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+/*  Lattice Boltzmann sample, written in C++, using the OpenLB
+ *  library
+ *
+ *  Copyright (C) 2019 Davide Dapelo
+ *  E-mail contact: info@openlb.net
+ *  The most recent release of OpenLB can be downloaded at
+ *  <http://www.openlb.net/>
+ *
+ *  This program 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 2
+ *  of the License, or (at your option) any later version.
+ *
+ *  This program 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, write to the Free
+ *  Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ *  Boston, MA  02110-1301, USA.
+ */
+
+/* Models for Lagrangian back-coupling methods -- header file.
+ */
+
+#ifndef LB_BACK_COUPLING_MODELS_H
+#define LB_BACK_COUPLING_MODELS_H
+
+#include "twoWayHelperFunctionals.h"
+
+namespace olb {
+
+/** Abstact base class for BaseBackCouplingModel.
+  * Its raison d'etre consists of not being templetized in Lattice.
+  */
+template<typename T, template<typename V> class Particle>
+class BackCouplingModel {
+public:
+  /// Class operator to apply the coupling, for overload.
+  virtual bool operator() (Particle<T>* p, int globic, int material, int subCycles=1)=0;
+  /// Resets external field
+  virtual void resetExternalField(int material)=0;
+};
+
+/** Abstact class for all the back-coupling models,
+  * viz., momentum coupling from particle to fluid.
+  * Input parameters in attice units.
+  */
+template<typename T, typename Lattice, template<typename V> class Particle>
+class BaseBackCouplingModel : public BackCouplingModel<T,Particle> {
+public:
+  /// Resets external field
+  virtual void resetExternalField(int material) override;
+protected:
+  /// Constructor
+  BaseBackCouplingModel ( UnitConverter<T, Lattice>& converter,
+                         SuperLattice3D<T, Lattice>& sLattice,
+                         SuperGeometry3D<T>& sGeometry );
+  UnitConverter<T, Lattice>& _converter; // reference to a UnitConverter
+  SuperGeometry3D<T>& _sGeometry;
+  SuperLattice3D<T, Lattice>& _sLattice; // reference to a lattice
+private:
+  // Pointers to functions to reset fluid force
+  std::shared_ptr<AnalyticalConst3D<T, T> > _zeroAnalytical;
+  std::shared_ptr<AnalyticalComposed3D<T, T> > _zeroField;
+};
+
+/** Back-coupling is performed on the cell containing the particle
+  * and its neighbours within a cube of one lattice spacing, as per in Maier et al. (2017).
+  * Input parameters in attice units.
+  */
+template<typename T, typename Lattice, template<typename V> class Particle>
+class CubicDeltaBackCouplingModel : public BaseBackCouplingModel<T,Lattice,Particle> {
+public:
+  /// Constructor
+  CubicDeltaBackCouplingModel ( UnitConverter<T, Lattice>& converter,
+                         SuperLattice3D<T, Lattice>& sLattice,
+                         SuperGeometry3D<T>& sGeometry );
+  /// Class operator to apply the coupling.
+  virtual bool operator() (Particle<T>* p, int globic, int material, int subCycles=1) override;
+protected:
+  int _range = 1;
+  T _delta[4][4][4] = { T() };
+  std::shared_ptr<SuperLatticeSmoothDiracDelta3D<T, Lattice> > _cubicDeltaFunctional;
+};
+
+/** Back-coupling is performed only on the cell containing the particle.
+  * Input parameters in attice units.
+  */
+template<typename T, typename Lattice, template<typename V> class Particle>
+class LocalBackCouplingModel : public BaseBackCouplingModel<T,Lattice,Particle> {
+public:
+  /// Constructor
+  LocalBackCouplingModel ( UnitConverter<T, Lattice>& converter,
+                         SuperLattice3D<T, Lattice>& sLattice,
+                         SuperGeometry3D<T>& sGeometry );
+  /// Class operator to apply the coupling.
+  virtual bool operator() (Particle<T>* p, int globic, int material, int subCycles=1) override;
+};
+
+/** Class for a generic non-local back-coupling model,
+  * viz., momentum coupling from particle to fluid.
+  * It reproduces the characteristics (viz., smoothing) of an input forward coupling model.
+  * Input parameters in attice units.
+  */
+template<typename T, typename Lattice, template<typename V> class Particle>
+class NonLocalBaseBackCouplingModel : public BaseBackCouplingModel<T,Lattice,Particle> {
+public:
+  /// Constructor
+  NonLocalBaseBackCouplingModel ( UnitConverter<T, Lattice>& converter,
+                         SuperLattice3D<T, Lattice>& sLattice,
+                         SuperGeometry3D<T>& sGeometry,
+                         SmoothingFunctional<T, Lattice>& smoothingFunctional );
+  /// Class operator to apply the coupling.
+  virtual bool operator() (Particle<T>* p, int globic, int material, int subCycles=1) override;
+protected:
+  SmoothingFunctional<T, Lattice>& _smoothingFunctional; // Functional to treat non-local smoothing
+};
+
+}
+
+#endif