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/**
* Copyright (C) 2018-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 <cstdint>
#include <limits>
#include "mongo/base/string_data.h"
#include "mongo/util/itoa.h"
namespace mongo {
/**
* Stores unsigned counter as well as its decimal ASCII representation, avoiding the need for
* separate binary to decimal conversions. This speeds up code that needs string representations
* for all counter values.
*/
template <typename T>
class DecimalCounter {
public:
static_assert(std::is_unsigned<T>::value, "DecimalCounter requires an unsigned type");
DecimalCounter(T start = 0) : _lastDigitIndex(_getLastDigitIndex(start)), _counter(start) {}
constexpr operator StringData() const {
return {_digits, static_cast<size_t>(_lastDigitIndex + 1)};
}
constexpr operator uint32_t() const {
return _counter;
}
/**
* Increments the counter and its decimal representation. The decimal representation wraps
* independently of the binary counter.
*/
DecimalCounter<T>& operator++() {
// Common case: just increment the last digit and we're done with the string part.
char* lastPtr = _digits + _lastDigitIndex;
if (MONGO_unlikely((*lastPtr)++ == '9')) {
// Let zeroPtr point at the first char in the string, such that it and all digits
// after need to change to zeroes.
char* zeroPtr = lastPtr;
while (zeroPtr > _digits && zeroPtr[-1] == '9')
--zeroPtr;
// If digits wasn't all nines, increment the first non-nine.
if (zeroPtr > _digits) {
zeroPtr[-1]++;
} else if (lastPtr < _digits + sizeof(_digits) - 2) {
// Rare case: new power of 10 increases string length, so start with a one.
*zeroPtr++ = '1';
_lastDigitIndex++;
lastPtr++;
}
// Zero out the rest.
do {
*zeroPtr++ = '0';
} while (zeroPtr <= lastPtr);
}
if (MONGO_unlikely(++_counter == 0))
*this = {};
return *this;
}
/**
* Post-inrement version of operator++. Typically slower than pre-increment due to the need
* to return the pre-image by value.
*/
DecimalCounter<T> operator++(int) {
auto pre = *this;
operator++();
return pre;
}
private:
uint8_t _getLastDigitIndex(T start) {
if (!start) {
return 0;
}
ItoA startItoA(start);
StringData startStr(startItoA);
std::memcpy(_digits, startStr.rawData(), startStr.size());
return startStr.size() - 1;
}
// Add 1, because digit10 is 1 less than the maximum number of digits, and 1 for the final '\0'.
static constexpr size_t kBufSize = std::numeric_limits<T>::digits10 + 2;
char _digits[kBufSize] = {'0'}; // Remainder is zero-initialized.
uint8_t _lastDigitIndex; // Indicates the last digit in _digits.
T _counter;
};
} // namespace mongo
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