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/**
* Copyright (C) 2023-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <cstddef>
#include <limits>
#include <string>
#include <vector>
#include "mongo/util/string_map.h"
namespace mongo {
class IndexedStringVector {
public:
static constexpr size_t npos = std::numeric_limits<size_t>::max();
private:
inline static size_t getLowestNBits(size_t val, size_t n) {
return val & ((1u << n) - 1u);
}
// This function assumes that 'length' is not 0.
inline static size_t computeFastHash1(const char* str, size_t len) {
// The lowest 5 bits of 'str[len - 1]' and the lowest 2 bits of 'len' are decent sources
// of entropy. Combine them to generate a pseudo-random number 'h' where 0 <= h <= 127.
size_t h = getLowestNBits(size_t(str[len - 1]) + (len << 5u), 7);
return h;
}
// This function assumes that 'length' is not 0.
inline static size_t computeFastHash2(const char* str, size_t len, size_t fastHash1) {
// The lowest 5 bits of 'str[0]' are a decent source of entropy. Using 'str[0]' and
// 'fastHash1', generate a pseudo-random number 'h' where 0 <= h <= 127 and where
// h != fastHash1.
size_t h = getLowestNBits(
fastHash1 + size_t(str[0]) + getLowestNBits(~size_t(str[0]) >> 4u, 1), 7);
return h;
}
public:
explicit IndexedStringVector(std::vector<std::string> strings)
: _strings(std::move(strings)), _stringToIndexMap(), _fastHt(buildFastHash()) {}
IndexedStringVector(const IndexedStringVector& other)
: _strings(other._strings), _stringToIndexMap(), _fastHt(buildFastHash()) {}
IndexedStringVector(IndexedStringVector&& other)
: _strings(std::move(other._strings)), _stringToIndexMap(), _fastHt(buildFastHash()) {}
IndexedStringVector& operator=(const IndexedStringVector& other) {
if (&other != this) {
_strings = other._strings;
_fastHt = buildFastHash();
}
return *this;
}
IndexedStringVector& operator=(IndexedStringVector&& other) {
if (&other != this) {
_strings = std::move(other._strings);
_fastHt = buildFastHash();
}
return *this;
}
inline size_t size() const {
return _strings.size();
}
inline std::vector<std::string>::const_iterator begin() const {
return _strings.cbegin();
}
inline std::vector<std::string>::const_iterator cbegin() const {
return _strings.cbegin();
}
inline std::vector<std::string>::const_iterator end() const {
return _strings.cend();
}
inline std::vector<std::string>::const_iterator cend() const {
return _strings.cend();
}
inline const std::string& operator[](size_t idx) const {
return _strings[idx];
}
inline const std::string& at(size_t idx) const {
return _strings.at(idx);
}
inline size_t findPos(StringData str) const {
size_t len = str.size();
size_t fastHash = computeFastHash1(str.rawData(), len);
size_t encodedIdx = 0;
if (useFastHash()) {
size_t numAttempts = 0;
// This 'for' loop will iterate at most two times. There is logic near the end of
// the loop body that breaks out of the loop once 'numAttempts' reaches 2.
for (;;) {
encodedIdx = _fastHt[fastHash];
if (encodedIdx > 1) {
// If encodedIdx >= 2, then there is a single string in _strings to compare
// with. Compare 'str' with this single string.
if (len != _strings[encodedIdx - 2].size() ||
memcmp(str.rawData(), _strings[encodedIdx - 2].data(), len) != 0) {
return npos;
}
return encodedIdx - 2;
} else if (MONGO_likely(encodedIdx < 1)) {
// If encodedIdx == 0, then we know 'str' is not present in _strings.
return npos;
}
++numAttempts;
if (numAttempts >= 2) {
// If this was our second attempt, we give up and search for 'str' in
// 'stringToIndexMap'. We break out of the loop early to avoid calling
// computeFastHash2() again.
break;
}
// If this was our first attempt, try again using computeFastHash2().
fastHash = computeFastHash2(str.rawData(), len, fastHash);
}
}
return findInMapImpl(str);
}
inline std::vector<std::string>::const_iterator find(StringData str) const {
auto pos = findPos(str);
return pos != npos ? _strings.cbegin() + pos : _strings.cend();
}
const auto& getUnderlyingVector() const {
return _strings;
}
const auto& getUnderlyingMap() const {
return _stringToIndexMap;
}
private:
bool useFastHash() const {
// If there are more than 64 strings in '_strings', don't bother with '_fastHt'.
return _strings.size() <= 64;
}
std::array<uint8_t, 128> buildFastHash();
size_t findInMapImpl(StringData str) const;
std::vector<std::string> _strings;
StringDataMap<size_t> _stringToIndexMap;
std::array<uint8_t, 128> _fastHt;
};
} // namespace mongo
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