data/src/OpenFOAM/fields/expression/expressionReduce.H

/*---------------------------------------------------------------------------*\
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  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
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    \\  /    A nd           | Copyright (C) 2026 OpenFOAM Foundation
     \\/     M anipulation  |
-------------------------------------------------------------------------------
License
    This file is part of OpenFOAM.

    OpenFOAM is free software: you can redistribute it and/or modify it
    under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    OpenFOAM 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 GNU General Public License
    for more details.

    You should have received a copy of the GNU General Public License
    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.

Classes
    Foam::expression::Reduce
    Foam::expression::ReduceIterable

Description
    Class which provides the ability to reduce an expression to a single value
    using a binary operation (probably either min, max or sum). This must be
    specialised for expressionable containers.

    In the case of iterable containers that specialisation can inherit a
    generic iterable implementation of reduction from the ReduceIterable class
    (also defined in this file).

    Non-iterable containers must implement their own reduction methods. This is
    likely to be straightforward as non-iterable containers are typically just
    wrappings or structured collections of iterable containers, so the
    non-iterable implementation can just forward to the iterable implementation
    and combine and return the result.

\*---------------------------------------------------------------------------*/

#ifndef expressionReduce_H
#define expressionReduce_H

#include "expressionResult.H"
#include "Pstream.H"
#include "Tuple2.H"
#include <numeric>

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

namespace Foam
{
namespace expression
{

/*---------------------------------------------------------------------------*\
                          Class Reduce Declaration
\*---------------------------------------------------------------------------*/

template<class Type>
struct Reduce
{
    static const bool globalReduce = false;

    template<class Op, class Value>
    static const Value& value
    (
        const Value& x,
        const bool globalReduce = false,
        const label comm = UPstream::worldComm
    )
    {
        return x;
    }
};

/*---------------------------------------------------------------------------*\
                      Class ReduceIterable Declaration
\*---------------------------------------------------------------------------*/

template<class Type>
struct ReduceIterable;

template<template<class ...> class Container, class Type, class ... Args>
struct ReduceIterable<Container<Type, Args ...>>
{
    static const bool globalReduce = false;

    template<class Op, class Expression>
    static typename ResultIterable<Container<Type, Args ...>>::
    template elementType<Expression> value
    (
        const Expression& e,
        const bool globalReduce = false,
        const label comm = UPstream::worldComm
    )
    {
        typedef typename ResultIterable<Container<Type, Args ...>>::template
            operateType<Expression> operateType;

        typedef typename ResultIterable<Container<Type, Args ...>>::template
            elementType<Expression> elementType;

        auto x =
            std::accumulate
            (
                begin(e),
                end(e),
                Op::template identity<elementType>(),
                [](const elementType& x, const operateType& y)
                {
                    return Op::value
                    (
                        x,
                        Reduce<Type>::template value<Op>
                        (
                            y,
                            false,
                            UPstream::worldComm
                        )
                    );
                }
            );

        if (globalReduce)
        {
            reduce(x, Op(), Pstream::msgType(), comm);
        }

        return x;
    }


    template<class Expression>
    static typename ResultIterable<Container<Type, Args ...>>::
    template elementType<Expression> average
    (
        const Expression& e,
        const bool globalReduce,
        const label comm = UPstream::worldComm
    )
    {
        const Size::type n = getFirst<Size>(e);

        if (globalReduce)
        {
            auto s = value<sumOp>(e, false);
            Tuple2<decltype(s), Size::type> sn(s, n);
            reduce(sn, sum2Op(), Pstream::msgType(), comm);

            if (sn.second() > 0)
            {
                return sn.first()/sn.second();
            }
            else
            {
                WarningInFunction
                    << "empty field, returning zero" << endl;

                return ElementType<Expression>(Zero);
            }
        }
        else
        {
            if (n)
            {
                return value<sumOp>(e, false)/n;
            }
            else
            {
                WarningInFunction
                    << "empty field, returning zero" << endl;

                return ElementType<Expression>(Zero);
            }
        }
    }

    template<class Expression, class WeightExpression>
    static typename ResultIterable<Container<Type, Args ...>>::
    template elementType<Expression> weightedAverage
    (
        const Expression& e,
        const WeightExpression& we,
        const bool globalReduce,
        const label comm = UPstream::worldComm
    )
    {
        auto ws = value<sumOp>(we*e, false);
        auto w = value<sumOp>(we, false);

        if (globalReduce)
        {
            Tuple2<decltype(ws), decltype(w)> wsw(ws, w);
            reduce(wsw, sum2Op(), Pstream::msgType(), comm);

            return wsw.first()/wsw.second();
        }
        else
        {
            return ws/w;
        }
    }
};

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

} // End namespace expression

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

template<class Expression>
using ReduceType = typename expression::Reduce
<
    typename expression::AllContainers<Expression>::type
>;

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

} // End namespace Foam

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

#endif

// ************************************************************************* //