Introduce ForwardsTextBuffer and BackwardsTextBuffer

third_party/WebKit/Source/core/editing/iterators/BackwardsTextBuffer.cpp

Index: third_party/WebKit/Source/core/editing/iterators/BackwardsTextBuffer.cpp
diff --git a/third_party/WebKit/Source/core/editing/iterators/BackwardsTextBuffer.cpp b/third_party/WebKit/Source/core/editing/iterators/BackwardsTextBuffer.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..9d3cc06c2938363c081e92719f289e76593852d8
--- /dev/null
+++ b/third_party/WebKit/Source/core/editing/iterators/BackwardsTextBuffer.cpp
@@ -0,0 +1,27 @@
+// 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/iterators/BackwardsTextBuffer.h"
+
+namespace blink {
+
+const UChar* BackwardsTextBuffer::data() const
+{
+    return bufferEnd() - size();
+}
+
+UChar* BackwardsTextBuffer::calcDestination(size_t length)
+{
+    ASSERT(size() + length <= capacity());
+    return bufferEnd() - size() - length;
+}
+
+void BackwardsTextBuffer::shiftData(size_t oldCapacity)
+{
+    ASSERT(oldCapacity <= capacity());
+    ASSERT(size() <= oldCapacity);
+    std::copy_backward(bufferBegin() + oldCapacity - size(), bufferBegin() + oldCapacity, bufferEnd());
+}
+
+} // namespace blink