test_that("kernel_regression runs and returns expected structure", {

  set.seed(123)

  f  <- function(x) 5*sin(x) + 23*(cos(x))^2
  n  <- 80
  X  <- runif(n, 5, 15)
  Y  <- f(X) + rnorm(n, 0, 3)

  out <- kernel_regression(
    X, Y,
    kernel      = "tricube",
    bandwidths  = c(1, 5, 10),
    n_repeats   = 5,
    train_ratio = 0.8
  )

  # Structure
  expect_type(out, "list")
  expect_true(all(c("results", "best_h", "x_grid", "y_smooth") %in% names(out)))

  # results must be data.frame
  expect_s3_class(out$results, "data.frame")

  # columns present
  expect_true(all(c("h", "Avg_Train_MSE", "Avg_Test_MSE") %in% names(out$results)))

  # bandwidth list not empty
  expect_gt(nrow(out$results), 0)

  # best_h is numeric
  expect_type(out$best_h, "double")

  # x_grid and y_smooth have same length
  expect_equal(length(out$x_grid), length(out$y_smooth))

  # no NaN in results
  expect_false(any(is.na(out$results$Avg_Train_MSE)))
  expect_false(any(is.na(out$results$Avg_Test_MSE)))
})