/*
 * heat diffusion example, as discussed in textbook.
 *
 * command line arguments are number of points, maximum number of
 *   iterations, and convergence threshold.
 *
 * parallel version with OpenMP, more complex SPMD-like approach.
 */
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <omp.h>

#define LEFTVAL 1.0
#define RIGHTVAL 10.0

void initialize(double uk[], double ukp1[], int nx) {
    int i;
    uk[0] = LEFTVAL; uk[nx-1] = RIGHTVAL;
    for (i = 1; i < nx-1; ++i)
        uk[i] = 0.0;
    for (i = 0; i < nx; ++i)
        ukp1[i] = uk[i];
}

int main(int argc, char *argv[]) {

    int nx;
    int maxsteps;
    double threshold;

    double *uk; 
    double *ukp1;
    double *temp; 
    double dx, dt;
    double start_time, end_time;
    double maxdiff;
    int step;

    int nthreads;

    if (argc < 4) {
        fprintf(stderr, 
                "usage is %s points max_iterations convergence_threshold\n", 
                argv[0]);
        exit(EXIT_FAILURE);
    }

    start_time = omp_get_wtime();

    nx = atoi(argv[1]);
    maxsteps = atoi(argv[2]);
    threshold = atof(argv[3]);

    #pragma omp parallel 
    {
        #pragma omp single 
        {
            nthreads = omp_get_num_threads();
        }
    }
    if ((nx % nthreads) != 0) {
        fprintf(stderr, "Number of threads must evenly divide %d\n", nx);
        exit(EXIT_FAILURE);
    }

    uk = malloc(sizeof(double) * nx);
    ukp1 = malloc(sizeof(double) * nx);
    if (!uk || !ukp1) {
        fprintf(stderr, "Unable to allocate memory\n");
        return EXIT_FAILURE;
    }
    dx = 1.0/nx;
    dt = 0.5*dx*dx;
    maxdiff = threshold;

    initialize(uk, ukp1, nx);

    #pragma omp parallel 
    {
        double diff;
        double local_maxdiff;
        double local_step; 

        int my_id;
        int loop_start;
        int loop_end;
        int i;

        my_id = omp_get_thread_num();
        loop_start = my_id * (nx/nthreads);
        if (my_id == 0) loop_start = 1;
        loop_end = (my_id + 1) * (nx/nthreads);
        if (my_id == nthreads-1) loop_end = nx-1;

        for (local_step = 0; (local_step < maxsteps) && (maxdiff >= threshold);
                ++local_step) {

            /* compute new values */
            /* for (i = 1; i < nx-1; ++i) */
            for (i = loop_start; i < loop_end; ++i) {
                ukp1[i]=uk[i]+ (dt/(dx*dx))*(uk[i+1]-2*uk[i]+uk[i-1]);
            }

            /* check for convergence */
            local_maxdiff = 0.0;
            #pragma omp single
            {
                maxdiff = 0.0;
            }
            /* for (i = 1; i < nx-1; ++i) */
            for (i = loop_start; i < loop_end; ++i) {
                diff = fabs(uk[i] - ukp1[i]);
                if (diff > local_maxdiff) local_maxdiff = diff;
            }
            #pragma omp critical
            {
                if (local_maxdiff > maxdiff) maxdiff = local_maxdiff;
            }

            /* "copy" ukp1 to uk by swapping pointers */
            #pragma omp barrier
            #pragma omp single
            {
                temp = ukp1; ukp1 = uk; uk = temp;
            }
        }

        /* save count of steps in shared variable */
        #pragma omp single
        {
            step = local_step;
        }
    }

    end_time = omp_get_wtime();
    printf("OpenMP program (%d threads):\n", nthreads);
    printf("nx = %d, maxsteps = %d, threshold = %g\n", nx, maxsteps, threshold);
    if (maxdiff < threshold) {
        printf("converged in %d iterations\n", step);
    }
    else {
        printf("failed to converge in %d iterations, maxdiff = %g\n", 
                step, maxdiff);
    }
    printf("execution time = %g\n", end_time - start_time);

    /* clean up and end */
    return EXIT_SUCCESS;
}