePosition::MaxTextOffset() const {
+  if (IsNullPosition())
+    return INVALID_INDEX;
+
+  const AXNode* anchor = GetAnchor();
+  DCHECK(anchor);
+  const std::string& value =
+      anchor->data().GetStringAttribute(ax::mojom::StringAttribute::kValue);
+  if (!value.empty())
+    return value.length();
+
+  if (anchor->IsText()) {
+    return anchor->data()
+        .GetStringAttribute(ax::mojom::StringAttribute::kName)
+        .length();
+  }
+
+  int max_text_offset = 0;
+  for (int i = 0; i < AnchorChildCount(); ++i)
+    max_text_offset += CreateChildPositionAt(i)->MaxTextOffset();
+
+  return max_text_offset;
+}
+
 void AXNodePosition::AnchorChild(int child_index,
                                  AXTreeID* tree_id,
                                  int32_t* child_id) const {

ui/accessibility/ax_node_position.h

@@ -27,6 +27,7 @@ class AX_EXPORT AXNodePosition : public AXPosition<AXNodePosition, AXNode> {
 
   AXPositionInstance Clone() const override;
 
+  int MaxTextOffset() const override;
   bool IsInLineBreak() const override;
   bool IsInTextObject() const override;
   bool IsInWhiteSpace() const override;

ui/accessibility/ax_node_position_unittest.cc

@@ -50,6 +50,38 @@ class AXPositionTest : public testing::Test {
   void SetUp() override;
   void TearDown() override;
 
+  void AssertTextLengthEquals(const AXTree* tree,
+                              int32_t node_id,
+                              int expected_text_length) {
+    TestPositionType text_position = AXNodePosition::CreateTextPosition(
+        tree->data().tree_id, node_id, 0 /* text_offset */,
+        ax::mojom::TextAffinity::kUpstream);
+    ASSERT_NE(nullptr, text_position);
+    ASSERT_TRUE(text_position->IsTextPosition());
+    ASSERT_EQ(expected_text_length, text_position->MaxTextOffset());
+    ASSERT_EQ(expected_text_length,
+              static_cast<int>(text_position->GetText().length()));
+  }
+
+  // Creates a new AXTree from a vector of nodes.
+  // Assumes the first node in the vector is the root.
+  std::unique_ptr<AXTree> CreateAXTree(
+      const std::vector<ui::AXNodeData>& nodes) {
+    ui::AXTreeUpdate update;
+    ui::AXTreeData tree_data;
+    tree_data.tree_id = ui::AXTreeID::CreateNewAXTreeID();
+    update.tree_data = tree_data;
+    update.has_tree_data = true;
+    update.root_id = nodes[0].id;
+    update.nodes = nodes;
+
+    tree_data.tree_id = ui::AXTreeID::CreateNewAXTreeID();
+    update.tree_data = tree_data;
+    std::unique_ptr<AXTree> tree = std::make_unique<AXTree>(update);
+    AXNodePosition::SetTreeForTesting(tree.get());
+    return tree;
+  }
+
   AXNodeData root_;
   AXNodeData button_;
   AXNodeData check_box_;
@@ -416,6 +448,49 @@ TEST_F(AXPositionTest, GetMaxTextOffsetFromLineBreak) {
   ASSERT_EQ(1, text_position->MaxTextOffset());
 }
 
+TEST_F(AXPositionTest, GetMaxTextOffsetUpdate) {
+  AXNodePosition::SetTreeForTesting(nullptr);
+
+  ui::AXNodeData root_data;
+  root_data.id = 1;
+  root_data.role = ax::mojom::Role::kRootWebArea;
+
+  ui::AXNodeData text_data;
+  text_data.id = 2;
+  text_data.role = ax::mojom::Role::kStaticText;
+  text_data.SetName("some text");
+
+  ui::AXNodeData more_text_data;
+  more_text_data.id = 3;
+  more_text_data.role = ax::mojom::Role::kStaticText;
+  more_text_data.SetName("more text");
+
+  root_data.child_ids = {2, 3};
+
+  std::unique_ptr<AXTree> new_tree =
+      CreateAXTree({root_data, text_data, more_text_data});
+
+  AssertTextLengthEquals(new_tree.get(), text_data.id, 9);
+  AssertTextLengthEquals(new_tree.get(), root_data.id, 18);
+
+  text_data.SetName("Adjusted line 1");
+  new_tree = CreateAXTree({root_data, text_data, more_text_data});
+  AssertTextLengthEquals(new_tree.get(), text_data.id, 15);
+  AssertTextLengthEquals(new_tree.get(), root_data.id, 24);
+
+  // Value should override name
+  text_data.SetValue("Value should override name");
+  new_tree = CreateAXTree({root_data, text_data, more_text_data});
+  AssertTextLengthEquals(new_tree.get(), text_data.id, 26);
+  AssertTextLengthEquals(new_tree.get(), root_data.id, 35);
+
+  // An empty value should fall back to name
+  text_data.SetValue("");
+  new_tree = CreateAXTree({root_data, text_data, more_text_data});
+  AssertTextLengthEquals(new_tree.get(), text_data.id, 15);
+  AssertTextLengthEquals(new_tree.get(), root_data.id, 24);
+}
+
 TEST_F(AXPositionTest, AtStartOfAnchorWithNullPosition) {
   TestPositionType null_position = AXNodePosition::CreateNullPosition();
   ASSERT_NE(nullptr, null_position);

ui/accessibility/ax_position.h

@@ -1777,7 +1777,7 @@ class AXPosition {
 
   // Returns the length of the text that is present inside the anchor node,
   // including any text found in descendant text nodes.
-  int MaxTextOffset() const {
+  virtual int MaxTextOffset() const {
     if (IsNullPosition())
       return INVALID_INDEX;
     return static_cast<int>(GetText().length());