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/**
* Copyright (C) 2016 MongoDB Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* 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
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* 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 GNU Affero General 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/platform/basic.h"
#include "summation.h"
#include <cmath>
#include "mongo/util/assert_util.h"
namespace mongo {
void DoubleDoubleSummation::addLong(long long x) {
// Split 64-bit integers into two doubles, so the sum remains exact.
int64_t high = x / (1ll << 32) * (1ll << 32);
int64_t low = x - high;
dassert(high + low == x && 1.0 * high == high && 1.0 * low == low);
addDouble(low);
addDouble(high);
}
/**
* Returns whether the sum is in range of the 64-bit signed integer long long type.
*/
bool DoubleDoubleSummation::fitsLong() const {
using limits = std::numeric_limits<long long>;
// Fast path: if the rounded _sum is strictly between the minimum and maximum long long value,
// it must be valid. This is the common case. Note that this is correct for NaNs/infinities.
if (_sum > limits::min() && _sum < limits::max())
return true;
// Now check the cases where the _sum equals one of the boundaries, and check the compensation
// amount to determine to what integer the value would round.
// If _sum is equal to limits::max() + 1, _addend must cause us to round down to a lower integer
// and thus be strictly less than -0.5. limits.max() rounds up to limits.max() + 1, as double
// precision does not have enough precision.
if (_sum == limits::max())
return _addend < -0.5;
// If _sum is equal to limits::min(), _addend must not cause us to round down and thus be
// greater than or equal to -0.5.
if (_sum == limits::min())
return _addend >= -0.5;
// The sum is out of range, an infinity or a NaN.
return false;
}
/**
* Returns result of sum rounded to nearest integer, rounding half-way cases away from zero.
*/
long long DoubleDoubleSummation::getLong() const {
uassert(ErrorCodes::Overflow, "sum out of range of a 64-bit signed integer", fitsLong());
if (_sum == std::numeric_limits<long long>::max()) {
// Can't directly convert, because _sum would overflow a signed 64-bit number.
dassert(_addend < -0.5 && -_sum == std::numeric_limits<long long>::min());
return llround(_addend) - std::numeric_limits<long long>::min();
}
long long sum = llround(_sum);
sum += llround((_sum - sum) + _addend);
return sum;
}
} // namespace mongo
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