gromacs/mdspan/mdspan.h

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/*
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/*! \libinternal \file
 * \brief Declares gmx::mdspan
 *
 * \author Christian Trott <crtrott@sandia.gov>
 * \author Ronan Keryell <ronan.keryell@xilinx.com>
 * \author Carter Edwards <hedwards@nvidia.com>
 * \author David Hollman <dshollm@sandia.gov>
 * \author Christian Blau <cblau@gwdg.de>
 * \inlibraryapi
 * \ingroup mdspan
 */
#ifndef MDSPAN_MDSPAN_H
#define MDSPAN_MDSPAN_H

#include <array>
#include <type_traits>

#include "accessor_policy.h"
#include "extents.h"
#include "layouts.h"

namespace gmx
{

/*! \libinternal \brief Multidimensional array indexing and memory access with flexible mapping and access model.
 *
 * \tparam ElementType Type of elemnt to be viewed
 * \tparam Extents The dimensions of the multidimenisonal array to view.
 * \tparam LayoutPolicy Describes is the memory layout of the multidimensional array; right by default.
 * \tparam AccessorPolicy Describes memory access model.
 */
template<class ElementType, class Extents, class LayoutPolicy = layout_right, class AccessorPolicy = accessor_basic<ElementType>>
class basic_mdspan
{
public:
    //! Expose type used to define the extents of the data.
    using extents_type = Extents;
    //! Expose type used to define the layout of the data.
    using layout_type = LayoutPolicy;
    //! Expose type used to define the memory access model of the data.
    using accessor_type = AccessorPolicy;
    //! Expose type used to map multidimensional indices to one-dimensioal indices.
    using mapping_type = typename layout_type::template mapping<extents_type>;
    //! Exposes the type of stored element.
    using element_type = typename accessor_type::element_type;
    //! Expose the underlying type of the stored elements.
    using value_type = std::remove_cv_t<element_type>;
    //! Expose the type used for indexing.
    using index_type = ptrdiff_t;
    //! Expose type for index differences.
    using difference_type = ptrdiff_t;
    //! Expose underlying pointer to data type.
    using pointer = typename accessor_type::pointer;
    //! Expose reference to data type.
    using reference = typename accessor_type::reference;

    //! Trivial constructor
    constexpr basic_mdspan() noexcept : acc_(), map_(), ptr_() {}
    //! Move constructor
    constexpr basic_mdspan(basic_mdspan&& other) noexcept = default;
    //! copy constructor
    constexpr basic_mdspan(const basic_mdspan& other) noexcept = default;
    //! Copy assignment
    basic_mdspan& operator=(const basic_mdspan& other) noexcept = default;
    //! Move assignment
    basic_mdspan& operator=(basic_mdspan&& other) noexcept = default;

    //! Copy constructor
    template<class OtherElementType, class OtherExtents, class OtherLayoutPolicy, class OtherAccessor>
    constexpr basic_mdspan(
            const basic_mdspan<OtherElementType, OtherExtents, OtherLayoutPolicy, OtherAccessor>& rhs) noexcept :
        acc_(rhs.acc_),
        map_(rhs.map_),
        ptr_(rhs.ptr_)
    {
    }
    //! Copy assignment constructor
    template<class OtherElementType, class OtherExtents, class OtherLayoutPolicy, class OtherAccessor>
    basic_mdspan&
    operator=(const basic_mdspan<OtherElementType, OtherExtents, OtherLayoutPolicy, OtherAccessor>& rhs) noexcept
    {
        acc_ = rhs.acc_;
        map_ = rhs.map_;
        ptr_ = rhs.ptr_;
        return *this;
    }

    /*!\brief Construct mdspan by setting the dynamic extents and pointer to data.
     * \param[in] ptr Pointer to data to be accessed by this span
     * \param[in] DynamicExtents
     * \tparam IndexType index type to describe dynamic extents
     */
    template<class... IndexType>
    explicit constexpr basic_mdspan(pointer ptr, IndexType... DynamicExtents) noexcept :
        acc_(accessor_type()),
        map_(extents_type(DynamicExtents...)),
        ptr_(ptr)
    {
    }
    /*! \brief Construct from array describing dynamic extents.
     * \param[in] ptr Pointer to data to be accessed by this span
     * \param[in] dynamic_extents Array the size of dynamic extents.
     */
    constexpr basic_mdspan(pointer                                                    ptr,
                           const std::array<ptrdiff_t, extents_type::rank_dynamic()>& dynamic_extents) :
        acc_(accessor_type()),
        map_(extents_type(dynamic_extents)),
        ptr_(ptr)
    {
    }
    /*! \brief Construct from pointer and mapping.
     * \param[in] ptr Pointer to data to be accessed by this span
     * \param[in] m Mapping from multidimenisonal indices to one-dimensional offset.
     */
    constexpr basic_mdspan(pointer ptr, const mapping_type& m) noexcept :
        acc_(accessor_type()),
        map_(m),
        ptr_(ptr)
    {
    }
    /*! \brief Construct with pointer, mapping and accessor.
     * \param[in] ptr Pointer to data to be accessed by this span
     * \param[in] m Mapping from multidimenisonal indices to one-dimensional offset.
     * \param[in] a Accessor implementing memory access model.
     */
    constexpr basic_mdspan(pointer ptr, const mapping_type& m, const accessor_type& a) noexcept :
        acc_(a),
        map_(m),
        ptr_(ptr)
    {
    }
    /*! \brief Construct mdspan from multidimensional arrays implemented with mdspan
     *
     * Requires the container to have a view_type describing the mdspan, which is
     * accessible through an asView() call
     *
     *  This allows functions to declare mdspans as arguments, but take e.g. multidimensional
     *  arrays implicitly during the function call
     * \tparam U container type
     * \param[in] other mdspan-implementing container
     */
    template<typename U, typename = std::enable_if_t<std::is_same_v<typename std::remove_reference_t<U>::view_type::element_type, ElementType>>>
    constexpr basic_mdspan(U&& other) : basic_mdspan(other.asView())
    {
    }
    /*! \brief Construct mdspan of const Elements from multidimensional arrays implemented with mdspan
     *
     * Requires the container to have a const_view_type describing the mdspan, which is
     * accessible through an asConstView() call
     *
     *  This allows functions to declare mdspans as arguments, but take e.g. multidimensional
     *  arrays implicitly during the function call
     * \tparam U container type
     * \param[in] other mdspan-implementing container
     */
    template<typename U, typename = std::enable_if_t<std::is_same_v<typename std::remove_reference_t<U>::const_view_type::element_type, ElementType>>>
    constexpr basic_mdspan(const U& other) : basic_mdspan(other.asConstView())
    {
    }
    /*! \brief Brace operator to access multidimensional array element.
     * \param[in] indices The multidimensional indices of the object.
     * Requires rank() == sizeof...(IndexType). Slicing is implemented via sub_span.
     * \returns reference to element at indices.
     */
    template<class... IndexType>
    constexpr std::enable_if_t<sizeof...(IndexType) == extents_type::rank(), reference>
    operator()(IndexType... indices) const noexcept
    {
        return acc_.access(ptr_, map_(indices...));
    }
    /*! \brief Canonical bracket operator for one-dimensional arrays.
     * Allows mdspan to act like array in one-dimension.
     * Enabled only when rank==1.
     * \param[in] i one-dimensional index
     * \returns reference to element stored at position i
     */
    template<class IndexType>
    constexpr std::enable_if_t<std::is_integral_v<IndexType> && extents_type::rank() == 1, reference>
    operator[](const IndexType& i) const noexcept
    {
        return acc_.access(ptr_, map_(i));
    }
    /*! \brief Bracket operator for multi-dimensional arrays.
     *
     * \note Prefer operator() for better compile-time and run-time performance
     *
     * Slices two- and higher-dimensional arrays along a given slice by
     * returning a new basic_mdspan that drops the first extent and indexes
     * the remaining extents
     *
     * \note Currently only implemented for layout_right
     * \note For layout_right this implementation has significant
     *       performance benefits over implementing a more general slicing
     *       operator with a strided layout
     * \note Enabled only when rank() > 1
     *
     * \tparam IndexType integral tyoe for the index that enables indexing
     *                   with, e.g., int or size_t
     * \param[in] index  one-dimensional index of the slice to be indexed
     *
     * \returns basic_mdspan that is sliced at the given index
     */
    template<class IndexType,
             typename sliced_mdspan_type = basic_mdspan<element_type, decltype(extents_type().sliced_extents()), LayoutPolicy, AccessorPolicy>>
    constexpr std::enable_if_t<std::is_integral_v<IndexType> && (extents_type::rank() > 1)
                                       && std::is_same_v<LayoutPolicy, layout_right>,
                               sliced_mdspan_type>
    operator[](const IndexType index) const noexcept
    {
        return sliced_mdspan_type(ptr_ + index * stride(0), extents().sliced_extents());
    }
    //! Report the rank.
    static constexpr int rank() noexcept { return extents_type::rank(); }
    //! Report the dynamic rank.
    static constexpr int rank_dynamic() noexcept { return extents_type::rank_dynamic(); }
    /*! \brief Return the static extent.
     * \param[in] k dimension to query for static extent
     * \returns static extent along specified dimension
     */
    constexpr index_type static_extent(size_t k) const noexcept
    {
        return map_.extents().static_extent(k);
    }

    /*! \brief Return the extent.
     * \param[in] k dimension to query for extent
     * \returns extent along specified dimension
     */
    constexpr index_type extent(int k) const noexcept { return map_.extents().extent(k); }

    //! Return all extents
    constexpr const extents_type& extents() const noexcept { return map_.extents(); }
    //! Report if mappings for this basic_span is always unique.
    static constexpr bool is_always_unique() noexcept { return mapping_type::is_always_unique(); }
    //! Report if mapping for this basic_span is always strided
    static constexpr bool is_always_strided() noexcept { return mapping_type::is_always_strided(); }
    //! Report if mapping for this basic_span is always is_contiguous
    static constexpr bool is_always_contiguous() noexcept
    {
        return mapping_type::is_always_contiguous();
    }
    //! Report if the currently applied map is unique
    constexpr bool is_unique() const noexcept { return map_.is_unique(); }
    //! Report if the currently applied map is strided
    constexpr bool is_strided() const noexcept { return map_.is_strided(); }
    //! Report if the currently applied map is contiguous
    constexpr bool is_contiguous() const noexcept { return map_.is_contiguous(); }
    //! Report stride along a specific rank.
    constexpr index_type stride(size_t r) const noexcept { return map_.stride(r); }
    //! Return the currently applied mapping.
    constexpr mapping_type mapping() const noexcept { return map_; }
    //! Return the memory access model.
    constexpr accessor_type accessor() const noexcept { return acc_; }
    //! Return pointer to underlying data
    constexpr pointer data() const noexcept { return ptr_; }

private:
    //! The memory access model
    accessor_type acc_;
    //! The transformation from multidimenisonal index to memory offset.
    mapping_type map_;
    //! Memory location handle
    pointer ptr_;
};

//! basic_mdspan with wrapped indices, basic_accessor policiy and right-aligned  memory layout.
template<class T, ptrdiff_t... Indices>
using mdspan = basic_mdspan<T, extents<Indices...>, layout_right, accessor_basic<T>>;

} // namespace gmx

#endif /* end of include guard: MDSPAN_MDSPAN_H */