local({
  previous_options <- options(warn = 2, digits = 8, stringsAsFactors = FALSE)
  on.exit(options(previous_options), add = TRUE)
  check_counts <- function(x, allow_missing = FALSE, allow_empty = FALSE) {
    if (!(is.integer(x) || is.double(x)) || is.object(x) || !is.null(dim(x))) {
      stop("Use a plain numeric vector")
    }
    if (length(x) > 256L || (!allow_empty && length(x) == 0L)) {
      stop("Use 1..256 counts unless empty input is explicitly allowed")
    }
    if (any(is.nan(x)) || (!allow_missing && anyNA(x))) {
      stop("Missing or undefined counts are not allowed here")
    }
    observed <- x[!is.na(x)]
    if (any(!is.finite(observed)) || any(observed < 0) ||
        any(observed > 1000000) || any(observed != floor(observed))) {
      stop("Counts must be finite whole numbers from 0 to 1000000")
    }
    invisible(x)
  }

  validate_sessions <- function(data) {
    if (!is.data.frame(data) || !identical(names(data), c("id", "room", "visits"))) {
      stop("Expected id, room and visits columns")
    }
    if (!is.character(data$id) || !is.character(data$room) || anyNA(data$id) ||
        anyNA(data$room) || any(!nzchar(trimws(data$id))) ||
        any(!nzchar(trimws(data$room))) || anyDuplicated(data$id)) {
      stop("Use unique ids and non-empty room labels")
    }
    check_counts(data$visits, allow_missing = TRUE)
    invisible(data)
  }
  sessions <- data.frame(id = c("S1", "S2"), room = c("East", "West"), visits = c(8, NA_real_))
  validate_sessions(sessions)
  cat("valid rows:", nrow(sessions), "unknown counts:", sum(is.na(sessions$visits)), "\n")
  invisible(NULL)
})