code: static final int nextSecondarySeed() {
      int r;
      if ((r = getThreadLocalRandomSecondarySeed()) != 0) {
        r ^= r << 13; // xorshift
        r ^= r >>> 17;
        r ^= r << 5;
      } else if ((r = mix32(seeder.getAndAdd(SEEDER_INCREMENT))) == 0) {
        r = 1; // avoid zero
      }
      setThreadLocalRandomSecondarySeed(r);
      return r;
    }
docs: Returns the pseudo-randomly initialized or updated secondary seed.