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+/*
+ * Copyright 2017 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// 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 IPHONE_FIRESTORE_PORT_ORDERED_CODE_H_
+#define IPHONE_FIRESTORE_PORT_ORDERED_CODE_H_
+
+#include <string>
+
+namespace leveldb {
+class Slice;
+}
+
+namespace Firestore {
+
+class OrderedCode {
+ public:
+  // -------------------------------------------------------------------
+  // Encoding routines: each one of the following routines append
+  // one item to "*dest".  The Write(Signed)NumIncreasing() and
+  // Write(Signed)NumDecreasing() routines differ in whether the resulting
+  // encoding is ordered by increasing number or decreasing number.
+  // Similarly, WriteString() and WriteStringDecreasing() differ in whether
+  // the resulting encoding is ordered by the original string in
+  // lexicographically increasing or decreasing order.  WriteString()
+  // is not called WriteStringIncreasing() for convenience and backward
+  // compatibility.
+
+  static void WriteString(std::string* dest, leveldb::Slice str);
+  static void WriteNumIncreasing(std::string* dest, uint64_t num);
+  static void WriteSignedNumIncreasing(std::string* dest, int64_t num);
+
+  // Creates an encoding for the "infinite string", a value considered to
+  // be lexicographically after any real string.  Note that in the case of
+  // WriteInfinityDecreasing(), this would come before any real string as
+  // the ordering puts lexicographically greater values first.
+  static void WriteInfinity(std::string* dest);
+
+  // Special string append that can only be used at the tail end of
+  // an encoded string -- blindly appends "str" to "*dest".
+  static void WriteTrailingString(std::string* dest, leveldb::Slice str);
+
+  // -------------------------------------------------------------------
+  // 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(leveldb::Slice* src, std::string* result);
+  static bool ReadNumIncreasing(leveldb::Slice* src, uint64_t* result);
+  static bool ReadSignedNumIncreasing(leveldb::Slice* src, int64_t* result);
+
+  static bool ReadInfinity(leveldb::Slice* src);
+  static bool ReadTrailingString(leveldb::Slice* src, std::string* result);
+
+  // REQUIRES: next item was encoded by WriteInfinity() or WriteString()
+  static bool ReadStringOrInfinity(leveldb::Slice* src, std::string* result, bool* inf);
+
+  // Helper for testing: corrupt "*str" by changing the kth item separator
+  // in the string.
+  static void TEST_Corrupt(std::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);
+
+  // Not an instantiable class, but the class exists to make it easy to
+  // use with a single using statement.
+  OrderedCode() = delete;
+  OrderedCode(const OrderedCode&) = delete;
+  OrderedCode& operator=(const OrderedCode&) = delete;
+};
+
+}  // namespace Firestore
+
+#endif  // IPHONE_FIRESTORE_PORT_ORDERED_CODE_H_