serial jacobi example

serial_jacobi.c

#include <stdio.h>
#include <sys/time.h>

#define N 100
#define MAX_ITER 5000

typedef float MATRIX_V[N+1][N+1];

int timeval_subtract(struct timeval *result, struct timeval *t2, struct timeval *t1);

int main(int argc, char* argv) {
  int i,j,k;
	MATRIX_V V, V_tmp;
    struct timeval start, end, diff;
	float *p_local, *p_local_tmp, *swap;
	
	// get starting time
	gettimeofday(&start, NULL);

	// initialize all points with reasonable starting values for the potential
	for (i = 1; i < N; i++)
		for (j = 1; j < N; j++)
			V[i][j] = 51.0;
                
	// boundary conditions
	for (j = 0; j <= N; j++) V[0][j] =  30;   /// left/
	for (j = 0; j <= N; j++) V[N][j] =  70;   /// right/
	for (i = 0; i <= N; i++) V[i][0] =   0;   /// bottom/
	for (i = 0; i <= N; i++) V[i][N] = 100;   /// top/
    

	p_local=&V[0][0];
	p_local_tmp=&V_tmp[0][0];

	// numerically solve Laplace's equation
	for (k = 0; k < MAX_ITER; k++) {
    	
		// calculate new values
		for (i = 1; i < N; i++){
      		for (j = 1; j < N; j++){
					*(p_local_tmp+i*N+j) = 0.25 * ( *(p_local+((i-1)*N)+j) + *(p_local+((i+1)*N)+j) + 
												    *(p_local+(i*N)+j-1) + *(p_local+(i*N)+j+1));
			}
							
		}
      
		// copy array
		swap = p_local_tmp; 
		p_local_tmp = p_local;
		p_local = swap;

	}
			
	// get end time
	gettimeofday(&end, NULL);
	timeval_subtract(&diff, &end, &start);
	
	printf("Time used in seconds: %ld.%06ld;%d\n", diff.tv_sec, diff.tv_usec,N);
	
}

int timeval_subtract(struct timeval *result, struct timeval *t2, struct timeval *t1)
{
    long int diff = (t2->tv_usec + 1000000 * t2->tv_sec) - (t1->tv_usec + 1000000 * t1->tv_sec);
    result->tv_sec = diff / 1000000;
    result->tv_usec = diff % 1000000;

    return (diff<0);
}