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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 FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_ORDERED_CODE_H_
#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_ORDERED_CODE_H_
#include <string>
#include "absl/strings/string_view.h"
namespace firebase {
namespace firestore {
namespace util {
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, absl::string_view 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, absl::string_view 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(absl::string_view* src, std::string* result);
static bool ReadNumIncreasing(absl::string_view* src, uint64_t* result);
static bool ReadSignedNumIncreasing(absl::string_view* src, int64_t* result);
static bool ReadInfinity(absl::string_view* src);
static bool ReadTrailingString(absl::string_view* src, std::string* result);
/** REQUIRES: next item was encoded by WriteInfinity() or WriteString(). */
static bool ReadStringOrInfinity(absl::string_view* 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 util
} // namespace firestore
} // namespace firebase
#endif // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_ORDERED_CODE_H_
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