UIA move-by-format performance improvements

This change introduces two main optimizations for move-by-format. Both
of these optimizations combined end up improving performance of Narrator
heading moves by 25% on NYTimes.com and 40% on a reduced test page.
I'm now seeing the format move cost dwarfed by the GetAttributeValue
cost for Narrator Scan Mode move-by-heading, which I'll look into next.

The first category of improvements in this change is from replacing
AsLeafTextPosition calls with AsLeafTreePosition. When we're moving
by format in intermediate steps, it doesn't matter whether the node
is a tree position or a text position, so this avoids expensive
MaxTextOffset calls.

The second category involves returning a reason why a format boundary
was determined. I saw that we were calling
Create(Next/Previous)TreePosition several times just to see if a move is
at the end of a document. So a more complex return value that provides
the reason of why a format boundary exists allows us to not have to do
that work multiple times. The AXBoundaryType enum class added with this
change provides this information, with the new method
'GetFormatEndBoundaryType' doing the work of determining format
boundaries and returning the appropriate AXBoundaryType.

Bug: 1029867
Change-Id: I12151ab2b4000227e1d5e2726223a7545d73bc2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1965865Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727375}

ui/accessibility/ax_position.h

@@ -52,6 +52,20 @@ enum class AXBoundaryBehavior {
   StopAtLastAnchorBoundary
 };
 
+// Describes in further detail what type of boundary a current position is on.
+// For complex boundaries such as format boundaries, it can be useful to know
+// why a particular boundary was chosen.
+enum class AXBoundaryType {
+  // Not at a unit boundary.
+  kNone,
+  // At a unit boundary (e.g. a format boundary).
+  kUnitBoundary,
+  // At the start of a document.
+  kDocumentStart,
+  // At the end of a document.
+  kDocumentEnd
+};
+
 // When converting to an unignored position, determines how to adjust the new
 // position in order to make it valid, either moving backwards or forwards in
 // the accessibility tree.
@@ -706,38 +720,54 @@ class AXPosition {
     }
   }
 
-  bool AtStartOfFormat() const {
+  AXBoundaryType GetFormatStartBoundaryType() const {
     // Since formats are stored on text anchors, the start of a format boundary
     // must be at the start of an anchor.
     if (IsNullPosition() || !AtStartOfAnchor())
-      return false;
+      return AXBoundaryType::kNone;
 
     // Treat the first iterable node as a format boundary.
     if (CreatePreviousLeafTreePosition()->IsNullPosition())
-      return true;
+      return AXBoundaryType::kDocumentStart;
 
     // Iterate over anchors until a format boundary is found. This will return a
     // null position upon crossing a boundary.
     AXPositionInstance previous_position = CreatePreviousLeafTreePosition(
         base::BindRepeating(&AbortMoveAtFormatBoundary));
-    return previous_position->IsNullPosition();
+
+    if (previous_position->IsNullPosition())
+      return AXBoundaryType::kUnitBoundary;
+
+    return AXBoundaryType::kNone;
   }
 
-  bool AtEndOfFormat() const {
+  bool AtStartOfFormat() const {
+    return GetFormatStartBoundaryType() != AXBoundaryType::kNone;
+  }
+
+  AXBoundaryType GetFormatEndBoundaryType() const {
     // Since formats are stored on text anchors, the end of a format break must
     // be at the end of an anchor.
     if (IsNullPosition() || !AtEndOfAnchor())
-      return false;
+      return AXBoundaryType::kNone;
 
     // Treat the last iterable node as a format boundary
     if (CreateNextLeafTreePosition()->IsNullPosition())
-      return true;
+      return AXBoundaryType::kDocumentEnd;
 
     // Iterate over anchors until a format boundary is found. This will return a
     // null position upon crossing a boundary.
     AXPositionInstance next_position = CreateNextLeafTreePosition(
         base::BindRepeating(&AbortMoveAtFormatBoundary));
-    return next_position->IsNullPosition();
+
+    if (next_position->IsNullPosition())
+      return AXBoundaryType::kUnitBoundary;
+
+    return AXBoundaryType::kNone;
+  }
+
+  bool AtEndOfFormat() const {
+    return GetFormatEndBoundaryType() != AXBoundaryType::kNone;
   }
 
   bool AtStartOfInlineBlock() const {
@@ -1968,12 +1998,11 @@ class AXPosition {
     if (IsNullPosition())
       return Clone();
 
-    // AtStartOfFormat() always returns true if we are at the first iterable
-    // position, i.e. CreatePreviousLeafTreePosition()->IsNullPosition().
-    if (AtStartOfFormat()) {
+    AXBoundaryType boundary_type = GetFormatStartBoundaryType();
+    if (boundary_type != AXBoundaryType::kNone) {
       if (boundary_behavior == AXBoundaryBehavior::StopIfAlreadyAtBoundary ||
           (boundary_behavior == AXBoundaryBehavior::StopAtLastAnchorBoundary &&
-           CreatePreviousLeafTreePosition()->IsNullPosition())) {
+           boundary_type == AXBoundaryType::kDocumentStart)) {
         AXPositionInstance clone = Clone();
         // In order to make equality checks simpler, affinity should be reset so
         // that we would get consistent output from this function regardless of
@@ -1981,7 +2010,7 @@ class AXPosition {
         clone->affinity_ = ax::mojom::TextAffinity::kDownstream;
         return clone;
       } else if (boundary_behavior == AXBoundaryBehavior::CrossBoundary &&
-                 CreatePreviousLeafTreePosition()->IsNullPosition()) {
+                 boundary_type == AXBoundaryType::kDocumentStart) {
         // If we're at a format boundary and there are no more text positions
         // to traverse, return a null position for cross-boundary moves.
         return CreateNullPosition();
@@ -2004,7 +2033,8 @@ class AXPosition {
 
     // The first position in the document is also a format start boundary, so we
     // should not return NullPosition unless we started from that location.
-    while (!previous_tree_position->IsNullPosition() &&
+    while (boundary_type != AXBoundaryType::kDocumentStart &&
+           !previous_tree_position->IsNullPosition() &&
            !tree_position->AtStartOfFormat()) {
       tree_position = std::move(previous_tree_position);
       previous_tree_position = tree_position->CreatePreviousLeafTreePosition();
@@ -2031,12 +2061,11 @@ class AXPosition {
     if (IsNullPosition())
       return Clone();
 
-    // AtEndOfFormat() always returns true if we are at the last iterable
-    // position, i.e. CreateNextLeafTreePosition()->IsNullPosition().
-    if (AtEndOfFormat()) {
+    AXBoundaryType boundary_type = GetFormatEndBoundaryType();
+    if (boundary_type != AXBoundaryType::kNone) {
       if (boundary_behavior == AXBoundaryBehavior::StopIfAlreadyAtBoundary ||
           (boundary_behavior == AXBoundaryBehavior::StopAtLastAnchorBoundary &&
-           CreateNextLeafTreePosition()->IsNullPosition())) {
+           boundary_type == AXBoundaryType::kDocumentEnd)) {
         AXPositionInstance clone = Clone();
         // In order to make equality checks simpler, affinity should be reset so
         // that we would get consistent output from this function regardless of
@@ -2044,7 +2073,7 @@ class AXPosition {
         clone->affinity_ = ax::mojom::TextAffinity::kDownstream;
         return clone;
       } else if (boundary_behavior == AXBoundaryBehavior::CrossBoundary &&
-                 CreateNextLeafTreePosition()->IsNullPosition()) {
+                 boundary_type == AXBoundaryType::kDocumentEnd) {
         // If we're at a format boundary and there are no more text positions
         // to traverse, return a null position for cross-boundary moves.
         return CreateNullPosition();
@@ -2068,7 +2097,8 @@ class AXPosition {
 
     // The last position in the document is also a format end boundary, so we
     // should not return NullPosition unless we started from that location.
-    while (!next_tree_position->IsNullPosition() &&
+    while (boundary_type != AXBoundaryType::kDocumentEnd &&
+           !next_tree_position->IsNullPosition() &&
            !tree_position->AtEndOfFormat()) {
       tree_position = std::move(next_tree_position);
       next_tree_position = tree_position->CreateNextLeafTreePosition()
@@ -3012,8 +3042,8 @@ class AXPosition {
     }
 
     // Stop moving when text styles differ.
-    return move_from.AsLeafTextPosition()->GetTextStyles() !=
-           move_to.AsLeafTextPosition()->GetTextStyles();
+    return move_from.AsLeafTreePosition()->GetTextStyles() !=
+           move_to.AsLeafTreePosition()->GetTextStyles();
   }
 
   // AbortMovePredicate function used to detect paragraph boundaries.