#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <getopt.h>
#include <errno.h>

/* matrix dimension */
static size_t dim;

/* options */
static struct option long_opts[] = {
    { "dim", required_argument, NULL, 'd' },
    { "1",   no_argument, NULL,       '1' },
    { "2",   no_argument, NULL,       '2' },
    { NULL,  0,                 NULL,  0  },
};

static int visit_every_element_1(const int *matrix, size_t dim)
{
    int sum = 0;

    /* row by row */
    for (int i = 0; i < dim; ++i) {
        for (int j = 0; j < dim; ++j) {
            sum += matrix[i * dim + j];
        }
    }

    return sum;
}

static int visit_every_element_2(const int *matrix, size_t dim)
{
    int sum = 0;

    /* column by column */
    for (int j = 0; j < dim; ++j) {
        for (int i = 0; i < dim; ++i) {
            sum += matrix[i * dim + j];
        }
    }

    return sum;
}

static inline void print_usage_and_die(void)
{
    fprintf(stderr, "usage: matrix [-1|-2] [-d <size>]\n");
    exit(EXIT_FAILURE);
}

int main(int argc, char **argv)
{
    int *matrix, sum;
    char *end;
    int c, choice = 1;

    dim = 10000;
    while ((c = getopt_long(argc, argv, "d:12", long_opts, NULL)) != -1) {
        switch (c) {
        case 'd':
            dim = strtoull(optarg, &end, 10);
            if (end == optarg || *end != '\0' || errno == ERANGE) {
                fprintf(stderr, "given dim is not valid\n");
                return EXIT_FAILURE;
            }
            break;
        case '1':
            choice = 1;
            break;
        case '2':
            choice = 2;
            break;
        default:
            print_usage_and_die();
        }
    }

    matrix = (int *)calloc(dim * dim, sizeof(int));
    if (!matrix) {
        perror("malloc() failed");
        return EXIT_FAILURE;
    }

    sum = choice == 1 ? visit_every_element_1(matrix, dim) :
        visit_every_element_2(matrix, dim);
    printf("Bogus result: %d\n", sum);

    return EXIT_SUCCESS;
}