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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 "mongo/bson/bsonelement.h"
#include "mongo/db/pipeline/accumulator.h"
#include "mongo/db/pipeline/expression.h"
namespace mongo {
/**
* Registers an AccumulatorState to have the name 'key'. When an accumulator with name '$key' is
* found during parsing, 'factory' will be called to construct the AccumulatorState.
*
* As an example, if your accumulator looks like {"$foo": <args>}, with a factory method 'create',
* you would add this line:
* REGISTER_ACCUMULATOR(foo, AccumulatorFoo::create);
*/
#define REGISTER_ACCUMULATOR(key, factory) \
REGISTER_ACCUMULATOR_WITH_MIN_VERSION(key, factory, boost::none)
#define REGISTER_ACCUMULATOR_WITH_MIN_VERSION(key, factory, minVersion) \
MONGO_INITIALIZER_GENERAL( \
addToAccumulatorFactoryMap_##key, ("default"), ("accumulatorParserMap")) \
(InitializerContext*) { \
AccumulationStatement::registerAccumulator("$" #key, (factory), (minVersion)); \
}
/**
* AccumulatorExpression represents the right-hand side of an AccumulationStatement. Note this is
* different from Expression; they are different nonterminals in the grammar.
*
* For example, in
* {$group: {
* _id: 1,
* count: {$sum: {$size: "$tags"}}
* }}
*
* we would say:
* The AccumulationStatement is count: {$sum: {$size: "$tags"}}
* The AccumulationExpression is {$sum: {$size: "$tags"}}
* The AccumulatorState::Factory is $sum
* The argument Expression is {$size: "$tags"}
* There is no initializer Expression.
*
* "$sum" corresponds to an AccumulatorState::Factory rather than AccumulatorState because
* AccumulatorState is an execution concept, not an AST concept: each instance of AccumulatorState
* contains intermediate values being accumulated.
*
* Like most accumulators, $sum does not require or accept an initializer Expression. At time of
* writing, only user-defined accumulators accept an initializer.
*
* For example, in:
* {$group: {
* _id: {cc: "$country_code"},
* top_stories: {$accumulator: {
* init: function(cc) { ... },
* initArgs: ["$cc"],
* accumulate: function(state, title, upvotes) { ... },
* accumulateArgs: ["$title", "$upvotes"],
* merge: function(state1, state2) { ... },
* lang: "js",
* }}
* }}
*
* we would say:
* The AccumulationStatement is top_stories: {$accumulator: ... }
* The AccumulationExpression is {$accumulator: ... }
* The argument Expression is ["$cc"]
* The initializer Expression is ["$title", "$upvotes"]
* The AccumulatorState::Factory holds all the other arguments to $accumulator.
*
*/
struct AccumulationExpression {
AccumulationExpression(boost::intrusive_ptr<Expression> initializer,
boost::intrusive_ptr<Expression> argument,
AccumulatorState::Factory factory)
: initializer(initializer), argument(argument), factory(factory) {
invariant(this->initializer);
invariant(this->argument);
}
// The expression to use to obtain the input to the accumulator.
boost::intrusive_ptr<Expression> initializer;
// An expression evaluated once per input document, and passed to AccumulatorState::process.
boost::intrusive_ptr<Expression> argument;
// Constructs an AccumulatorState to do actual accumulation.
boost::intrusive_ptr<AccumulatorState> makeAccumulator() const;
// A no argument function object that can be called to create an AccumulatorState.
const AccumulatorState::Factory factory;
};
/**
* A default parser for any accumulator that only takes a single expression as an argument. Returns
* the expression to be evaluated by the accumulator and an AccumulatorState::Factory.
*/
template <class AccName>
AccumulationExpression genericParseSingleExpressionAccumulator(ExpressionContext* const expCtx,
BSONElement elem,
VariablesParseState vps) {
auto initializer = ExpressionConstant::create(expCtx, Value(BSONNULL));
auto argument = Expression::parseOperand(expCtx, elem, vps);
return {initializer, argument, [expCtx]() { return AccName::create(expCtx); }};
}
/**
* A parser that desugars { $count: {} } to { $sum: 1 }.
*/
inline AccumulationExpression parseCountAccumulator(ExpressionContext* const expCtx,
BSONElement elem,
VariablesParseState vps) {
uassert(ErrorCodes::TypeMismatch,
"$count takes no arguments, i.e. $count:{}",
elem.type() == BSONType::Object && elem.Obj().isEmpty());
auto initializer = ExpressionConstant::create(expCtx, Value(BSONNULL));
auto argument = ExpressionConstant::create(expCtx, Value(1));
return {initializer, argument, [expCtx]() { return AccumulatorSum::create(expCtx); }};
}
/**
* A class representing a user-specified accumulation, including the field name to put the
* accumulated result in, which accumulator to use, and the expression used to obtain the input to
* the AccumulatorState.
*/
class AccumulationStatement {
public:
using Parser = std::function<AccumulationExpression(
ExpressionContext* const, BSONElement, VariablesParseState)>;
AccumulationStatement(std::string fieldName, AccumulationExpression expr)
: fieldName(std::move(fieldName)), expr(std::move(expr)) {}
/**
* Parses a BSONElement that is an accumulated field, and returns an AccumulationStatement for
* that accumulated field.
*
* Throws an AssertionException if parsing fails.
*/
static AccumulationStatement parseAccumulationStatement(ExpressionContext* const expCtx,
const BSONElement& elem,
const VariablesParseState& vps);
/**
* Registers an AccumulatorState with a parsing function, so that when an accumulator with the
* given name is encountered during parsing, we will know to call 'factory' to construct that
* AccumulatorState.
*
* DO NOT call this method directly. Instead, use the REGISTER_ACCUMULATOR macro defined in this
* file.
*/
static void registerAccumulator(
std::string name,
Parser parser,
boost::optional<ServerGlobalParams::FeatureCompatibility::Version> requiredMinVersion);
/**
* Retrieves the Parser for the accumulator specified by the given name, and raises an error if
* there is no such Parser registered, or the Parser is registered under an FCV greater than the
* specified maximum allowed FCV.
*/
static Parser& getParser(
StringData name,
boost::optional<ServerGlobalParams::FeatureCompatibility::Version> allowedMaxVersion);
// The field name is used to store the results of the accumulation in a result document.
std::string fieldName;
AccumulationExpression expr;
// Constructs an AccumulatorState to do actual accumulation.
boost::intrusive_ptr<AccumulatorState> makeAccumulator() const;
};
} // namespace mongo
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