code: public static int getTileHash (int x, int y)
      {
          long seed = (((x << 2) ^ y) ^ MULTIPLIER) & MASK;
          long hash = (seed * MULTIPLIER + ADDEND) & MASK;
          return (int) (hash >>> 30);
      }
docs: Compute some hash value for "randomizing" tileset picks
  based on x and y coordinates.
  
  @return a positive, seemingly random number based on x and y.