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+// This module provides routines for encoding a sequence of typed
+// entities into a string.  The resulting strings can be
+// lexicographically compared to yield the same comparison value that
+// would have been generated if the encoded items had been compared
+// one by one according to their type.
+//
+// More precisely, suppose:
+//  1. string A is generated by encoding the sequence of items [A_1..A_n]
+//  2. string B is generated by encoding the sequence of items [B_1..B_n]
+//  3. The types match; i.e., for all i: A_i was encoded using
+//     the same routine as B_i
+// Then:
+//    Comparing A vs. B lexicographically is the same as comparing
+//    the vectors [A_1..A_n] and [B_1..B_n] lexicographically.
+//
+// Furthermore, if n < m, the encoding of [A_1..A_n] is a strict prefix of
+// [A_1..A_m] (unless m = n+1 and A_m is the empty string encoded with
+// WriteTrailingString, in which case the encodings are equal).
+//
+// This module is often useful when generating multi-part sstable
+// keys that have to be ordered in a particular fashion.
+
+#ifndef TENSORFLOW_LIB_STRINGS_ORDERED_CODE_H__
+#define TENSORFLOW_LIB_STRINGS_ORDERED_CODE_H__
+
+#include <string>
+#include "tensorflow/core/platform/port.h"
+
+namespace tensorflow {
+class StringPiece;
+
+namespace strings {
+
+class OrderedCode {
+ public:
+  // -------------------------------------------------------------------
+  // Encoding routines: each one of the following routines append
+  // one item to "*dest" in an encoding where larger values are
+  // ordered lexicographically after smaller values.
+  static void WriteString(string* dest, StringPiece str);
+  static void WriteNumIncreasing(string* dest, uint64 num);
+  static void WriteSignedNumIncreasing(string* dest, int64 num);
+
+  // -------------------------------------------------------------------
+  // Decoding routines: these extract an item earlier encoded using
+  // the corresponding WriteXXX() routines above.  The item is read
+  // from "*src"; "*src" is modified to point past the decoded item;
+  // and if "result" is non-NULL, "*result" is modified to contain the
+  // result.  In case of string result, the decoded string is appended to
+  // "*result".  Returns true if the next item was read successfully, false
+  // otherwise.
+  static bool ReadString(StringPiece* src, string* result);
+  static bool ReadNumIncreasing(StringPiece* src, uint64* result);
+  static bool ReadSignedNumIncreasing(StringPiece* src, int64* result);
+
+  // Helper for testing: corrupt "*str" by changing the kth item separator
+  // in the string.
+  static void TEST_Corrupt(string* str, int k);
+
+  // Helper for testing.
+  // SkipToNextSpecialByte is an internal routine defined in the .cc file
+  // with the following semantics. Return a pointer to the first byte
+  // in the range "[start..limit)" whose value is 0 or 255.  If no such
+  // byte exists in the range, returns "limit".
+  static const char* TEST_SkipToNextSpecialByte(const char* start,
+                                                const char* limit);
+
+ private:
+  // This has only static methods, so disallow construction entirely
+  OrderedCode();
+  TF_DISALLOW_COPY_AND_ASSIGN(OrderedCode);
+};
+
+}  // namespace strings
+}  // namespace tensorflow
+
+#endif  // TENSORFLOW_LIB_STRINGS_ORDERED_CODE_H__