public static void FindExplorePath2Opt(ExploreCell start_cell, List <ExploreCell> explore_cells, CellsDistancer distances, ref List <ExploreCell> path)
{
using (new Profiler($"2-Opt %t"))
{
path = path ?? new List <ExploreCell>();
path.Clear();
if (start_cell == null)
{
return;
}
//List<int> cells_id_in_start_cell_network = distances.GetConnectedTo(start_cell.GlobalId);
//List<ExploreCell> best_tour_old = explore_cells.FindAll(c => cells_id_in_start_cell_network.Contains(c.GlobalId));
// reordering cell in breadth-first order seems to help with 2-opt backtracking
var collect_cells_visitor = new CollectVisitor();
Algorihms.VisitBreadth(start_cell, visitor: collect_cells_visitor);
var best_tour = collect_cells_visitor.cells.Select(x => x as ExploreCell).Intersect(explore_cells).ToList();
best_tour.RemoveAll(c => c.Explored || c.Neighbours.Count == 0);
if (start_cell.Explored) // re-insert start cell if needed
{
best_tour.Insert(0, start_cell);
}
if (best_tour.Count == 1)
{
path.Add(best_tour[0]);
return;
}
float best_dist = GetTravelDistance(best_tour, distances);
if (best_tour.Count < 90)
{
// based on http://en.wikipedia.org/wiki/2-opt
while (true)
{
bool better_found = false;
for (int i = 1; i < best_tour.Count - 1; ++i)
{
for (int k = i + 1; k < best_tour.Count; ++k)
{
float new_dist = Swap2OptDistance(best_tour, distances, i, k);
if (new_dist < best_dist)
{
Swap2Opt(ref best_tour, i, k);
best_dist = new_dist;
better_found = true;
break;
}
}
if (better_found)
{
break;
}
}
if (!better_found)
{
break;
}
}
}
else // greedy
{
// based on http://on-demand.gputechconf.com/gtc/2014/presentations/S4534-high-speed-2-opt-tsp-solver.pdf
while (true)
{
float min_change = 0;
int min_i = -1;
int min_j = -1;
for (int i = 0; i < best_tour.Count - 2; ++i)
{
for (int j = i + 2; j < best_tour.Count - 1; ++j)
{
int city_a = best_tour[i].GlobalId;
int city_b = best_tour[i + 1].GlobalId;
int city_c = best_tour[j].GlobalId;
int city_d = best_tour[j + 1].GlobalId;
float change = (distances.GetDistance(city_a, city_c) + distances.GetDistance(city_b, city_d)) -
(distances.GetDistance(city_a, city_b) + distances.GetDistance(city_c, city_d));
if (change < min_change)
{
min_change = change;
min_i = i + 1;
min_j = j;
}
}
}
if (min_change >= 0)
{
break;
}
// apply min_i/min_j move
ExploreCell t = best_tour[min_i];
best_tour[min_i] = best_tour[min_j];
best_tour[min_j] = t;
}
}
Trace.WriteLine("[FindExplorePath 2-Opt] Final solution distance: " + GetTravelDistance(best_tour, distances));
foreach (ExploreCell cell in best_tour)
{
path.Add(cell);
}
if (start_cell.Explored)
{
path.RemoveAt(0);
}
}
}
