test_that("get_xy extracts x,y from data.frame, matrix, and field_layout", {
  df <- data.frame(x = 1:3, y = 4:6, other = 7:9)
  expect_equal(get_xy(df), cbind(x = 1:3, y = 4:6))

  m <- matrix(c(1, 2, 3, 4, 5, 6), ncol = 2)
  got <- get_xy(m)
  expect_equal(dim(got), c(3L, 2L))

  expect_error(get_xy(data.frame(a = 1, b = 2)), "x.*y")
})

test_that("with_seed is reproducible and restores RNG state", {
  set.seed(999)
  before <- runif(1)
  a <- with_seed(42, runif(3))
  b <- with_seed(42, runif(3))
  expect_equal(a, b)                       # reproducible
  after <- runif(1)                        # RNG stream continues as if untouched
  set.seed(999); runif(1)
  expect_equal(after, runif(1))            # state was restored
})