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// This file is copied from

// https://github.com/ClickHouse/ClickHouse/blob/master/src/AggregateFunctions/IAggregateFunctionCombinator.h

// and modified by Doris

#pragma once

#include <memory>

#include "core/data_type/data_type.h"

#include "exprs/aggregate/aggregate_function.h"

namespace doris {

/** Aggregate function combinator allows to take one aggregate function

  *  and transform it to another aggregate function.

  *

  * In SQL language they are used as suffixes for existing aggregate functions.

  *

  * Example: -If combinator takes an aggregate function and transforms it

  *  to aggregate function with additional argument at end (condition),

  *  that will pass values to original aggregate function when the condition is true.

  *

  * More examples:

  *

  * sum(x) - calculate sum of x

  * sumIf(x, cond) - calculate sum of x for rows where condition is true.

  * sumArray(arr) - calculate sum of all elements of arrays.

  *

  * PS. Please don't mess it with so called "combiner" - totally unrelated notion from Hadoop world.

  * "combining" - merging the states of aggregate functions - is supported naturally in ClickHouse.

  */

class IAggregateFunctionCombinator {

public:

    virtual String get_name() const = 0;

    /** From the arguments for combined function (ex: UInt64, UInt8 for sumIf),

      *  get the arguments for nested function (ex: UInt64 for sum).

      * If arguments are not suitable for combined function, throw an exception.

      */

    virtual DataTypes transform_arguments(const DataTypes& arguments) const { return arguments; }

    /** Create combined aggregate function (ex: sumIf)

      *  from nested function (ex: sum)

      *  and arguments for combined aggregate function (ex: UInt64, UInt8 for sumIf).

      * It's assumed that function transform_arguments was called before this function and 'arguments' are validated.

      */

    virtual AggregateFunctionPtr transform_aggregate_function(

            const AggregateFunctionPtr& nested_function, const DataTypes& arguments,

            const bool result_is_nullable, const AggregateFunctionAttr& attr) const = 0;

    virtual ~IAggregateFunctionCombinator() = default;

};

} // namespace doris