interpolation_stencil.hh

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#pragma once
 
// standard library
#include <cstddef>
 
// DiFfRG
#include <DiFfRG/common/kokkos.hh>
#include <DiFfRG/discretization/coordinates/coordinates.hh>
 
namespace DiFfRG
{
  template <typename CT> struct InterpolationStencil {
    size_t lower, upper;
    CT t;
  };
 
  template <bool periodic, typename CT>
  KOKKOS_FORCEINLINE_FUNCTION InterpolationStencil<CT> make_interpolation_stencil(CT idx, const size_t n)
  {
    using Kokkos::floor, Kokkos::max, Kokkos::min;
    if constexpr (periodic) {
      // backward() folds into [0, n), the min/max only guard against rounding at the seam
      const size_t lower = min(size_t(max(static_cast<CT>(0), floor(idx))), n - 1);
      return {lower, (lower + 1) % n, idx - static_cast<CT>(lower)};
    } else {
      idx = max(static_cast<CT>(0), min(idx, static_cast<CT>(n - 1)));
      const size_t lower = min(size_t(floor(idx)), n - 2);
      return {lower, lower + 1, idx - static_cast<CT>(lower)};
    }
  }
} // namespace DiFfRG