code: static int rehash(int h) {
      // Spread bits to regularize both segment and index locations,
      // using variant of single-word Wang/Jenkins hash.
      // TODO(kevinb): use Hashing/move this to Hashing?
      h += (h << 15) ^ 0xffffcd7d;
      h ^= (h >>> 10);
      h += (h << 3);
      h ^= (h >>> 6);
      h += (h << 2) + (h << 14);
      return h ^ (h >>> 16);
    }
docs: Applies a supplemental hash function to a given hash code, which defends against poor quality
  hash functions. This is critical when the concurrent hash map uses power-of-two length hash
  tables, that otherwise encounter collisions for hash codes that do not differ in lower or upper
  bits.
  
  @param h hash code