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poisson.cc
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	/* -------------------------------------------------------------------------- */
	#include "simulation.hh"
	/* -------------------------------------------------------------------------- */
	#include <chrono>
	#include <iostream>
	#include <sstream>
	#include <tuple>
	#include <chrono>
	/* -------------------------------------------------------------------------- */
	#include <mpi.h>
	/* -------------------------------------------------------------------------- */

	#define EPSILON 0.005

	typedef std::chrono::high_resolution_clock clk;
	typedef std::chrono::duration<double> second;

	static void usage(const std::string & prog_name) {
	std::cerr << prog_name << " <grid_size>" << std::endl;
	exit(0);
	}

	int main(int argc, char * argv[]) {
	MPI_Init(&argc, &argv);
	int prank, psize;

	MPI_Comm_rank(MPI_COMM_WORLD, &prank);
	MPI_Comm_size(MPI_COMM_WORLD, &psize);

	if (argc != 2) usage(argv[0]);

	std::stringstream args(argv[1]);
	int N;
	args >> N;

	if(args.fail()) usage(argv[0]);


	// Divide the total size by the number of processes
	int m_local = N / psize;

	// Add 1 or 2 ghosts depending on the prank
	m_local +=
	(psize == 1) ? 0 :
	(prank == 0 or prank == psize -1) ? 1 : 2;

	Simulation simu(m_local, N);

	simu.set_initial_conditions();

	simu.set_epsilon(EPSILON);

	float l2;
	int k;

	auto start = clk::now();
	std::tie(l2, k) = simu.compute();
	auto end = clk::now();

	second time = end - start;

	if(prank == 0)
	std::cout << psize << " " << N << " "
	<< k << " " << std::scientific << l2 << " "
	<< time.count() / k << std::endl;

	MPI_Finalize();

	return 0;
	}