layout.hpp

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#pragma once
 
#include <uni20/mdspan/mdspan.hpp>
 
#include <array>
#include <concepts>
#include <cstddef>
#include <type_traits>
#include <utility>
 
namespace uni20::layout
{
 
template <typename LayoutPolicy, typename Extents>
using mapping_for_t = typename LayoutPolicy::template mapping<Extents>;
 
template <typename Builder, typename LayoutPolicy, typename Extents>
concept mapping_builder_for = requires(Builder&& builder, Extents const& exts) {
  { std::forward<Builder>(builder)(exts) } -> std::same_as<mapping_for_t<LayoutPolicy, Extents>>;
};
namespace detail
{
 
template <typename Extents>
constexpr auto layout_right_strides(Extents const& exts) -> std::array<typename Extents::index_type, Extents::rank()>
{
  using index_type = typename Extents::index_type;
  std::array<index_type, Extents::rank()> strides{};
  index_type run = 1;
  for (int d = static_cast<int>(Extents::rank()) - 1; d >= 0; --d)
  {
    strides[d] = run;
    run *= exts.extent(static_cast<std::size_t>(d));
  }
  return strides;
}
 
template <typename Extents>
constexpr auto layout_left_strides(Extents const& exts) -> std::array<typename Extents::index_type, Extents::rank()>
{
  using index_type = typename Extents::index_type;
  std::array<index_type, Extents::rank()> strides{};
  index_type run = 1;
  for (std::size_t d = 0; d < Extents::rank(); ++d)
  {
    strides[d] = run;
    run *= exts.extent(d);
  }
  return strides;
}
 
} // namespace detail
 
struct LayoutRight
{
    template <typename Extents>
    constexpr auto operator()(Extents const& exts) const -> mapping_for_t<stdex::layout_stride, Extents>
    {
      return {exts, detail::layout_right_strides(exts)};
    }
};
 
struct LayoutLeft
{
    template <typename Extents>
    constexpr auto operator()(Extents const& exts) const -> mapping_for_t<stdex::layout_stride, Extents>
    {
      return {exts, detail::layout_left_strides(exts)};
    }
};
 
template <typename LayoutPolicy, typename Extents>
constexpr auto make_mapping(Extents const& exts) -> mapping_for_t<LayoutPolicy, Extents>
{
  if constexpr (std::is_same_v<LayoutPolicy, stdex::layout_stride>)
  {
    return LayoutRight{}(exts);
  }
  else
  {
    return mapping_for_t<LayoutPolicy, Extents>{exts};
  }
}
 
} // namespace uni20::layout