code: public static int add(int key, int x, int y, int z) {
          int offset = key(x, y, z);
          return (key + offset) & mask;
      }
docs: Adds the given offset to the packed key
  
  @param key the base key
  @param x   the x offset
  @param y   the y offset
  @param z   the z offset
  
  @return the new key