code: function _grad(hash, x, y, z) {
      var h, u, v;
      h = hash & 15;
      u = h < 8 ? x : y;
      v = h < 4 ? y : h === 12 || h === 14 ? x : z;
      return ((h & 1) === 0 ? u : -u) + ((h & 2) === 0 ? v : -v);
  }
docs: Convert low 4 bits of hash code into 12 gradient directions.