Introduce BackwardCodePointStateMachine to traverse chars in code points

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

+// 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.
+
+#include "core/editing/state_machines/BackwardCodePointStateMachine.h"
+
+namespace blink {
+
+enum class BackwardCodePointStateMachine::BackwardCodePointState {
+  NotSurrogate,
+  TrailSurrogate,
+  Invalid,
+};
+
+BackwardCodePointStateMachine::BackwardCodePointStateMachine()
+    : m_state(BackwardCodePointState::NotSurrogate) {}
+
+TextSegmentationMachineState
+BackwardCodePointStateMachine::feedPrecedingCodeUnit(UChar codeUnit) {
+  switch (m_state) {
+    case BackwardCodePointState::NotSurrogate:
+      if (U16_IS_LEAD(codeUnit)) {
+        m_codeUnitsToBeDeleted = 0;
+        m_state = BackwardCodePointState::Invalid;
+        return TextSegmentationMachineState::Invalid;
+      }
+      ++m_codeUnitsToBeDeleted;
+      if (U16_IS_TRAIL(codeUnit)) {
+        m_state = BackwardCodePointState::TrailSurrogate;
+        return TextSegmentationMachineState::NeedMoreCodeUnit;
+      }
+      return TextSegmentationMachineState::Finished;
+    case BackwardCodePointState::TrailSurrogate:
+      if (U16_IS_LEAD(codeUnit)) {
+        ++m_codeUnitsToBeDeleted;
+        m_state = BackwardCodePointState::NotSurrogate;
+        return TextSegmentationMachineState::Finished;
+      }
+      m_codeUnitsToBeDeleted = 0;
+      m_state = BackwardCodePointState::Invalid;
+      return TextSegmentationMachineState::Invalid;
+    case BackwardCodePointState::Invalid:
+      m_codeUnitsToBeDeleted = 0;
+      return TextSegmentationMachineState::Invalid;
+  }
+  NOTREACHED();
+  return TextSegmentationMachineState::Invalid;
+}
+
+TextSegmentationMachineState
+BackwardCodePointStateMachine::feedFollowingCodeUnit(UChar codeUnit) {
+  NOTREACHED();
+  return TextSegmentationMachineState::Invalid;
+}
+
+bool BackwardCodePointStateMachine::atCodePointBoundary() {
+  return m_state == BackwardCodePointState::NotSurrogate;
+}
+
+int BackwardCodePointStateMachine::getBoundaryOffset() {
+  return -m_codeUnitsToBeDeleted;
+}
+
+void BackwardCodePointStateMachine::reset() {
+  m_codeUnitsToBeDeleted = 0;
+  m_state = BackwardCodePointState::NotSurrogate;
+}
+
+}  // namespace blink