// Author: Wes Kendall
// Copyright 2013 www.mpitutorial.com
// This code is provided freely with the tutorials on mpitutorial.com. Feel
// free to modify it for your own use. Any distribution of the code must
// either provide a link to www.mpitutorial.com or keep this header intact.
//
// Program that computes the standard deviation of an array of elements in parallel using
// MPI_Reduce.
//
#include <assert.h>
#include <math.h>
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

// Creates an array of random numbers. Each number has a value from 0 - 1
float*
create_rand_nums(int num_elements)
{
    float* rand_nums = (float*) malloc(sizeof(float) * num_elements);
    assert(rand_nums != NULL);
    int i;
    for(i = 0; i < num_elements; i++)
    {
        rand_nums[i] = (rand() / (float) RAND_MAX);
    }
    return rand_nums;
}

int
main(int argc, char** argv)
{
    if(argc != 2)
    {
        fprintf(stderr, "Usage: avg num_elements_per_proc\n");
        exit(1);
    }

    int num_elements_per_proc = atoi(argv[1]);

    MPI_Init(NULL, NULL);

    int world_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
    int world_size;
    MPI_Comm_size(MPI_COMM_WORLD, &world_size);

    // Create a random array of elements on all processes.
    srand(time(NULL) *
          world_rank);  // Seed the random number generator of processes uniquely
    float* rand_nums = NULL;
    rand_nums        = create_rand_nums(num_elements_per_proc);

    // Sum the numbers locally
    float local_sum = 0;
    int   i;
    for(i = 0; i < num_elements_per_proc; i++)
    {
        local_sum += rand_nums[i];
    }

    // Reduce all of the local sums into the global sum in order to
    // calculate the mean
    float global_sum;
    MPI_Allreduce(&local_sum, &global_sum, 1, MPI_FLOAT, MPI_SUM, MPI_COMM_WORLD);
    float mean = global_sum / (num_elements_per_proc * world_size);

    // Compute the local sum of the squared differences from the mean
    float local_sq_diff = 0;
    for(i = 0; i < num_elements_per_proc; i++)
    {
        local_sq_diff += (rand_nums[i] - mean) * (rand_nums[i] - mean);
    }

    // Reduce the global sum of the squared differences to the root process
    // and print off the answer
    float global_sq_diff;
    MPI_Reduce(&local_sq_diff, &global_sq_diff, 1, MPI_FLOAT, MPI_SUM, 0, MPI_COMM_WORLD);

    // The standard deviation is the square root of the mean of the squared
    // differences.
    if(world_rank == 0)
    {
        float stddev = sqrt(global_sq_diff / (num_elements_per_proc * world_size));
        printf("Mean - %f, Standard deviation = %f\n", mean, stddev);
    }

    // Clean up
    free(rand_nums);

    MPI_Barrier(MPI_COMM_WORLD);
    MPI_Finalize();
}