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#######################################################################
# 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.
## nearest neighbor algorithm
tsp_nn <- function(x, control = NULL) {
## parameter x comes checked from solve_TSP/solve_ATSP
n <- n_of_cities(x)
## we use a matrix for now (coveres TSP and ATSP)
x <- as.matrix(x)
control <- .get_parameters(control, list(
start = sample(n, 1)
))
start <- control$start
if(start < 0 || start > n)
stop(paste("illegal value for", sQuote("start")))
placed <- logical(n)
order <- integer(n)
## place first city
current <- start
order[1L] <- current
placed[current] <- TRUE
while(length(rest <- which(!placed)) > 0L) {
## nearest <- rest[which.min(x[current,rest])]
## which.min has problems with Inf
## so we can break ties randomly now too
x_sub <- x[current, rest]
current <- rest[which(x_sub == min(x_sub, na.rm = TRUE))]
if(length(current) > 1L) current <- sample(current, 1)
## place city
order[n + 1L - length(rest)] <- current
placed[current] <- TRUE
}
order
}
## repetitive NN
tsp_repetitive_nn <- function(x, control){
n <- n_of_cities(x)
#tours <- lapply(1:n, function(i) tsp_nn(x, control = list(start = i)))
## no backend would warn!
i <- 0L ## for R CMD check (no global binding for i)
suppressWarnings(
tours <- foreach(i = 1:n) %dopar% tsp_nn(x, control = list(start = i))
)
lengths <- sapply(tours, FUN = function(i) tour_length(x, i))
tours[[which.min(lengths)]]
}
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