kspace.hpp

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// SPDX-FileCopyrightText: 2025 VTT Technical Research Centre of Finland Ltd
// SPDX-License-Identifier: AGPL-3.0-or-later
 
#pragma once
 
#include <array>
#include <cmath>
#include <openpfc/constants.hpp>
#include <openpfc/core/world.hpp>
 
namespace pfc {
namespace fft {
namespace kspace {
 
template <typename T>
inline std::array<double, 3>
k_frequency_scaling(const world::World<T> &world) noexcept {
  const auto spacing = world::get_spacing(world);
  const auto size = world::get_size(world);
  return {two_pi / (spacing[0] * size[0]), two_pi / (spacing[1] * size[1]),
          two_pi / (spacing[2] * size[2])};
}
 
inline double k_component(int index, int size, double freq_scale) noexcept {
  return (index <= size / 2) ? index * freq_scale : (index - size) * freq_scale;
}
 
inline double k_laplacian_value(double ki, double kj, double kk) noexcept {
  return -(ki * ki + kj * kj + kk * kk);
}
 
inline double k_squared_value(double ki, double kj, double kk) noexcept {
  return ki * ki + kj * kj + kk * kk;
}
 
} // namespace kspace
} // namespace fft
} // namespace pfc