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

#pragma once

#include <DB/IO/WriteHelpers.h>
#include <DB/IO/ReadHelpers.h>

#include <DB/DataTypes/DataTypesNumber.h>
#include <DB/Columns/ColumnsNumber.h>

#include <DB/AggregateFunctions/IUnaryAggregateFunction.h>


namespace DB
{


template <typename T>
struct AggregateFunctionAvgData
{
	T sum;
	UInt64 count;

	AggregateFunctionAvgData() : sum(0), count(0) {}
};


/// Calculates arithmetic mean of numbers.
template <typename T>
class AggregateFunctionAvg final : public IUnaryAggregateFunction<AggregateFunctionAvgData<typename NearestFieldType<T>::Type>, AggregateFunctionAvg<T> >
{
public:
	String getName() const override { return "avg"; }

	DataTypePtr getReturnType() const override
	{
		return std::make_shared<DataTypeFloat64>();
	}

	void setArgument(const DataTypePtr & argument)
	{
		if (!argument->isNumeric())
			throw Exception("Illegal type " + argument->getName() + " of argument for aggregate function " + getName(),
				ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
	}


	void addImpl(AggregateDataPtr place, const IColumn & column, size_t row_num, Arena *) const
	{
		this->data(place).sum += static_cast<const ColumnVector<T> &>(column).getData()[row_num];
		++this->data(place).count;
	}

	void merge(AggregateDataPtr place, ConstAggregateDataPtr rhs, Arena * arena) const override
	{
		this->data(place).sum += this->data(rhs).sum;
		this->data(place).count += this->data(rhs).count;
	}

	void serialize(ConstAggregateDataPtr place, WriteBuffer & buf) const override
	{
		writeBinary(this->data(place).sum, buf);
		writeVarUInt(this->data(place).count, buf);
	}

	void deserialize(AggregateDataPtr place, ReadBuffer & buf, Arena *) const override
	{
		readBinary(this->data(place).sum, buf);
		readVarUInt(this->data(place).count, buf);
	}

	void insertResultInto(ConstAggregateDataPtr place, IColumn & to) const override
	{
		static_cast<ColumnFloat64 &>(to).getData().push_back(
			static_cast<Float64>(this->data(place).sum) / this->data(place).count);
	}
};


}