//
// Written for Computer Networks and Systems lab classes
// AUTHOR : Sergiusz Warga

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>

#define N_OF_ELEMENTS 1e6
#define N_OF_CHILDREN 10
#define SPAN (int)(N_OF_ELEMENTS/N_OF_CHILDREN)


int child_task(double * array, int start, int pipe) {
    double sum;
    for (int i = start; i < start + SPAN; ++i ) {
        sum += array[i];
    }

    write(pipe, &sum, sizeof(sum));
    close(pipe);
    return 0;
}

double avg(double * array) {
    double sum = 0;
    for (int i = 0; i < N_OF_CHILDREN; ++i) {
        sum += array[i];
    }
    return sum/N_OF_ELEMENTS;
}

int main() {
    
    int children_pids[N_OF_CHILDREN];
    pid_t pid;
    
    double sums[N_OF_CHILDREN] = {0};
    double * array = malloc(N_OF_ELEMENTS * sizeof(double));

    srandom(time(NULL));
    for (int i = 0; i < N_OF_ELEMENTS; ++i) {
        array[i] = 2*(double)random()/RAND_MAX - 1;  // Nasty
    }

    int child_to_parent[N_OF_CHILDREN][2];

    for (int i = 0; i < N_OF_CHILDREN; ++i) {
        if (pipe(child_to_parent[i]) < 0 ){
            fprintf(stderr,"Something went wrong!\n");
            return -1;
        }
        pid = fork();
        if (pid == -1) {    // If something went wrong.
            perror("Fork: ");
            exit(EXIT_FAILURE);
        }
        if (pid == 0){      // If this is a child process.
            child_task(array, (int)(i*SPAN), child_to_parent[i][1]);
            exit(EXIT_SUCCESS);
        }
        children_pids[i] = pid;
    }

    for (int i = 0; i < N_OF_CHILDREN; ++i) {

        read(child_to_parent[i][0], &sums[i], sizeof(double));    
        close(child_to_parent[i][0]);
    }

    printf("avg = %f\n", avg(sums));

    return 0;
}