ClickHouse/dbms/include/DB/AggregateFunctions/IAggregateFunction.h

#pragma once

#include <memory>

#include <DB/Core/Row.h>
#include <DB/DataTypes/IDataType.h>
#include <DB/Common/typeid_cast.h>
#include <DB/Common/Arena.h>


namespace DB
{

namespace ErrorCodes
{
	extern const int AGGREGATE_FUNCTION_DOESNT_ALLOW_PARAMETERS;
	extern const int NUMBER_OF_ARGUMENTS_DOESNT_MATCH;
	extern const int ILLEGAL_TYPE_OF_ARGUMENT;
	extern const int ARGUMENT_OUT_OF_BOUND;
}

using AggregateDataPtr = char *;
using ConstAggregateDataPtr = const char *;


/** Aggregate functions interface.
  * Instances of classes with this interface do not contain the data itself for aggregation,
  *  but contain only metadata (description) of the aggregate function,
  *  as well as methods for creating, deleting and working with data.
  * The data resulting from the aggregation (intermediate computing states) is stored in other objects
  *  (which can be created in some pool),
  *  and IAggregateFunction is the external interface for manipulating them.
  */
class IAggregateFunction
{
public:
	/// Get main function name.
	virtual String getName() const = 0;

	/** Specify the types of arguments. If the function does not apply to these arguments throw an exception.
	  * You must call before other calls.
	  */
	virtual void setArguments(const DataTypes & arguments) = 0;

	/** Specify parameters for parametric aggregate functions.
	  * If no parameters are provided, or the passed parameters are not valid, throw an exception.
	  * If there are parameters - it is necessary to call before other calls, otherwise - do not call.
	  */
	virtual void setParameters(const Array & params)
	{
		throw Exception("Aggregate function " + getName() + " doesn't allow parameters.",
			ErrorCodes::AGGREGATE_FUNCTION_DOESNT_ALLOW_PARAMETERS);
	}

	/// Get the result type.
	virtual DataTypePtr getReturnType() const = 0;

	virtual ~IAggregateFunction() {};


	/** Data functions. */

	/** Create empty data for aggregation with `placement new` at the specified location.
	  * You will have to destroy them using the `destroy` method.
	  */
	virtual void create(AggregateDataPtr place) const = 0;

	/// Delete data for aggregation.
	virtual void destroy(AggregateDataPtr place) const noexcept = 0;

	/// It is not necessary to delete data.
	virtual bool hasTrivialDestructor() const = 0;

	/// Get `sizeof` of structure with data.
	virtual size_t sizeOfData() const = 0;

	/// How the data structure should be aligned. NOTE: Currently not used (structures with aggregation state are put without alignment).
	virtual size_t alignOfData() const = 0;

	/** Adds a value into aggregation data on which place points to.
	 *  columns points to columns containing arguments of aggregation function.
	 *  row_num is number of row which should be added.
	 *  Additional parameter arena should be used instead of standard memory allocator if the addition requires memory allocation.
	 */
	virtual void add(AggregateDataPtr place, const IColumn ** columns, size_t row_num, Arena * arena) const = 0;

	/// Merges state (on which place points to) with other state of current aggregation function.
	virtual void merge(AggregateDataPtr place, ConstAggregateDataPtr rhs, Arena * arena) const = 0;

	/// Serializes state (to transmit it over the network, for example).
	virtual void serialize(ConstAggregateDataPtr place, WriteBuffer & buf) const = 0;

	/// Deserializes state. This function is called only for empty (just created) states.
	virtual void deserialize(AggregateDataPtr place, ReadBuffer & buf, Arena * arena) const = 0;

	/// Returns true if a function requires Arena to handle own states (see add(), merge(), deserialize()).
	virtual bool allocatesMemoryInArena() const
	{
		return false;
	}

	/// Inserts results into a column.
	virtual void insertResultInto(ConstAggregateDataPtr place, IColumn & to) const = 0;

	/** Returns true for aggregate functions of type -State.
	  * They are executed as other aggregate functions, but not finalized (return an aggregation state that can be combined with another).
	  */
	virtual bool isState() const { return false; }


	/** The inner loop that uses the function pointer is better than using the virtual function.
	  * The reason is that in the case of virtual functions GCC 5.1.2 generates code,
	  *  which, at each iteration of the loop, reloads the function address (the offset value in the virtual function table) from memory to the register.
	  * This gives a performance drop on simple queries around 12%.
	  * After the appearance of better compilers, the code can be removed.
	  */
	using AddFunc = void (*)(const IAggregateFunction *, AggregateDataPtr, const IColumn **, size_t, Arena *);
	virtual AddFunc getAddressOfAddFunction() const = 0;
};


/// Implements several methods. T - type of structure with data for aggregation.
template <typename T>
class IAggregateFunctionHelper : public IAggregateFunction
{
protected:
	using Data = T;

	static Data & data(AggregateDataPtr place) 				{ return *reinterpret_cast<Data*>(place); }
	static const Data & data(ConstAggregateDataPtr place) 	{ return *reinterpret_cast<const Data*>(place); }

public:
	void create(AggregateDataPtr place) const override
	{
		new (place) Data;
	}

	void destroy(AggregateDataPtr place) const noexcept override
	{
		data(place).~Data();
	}

	bool hasTrivialDestructor() const override
	{
		return __has_trivial_destructor(Data);
	}

	size_t sizeOfData() const override
	{
		return sizeof(Data);
	}

	/// NOTE: Currently not used (structures with aggregation state are put without alignment).
	size_t alignOfData() const override
	{
		return __alignof__(Data);
	}
};


using AggregateFunctionPtr = std::shared_ptr<IAggregateFunction>;

}