devtools::install_github("FinnishCancerRegistry/stabli@release")

# stabli::stat_table_make_from_expr
my_stat_fun_1 <- function(
  x,
  by = NULL,
  subset = NULL,
  by_style = NULL
) {
  out <- stabli:::stat_table_make_from_expr(
    expr = quote(list(n = .N, mu = mean(b))),
    meta_expr = quote(list(
      stratum_col_nms = as.character(names(by)),
      value_col_nms = c("n", "mu")
    )),
    dataset_nm = "x"
  )
  return(out[])
}

my_stat_fun_2 <- function(
  x,
  by = NULL,
  subset = NULL,
  by_style = NULL
) {
  out <- stabli:::stat_table_make_from_expr(
    expr = quote({
      dt <- data.table::data.table(
        interval_no = 1:5,
        time_lo = 0:4,
        time_up = 1:5,
        n = NA_integer_,
        d = NA_integer_
      )
      if (.N > 0) {
        data.table::set(
          dt,
          j = c("n", "d"),
          value = list(
            n = sample(size = 5, x = 10L, replace = TRUE),
            d = sample(size = 5, x = 3L, replace = TRUE)
          )
        )
      }
      dt[]
    }),
    meta_expr = quote(list(
      stratum_col_nms = c(names(by), "interval_no"),
      value_col_nms = c("n", "d")
    )),
    dataset_nm = "x"
  )
  return(out[])
}

# If you don't some args in your function
my_stat_fun_3 <- function(
  x,
  by = NULL
) {
  subset <- NULL
  by_style <- NULL
  out <- stabli:::stat_table_make_from_expr(
    expr = quote(list(n = .N, mu = mean(b))),
    meta_expr = quote(list(
      stratum_col_nms = as.character(names(by)),
      value_col_nms = c("n", "mu")
    )),
    dataset_nm = "x"
  )
  return(out[])
}

my_dataset <- data.table::data.table(
  a = sample(1:5, size = 1e6L, replace = TRUE)
)
my_dataset[j = "b" := a + runif(n = 1e6L)]

st_1 <- my_stat_fun_1(
  x = my_dataset,
  by = "a",
  subset = data.table::data.table(a = 3:5),
  by_style = "keep_empty"
)
stopifnot(nrow(st_1) == 5)

st_2 <- my_stat_fun_2(
  x = my_dataset,
  by = "a",
  subset = data.table::data.table(a = 3:5),
  by_style = "keep_empty"
)
stopifnot(nrow(st_2) == 5 * 5)

st_3 <- my_stat_fun_2(
  x = my_dataset,
  by = NULL,
  subset = data.table::data.table(a = 3:5),
  by_style = "keep_empty"
)
stopifnot(nrow(st_3) == 5)

st_4 <- my_stat_fun_1(
  x = my_dataset,
  by = NULL,
  subset = data.table::data.table(a = 3:5),
  by_style = "keep_empty"
)
stopifnot(nrow(st_4) == 1)

st_5 <- my_stat_fun_1(
  x = my_dataset,
  by = list(a = 1:6),
  subset = data.table::data.table(a = 3:5),
  by_style = "keep_empty"
)
stopifnot(nrow(st_5) == 6)

st_6 <- my_stat_fun_2(
  x = my_dataset,
  by = list(a = 1:6),
  subset = data.table::data.table(a = 3:5),
  by_style = "keep_empty"
)
stopifnot(nrow(st_6) == 6 * 5)

st_7 <- my_stat_fun_3(
  x = my_dataset,
  by = "a"
)
stopifnot(nrow(st_7) == 5)