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/**
* Copyright (C) 2021-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.
*/
#include "mongo/bson/util/simple8b.h"
namespace mongo {
namespace {
static constexpr uint8_t _maxSelector = 15;
static constexpr uint8_t _minSelector = 2;
static constexpr uint64_t _selectorMask = 0xF000000000000000;
static constexpr uint8_t _selectorSize = 4;
static constexpr uint8_t _dataSize = 60;
// Pass the selector as the index to get the corresponding mask.
// Get the maskSize by getting the number of bits for the selector. Then 2^maskSize - 1.
const uint64_t _selectorForMask[16] = {0, 0, 1, 3, 7, 15, 31, 63, 127, 255, 1023, 4095, 32767, 1048575, 1073741823, 1152921504606846975};
// Pass the selector value as the index to get the number of bits per integer in the Simple8b block.
const uint8_t _selectorForBitsPerInteger[16] = {0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 15, 20, 30, 60};
// Pass the selector value as the index to get the number of integers coded in the Simple8b block.
const uint8_t _selectorForIntegersCoded[16] = {240, 120, 60, 30, 20, 15, 12, 10, 8, 7, 6, 5, 4, 3, 2, 1};
}
uint64_t Simple8b::encodeSimple8b(uint8_t selector, const std::vector<uint64_t>& values) {
if (selector > _maxSelector || selector < _minSelector)
return errCode;
uint8_t bitsPerInteger = _selectorForBitsPerInteger[selector];
uint8_t integersCoded = _selectorForIntegersCoded[selector];
uint64_t encodedWord = (uint64_t) selector << _dataSize;
for (uint8_t i = 0; i < integersCoded; ++i) {
uint8_t shiftSize = _dataSize - bitsPerInteger * (i + 1);
encodedWord += values[i] << shiftSize;
}
return encodedWord;
}
std::vector<uint64_t> Simple8b::decodeSimple8b(const uint64_t simple8bWord) {
std::vector<uint64_t> values;
uint8_t selector = (simple8bWord & _selectorMask) >> _dataSize;
if (selector < _minSelector)
return values;
uint8_t bitsPerInteger = _selectorForBitsPerInteger[selector];
uint8_t integersCoded = _selectorForIntegersCoded[selector];
for (int8_t i = integersCoded - 1; i >= 0; --i) {
uint8_t startIdx = bitsPerInteger * i;
uint64_t mask = _selectorForMask[selector] << startIdx;
uint64_t value = (simple8bWord & mask) >> startIdx;
values.push_back(value);
}
return values;
}
} // namespace mongo
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