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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/db/pipeline/window_function/window_function_exec_derivative.h"
namespace mongo {
Value WindowFunctionExecDerivative::getNext() {
auto endpoints = _iter.getEndpoints(_bounds);
if (!endpoints)
return kDefault;
auto [leftOffset, rightOffset] = *endpoints;
const Document leftDoc = *(_iter)[leftOffset];
const Document rightDoc = *(_iter)[rightOffset];
// Conceptually, $derivative computes 'rise/run' where 'rise' is dimensionless and 'run' is
// a time. The result has dimension 1/time, which doesn't correspond to any BSON type, so
// 'unit' tells us how to express the result as a dimensionless BSON number.
//
// However, BSON also can't represent a time (duration) directly. BSONType::Date represents
// a point in time, but there is no type that represents an amount of time. Subtracting two
// Date values implicitly converts them to milliseconds.
// So, when we compute 'rise/run', the answer is expressed in units '1/millisecond'. If an
// 'unit' is specified, we scale the answer by 'millisecond/unit' to
// re-express it in '1/unit'.
Value leftTime = _time->evaluate(leftDoc, &_time->getExpressionContext()->variables);
Value rightTime = _time->evaluate(rightDoc, &_time->getExpressionContext()->variables);
if (_unitMillis) {
// If a unit is specified, we require both endpoints to be dates. We don't
// want to interpret bare numbers as milliseconds, when we don't know what unit they
// really represent.
//
// For example: imagine the '_time' field contains floats representing seconds: then
// 'rise/run' will already be expressed in units of 1/second. If you think "my data is
// seconds" and write 'unit: "second"', and we applied the scale factor of
// 'millisecond/unit', then the final answer would be wrong by a factor of 1000.
uassert(5624900,
"$derivative with 'unit' expects the sortBy field to be a Date",
leftTime.getType() == BSONType::Date && rightTime.getType() == BSONType::Date);
} else {
// Without unit, we require both time values to be numeric.
uassert(5624901,
"$derivative where the sortBy is a Date requires an 'unit'",
leftTime.getType() != BSONType::Date && rightTime.getType() != BSONType::Date);
uassert(5624902,
"$derivative (with no 'unit') expects the sortBy field to be numeric",
leftTime.numeric() && rightTime.numeric());
}
// Now leftTime and rightTime are either both numeric, or both dates.
// $subtract on two dates gives us the difference in milliseconds.
Value run =
uassertStatusOK(ExpressionSubtract::apply(std::move(rightTime), std::move(leftTime)));
Value rise = uassertStatusOK(ExpressionSubtract::apply(
_position->evaluate(rightDoc, &_position->getExpressionContext()->variables),
_position->evaluate(leftDoc, &_position->getExpressionContext()->variables)));
uassert(5624903, "$derivative input must not be null or missing", !rise.nullish());
auto divideStatus = ExpressionDivide::apply(std::move(rise), std::move(run));
if (divideStatus.getStatus().code() == ErrorCodes::BadValue) {
// Divide by zero can't be an error. On the first document of a partition, a window like
// 'documents: [-1, 0]' contains only one document, so 'run' is zero.
return kDefault;
}
Value result = uassertStatusOK(divideStatus);
if (_unitMillis) {
// 'result' has units 1/millisecond; scale by millisecond/unit to express in
// 1/unit.
// tassert because at this point the result should already be numeric, so if
// ExpressionMultiply returns a non-OK Status then something has gone wrong.
auto statusWithResult = ExpressionMultiply::apply(result, Value(*_unitMillis));
tassert(statusWithResult);
result = statusWithResult.getValue();
}
return result;
}
} // namespace mongo
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