[LayoutNG] Avoid CreateShapeResult when `text-overflow: ellipsis`

This patch changes `text-overflow: ellipsis` to create the
truncated ShapeResultView from ShapeResultView without
creating intermediate ShapeResult, using the mechanism added
in r633612 (CL:1477141). This saves memory by not creating
actual sub RunInfo.

This change also fixes an issue in offset computation in
ShapeResultView::ForEachGraphemeClusters. This is discovered
by this change because before this change, RunInfoPart always
points to the full range of RunInfo.

Some utility functions to compute character index are added.
I hope they can be better structured to abstract differences
between ShapeResult and ShapeResultView, but doing so is
deferred to future patches as I understand it better.

Bug: 636993
Change-Id: Ib2e501829051ca44f7aaccac4856ffce5618ba23
Reviewed-on: https://chromium-review.googlesource.com/c/1478319
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635758}

third_party/blink/renderer/core/layout/ng/inline/ng_line_truncator.cc

@@ -216,6 +216,7 @@ bool NGLineTruncator::TruncateChild(LayoutUnit space_for_child,
       IsLtr(line_direction_) ? space_for_child
                              : shape_result->Width() - space_for_child,
       line_direction_);
+  DCHECK_LE(new_length, fragment.Length());
   if (!new_length || new_length == fragment.Length()) {
     if (!is_first_child)
       return false;

third_party/blink/renderer/core/layout/ng/inline/ng_physical_text_fragment.cc

@@ -288,14 +288,10 @@ scoped_refptr<const NGPhysicalFragment> NGPhysicalTextFragment::TrimText(
   DCHECK_GE(new_start_offset, StartOffset());
   DCHECK_GT(new_end_offset, new_start_offset);
   DCHECK_LE(new_end_offset, EndOffset());
-  // TODO(layout-dev): Add sub-range version of CreateShapeResult to avoid
-  // this double copy.
-  scoped_refptr<ShapeResult> new_shape_result =
-      shape_result_->CreateShapeResult()->SubRange(new_start_offset,
-                                                   new_end_offset);
+  scoped_refptr<ShapeResultView> new_shape_result = ShapeResultView::Create(
+      shape_result_.get(), new_start_offset, new_end_offset);
   return base::AdoptRef(new NGPhysicalTextFragment(
-      *this, new_start_offset, new_end_offset,
-      ShapeResultView::Create(new_shape_result.get())));
+      *this, new_start_offset, new_end_offset, std::move(new_shape_result)));
 }
 
 unsigned NGPhysicalTextFragment::TextOffsetForPoint(

third_party/blink/renderer/platform/fonts/shaping/shape_result_view.cc

@@ -42,6 +42,13 @@ struct ShapeResultView::RunInfoPart {
   const HarfBuzzRunGlyphData& GlyphAt(unsigned index) const {
     return *(range_.begin + index);
   }
+  // The end character index of |this| without considering offsets in
+  // |ShapeResultView|. This is analogous to:
+  //   GlyphAt(Rtl() ? -1 : NumGlyphs()).character_index
+  // if such |HarfBuzzRunGlyphData| is available.
+  unsigned CharacterIndexOfEndGlyph() const {
+    return num_characters_ + offset_;
+  }
 
   bool Rtl() const { return run_->Rtl(); }
   bool IsHorizontal() const { return run_->IsHorizontal(); }
@@ -50,9 +57,6 @@ struct ShapeResultView::RunInfoPart {
   float Width() const { return width_; }
 
   unsigned PreviousSafeToBreakOffset(unsigned offset) const;
-  size_t GlyphToCharacterIndex(size_t i) const {
-    return run_->GlyphToCharacterIndex(i);
-  }
 
   // Common signatures with RunInfo, to templatize algorithms.
   const ShapeResult::RunInfo* GetRunInfo() const { return run_.get(); }
@@ -100,6 +104,13 @@ unsigned ShapeResultView::RunInfoPart::PreviousSafeToBreakOffset(
   return 0;
 }
 
+// The offset to add to |HarfBuzzRunGlyphData.character_index| to compute the
+// character index of the source string.
+unsigned ShapeResultView::CharacterIndexOffsetForGlyphData(
+    const RunInfoPart& part) const {
+  return part.start_index_ + char_index_offset_ - part.offset_;
+}
+
 template <class ShapeResultType>
 ShapeResultView::ShapeResultView(const ShapeResultType* other)
     : primary_font_(other->primary_font_),
@@ -378,10 +389,11 @@ float ShapeResultView::ForEachGlyph(float initial_advance,
     const auto& run = part->run_;
     bool is_horizontal = HB_DIRECTION_IS_HORIZONTAL(run->direction_);
     const SimpleFontData* font_data = run->font_data_.get();
+    const unsigned character_index_offset_for_glyph_data =
+        CharacterIndexOffsetForGlyphData(*part);
     for (const auto& glyph_data : *part) {
-      unsigned character_index = glyph_data.character_index +
-                                 part->start_index_ + char_index_offset_ -
-                                 part->offset_;
+      unsigned character_index =
+          glyph_data.character_index + character_index_offset_for_glyph_data;
       glyph_callback(context, character_index, glyph_data.glyph,
                      glyph_data.offset, total_advance, is_horizontal,
                      run->canvas_rotation_, font_data);
@@ -404,12 +416,13 @@ float ShapeResultView::ForEachGlyph(float initial_advance,
     const auto& run = part->run_;
     bool is_horizontal = HB_DIRECTION_IS_HORIZONTAL(run->direction_);
     const SimpleFontData* font_data = run->font_data_.get();
+    const unsigned character_index_offset_for_glyph_data =
+        CharacterIndexOffsetForGlyphData(*part);
 
     if (!run->Rtl()) {  // Left-to-right
       for (const auto& glyph_data : *part) {
-        unsigned character_index = glyph_data.character_index +
-                                   part->start_index_ + char_index_offset_ -
-                                   part->offset_;
+        unsigned character_index =
+            glyph_data.character_index + character_index_offset_for_glyph_data;
         if (character_index >= to)
           break;
         if (character_index >= from) {
@@ -422,9 +435,8 @@ float ShapeResultView::ForEachGlyph(float initial_advance,
 
     } else {  // Right-to-left
       for (const auto& glyph_data : *part) {
-        unsigned character_index = glyph_data.character_index +
-                                   part->start_index_ + char_index_offset_ -
-                                   part->offset_;
+        unsigned character_index =
+            glyph_data.character_index + character_index_offset_for_glyph_data;
         if (character_index < from)
           break;
         if (character_index < to) {
@@ -460,20 +472,24 @@ float ShapeResultView::ForEachGraphemeClusters(const StringView& text,
     // broken down further from a text shaping point of view.  A cluster can
     // contain multiple glyphs and grapheme clusters, with mutually overlapping
     // boundaries.
-    uint16_t cluster_start = static_cast<uint16_t>(
-        rtl ? run->start_index_ + run->num_characters_ + run_offset
-            : run->GlyphToCharacterIndex(0) + run_offset);
+    const unsigned character_index_offset_for_glyph_data =
+        CharacterIndexOffsetForGlyphData(*part) + run_offset;
+    uint16_t cluster_start =
+        static_cast<uint16_t>(rtl ? part->CharacterIndexOfEndGlyph() +
+                                        character_index_offset_for_glyph_data
+                                  : part->GlyphAt(0).character_index +
+                                        character_index_offset_for_glyph_data);
 
     const unsigned num_glyphs = part->NumGlyphs();
     for (unsigned i = 0; i < num_glyphs; ++i) {
       const HarfBuzzRunGlyphData& glyph_data = part->GlyphAt(i);
-      uint16_t current_character_index = glyph_data.character_index +
-                                         part->start_index_ +
-                                         char_index_offset_ - part->offset_;
+      uint16_t current_character_index =
+          glyph_data.character_index + character_index_offset_for_glyph_data;
 
       bool is_run_end = (i + 1 == num_glyphs);
       bool is_cluster_end =
-          is_run_end || (run->GlyphToCharacterIndex(i + 1) + run_offset !=
+          is_run_end || (part->GlyphAt(i + 1).character_index +
+                             character_index_offset_for_glyph_data !=
                          current_character_index);
 
       if ((rtl && current_character_index >= to) ||
@@ -496,8 +512,10 @@ float ShapeResultView::ForEachGraphemeClusters(const StringView& text,
           cluster_end = current_character_index;
         } else {
           cluster_end = static_cast<uint16_t>(
-              is_run_end ? run->start_index_ + run->num_characters_ + run_offset
-                         : run->GlyphToCharacterIndex(i + 1) + run_offset);
+              is_run_end ? part->CharacterIndexOfEndGlyph() +
+                               character_index_offset_for_glyph_data
+                         : part->GlyphAt(i + 1).character_index +
+                               character_index_offset_for_glyph_data);
         }
         graphemes_in_cluster = ShapeResult::CountGraphemesInCluster(
             text.Characters16(), text.length(), cluster_start, cluster_end);

third_party/blink/renderer/platform/fonts/shaping/shape_result_view.h

@@ -155,6 +155,8 @@ class PLATFORM_EXPORT ShapeResultView final
   template <bool is_horizontal_run>
   void ComputeBoundsForPart(const RunInfoPart&, float origin);
 
+  unsigned CharacterIndexOffsetForGlyphData(const RunInfoPart&) const;
+
   // Common signatures with ShapeResult, to templatize algorithms.
   const Vector<std::unique_ptr<RunInfoPart>, 4>& RunsOrParts() const {
     return parts_;