feat: add count_reachable() function to R (#2412)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: krlmlr <[email protected]>
Co-authored-by: Kirill Müller <[email protected]>

Author: Copilot

NAMESPACE

@@ -275,6 +275,7 @@ export(count_isomorphisms)
 export(count_max_cliques)
 export(count_motifs)
 export(count_multiple)
+export(count_reachable)
 export(count_subgraph_isomorphisms)
 export(count_triangles)
 export(create.communities)

R/structural-properties.R

@@ -3161,6 +3161,57 @@ count_components <- function(graph, mode = c("weak", "strong")) {
   .Call(Rx_igraph_no_components, graph, mode)
 }
 
+#' Count reachable vertices
+#'
+#' `r lifecycle::badge("experimental")`
+#'
+#' Counts the number of vertices reachable from each vertex in the graph.
+#'
+#' For each vertex in the graph, this function counts how many vertices
+#' are reachable from it, including the vertex itself.
+#' A vertex is reachable from another if there is a directed path between them.
+#' For undirected graphs, two vertices are reachable from each other if they
+#' are in the same connected component.
+#'
+#' @param graph The input graph.
+#' @param mode Character constant, defines how edge directions are considered
+#'   in directed graphs.
+#'   `"out"` counts vertices reachable via outgoing edges,
+#'   `"in"` counts vertices from which the current vertex is reachable via
+#'   incoming edges,
+#'   `"all"` or `"total"` ignores edge directions.
+#'   This parameter is ignored for undirected graphs.
+#' @return An integer vector of length `vcount(graph)`.
+#'   The i-th element is the number of vertices reachable from vertex i
+#'   (including vertex i itself).
+#' @author Gabor Csardi \email{csardi.gabor@@gmail.com}
+#' @seealso [components()], [subcomponent()], [is_connected()]
+#' @family components
+#' @export
+#' @keywords graphs
+#' @examples
+#'
+#' # In a directed path graph, the reachability depends on direction
+#' g <- make_graph(~ 1 -+ 2 -+ 3 -+ 4 -+ 5)
+#' count_reachable(g, mode = "out")
+#' count_reachable(g, mode = "in")
+#'
+#' # In an undirected graph, reachability is the same in all directions
+#' g2 <- make_graph(~ 1 - 2 - 3 - 4 - 5)
+#' count_reachable(g2, mode = "out")
+#'
+#' # A graph with multiple components
+#' g3 <- make_graph(~ 1 - 2 - 3, 4 - 5, 6)
+#' count_reachable(g3, mode = "all")
+#'
+#' @cdocs igraph_count_reachable
+count_reachable <- function(graph, mode = c("out", "in", "all", "total")) {
+  count_reachable_impl(
+    graph = graph,
+    mode = mode
+  )
+}
+
 #' Convert a general graph into a forest
 #'
 #' Perform a breadth-first search on a graph and convert it into a tree or

man/articulation_points.Rd

@@ -730,6 +730,50 @@ test_that("a null graph has zero components", {
   expect_equal(count_components(g), 0L)
 })
 
+test_that("count_reachable() works for directed graphs", {
+  # Directed path graph: 1 -> 2 -> 3 -> 4 -> 5
+  g <- make_graph(~ 1 -+ 2 -+ 3 -+ 4 -+ 5)
+
+  # Out mode: count vertices reachable via outgoing edges
+  out_counts <- count_reachable(g, mode = "out")
+  expect_equal(out_counts, c(5, 4, 3, 2, 1))
+
+  # In mode: count vertices from which current vertex is reachable
+  in_counts <- count_reachable(g, mode = "in")
+  expect_equal(in_counts, c(1, 2, 3, 4, 5))
+
+  # All mode: treat as undirected
+  all_counts <- count_reachable(g, mode = "all")
+  expect_equal(all_counts, rep(5, 5))
+})
+
+test_that("count_reachable() works for undirected graphs", {
+  # Undirected path graph
+  g <- make_graph(~ 1 - 2 - 3 - 4 - 5)
+
+  # All vertices are reachable from each other in an undirected path
+  counts <- count_reachable(g, mode = "out")
+  expect_equal(counts, rep(5, 5))
+})
+
+test_that("count_reachable() works for graphs with multiple components", {
+  # Graph with three components: 1-2-3, 4-5, 6
+  g <- make_graph(~ 1 - 2 - 3, 4 - 5, 6)
+
+  counts <- count_reachable(g, mode = "all")
+  expect_equal(counts, c(3, 3, 3, 2, 2, 1))
+})
+
+test_that("count_reachable() works for empty and single-vertex graphs", {
+  # Empty graph
+  g0 <- make_empty_graph(0)
+  expect_equal(count_reachable(g0, mode = "out"), integer(0))
+
+  # Single vertex
+  g1 <- make_empty_graph(1)
+  expect_equal(count_reachable(g1, mode = "out"), 1L)
+})
+
 test_that("girth() works", {
   ## No circle in a tree
   g <- make_tree(1000, 3)