/*
* Compares node to its parent.
* If the node is smaller than its parent they are swapped.
* As it sorts this method goes through at most log n levels in the heap.
* Therefore the Big-O is O(logn).
*/
private void BubbleUp(PartialPath node)
{
while (positions[node] != 1 && node.Priority < Parent(node).Priority)
{
Swap(node, Parent(node));
}
}
/*
* Compares node to its two children.
* If it is larger than a child, it swaps with the smallest child.
* As it sorts this method goes through at most log n levels in the heap.
* Therefore the Big-O is O(logn).
*/
private void SiftDown(PartialPath node)
{
while (SmallestChild(node) != null && node.Priority > SmallestChild(node).Priority)
{
Swap(node, SmallestChild(node));
}
}
/*
* Swaps two nodes in the array.
* Also swaps their positions in the positions array (O(1)).
*/
private void Swap(PartialPath node1, PartialPath node2)
{
int pos1 = positions[node1];
int pos2 = positions[node2];
nodes[pos1] = node2;
nodes[pos2] = node1;
positions[node1] = pos2;
positions[node2] = pos1;
}
/*
* Generates all child partial paths.
* It creates one for each non-infinte edge
* leaving the last city visited.
* If the new node's lower bound < bssf, it's added to the queue.
*/
private void GenerateChildren(PartialPath currentNode, PriorityQueue queue)
{
for (int i = 1; i < Cities.Length; i++)
{
if (currentNode.Matrix[currentNode.LastCityVisited, i] != double.PositiveInfinity)
{
PartialPath child = new PartialPath(currentNode, i);
if (child.LowerBound < costOfBssf())
{
queue.Insert(child);
}
}
}
}
/*
* Removes the root node (the minimum).
* Replaces the root with the lowest right-most node.
* This node is then "sifted" down until the queue is sorted.
* O(logn) since it has to go through log n levels in the heap.
*/
public PartialPath DeleteMin()
{
PartialPath min = nodes[1];
PartialPath last = nodes[nodes.Count - 1];
positions[last] = 1;
positions[min] = -1;
nodes.Remove(last);
if (!Empty())
{
nodes[1] = last;
SiftDown(nodes[1]);
}
return(min);
}
/// <summary>
/// performs a Branch and Bound search of the state space of partial tours
/// stops when time limit expires and uses BSSF as solution
/// </summary>
/// <returns>results array for GUI that contains three ints: cost of solution, time spent to find solution, number of solutions found during search (not counting initial BSSF estimate)</returns>
public string[] bBSolveProblem()
{
string[] results = new string[3];
int count = 0;
Stopwatch timer = new Stopwatch();
PriorityQueue queue = new PriorityQueue();
timer.Start();
greedySolveProblem();
queue.Insert(new PartialPath(CalculateInitialMatrix(), Cities));
/* While there are still partial paths on the queue,
* the best one is popped off and its children are generated.
* Those that are better than the bssf are added to the queue.
*/
while (timer.Elapsed.TotalMilliseconds < time_limit && !queue.Empty())
{
PartialPath currentNode = queue.DeleteMin();
if (currentNode.FullPath && currentNode.CompletePath)
{
TSPSolution potentialSolution = new TSPSolution(currentNode.Path);
if (potentialSolution.costOfRoute() < costOfBssf())
{
bssf = potentialSolution;
count++;
}
}
else if (currentNode.LowerBound < costOfBssf())
{
GenerateChildren(currentNode, queue);
}
}
timer.Stop();
results[COST] = costOfBssf().ToString();
results[TIME] = timer.Elapsed.ToString();
results[COUNT] = count.ToString();
Console.WriteLine("Branch & Bound Size: " + Cities.Length + " Random: " + Seed + " Path: " + costOfBssf().ToString() + " Time: " + timer.Elapsed.ToString());
return(results);
}
/*
* Returns the smallest child of a given node.
* If the node has no children 0 is returned (O(1)).
*/
private PartialPath SmallestChild(PartialPath node)
{
int posFirstChild = positions[node] * 2;
int posSecondChild = posFirstChild + 1;
if (posFirstChild > nodes.Count - 1)
{
return(null); //no children
}
else if (posSecondChild > nodes.Count - 1)
{
return(nodes[posFirstChild]); //only one child
}
else
{
return(nodes[posFirstChild].Priority < nodes[posSecondChild].Priority ?
nodes[posFirstChild] : nodes[posSecondChild]); //return whichever child is smaller
}
}
/*
* Constructor used for child nodes.
* The data from the parent node is copied.
* After this the edge to the next city is added.
* During this process the matrix is reduced.
* The lower bound is recalculated as well.
*/
public PartialPath(PartialPath parent, int cityToVisit)
{
lowerBound = parent.lowerBound;
cities = parent.cities;
path = new List <int>();
for (int i = 0; i < parent.path.Count; i++)
{
path.Add(parent.path[i]);
}
matrix = new double[cities.Length, cities.Length];
for (int i = 0; i < cities.Length; i++)
{
for (int j = 0; j < cities.Length; j++)
{
matrix[i, j] = parent.matrix[i, j];
}
}
AddCity(LastCityVisited, cityToVisit);
}
/*
* Returns the parent of a given node (O(1)).
*/
private PartialPath Parent(PartialPath node)
{
return(nodes[positions[node] / 2]);
}
/*
* Inserts a node at the lowest right-most spot.
* BubbleUp is then called which resorts the queue.
* O(logn) since it has to go through log n levels in the heap.
*/
public void Insert(PartialPath node)
{
nodes.Add(node);
positions.Add(node, nodes.Count - 1);
BubbleUp(node);
}