Introduce BackwardCodePointStateMachine to traverse chars in code points

third_party/WebKit/Source/core/editing/state_machines/BackwardCodePointStateMachine.h

+// Copyright 2017 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 BackwardCodePointStateMachine_h
+#define BackwardCodePointStateMachine_h
+
+#include "core/CoreExport.h"
+#include "core/editing/state_machines/TextSegmentationMachineState.h"
+#include "wtf/Allocator.h"
+#include "wtf/Noncopyable.h"
+#include "wtf/text/Unicode.h"
+
+namespace blink {
+
+class CORE_EXPORT BackwardCodePointStateMachine {
+  STACK_ALLOCATED();
+  WTF_MAKE_NONCOPYABLE(BackwardCodePointStateMachine);
+
+ public:
+  BackwardCodePointStateMachine();
+  ~BackwardCodePointStateMachine() = default;
+
+  // Prepares by feeding preceding text.
+  TextSegmentationMachineState feedPrecedingCodeUnit(UChar codeUnit);
+
+  // Finds boundary offset by feeding following text.
+  TextSegmentationMachineState feedFollowingCodeUnit(UChar codeUnit);
+
+  // Returns true if we are at code point boundary.
+  bool atCodePointBoundary();
+
+  // Returns the next boundary offset.
+  int getBoundaryOffset();
+
+  // Resets the internal state to the initial state.
+  void reset();
+
+ private:
+  enum class BackwardCodePointState;
+
+  // The number of code units to be deleted.
+  // Nothing to delete if there is an invalid surrogate pair.
+  int m_codeUnitsToBeDeleted = 0;
+
+  // The internal state.
+  BackwardCodePointState m_state;
+};
+
+}  // namespace blink
+
+#endif