Shuffle an array

main.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

void shuffle(int array[], size_t n) {
    if (n > 1) {
        size_t i;
        for (i = n - 1; i > 0; i--) {
          size_t j = rand() % (i + 1);
          int tmp = array[j];
          array[j] = array[i];
          array[i] = tmp;
        }
    }
}

void test_uniformity() {
    int counts[5] = {0, 0, 0, 0, 0};
    for (int i = 0; i < 1e8; i++) {
        // always start from the same array to measure uniformity
        int ns[] = {1, 2, 3, 4, 5};
        shuffle(ns, 5);
        for (int j = 0; j < 5; j++) {
            counts[j] += ns[j];
        }
    }
    int min = counts[0];
    int max = counts[0];
    for (int i = 0; i < 5; i++) {
        if (counts[i] < min) {
            min = counts[i];
        }
        if (counts[i] > max) {
            max = counts[i];
        }
    }
    double ratio = (double) max / min;
    printf("test_uniformity:  %f (0 = perfect)\n", ratio - 1);
}

void test_performance() {
    struct timespec start_time, end_time;
    // initialize only once to measure performance
    int ns[] = {1, 2, 3, 4, 5};
    int ops = 1e8;
    int start = clock();
    for (int i = 0; i < ops; i++) {
        shuffle(ns, 5);
    }
    int end = clock();
    double sec_tot = (double) (end - start) / CLOCKS_PER_SEC;
    double nsec_op = sec_tot / ops * 1e9;
    printf("test_performance: %fns/op (%fs overall)\n", nsec_op, sec_tot);
}

int main() {
    srand(time(NULL)); // seed the rng
    test_uniformity();
    test_performance();
    return 0;
}