cat_tab <- function(x, var, digits = 2, na.rm = TRUE, drop_empty = TRUE, trim = TRUE){
  x <- as.character(x)
  if (trim) x <- trimws(x)
  if (na.rm) x <- x[!is.na(x)]
  if (drop_empty) x <- x[x != ""]

  tab <- as.data.frame(table(x), stringsAsFactors = FALSE)
  names(tab) <- c(var, "n")
  tab$n <- as.integer(tab$n)
  tab <- tab[order(-tab$n, tab[[var]]), , drop = FALSE]
  tab$prc <- round(tab$n / sum(tab$n) * 100, digits)

  tab
}

make_key <- function(z){
  z2 <- iconv(z, from = "", to = "ASCII//TRANSLIT")
  if (is.na(z2)) z2 <- z
  z2 <- tolower(z2)
  z2 <- gsub("[^a-z0-9]+", "_", z2)
  z2 <- gsub("^_+|_+$", "", z2)
  z2
}

cat_add <- function(lista, x, var,
                    type = c("levels", "topk"),
                    levels = NULL, k = Inf,
                    digits = 2,
                    na.rm = TRUE, drop_empty = TRUE, trim = TRUE,
                    sep = "_",
                    prefix_label = "var",
                    add_nvalid = TRUE){

  type <- match.arg(type)

  tab <- cat_tab(x, var = var, digits = digits, na.rm = na.rm, drop_empty = drop_empty, trim = trim)

  if (add_nvalid) {
    lista[[paste0("nvalid", sep, var)]] <- sum(tab$n)
  }

  if (type == "levels") {
    if (is.null(levels)) levels <- tab[[var]]

    for (lv in levels) {
      i <- match(lv, tab[[var]])
      key <- make_key(lv)

      lista[[paste0(var, sep, key, sep, "n")]]   <- if (is.na(i)) NA_integer_ else tab$n[i]
      lista[[paste0(var, sep, key, sep, "prc")]] <- if (is.na(i)) NA_real_    else tab$prc[i]
    }

  } else {  # topk
    if (is.infinite(k)) k <- nrow(tab)
    k <- min(as.integer(k), nrow(tab))
    suffix <- paste0(sep, var)

    for (j in seq_len(k)) {
      lista[[paste0(prefix_label, j, suffix)]] <- tab[[var]][j]
      lista[[paste0("n", j, suffix)]]          <- tab$n[j]
      lista[[paste0("prc", j, suffix)]]        <- tab$prc[j]
    }
  }

  lista
}