OpenFPM_pdata  1.1.0
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Stencil example

Stencil example and ghost

This example show how to move grid_key in order to create a Laplacian stencil, be careful, the function move is convenient, but not the fastest implementation. We also show how to do ghost communications

Define some convenient constants and types

constexpr size_t x = 0;
constexpr size_t y = 1;
constexpr size_t z = 2;
constexpr size_t A = 0;
constexpr size_t B = 0;

Initialization

Initialize the library and several objects

See Also
Initialization
openfpm_init(&argc,&argv);
// domain
Box<3,float> domain({0.0,0.0,0.0},{1.0,1.0,1.0});
// grid sizes
size_t sz[3] = {100,100,100};
// ghost extension
Ghost<3,float> g(0.03);

Grid create

Create a distributed grid in 3D. With typedef we create an alias name for aggregate<float[3],float[3]>. In practice the type of grid_point == aggregate<float[3],float[3]>

See Also
Grid instantiation
// a convenient alias for aggregate<...>
typedef aggregate<float,float> grid_point;

Loop over grid points

Get an iterator that go through the point of the domain (No ghost)

See Also
Loop over grid points
auto dom = g_dist.getDomainIterator();
while (dom.isNext())
{
++dom;
}

Inside the cycle we get the local grid key

See Also
Grid coordinates
auto key = dom.get();

We convert the local grid position, into global position, key_g contain 3 integers that identify the position of the grid point in global coordinates

See Also
Grid coordinates
auto key_g = g_dist.getGKey(key);

we write on the grid point of position (i,j,k) the value i*i + j*j + k*k on the property A. Mathematically is equivalent to the function

\( f(x,y,z) = x^2 + y^2 + z^2 \)

g_dist.template get<A>(key) = key_g.get(0)*key_g.get(0) + key_g.get(1)*key_g.get(1) + key_g.get(2)*key_g.get(2);

Ghost

Each sub-domain has an extended part, that is materially contained into another processor. In general is not synchronized ghost_get synchronize the property A in the ghost part

g_dist.template ghost_get<A>();

Get again another iterator, iterate across all the domain points, calculating a Laplace stencil. Write the result on B

auto dom2 = g_dist.getDomainIterator();
while (dom2.isNext())
{
auto key = dom2.get();
// Laplace stencil
g_dist.template get<B>(key) = g_dist.template get<A>(key.move(x,1)) + g_dist.template get<A>(key.move(x,-1)) +
g_dist.template get<A>(key.move(y,1)) + g_dist.template get<A>(key.move(y,-1)) +
g_dist.template get<A>(key.move(z,1)) + g_dist.template get<A>(key.move(z,-1)) -
6*g_dist.template get<A>(key);
++dom2;
}

Finally we want a nice output to visualize the information stored by the distributed grid

See Also
VTK and visualization
g_dist.write("output");

Deinitialize the library

openfpm_finalize();

Full code

#include "Grid/grid_dist_id.hpp"
#include "data_type/aggregate.hpp"
#include "Decomposition/CartDecomposition.hpp"
constexpr size_t x = 0;
constexpr size_t y = 1;
constexpr size_t z = 2;
constexpr size_t A = 0;
constexpr size_t B = 0;
int main(int argc, char* argv[])
{
openfpm_init(&argc,&argv);
// domain
Box<3,float> domain({0.0,0.0,0.0},{1.0,1.0,1.0});
// grid sizes
size_t sz[3] = {100,100,100};
// ghost extension
Ghost<3,float> g(0.03);
// a convenient alias for aggregate<...>
typedef aggregate<float,float> grid_point;
auto dom = g_dist.getDomainIterator();
while (dom.isNext())
{
auto key = dom.get();
auto key_g = g_dist.getGKey(key);
g_dist.template get<A>(key) = key_g.get(0)*key_g.get(0) + key_g.get(1)*key_g.get(1) + key_g.get(2)*key_g.get(2);
++dom;
}
g_dist.template ghost_get<A>();
auto dom2 = g_dist.getDomainIterator();
while (dom2.isNext())
{
auto key = dom2.get();
// Laplace stencil
g_dist.template get<B>(key) = g_dist.template get<A>(key.move(x,1)) + g_dist.template get<A>(key.move(x,-1)) +
g_dist.template get<A>(key.move(y,1)) + g_dist.template get<A>(key.move(y,-1)) +
g_dist.template get<A>(key.move(z,1)) + g_dist.template get<A>(key.move(z,-1)) -
6*g_dist.template get<A>(key);
++dom2;
}
g_dist.write("output");
openfpm_finalize();
}