from_diagonal.hpp

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// SPDX-License-Identifier: BSD-2-Clause
 
#ifndef pRC_CORE_TENSOR_OPERATOR_VIEWS_FROM_DIAGONAL_H
#define pRC_CORE_TENSOR_OPERATOR_VIEWS_FROM_DIAGONAL_H
 
#include <prc/core/basic/sequence.hpp>
#include <prc/core/tensor/declarations.hpp>
 
namespace pRC::TensorViews
{
    template<class T, class N, class V>
        requires IsTensorView<V>
    class FromDiagonal : public View<T, N, FromDiagonal<T, N, V>>
    {
    private:
        using Base = View<T, N, FromDiagonal>;
 
    public:
        template<class X>
            requires IsSame<V, RemoveReference<X>>
        FromDiagonal(X &&a)
            : mA(forward<X>(a))
        {
        }
 
        template<IsConvertible<Index>... Is>
            requires(sizeof...(Is) == Base::Dimension)
        constexpr decltype(auto) operator()(Is const... indices)
        {
            return this->call(indices...);
        }
 
        template<IsConvertible<Index>... Is>
            requires(sizeof...(Is) == Base::Dimension)
        constexpr decltype(auto) operator()(Is const... indices) const
        {
            return this->call(indices...);
        }
 
        constexpr decltype(auto) operator()(
            typename Base::Subscripts const &subscripts)
        {
            return expand(makeSeries<Index, Base::Dimension / 2>(),
                [this, &subscripts](auto const... seq) -> T
                {
                    if(((subscripts[seq] ==
                            subscripts[Base::Dimension / 2 + seq]) &&
                           ...))
                    {
                        return mA(subscripts[seq]...);
                    }
 
                    return zero();
                });
        }
 
        constexpr decltype(auto) operator()(
            typename Base::Subscripts const &subscripts) const
        {
            return expand(makeSeries<Index, Base::Dimension / 2>(),
                [this, &subscripts](auto const... seq) -> T
                {
                    if(((subscripts[seq] ==
                            subscripts[Base::Dimension / 2 + seq]) &&
                           ...))
                    {
                        return mA(subscripts[seq]...);
                    }
 
                    return zero();
                });
        }
 
        constexpr decltype(auto) operator[](Index const index) = delete;
        constexpr decltype(auto) operator[](Index const index) const = delete;
 
    private:
        V mA;
    };
}
#endif // pRC_CORE_TENSOR_OPERATOR_VIEWS_FROM_DIAGONAL_H