sqpi.c

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <errno.h>

struct worker_ctx {
    pthread_t thr;
    size_t n;
    size_t stride;
    size_t offset;
    size_t result;
};

static size_t
calc_part(size_t n, size_t stride, size_t offset)
{
    size_t x, y, c = 0, r = n * n;

    for (y = offset; y < n; y += stride) {
        for (x = 0; x < n; x++) {
            if (x * x + y * y < r) {
                c++;
            }
        }
    }

    return c;
}

static void*
thread_main(void* ptr)
{
    struct worker_ctx* ctx = ptr;

    ctx->result = calc_part(ctx->n, ctx->stride, ctx->offset);

    return NULL;
}

int
main(int argc, const char** argv)
{
    size_t n = 0, w = 0, c = 0, i;

    if (argc < 3) {
        fprintf(stderr, "usage: sqpi n worker_count\n");
        return 1;
    }

    sscanf(argv[1], "%zu", &n);

    if (n == 0) {
        fprintf(stderr, "n must > 0\n");
        return 1;
    }

    sscanf(argv[2], "%zu", &w);

    if (w == 0) {
        fprintf(stderr, "worker_count must > 0\n");
        return 1;
    }

    struct worker_ctx* workers = calloc(n, sizeof(*workers));

    if (!workers) {
        perror("could not allocate workers");
        return 1;
    }

    for (i = 0; i < w; i++) {
        workers[i].n = n;
        workers[i].stride = w;
        workers[i].offset = i;

        if (pthread_create(&workers[i].thr, NULL, thread_main, &workers[i])) {
            perror("could not spawn worker thread");
            return 1;
        }
    }

    for (i = 0; i < w; i++) {
        if (pthread_join(workers[i].thr, NULL)) {
            perror("could not join worker thread");
            return 1;
        }
    }

    for (i = 0; i < n; i++) {
        c += workers[i].result;
    }

    double result = ((double)c / (double)(n * n)) * 4.0;

    printf("%lf\n", result);
    
    return 0;
}