src/gromacs/mdspan/layouts.h

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  40 /*
  41  *                          Kokkos v. 2.0
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  78 /*! \libinternal \file
  79  * \brief Declares gmx::layout_right for mdspan.
  80  *
  81  * \author David Hollman <dshollm@sandia.gov>
  82  * \author Christian Blau <cblau@gwdg.de>
  83  * \ingroup mdspan
  84  */
  85 #ifndef MDSPAN_LAYOUTS_H
  86 #define MDSPAN_LAYOUTS_H
  87 #include <cstddef>
  88
  89 #include <type_traits>
  90
  91 namespace gmx
  92 {
  93
  94 /*! \libinternal \brief Right-aligned array layout indexer.
  95  * Carries the mapping class performing the translation from multidimensional
  96  * index to one-dimensional number.
  97  */
  98 class layout_right
  99 {
 100
 101 public:
 102     /*! \libinternal \brief Mapping from multidimensional indices within extents to 1D index.
 103      * \tparam Extents the extents of the multidimensional integers for the mapping.
 104      */
 105     template<class Extents>
 106     class mapping
 107     {
 108     private:
 109         //! The extents.
 110         Extents m_extents;
 111
 112     public:
 113         //! exposing the type of indices
 114         using index_type = ptrdiff_t;
 115         //! exposing the type of the extents
 116         using extents_type = Extents;
 117         //! Default constructor.
 118         constexpr mapping() noexcept = default;
 119         //! Default move constructor.
 120         constexpr mapping(mapping&&) noexcept = default;
 121         //! Default copy constructor.
 122         constexpr mapping(const mapping&) noexcept = default;
 123         //! Default move assignment
 124         mapping& operator=(mapping&&) noexcept = default;
 125         //! Default copy assignment
 126         mapping& operator=(const mapping&) noexcept = default;
 127         /*! \brief Construct mapping, setting extents
 128          * \param[in] ext the extents
 129          */
 130         constexpr mapping(const Extents& ext) noexcept : m_extents(ext) {}
 131         /*! \brief Return the extents.
 132          * \returns extents
 133          */
 134         constexpr const Extents& extents() const noexcept { return m_extents; }
 135
 136     private:
 137         /* \brief End recursion helper function for static offset calculation.
 138          * \param[in] sum The accumulated offset over all dimensions
 139          * \returns The offset.
 140          */
 141         static constexpr index_type offset(const size_t /*r*/, const ptrdiff_t sum) { return sum; }
 142
 143         /* \brief Statically calculate offset from index and extent.
 144          * For a multidimensional index (i0,i1,..,in), in a right memory
 145          * layout, 'i0' denotes the slowest moving dimension and
 146          * 'in' the fastest moving dimension.
 147          * The overall offset within extents N = (N0,..,Nn) is then
 148          * offest = i0 * N1 * .. * Nn + i1 * N2 * .. * Nn + in-1 * Nn + in
 149          *        = (((i0*N1+i1)*N2+i2)*N3+i3) ...
 150          * \param[in] r current rank
 151          * \param[in] sum current sum up to this rank
 152          * \param[in] i index
 153          * \oaram[in] indices The rest of the paramter pack.
 154          * \returns The offset.
 155          */
 156         template<class... Indices>
 157         inline constexpr index_type
 158         offset(const size_t r, ptrdiff_t sum, const index_type i, Indices... indices) const noexcept
 159         {
 160             return offset(r + 1, sum * m_extents.extent(r) + i, indices...);
 161         }
 162
 163     public:
 164         /*! \brief Return the size of the underlying one-dimensional
 165          * data structure, so that the mapping is always valid.
 166          *
 167          * \returns number of span elements
 168          */
 169         constexpr index_type required_span_size() const noexcept
 170         {
 171             index_type size = 1;
 172             for (size_t r = 0; r < m_extents.rank(); r++)
 173             {
 174                 size *= m_extents.extent(r);
 175             }
 176             return size;
 177         }
 178
 179         /*! \brief Map the multidimensional indices to 1D.
 180          * Requires number of indicies have the same dimensionality as the mapping.
 181          * \tparam Indices type of the indices to be mapped
 182          * \param[in] indices the indices to be mapped
 183          * \returns One-dimensional integer index.
 184          */
 185         template<class... Indices>
 186         std::enable_if_t<sizeof...(Indices) == Extents::rank(), index_type> constexpr
 187         operator()(Indices... indices) const noexcept
 188         {
 189             return offset(0, 0, indices...);
 190         }
 191
 192         //! Report that this mapping is always unique.
 193         static constexpr bool is_always_unique() noexcept { return true; }
 194         //! Report that this mapping is always contiguous.
 195         static constexpr bool is_always_contiguous() noexcept { return true; }
 196         //! Report that this mapping is always strided.
 197         static constexpr bool is_always_strided() noexcept { return true; }
 198
 199         //! Report that this mapping is unique.
 200         constexpr bool is_unique() const noexcept { return true; }
 201         //! Report that this mapping is contiguous.
 202         constexpr bool is_contiguous() const noexcept { return true; }
 203         //! Report that this mapping is strided.
 204         constexpr bool is_strided() const noexcept { return true; }
 205         /*!\brief Return the stride of dimension r.
 206          * \param[in] R rank of the stride to be queried.
 207          * \returns the stride along dimension r.
 208          */
 209         constexpr index_type stride(const size_t R) const noexcept
 210         {
 211             ptrdiff_t stride = 1;
 212             for (size_t r = m_extents.rank() - 1; r > R; r--)
 213             {
 214                 stride *= m_extents.extent(r);
 215             }
 216             return stride;
 217         }
 218
 219     }; // class mapping
 220
 221 }; // class layout_right
 222
 223 } // namespace gmx
 224 #endif /* end of include guard: MDSPAN_LAYOUTS_H */