R/tranpose.R

In simplegraph: Simple Graph Data Types and Basic Algorithms

#' Transpose a graph
#'
#' The transposed graph have the same vertices, and the same number
#' of edges, but all edge directions are opposite comparated to the
#' original graph.
#'
#' @param graph Input graph
#' @return Transposed graph.
#'
#' @export
#' @examples
#' funcs <- graph(list(
#'   drop_internal = character(0),
#'   get_deps = c("get_description", "parse_deps",
#'     "%||%", "drop_internal"),
#'   get_description = "pkg_from_filename",
#'   parse_deps = "str_trim",
#'   cran_file = c("get_pkg_type", "r_minor_version", "cran_file"),
#'   download_urls = c("split_pkg_names_versions", "cran_file"),
#'   filename_from_url = character(0),
#'   get_pkg_type = character(0),
#'   pkg_download = c("dir_exists", "download_urls",
#'     "filename_from_url", "try_download"),
#'   r_minor_version = character(0),
#'   try_download = character(0),
#'   drop_missing_deps = character(0),
#'   install_order = character(0),
#'   restore = c("pkg_download", "drop_missing_deps",
#'     "install_order", "get_deps"),
#'   snap = character(0),
#'   `%||%` = character(0),
#'   data_frame = character(0),
#'   dir_exists = character(0),
#'   pkg_from_filename = character(0),
#'   split_pkg_names_versions = "data_frame",
#'   str_trim = character(0)
#' ))
#' edges(transpose(funcs))

transpose <- function(graph)
  UseMethod("transpose")

#' @method transpose simplegraph_df
#' @export

transpose.simplegraph_df <- function(graph) {
  new_graph <- graph
  new_graph$edges[[1]] <- graph$edges[[2]]
  new_graph$edges[[2]] <- graph$edges[[1]]
  new_graph
}

#' @method transpose simplegraph_adjlist
#' @export

transpose.simplegraph_adjlist <- function(graph) {
  res <- structure(
    replicate(length(graph), character()),
    names = names(graph)
  )

  for (v in names(graph)) {
    for (w in graph[[v]]) {
      res[[w]] <- c(res[[w]], v)
    }
  }

  graph(res)
}