test_df <- data.frame(x = c(NA,2,3),
                      y = c(1,NA,3),
                      z = c(1,2,3),
                      xi = factor(c(4,5,6)))

df_prop_miss <- data.frame(prop_miss_all = c(1/3,
                                             1/3,
                                             0))

test_df_answer <- dplyr::bind_cols(test_df,df_prop_miss)

test_that("add_shadow_shift returns a tibble",{
          expect_s3_class(add_shadow_shift(test_df), "tbl_df")
  })

test_that("add_shadow_shift doubles the number of columns when default used",{
  expect_equal(ncol(test_df) * 2, ncol(add_shadow_shift(test_df)))
})

test_that("add_shadow_shift adds one extra columns when one var used",{
  expect_equal(ncol(test_df) + 1, ncol(add_shadow_shift(test_df, xi)))
})

test_that("add_shadow_shift adds a column named 'xi_shift'",{
  expect_equal(names(add_shadow_shift(test_df, xi)),
               c(names(test_df),"xi_shift"))
})

test_that("add_shadow_shift adds two columns named 'xi_shift' and 'x_shift' ",{
  expect_equal(names(add_shadow_shift(test_df, xi, x)),
               c(names(test_df),"xi_shift", "x_shift"))
})