int simple_add(int* sum) {
  (*sum)++;  // attention: ++ is prior to *
}

int atomic_add(int* sum) {
  __sync_fetch_and_add(sum, 1);
}

peterson_mutex_t peterson_mutex;

int peterson_mutex_init(peterson_mutex_t* mutex) {
  memset(mutex->level, -1, N_THREAD * sizeof(int));
  memset(mutex->waiting, -1, N_THREAD * sizeof(int));
}

int peterson_mutex_lock(peterson_mutex_t* mutex, int i){
  for (int l = 0; l < N_THREAD - 1; ++l) {
    mutex->level[i] = l;
    mutex->waiting[l] = i;
    while (mutex->waiting[l] == i && !is_highest_level(mutex, i, l)) {
    }
  }
}
int peterson_mutex_unlock(peterson_mutex_t* mutex, int i) {
  peterson_mutex_state[i] = 0;
  mutex->level[i] = -1;
}