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diff --git a/src/rocksdb/utilities/merge_operators/string_append/stringappend2.h b/src/rocksdb/utilities/merge_operators/string_append/stringappend2.h
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+// Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+/**
+ * A TEST MergeOperator for rocksdb that implements string append.
+ * It is built using the MergeOperator interface rather than the simpler
+ * AssociativeMergeOperator interface. This is useful for testing/benchmarking.
+ * While the two operators are semantically the same, all production code
+ * should use the StringAppendOperator defined in stringappend.{h,cc}. The
+ * operator defined in the present file is primarily for testing.
+ *
+ * @author Deon Nicholas (dnicholas@fb.com)
+ * Copyright 2013 Facebook
+ */
+
+#pragma once
+#include <deque>
+#include <string>
+
+#include "rocksdb/merge_operator.h"
+#include "rocksdb/slice.h"
+
+namespace ROCKSDB_NAMESPACE {
+
+class StringAppendTESTOperator : public MergeOperator {
+ public:
+  // Constructor with delimiter
+  explicit StringAppendTESTOperator(char delim_char);
+  explicit StringAppendTESTOperator(const std::string& delim);
+
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override;
+
+  virtual bool PartialMergeMulti(const Slice& key,
+                                 const std::deque<Slice>& operand_list,
+                                 std::string* new_value,
+                                 Logger* logger) const override;
+
+  static const char* kClassName() { return "StringAppendTESTOperator"; }
+  static const char* kNickName() { return "stringappendtest"; }
+  const char* Name() const override { return kClassName(); }
+  const char* NickName() const override { return kNickName(); }
+
+ private:
+  // A version of PartialMerge that actually performs "partial merging".
+  // Use this to simulate the exact behaviour of the StringAppendOperator.
+  bool _AssocPartialMergeMulti(const Slice& key,
+                               const std::deque<Slice>& operand_list,
+                               std::string* new_value, Logger* logger) const;
+
+  std::string delim_;  // The delimiter is inserted between elements
+};
+
+}  // namespace ROCKSDB_NAMESPACE