/src/rocksdb/util/comparator.cc

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//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#include "rocksdb/comparator.h"

#include <algorithm>
#include <cstdint>
#include <memory>
#include <mutex>
#include <sstream>

#include "db/dbformat.h"
#include "port/lang.h"
#include "port/port.h"
#include "rocksdb/convenience.h"
#include "rocksdb/slice.h"
#include "rocksdb/utilities/customizable_util.h"
#include "rocksdb/utilities/object_registry.h"
#include "util/coding.h"

namespace ROCKSDB_NAMESPACE {

namespace {
class BytewiseComparatorImpl : public Comparator {
 public:
  BytewiseComparatorImpl() = default;
  static const char* kClassName() { return "leveldb.BytewiseComparator"; }
  const char* Name() const override { return kClassName(); }

  int Compare(const Slice& a, const Slice& b) const override {
    return a.compare(b);
  }

  bool Equal(const Slice& a, const Slice& b) const override { return a == b; }

  void FindShortestSeparator(std::string* start,
                             const Slice& limit) const override {
    // Find length of common prefix
    size_t min_length = std::min(start->size(), limit.size());
    size_t diff_index = 0;
    while ((diff_index < min_length) &&
           ((*start)[diff_index] == limit[diff_index])) {
      diff_index++;
    }

    if (diff_index >= min_length) {
      // Do not shorten if one string is a prefix of the other
    } else {
      uint8_t start_byte = static_cast<uint8_t>((*start)[diff_index]);
      uint8_t limit_byte = static_cast<uint8_t>(limit[diff_index]);
      if (start_byte >= limit_byte) {
        // Cannot shorten since limit is smaller than start or start is
        // already the shortest possible.
        return;
      }
      assert(start_byte < limit_byte);

      if (diff_index < limit.size() - 1 || start_byte + 1 < limit_byte) {
        (*start)[diff_index]++;
        start->resize(diff_index + 1);
      } else {
        //     v
        // A A 1 A A A
        // A A 2
        //
        // Incrementing the current byte will make start bigger than limit, we
        // will skip this byte, and find the first non 0xFF byte in start and
        // increment it.
        diff_index++;

        while (diff_index < start->size()) {
          // Keep moving until we find the first non 0xFF byte to
          // increment it
          if (static_cast<uint8_t>((*start)[diff_index]) <
              static_cast<uint8_t>(0xff)) {
            (*start)[diff_index]++;
            start->resize(diff_index + 1);
            break;
          }
          diff_index++;
        }
      }
      assert(Compare(*start, limit) < 0);
    }
  }

  void FindShortSuccessor(std::string* key) const override {
    // Find first character that can be incremented
    size_t n = key->size();
    for (size_t i = 0; i < n; i++) {
      const uint8_t byte = (*key)[i];
      if (byte != static_cast<uint8_t>(0xff)) {
        (*key)[i] = byte + 1;
        key->resize(i + 1);
        return;
      }
    }
    // *key is a run of 0xffs.  Leave it alone.
  }

  bool IsSameLengthImmediateSuccessor(const Slice& s,
                                      const Slice& t) const override {
    if (s.size() != t.size() || s.size() == 0) {
      return false;
    }
    size_t diff_ind = s.difference_offset(t);
    // same slice
    if (diff_ind >= s.size()) {
      return false;
    }
    uint8_t byte_s = static_cast<uint8_t>(s[diff_ind]);
    uint8_t byte_t = static_cast<uint8_t>(t[diff_ind]);
    // first different byte must be consecutive, and remaining bytes must be
    // 0xff for s and 0x00 for t
    if (byte_s != uint8_t{0xff} && byte_s + 1 == byte_t) {
      for (size_t i = diff_ind + 1; i < s.size(); ++i) {
        byte_s = static_cast<uint8_t>(s[i]);
        byte_t = static_cast<uint8_t>(t[i]);
        if (byte_s != uint8_t{0xff} || byte_t != uint8_t{0x00}) {
          return false;
        }
      }
      return true;
    } else {
      return false;
    }
  }

  bool CanKeysWithDifferentByteContentsBeEqual() const override {
    return false;
  }

  using Comparator::CompareWithoutTimestamp;
  int CompareWithoutTimestamp(const Slice& a, bool /*a_has_ts*/, const Slice& b,
                              bool /*b_has_ts*/) const override {
    return a.compare(b);
  }

  bool EqualWithoutTimestamp(const Slice& a, const Slice& b) const override {
    return a == b;
  }
};

class ReverseBytewiseComparatorImpl : public BytewiseComparatorImpl {
 public:
  ReverseBytewiseComparatorImpl() = default;

  static const char* kClassName() {
    return "rocksdb.ReverseBytewiseComparator";
  }
  const char* Name() const override { return kClassName(); }

  int Compare(const Slice& a, const Slice& b) const override {
    return -a.compare(b);
  }

  void FindShortestSeparator(std::string* start,
                             const Slice& limit) const override {
    // Find length of common prefix
    size_t min_length = std::min(start->size(), limit.size());
    size_t diff_index = 0;
    while ((diff_index < min_length) &&
           ((*start)[diff_index] == limit[diff_index])) {
      diff_index++;
    }

    assert(diff_index <= min_length);
    if (diff_index == min_length) {
      // Do not shorten if one string is a prefix of the other
      //
      // We could handle cases like:
      //     V
      // A A 2 X Y
      // A A 2
      // in a similar way as BytewiseComparator::FindShortestSeparator().
      // We keep it simple by not implementing it. We can come back to it
      // later when needed.
    } else {
      uint8_t start_byte = static_cast<uint8_t>((*start)[diff_index]);
      uint8_t limit_byte = static_cast<uint8_t>(limit[diff_index]);
      if (start_byte > limit_byte && diff_index < start->size() - 1) {
        // Case like
        //     V
        // A A 3 A A
        // A A 1 B B
        //
        // or
        //     v
        // A A 2 A A
        // A A 1 B B
        // In this case "AA2" will be good.
#ifndef NDEBUG
        std::string old_start = *start;
#endif
        start->resize(diff_index + 1);
#ifndef NDEBUG
        assert(old_start >= *start);
#endif
        assert(Slice(*start).compare(limit) > 0);
      }
    }
  }

  void FindShortSuccessor(std::string* /*key*/) const override {
    // Don't do anything for simplicity.
  }

  bool IsSameLengthImmediateSuccessor(const Slice& s,
                                      const Slice& t) const override {
    // Always returning false to prevent surfacing design flaws in
    // auto_prefix_mode
    (void)s, (void)t;
    return false;
    // "Correct" implementation:
    // return BytewiseComparatorImpl::IsSameLengthImmediateSuccessor(t, s);
  }

  bool CanKeysWithDifferentByteContentsBeEqual() const override {
    return false;
  }

  using Comparator::CompareWithoutTimestamp;
  int CompareWithoutTimestamp(const Slice& a, bool /*a_has_ts*/, const Slice& b,
                              bool /*b_has_ts*/) const override {
    return -a.compare(b);
  }
};

// Comparator with 64-bit integer timestamp.
// We did not performance test this yet.
template <typename TComparator>
class ComparatorWithU64TsImpl : public Comparator {
  static_assert(std::is_base_of<Comparator, TComparator>::value,
                "template type must be a inherited type of comparator");

 public:
  explicit ComparatorWithU64TsImpl() : Comparator(/*ts_sz=*/sizeof(uint64_t)) {
    assert(cmp_without_ts_.timestamp_size() == 0);
  }

  static const char* kClassName() {
    static std::string class_name = kClassNameInternal();
    return class_name.c_str();
  }

  const char* Name() const override { return kClassName(); }

  // The comparator that compares the user key without timestamp part is treated
  // as the root comparator.
  const Comparator* GetRootComparator() const override {
    return &cmp_without_ts_;
  }

  void FindShortSuccessor(std::string*) const override {}
  void FindShortestSeparator(std::string*, const Slice&) const override {}
  int Compare(const Slice& a, const Slice& b) const override {
    int ret = CompareWithoutTimestamp(a, b);
    size_t ts_sz = timestamp_size();
    if (ret != 0) {
      return ret;
    }
    // Compare timestamp.
    // For the same user key with different timestamps, larger (newer) timestamp
    // comes first.
    return -CompareTimestamp(ExtractTimestampFromUserKey(a, ts_sz),
                             ExtractTimestampFromUserKey(b, ts_sz));
  }

  Slice GetMaxTimestamp() const override { return MaxU64Ts(); }

  Slice GetMinTimestamp() const override { return MinU64Ts(); }

  std::string TimestampToString(const Slice& timestamp) const override {
    assert(timestamp.size() == sizeof(uint64_t));
    uint64_t ts = 0;
    DecodeU64Ts(timestamp, &ts).PermitUncheckedError();
    return std::to_string(ts);
  }

  using Comparator::CompareWithoutTimestamp;
  int CompareWithoutTimestamp(const Slice& a, bool a_has_ts, const Slice& b,
                              bool b_has_ts) const override {
    const size_t ts_sz = timestamp_size();
    assert(!a_has_ts || a.size() >= ts_sz);
    assert(!b_has_ts || b.size() >= ts_sz);
    Slice lhs = a_has_ts ? StripTimestampFromUserKey(a, ts_sz) : a;
    Slice rhs = b_has_ts ? StripTimestampFromUserKey(b, ts_sz) : b;
    return cmp_without_ts_.Compare(lhs, rhs);
  }
  int CompareTimestamp(const Slice& ts1, const Slice& ts2) const override {
    assert(ts1.size() == sizeof(uint64_t));
    assert(ts2.size() == sizeof(uint64_t));
    uint64_t lhs = DecodeFixed64(ts1.data());
    uint64_t rhs = DecodeFixed64(ts2.data());
    if (lhs < rhs) {
      return -1;
    } else if (lhs > rhs) {
      return 1;
    } else {
      return 0;
    }
  }

 private:
  static std::string kClassNameInternal() {
    std::stringstream ss;
    ss << TComparator::kClassName() << ".u64ts";
    return ss.str();
  }

  TComparator cmp_without_ts_;
};

}  // namespace

const Comparator* BytewiseComparator() {
  STATIC_AVOID_DESTRUCTION(BytewiseComparatorImpl, bytewise);
  return &bytewise;
}

const Comparator* ReverseBytewiseComparator() {
  STATIC_AVOID_DESTRUCTION(ReverseBytewiseComparatorImpl, rbytewise);
  return &rbytewise;
}

const Comparator* BytewiseComparatorWithU64Ts() {
  STATIC_AVOID_DESTRUCTION(ComparatorWithU64TsImpl<BytewiseComparatorImpl>,
                           comp_with_u64_ts);
  return &comp_with_u64_ts;
}

const Comparator* ReverseBytewiseComparatorWithU64Ts() {
  STATIC_AVOID_DESTRUCTION(
      ComparatorWithU64TsImpl<ReverseBytewiseComparatorImpl>, comp_with_u64_ts);
  return &comp_with_u64_ts;
}

Status DecodeU64Ts(const Slice& ts, uint64_t* int_ts) {
  if (ts.size() != sizeof(uint64_t)) {
    return Status::InvalidArgument("U64Ts timestamp size mismatch.");
  }
  *int_ts = DecodeFixed64(ts.data());
  return Status::OK();
}

Slice EncodeU64Ts(uint64_t ts, std::string* ts_buf) {
  char buf[sizeof(ts)];
  EncodeFixed64(buf, ts);
  ts_buf->assign(buf, sizeof(buf));
  return Slice(*ts_buf);
}

Slice MaxU64Ts() {
  static constexpr char kTsMax[] = "\xff\xff\xff\xff\xff\xff\xff\xff";
  return Slice(kTsMax, sizeof(uint64_t));
}

Slice MinU64Ts() {
  static constexpr char kTsMin[] = "\x00\x00\x00\x00\x00\x00\x00\x00";
  return Slice(kTsMin, sizeof(uint64_t));
}

static int RegisterBuiltinComparators(ObjectLibrary& library,
                                      const std::string& /*arg*/) {
  library.AddFactory<const Comparator>(
      BytewiseComparatorImpl::kClassName(),
      [](const std::string& /*uri*/,
         std::unique_ptr<const Comparator>* /*guard*/,
         std::string* /*errmsg*/) { return BytewiseComparator(); });
  library.AddFactory<const Comparator>(
      ReverseBytewiseComparatorImpl::kClassName(),
      [](const std::string& /*uri*/,
         std::unique_ptr<const Comparator>* /*guard*/,
         std::string* /*errmsg*/) { return ReverseBytewiseComparator(); });
  library.AddFactory<const Comparator>(
      ComparatorWithU64TsImpl<BytewiseComparatorImpl>::kClassName(),
      [](const std::string& /*uri*/,
         std::unique_ptr<const Comparator>* /*guard*/,
         std::string* /*errmsg*/) { return BytewiseComparatorWithU64Ts(); });
  library.AddFactory<const Comparator>(
      ComparatorWithU64TsImpl<ReverseBytewiseComparatorImpl>::kClassName(),
      [](const std::string& /*uri*/,
         std::unique_ptr<const Comparator>* /*guard*/,
         std::string* /*errmsg*/) {
        return ReverseBytewiseComparatorWithU64Ts();
      });
  return 4;
}

Status Comparator::CreateFromString(const ConfigOptions& config_options,
                                    const std::string& value,
                                    const Comparator** result) {
  static std::once_flag once;
  std::call_once(once, [&]() {
    RegisterBuiltinComparators(*(ObjectLibrary::Default().get()), "");
  });
  std::string id;
  std::unordered_map<std::string, std::string> opt_map;
  Status status = Customizable::GetOptionsMap(config_options, *result, value,
                                              &id, &opt_map);
  if (!status.ok()) {  // GetOptionsMap failed
    return status;
  }
  if (id == BytewiseComparatorImpl::kClassName()) {
    *result = BytewiseComparator();
  } else if (id == ReverseBytewiseComparatorImpl::kClassName()) {
    *result = ReverseBytewiseComparator();
  } else if (id ==
             ComparatorWithU64TsImpl<BytewiseComparatorImpl>::kClassName()) {
    *result = BytewiseComparatorWithU64Ts();
  } else if (id == ComparatorWithU64TsImpl<
                       ReverseBytewiseComparatorImpl>::kClassName()) {
    *result = ReverseBytewiseComparatorWithU64Ts();
  } else if (value.empty()) {
    // No Id and no options.  Clear the object
    *result = nullptr;
    return Status::OK();
  } else if (id.empty()) {  // We have no Id but have options.  Not good
    return Status::NotSupported("Cannot reset object ", id);
  } else {
    status = config_options.registry->NewStaticObject(id, result);
    if (!status.ok()) {
      if (config_options.ignore_unsupported_options &&
          status.IsNotSupported()) {
        return Status::OK();
      } else {
        return status;
      }
    } else {
      Comparator* comparator = const_cast<Comparator*>(*result);
      status =
          Customizable::ConfigureNewObject(config_options, comparator, opt_map);
    }
  }
  return status;
}
}  // namespace ROCKSDB_NAMESPACE

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Created: 2025-07-23 07:17