test_that("fit_temkin converges and returns sensible R2", {
  Ce <- c(2, 5, 10, 20, 35, 50, 70)
  qe <- c(0.8, 1.6, 2.3, 3.0, 3.6, 4.0, 4.4)
  fit <- fit_temkin(Ce, qe)
  expect_true(fit$R2 > 0.9)
  expect_true(fit$KT > 0)
})

test_that("fit_dr converges and E is in a plausible range", {
  Ce <- c(2, 5, 10, 20, 35, 50, 70)
  qe <- c(0.8, 1.6, 2.3, 3.0, 3.6, 4.0, 4.4)
  fit <- fit_dr(Ce, qe)
  expect_true(fit$qs > 0)
  expect_true(is.finite(fit$E))
})

test_that("fit_sips recovers parameters close to Langmuir when n approx 1", {
  set.seed(5)
  Ce <- c(2, 5, 10, 20, 35, 50, 70, 90)
  qe <- (5 * 0.15 * Ce) / (1 + 0.15 * Ce) + stats::rnorm(length(Ce), 0, 0.02)
  fit <- fit_sips(Ce, qe)
  expect_true(fit$R2 > 0.95)
  expect_equal(fit$n, 1, tolerance = 0.5)
})

test_that("fit_sips errors with too few points", {
  expect_error(fit_sips(c(1, 2, 3), c(1, 2, 3)), "at least 4")
})