I am trying to convert my 2-opt implementation of the travelling salesman problem into a 3-opt. I understand that it is, in comparison to 2-opt, removing 3 edges and replacing them to hopefully get a better distance. I am having issue figuring out what to change/add to my 2-opt swap to make it three opt. The main problem I am having is how to make sure all 8 varieties of swaps are accounted for in a single swap function. Thank you
2-opt code:
private static int[] TwoOpt(int[] TSP) {
double best = totalDistance;
int numCities = TSP.length;
int visited = 0;
int current = 0;
int[] newTour = TSP;
while (visited < numCities) {
for (int i = 0; i < numCities - 1; i++) {
for (int j = i + 1; j < numCities; j++) {
int[] newerTour = Swap(i, j, newTour);
int newDistance = distance(newerTour);
if (newDistance < best) {
visited = 0;
best = newDistance;
newTour = newerTour;
}
}
}
visited++;
}
return newTour;
}
private static int distance(int[] newTour) {
int distance = 0;
for (int i = 0; i < newTour.length - 1; i++) {
distance += mstList.get(i).get(i + 1).p;
}
distance += mstList.get(newTour.length).get(0).p;
return distance;
}
private static int[] Swap(int i, int j, int[] tour) {
int size = tour.length;
int[] newerTour = new int[tour.length];
for (int c = 0; c <= i - 1; c++) {
newerTour[c] = tour[c];
}
int change = 0;
for (int d = i; d <= j; d++) {
newerTour[d] = tour[d - change];
change++;
}
for (int e = j + 1; e < size; e++) {
newerTour[e] = tour[e];
}
return newerTour;
}
and this is what I have for the three-opt, no swap implemented yet.
private static int[] ThreeOpt(int[] TSP) {
double best = totalDistance;
int numCities = TSP.length;
int visited = 0;
int current = 0;
int[] newTour = TSP;
while (visited < numCities) {
for (int i = 0; i < numCities - 2; i++) {
for (int j = i + 1; j < numCities - 1; j++) {
for (int k = j + 1; k < numCities; k++) {
int[] newerTour = Swap(i, j, k, newTour);
int newDistance = distance(newTour);
if (newDistance < best) {
visited = 0;
best = newDistance;
newTour = newerTour;
}
}
}
}
visited++;
}
return newTour;
}
