InterpolateAndShiftAtBoundary.hpp

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/*
 * Copyright (C) 2021-2026 The ESPResSo project
 *
 * This file is part of ESPResSo.
 *
 * ESPResSo is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * ESPResSo is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#pragma once
 
#include <walberla_bridge/lattice_boltzmann/LeesEdwardsPack.hpp>
 
#include <blockforest/StructuredBlockForest.h>
#include <domain_decomposition/all.h>
#include <stencil/D3Q19.h>
 
#include <cmath>
#include <functional>
#include <memory>
#include <stdexcept>
#include <utility>
 
namespace walberla {
 
/**
 * @brief Lees-Edwards sweep.
 *
 * @todo Currently is constrained by the blockforest domain decomposition.
 * It only works if the structured block forest domain decomposition doesn't
 * partition along the shear direction or the normal direction.
 * The normal direction cannot be sliced, since we need full access to the
 * sheared layer population on the opposite side of the box during the
 * interpolation (we don't use the ghost populations).
 * The shear direction cannot be sliced, because the ghost layer might not
 * contain the data if the offset is larger than the ghost layer thickness.
 *
 * As a practical example, consider a simulation where the shear direction is
 * the z-axis, it is possible to run on 2 MPI ranks with MPI Cartesian topology
 * [2, 1, 1].
 * At the moment, ESPResSo requires system.cell_system.node_grid to be in
 * decreasing order, therefore parallelization requires a shear direction
 * along the z-axis and a MPI node_grid of [x, y, 1] with x >= y.
 */
template <class FieldType, typename FloatType>
class InterpolateAndShiftAtBoundary {
public:
  InterpolateAndShiftAtBoundary(
      std::shared_ptr<StructuredBlockForest> blocks, BlockDataID field_id,
      BlockDataID tmp_field_id, unsigned int n_ghost_layers,
      unsigned int shear_direction, unsigned int shear_plane_normal,
      std::function<double()> get_pos_offset,
      std::function<double()> get_shift = []() { return 0.0; })
      : m_blocks(std::move(blocks)), m_field_id(field_id),
        m_tmp_field_id(tmp_field_id), m_n_ghost_layers(uint_c(n_ghost_layers)),
        m_shear_direction(uint_c(shear_direction)),
        m_shear_plane_normal(uint_c(shear_plane_normal)),
        m_get_pos_offset(std::move(get_pos_offset)),
        m_get_shift(std::move(get_shift)) {
    if (m_n_ghost_layers != 1u) {
      throw std::domain_error("The Lees-Edwards sweep is implemented "
                              "for a ghost layer of thickness 1");
    }
    if (m_shear_plane_normal == 0u) {
      m_slab_min = stencil::W;
      m_slab_max = stencil::E;
    } else if (m_shear_plane_normal == 1u) {
      m_slab_min = stencil::S;
      m_slab_max = stencil::N;
    } else if (m_shear_plane_normal == 2u) {
      m_slab_min = stencil::B;
      m_slab_max = stencil::T;
    }
  }
 
  FloatType get_pos_offset() const {
    return numeric_cast<FloatType>(m_get_pos_offset());
  }
 
  FloatType get_shift() const { return numeric_cast<FloatType>(m_get_shift()); }
 
  void operator()(IBlock *block) {
    kernel(block, m_slab_min);
    kernel(block, m_slab_max);
  }
 
private:
  void kernel(IBlock *block, stencil::Direction slab_dir) {
    // setup lengths
    assert(m_blocks->getNumberOfCells(*block, m_shear_plane_normal) >= 2u);
    auto const dir = m_shear_direction;
    auto const dim = cell_idx_c(m_blocks->getNumberOfCells(*block, dir));
    auto const length = numeric_cast<FloatType>(dim);
 
    // setup slab
    auto field = block->template getData<FieldType>(m_field_id);
    auto tmp_field = block->template getData<FieldType>(m_tmp_field_id);
 
    CellInterval ci;
    field->getGhostRegion(slab_dir, ci, cell_idx_t{1}, true);
 
    // shift
    auto const shift = get_shift();
    // Note that the offset is applied to the interpolation source rather than
    // the target
    auto const prefactor =
        ((slab_dir == m_slab_max) ? FloatType{-1} : FloatType{1});
    auto const offset = static_cast<FloatType>(get_pos_offset()) * prefactor;
    auto const folded_offset = modulo(offset, length);
    // 0<=folded_offset<length
    auto const weight1 = FloatType{1} - std::fmod(folded_offset, FloatType{1});
    auto const weight2 = std::fmod(folded_offset, FloatType{1});
    for (auto const &&cell : ci) {
      Cell source1 = cell;
      Cell source2 = cell;
      auto const source_pos = static_cast<FloatType>(cell[dir]) + folded_offset;
      auto const folded_source_pos = modulo(source_pos, length);
      // 0 <= folded_source_pos < length
      source1[dir] = cell_idx_c(std::floor(folded_source_pos));
      // 0 <= source1[dir] < length, i.e. integer value up to length-1 inclusive
      source2[dir] = cell_idx_c(modulo(FloatType(source1[dir] + 1), length));
      // integer value between 0 and length -1 inclusive
      for (uint_t q = 0u; q < FieldType::F_SIZE; ++q) {
        tmp_field->get(cell, q) =
            field->get(source1, q) * weight1 + field->get(source2, q) * weight2;
      }
      tmp_field->get(cell, m_shear_direction) -= prefactor * shift;
    }
 
    // swap
    for (auto const &&cell : ci) {
      for (uint_t f = 0; f < FieldType::F_SIZE; ++f) {
        field->get(cell, f) = tmp_field->get(cell, f);
      }
    }
  }
 
  FloatType modulo(FloatType a, FloatType b) const {
    auto const res = std::fmod(a, b);
    return (res < FloatType{0}) ? res + b : res;
  }
 
private:
  std::shared_ptr<StructuredBlockForest> m_blocks;
  BlockDataID m_field_id;
  BlockDataID m_tmp_field_id;
  uint_t m_n_ghost_layers;
  uint_t m_shear_direction;
  uint_t m_shear_plane_normal;
  std::function<double()> m_get_pos_offset;
  std::function<double()> m_get_shift;
  stencil::Direction m_slab_min;
  stencil::Direction m_slab_max;
};
 
} // namespace walberla