Implement backspace state machine for complex emoji sequence.

third_party/WebKit/Source/core/editing/BackspaceStateMachine.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BackspaceStateMachine_h
 #define BackspaceStateMachine_h
 
 #include "core/CoreExport.h"
 #include "wtf/Allocator.h"
 #include "wtf/Noncopyable.h"
@@ skipping 21 matching lines @@
     // This method must be called after feedPrecedingCodeUnit() returns PreparationDone.
     State feedFollowingCodeUnit(UChar codeUnit);
 
     // Returns the next boundary offset. This method finalizes the state machine if it is not finished.
     int finalizeAndGetBoundaryOffset();
 
     // Resets the internal state to the initial state.
     void reset();
 
 private:
-    int m_codeUnitsToBeDeleted = 0;
+    enum class BackspaceState {
+        Start, // Initial state
+        BeforeLF, // Indicates the current offset is just before line feed.
+        BeforeKeycap, // Indicates the current offset is just before keycap.
+        BeforeVSAndKeycap, // Indicates the current offset is just before variation selector and keycap.
+        BeforeEmojiModifier, // Indicates the current offset is just before emoji modifier.
+        BeforeVSAndEmojiModifier, // Indicates the current offset is just before variation selector and emoji modifier.
+        BeforeVS, // Indicates the current offset is just before variation sequence.
+        BeforeZWJEmoji, // Indicates the current offset is just before ZWJ emoji.
+        BeforeZWJ, // Indicates the current offset is just before ZWJ.
+        BeforeVSAndZWJ, // Indicates the current offset is just before variation selector and ZWJ.
+        OddNumberedRIS, // Indicates that there are odd numbered RIS from the beggining.
+        EvenNumberedRIS, // Indicates that there are even numbered RIS from the begging.
+        Finished, // Indicates this state machine has finished.
+    } m_state = BackspaceState::Start;
 
     // Used for composing supplementary code point with surrogate pairs.
     UChar m_trailSurrogate = 0;
+
+    // The number of code units to be deleted.
+    int m_codeUnitsToBeDeleted = 0;
+
+    // The length of the previously seen variation selector.
+    int m_lastSeenVSCodeUnits = 0;
 };
 
 } // namespace blink
 
 #endif