Add volume functions. (#18)

* Add volume functions.

* Bump development version number.

DESCRIPTION

@@ -1,6 +1,6 @@
 Package: wallis
 Title: Room squares in R
-Version: 0.0.0.9001
+Version: 0.0.0.9002
 Authors@R: 
     person("Matthew", "Henderson", , "matthew.james.henderson@gmail.com", role = c("aut", "cre"),
            comment = c(ORCID = "0000-0001-7949-8208"))

NAMESPACE

@@ -1,3 +1,5 @@
 # Generated by roxygen2: do not edit by hand
 
+export(n_filled_cells)
 export(plot_room_square)
+export(volume)

R/volume.R

@@ -0,0 +1,21 @@
+#' Number of filled cells in a partial Room square
+#'
+#' @param R A partial Room square
+#'
+#' @return The number of filled cells in R.
+#' @export
+n_filled_cells <- function(R) {
+  R |>
+    dplyr::filter(!is.na(first)) |> 
+    nrow()
+}
+
+#' Volume of a partial Room square
+#'
+#' @param R A partial Room square.
+#'
+#' @return The volume of R.
+#' @export
+volume <- function(R) {
+  round(n_filled_cells(R)/choose(max(R$col) + 1, 2), 6)
+}