Introduce BackspaceStateMachine

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

+// 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.
+
+#include "core/editing/BackspaceStateMachine.h"
+
+#include "wtf/text/Unicode.h"
+
+namespace blink {
+
+int BackspaceStateMachine::finalizeAndGetCodeUnitCountToBeDeleted()
+{
+    if (m_trailSurrogate != 0) {
+        // Unpaired trail surrogate. Removing broken surrogate.
+        ++m_codeUnitsToBeDeleted;
+        m_trailSurrogate = 0;
+    }
+    return m_codeUnitsToBeDeleted;
+}
+
+bool BackspaceStateMachine::updateState(UChar codeUnit)
+{
+    uint32_t codePoint = codeUnit;
+    if (U16_IS_LEAD(codeUnit)) {
+        if (m_trailSurrogate == 0) {
+            // Unpaired lead surrogate. Aborting with deleting broken surrogate.
+            ++m_codeUnitsToBeDeleted;
+            return true;
+        }
+        codePoint = U16_GET_SUPPLEMENTARY(codeUnit, m_trailSurrogate);
+        m_trailSurrogate = 0;
+    } else if (U16_IS_TRAIL(codeUnit)) {
+        if (m_trailSurrogate != 0) {
+            // Unpaired trail surrogate. Aborting with deleting broken
+            // surrogate.
+            return true;
+        }
+        m_trailSurrogate = codeUnit;
+        return false; // Needs surrogate lead.
+    } else {
+        if (m_trailSurrogate != 0) {
+            // Unpaired trail surrogate. Aborting with deleting broken
+            // surrogate.
+            return true;
+        }
+    }
+
+    // TODO(nona): Handle emoji sequences.
+    m_codeUnitsToBeDeleted = U16_LENGTH(codePoint);
+    return true;
+}
+
+void BackspaceStateMachine::reset()
+{
+    m_codeUnitsToBeDeleted = 0;
+    m_trailSurrogate = 0;
+}
+
+} // namespace blink