Random long, can it be twice in a row the same number

Question

I was wondering with the current java 1.7 implementation of the Random class, is it possible for the code below to generate two times the same random long?

Rando         


        

Answer 1

No, getting two identical longs in a row with this algorithm is impossible.

While people were writing long posts about math and other wizardry, I went the code monkey route and brute forced the 2^48 possible seeds over the weekend. No two longs produced in a row were ever equal for any seed.

---

long int seed = 0;

int next(int bits){
    seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);
    return (int) (seed >> (48 - bits));
}
long int nextLong(){
    return ((long int) next(32) << 32) + next(32);
}

int main(int argc, char** argv) {
  long int step = atoi(argv[1]);
  long int i = step << 32;
  long int end = (step+1) << 32;
  while(i < end) {
    seed = i;
    if(nextLong() == nextLong()) {
      printf("Found seed %ld\n", i);
      return 0;
    }
    ++i;
  }
  printf("No seed in %ld\n", step);
  return 1;
}

then

echo {0..65535} | xargs -n 1 -P 12 ./executable