EXPERIMENT TextIterator built on Layout NG offset mapping

third_party/WebKit/Source/core/layout/ng/inline/ng_offset_mapping_builder.cc

Index: third_party/WebKit/Source/core/layout/ng/inline/ng_offset_mapping_builder.cc
diff --git a/third_party/WebKit/Source/core/layout/ng/inline/ng_offset_mapping_builder.cc b/third_party/WebKit/Source/core/layout/ng/inline/ng_offset_mapping_builder.cc
index 46c551c224066683e16223b7fb2e9dc0cd94b9dd..7b6c4746b428b655bb101a10ca4ed6425d152f6b 100644
--- a/third_party/WebKit/Source/core/layout/ng/inline/ng_offset_mapping_builder.cc
+++ b/third_party/WebKit/Source/core/layout/ng/inline/ng_offset_mapping_builder.cc
@@ -4,6 +4,9 @@
 
 #include "core/layout/ng/inline/ng_offset_mapping_builder.h"
 
+#include <algorithm>
+#include "core/editing/VisibleUnits.h"
+#include "core/layout/LayoutText.h"
 #include "core/layout/ng/inline/ng_offset_mapping_result.h"
 #include "platform/wtf/text/StringBuilder.h"
 
@@ -45,6 +48,34 @@ std::pair<NGOffsetMappingUnitType, unsigned> GetMappingUnitTypeAndEnd(
 
 }  // namespace
 
+const NGOffsetMappingUnit* NGOffsetMappingResult::GetMappingUnitForDOMOffset(
+    const Node& node,
+    unsigned offset) {
+  const LayoutObject* layout_object = AssociatedLayoutObjectOf(node, offset);
+  if (!layout_object || !layout_object->IsText() ||
+      !ranges.Contains(ToLayoutText(layout_object)))
+    return nullptr;
+
+  unsigned range_start;
+  unsigned range_end;
+  std::tie(range_start, range_end) = ranges.at(ToLayoutText(layout_object));
+
+  if (range_start == range_end || units[range_start].dom_start > offset)
+    return nullptr;
+
+  // Find the last unit where unit.dom_start <= offset
+  unsigned unit_index =
+      std::upper_bound(units.begin() + range_start, units.begin() + range_end,
+                       offset,
+                       [](unsigned offset, const NGOffsetMappingUnit& unit) {
+                         return offset < unit.dom_start;
+                       }) -
+      units.begin() - 1;
+  if (units[unit_index].dom_end < offset)
+    return nullptr;
+  return &units[unit_index];
+}
+
 NGOffsetMappingBuilder::NGOffsetMappingBuilder() {
   mapping_.push_back(0);
 }