test_that("border_weights is symmetric, hollow, and sums to 1", {
  coords <- expand.grid(x = 0:2, y = 0:2)
  W <- border_weights(coords)
  expect_s3_class(W, "border_weights")
  expect_equal(diag(W), rep(0, nrow(W)))         # hollow
  expect_equal(W, t(W))                          # symmetric
  expect_equal(sum(W), 1)                        # standardized
})

test_that("border_weights matches inverse-distance definition", {
  coords <- data.frame(x = c(0, 1), y = c(0, 0))
  W <- border_weights(coords, standardize = FALSE)
  expect_equal(W[1, 2], 1)                       # 1 / dist(=1)
  expect_equal(diag(W), c(0, 0))
})

test_that("border_weights errors on degenerate coords", {
  expect_error(border_weights(data.frame(x = c(0, 0), y = c(0, 0))), "distance")
})