protected def grad(beta: (Double, Double))(X: Data, y: Data): (Double, Double) =
    beta match {
      case (b0, b1) =>
        X.zip(y)
          .map { case (i, j) => j - b1 * i - b0 }
          .zip(X)
          .par
          .fold((0, 0)) {
            case ((acc0, acc1), (score, x)) => (acc0 + score, acc1 + x * score)
          } match {
          case (i, j) => (-2 * i / X.length, -2 * j / X.length)
        }
    }