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//                        Kokkos v. 2.0
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// This example simulates one timestep of an explicit
// finite-difference discretization of a time-dependent partial
// differential equation (PDE).  It shows how to take subviews of the
// mesh in order to represent particular boundaries or the interior of
// the mesh.

#include <Kokkos_Core.hpp>
#include <impl/Kokkos_Timer.hpp>
#include <cstdio>

typedef Kokkos::View<double***, Kokkos::LayoutRight> mesh_type;

// These View types represent subviews of the mesh.  Some of the Views
// have layout LayoutStride, meaning that they have run-time "strides"
// in each dimension which may differ from that dimension.  For
// example, inner_mesh_type (which represents the interior of the
// mesh) has to skip over the boundaries when computing its stride;
// the dimensions of the interior mesh differ from these strides.  You
// may safely always use a LayoutStride layout when taking a subview
// of a LayoutRight or LayoutLeft subview, but strided accesses may
// cost a bit more, especially for 1-D Views.
typedef Kokkos::View<double**, Kokkos::LayoutStride> xz_plane_type;
typedef Kokkos::View<double**, Kokkos::LayoutRight> yz_plane_type;
typedef Kokkos::View<double**, Kokkos::LayoutStride> xy_plane_type;
typedef Kokkos::View<double***, Kokkos::LayoutStride> inner_mesh_type;

// Functor to set all entries of a boundary of the mesh to a constant
// value.  The functor is templated on ViewType because different
// boundaries may have different layouts.
template<class ViewType>
struct set_boundary {
  ViewType a;
  double value;

  set_boundary (ViewType a_, double value_) :
    a (a_), value (value_)
  {}

  KOKKOS_INLINE_FUNCTION
  void operator() (const typename ViewType::size_type i) const {
    for (typename ViewType::size_type j = 0; j < a.extent(1); ++j) {
      a(i,j) = value;
    }
  }
};

// Functor to set all entries of a boundary of the mesh to a constant
// value.  The functor is templated on ViewType because different
// boundaries may have different layouts.
template<class ViewType>
struct set_inner {
  ViewType a;
  double value;

  set_inner (ViewType a_, double value_) :
    a (a_), value (value_)
  {}

  KOKKOS_INLINE_FUNCTION
  void operator () (const typename ViewType::size_type i) const {
    typedef typename ViewType::size_type size_type;
    for (size_type j = 0; j < a.extent(1); ++j) {
      for (size_type k = 0; k < a.extent(2); ++k) {
        a(i,j,k) = value;
      }
    }
  }
};

// Update the interior of the mesh.  This simulates one timestep of a
// finite-difference method.
template<class ViewType>
struct update {
  ViewType a;
  const double dt;

  update (ViewType a_, const double dt_) :
    a (a_), dt (dt_)
  {}

  KOKKOS_INLINE_FUNCTION
  void operator() (typename ViewType::size_type i) const {
    typedef typename ViewType::size_type size_type;
    i++;
    for (size_type j = 1; j < a.extent(1)-1; j++) {
      for (size_type k = 1; k < a.extent(2)-1; k++) {
        a(i,j,k) += dt* (a(i,j,k+1) - a(i,j,k-1) +
                         a(i,j+1,k) - a(i,j-1,k) +
                         a(i+1,j,k) - a(i-1,j,k));
      }
    }
  }
};


int main (int narg, char* arg[]) {
  using Kokkos::ALL;
  using Kokkos::pair;
  using Kokkos::parallel_for;
  using Kokkos::subview;
  typedef mesh_type::size_type size_type;

  Kokkos::initialize (narg, arg);

  {
    // The number of mesh points along each dimension of the mesh, not
    // including boundaries.
    const size_type size = 100;

    // A is the full cubic 3-D mesh, including the boundaries.
    mesh_type A ("A", size+2, size+2, size+2);
    // Ai is the "inner" part of A, _not_ including the boundaries.
    //
    // A pair of indices in a particular dimension means the contiguous
    // zero-based index range in that dimension, including the first
    // entry of the pair but _not_ including the second entry.
    inner_mesh_type Ai = subview(A, pair<size_type, size_type> (1, size+1),
                                    pair<size_type, size_type> (1, size+1),
                                    pair<size_type, size_type> (1, size+1));
    // A has six boundaries, one for each face of the cube.
    // Create a View of each of these boundaries.
    // ALL() means "select all indices in that dimension."
    xy_plane_type Zneg_halo = subview(A, ALL (), ALL (), 0);
    xy_plane_type Zpos_halo = subview(A, ALL (), ALL (), 101);
    xz_plane_type Yneg_halo = subview(A, ALL (), 0, ALL ());
    xz_plane_type Ypos_halo = subview(A, ALL (), 101, ALL ());
    yz_plane_type Xneg_halo = subview(A, 0, ALL (), ALL ());
    yz_plane_type Xpos_halo = subview(A, 101, ALL (), ALL ());

    // Set the boundaries to their initial conditions.
    parallel_for (Zneg_halo.extent(0), set_boundary<xy_plane_type> (Zneg_halo,  1));
    parallel_for (Zpos_halo.extent(0), set_boundary<xy_plane_type> (Zpos_halo, -1));
    parallel_for (Yneg_halo.extent(0), set_boundary<xz_plane_type> (Yneg_halo,  2));
    parallel_for (Ypos_halo.extent(0), set_boundary<xz_plane_type> (Ypos_halo, -2));
    parallel_for (Xneg_halo.extent(0), set_boundary<yz_plane_type> (Xneg_halo,  3));
    parallel_for (Xpos_halo.extent(0), set_boundary<yz_plane_type> (Xpos_halo, -3));

    // Set the interior of the mesh to its initial condition.
    parallel_for (Ai.extent(0), set_inner<inner_mesh_type> (Ai, 0));

    // Update the interior of the mesh.
    // This simulates one timestep with dt = 0.1.
    parallel_for (Ai.extent(0), update<mesh_type> (A, 0.1));

    Kokkos::fence();
    printf ("Done\n");
  }
  Kokkos::finalize ();
}