R/cut_tour.R

In TSP: Traveling Salesperson Problem (TSP)

#######################################################################
# TSP - Traveling Salesperson Problem
# Copyrigth (C) 2011 Michael Hahsler and Kurt Hornik
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License along
# with this program; if not, write to the Free Software Foundation, Inc.,
# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.

#' Cut a tour to form a path
#'
#' Cuts a tour at a specified city to form a path.
#'
#' @family TOUR
#'
#' @param x an object of class [TOUR].
#' @param cut the index or label of the city/cities to cut the tour.
#' @param exclude_cut exclude the city where we cut? If `FALSE`, the city
#' at the cut is included in the path as the first city.
#' @return Returns a named vector with city ids forming the path. If multiple
#' cuts are used then a list with paths is returned.
#' @author Michael Hahsler
#' @keywords optimize
#' @examples
#' data("USCA50")
#'
#' ## find a path starting at Austin, TX
#' tour <- solve_TSP(USCA50)
#' path <- cut_tour(tour, cut = "Austin, TX", exclude_cut = FALSE)
#' path
#'
#' ## cut the tours at two cities
#' tour <- solve_TSP(USCA50)
#' path <- cut_tour(tour, cut = c("Austin, TX", "Cambridge, MA"), exclude_cut = FALSE)
#' path
#'
#' ## cut a tour at the largest gap using a dummy city
#' tsp <- insert_dummy(USCA50, label = "cut")
#' tour <- solve_TSP(tsp)
#'
#' ## cut tour into path at the dummy city
#' path <- cut_tour(tour, "cut")
#' path
#' @export
cut_tour <- function(x, cut, exclude_cut = TRUE)
  UseMethod("cut_tour")

#' @rdname cut_tour
#' @export
cut_tour.TOUR <- function(x, cut, exclude_cut = TRUE) {
  exclude_cut <- as.integer(exclude_cut)

  ## city label
  if (is.character(cut)) {
    cut <-
      which(as.logical(apply(
        sapply(cut, "==", labels(x)), MARGIN = 1, sum
      )))
    if (length(cut) < 1)
      stop("cut has to exist")

    ## city id
  } else {
    if (any(is.na(cut <- match(cut, x))))
      stop("cut has to exist")
  }

  if (length(cut) == 1L) {
    ## single path
    if (exclude_cut && length(x) <= 1)
      path <- integer(0)
    else
      path <-
        c(x, x)[(cut + exclude_cut):(length(x) + cut - 1L)]

  } else {
    ## multiple paths, return as a list
    path <- replicate(length(cut), integer(0))
    path[[1L]] <- c(if ((tail(cut, 1) + exclude_cut) <= length(x))
      (tail(cut, 1) + exclude_cut):length(x)
      else
        NULL,
      if (cut[1] > 1)
        1:(cut[1] - 1L)
      else
        NULL)
    for (i in seq_len(length(cut) - 1L)) {
      if ((cut[i] + exclude_cut) <= (cut[i + 1L] - 1L))
        path[[i + 1L]] <-
          (cut[i] + exclude_cut):(cut[i + 1L] - 1L)
    }

    path <- lapply(
      path,
      FUN = function(i)
        x[i]
    )
    if (exclude_cut)
      names(path) <- labels(x)[cut]
  }

  path
}