Find seeds that produce sequential states in rand(). e.g. find the seed that produces the same state as `srand(1); rand();`. In other words, find `x` such that the following prints `1`: `srand(1); rand(); int a = rand(); srand(x); int b = rand(); printf("%i", a == b);

seed_sequence.c

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

unsigned find_following_seed(unsigned seed) {
    srand(seed);
    (void)rand();
    int arr[] = {rand(), rand(), rand(), rand()};

    for (long long next_seed = 0; next_seed <= UINT_MAX; ++next_seed) {
        srand(next_seed);
        if (rand() == arr[0] && rand() == arr[1] && rand() == arr[2] &&
            rand() == arr[3]) {
            return next_seed;
        }
    }
    fprintf(stderr, "Error: no seed following %u found.\n", seed);
    abort();
}

int main() {
    for (unsigned i = 0, next_seed, seed = 1; i < 10; ++i, seed = next_seed) {
        next_seed = find_following_seed(seed);
        printf("Seed following %u is %u.\n", seed, next_seed);
    }
}