Skip to content
Permalink
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
90 lines (76 sloc) 2.5 KB
// 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 <algorithm>
#include <stdint.h>
#include "leveldb/comparator.h"
#include "leveldb/slice.h"
#include "port/port.h"
#include "util/logging.h"
namespace leveldb {
Comparator::~Comparator() { }
namespace {
class BytewiseComparatorImpl : public Comparator {
public:
BytewiseComparatorImpl() { }
virtual const char* Name() const {
return "leveldb.BytewiseComparator";
}
virtual int Compare(const Slice& a, const Slice& b) const {
return a.compare(b);
}
//比较start, limit,得到满足>start && <= limit的第一个字符串
//这个字符串的好处是比start长度要小
//如果找不到,那么不修改start
//(abcdefg, abcdxyz) -> *start = abcdf
//(abcdxyz, abcdefg) -> *start = abcdxyz
//(abcdefg, abcdffg) -> *start = abcdefg
virtual void FindShortestSeparator(
std::string* start,
const Slice& limit) const {
// Find length of common prefix
size_t min_length = std::min(start->size(), limit.size());
size_t diff_index = 0;
//diff_index指向start与limit第一个不同的字符
while ((diff_index < min_length) &&
((*start)[diff_index] == limit[diff_index])) {
diff_index++;
}
//前缀
if (diff_index >= min_length) {
// Do not shorten if one string is a prefix of the other
} else {
uint8_t diff_byte = static_cast<uint8_t>((*start)[diff_index]);
if (diff_byte < static_cast<uint8_t>(0xff) &&
diff_byte + 1 < static_cast<uint8_t>(limit[diff_index])) {
(*start)[diff_index]++;
start->resize(diff_index + 1);
assert(Compare(*start, limit) < 0);
}
}
}
virtual void FindShortSuccessor(std::string* key) const {
// 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.
}
};
} // namespace
static port::OnceType once = LEVELDB_ONCE_INIT;
static const Comparator* bytewise;
static void InitModule() {
bytewise = new BytewiseComparatorImpl;
}
const Comparator* BytewiseComparator() {
port::InitOnce(&once, InitModule);
return bytewise;
}
} // namespace leveldb
You can’t perform that action at this time.