/// <summary>
/// Find the connected components in the subtree
/// </summary>
/// <returns></returns>
private List <List <Node> > FindConnectedComponents()
{
List <List <Node> > connectedComponents = new List <List <Node> >();
List <Node> subNodes = splitNode.AllNodesInSubtree;
subNodes.Remove(splitNode.Value);
HashSet <Node> beenList = new HashSet <Node>(splitNode.AncestorNodes)
{
splitNode.Value
};
// Use DFS to find all connected components
for (int i = 0; i < subNodes.Count; i++)
{
if (beenList.Contains(subNodes[i]))
{
continue;
}
List <Node> connectedNodes = DFS.All(subNodes[i], (n) =>
{
// When a node is explored, add the index of the component it is in to the dictionary
componentIndices[n] = connectedComponents.Count;
return(true);
}, beenList);
connectedComponents.Add(connectedNodes);
}
return(connectedComponents);
}
public override void Apply()
{
// Find all the connected components in this subtree
List <List <Node> > connectedComponents = FindConnectedComponents();
// If the number of connected components is equal to the number of children, we cannot split the tree even further, so return
if (connectedComponents.Count == splitNode.Children.Count)
{
return;
}
// Consider a subtree where the middle part is a connected component. We want that part as a new child, and the rest as a new child, but connected to each other
// Thus, if we remove the middle part of a connected component, we want to reconnect this component
// In each component, find the new children of the splitnode, and possibly new parents for nodes that were cut off
bool[] foundComponent = new bool[connectedComponents.Count];
List <Tuple <RecursiveTree <Node>, RecursiveTree <Node> > > newParents = new List <Tuple <RecursiveTree <Node>, RecursiveTree <Node> > >();
List <RecursiveTree <Node> > newChildren = DFS.All(splitNode, (n) =>
{
if (n == splitNode)
{
return(false);
}
int index = componentIndices[n.Value];
if (foundComponent[index])
{
// If we have already found this component, find the first parent that is also part of this component. This node is now namely divided from the rest of this component
RecursiveTree <Node> parent = n.Parent;
while (componentIndices[parent.Value] != index)
{
parent = parent.Parent;
}
newParents.Add(new Tuple <RecursiveTree <Node>, RecursiveTree <Node> >(n, parent));
return(false);
}
foundComponent[index] = true;
return(true);
});
// Reconnect the divided components
foreach (Tuple <RecursiveTree <Node>, RecursiveTree <Node> > nodePair in newParents)
{
operations.Push(new RemoveChildOperation(nodePair.Item1.Parent, nodePair.Item1));
operations.Push(new ChangeParentOperation(nodePair.Item1, nodePair.Item2));
operations.Push(new AddChildOperation(nodePair.Item2, nodePair.Item1));
}
// Connect the new children to the split node
foreach (RecursiveTree <Node> newChild in newChildren)
{
if (newChild.Parent == splitNode)
{
continue;
}
operations.Push(new RemoveChildOperation(newChild.Parent, newChild));
operations.Push(new ChangeParentOperation(newChild, splitNode));
operations.Push(new AddChildOperation(splitNode, newChild));
}
}
/// <summary>
/// Finds all the articulation points in the given nodes
/// </summary>
/// <param name="allNodes">The nodes to look for articulation points in</param>
private static void CalculateArticulationPoints(Node[] allNodes)
{
List <Node> articulationPoints = DFS.ArticulationPoints(allNodes.Length, allNodes[0]);
foreach (Node node in articulationPoints)
{
node.ArticulationPointValue = 1;
}
}
/// <summary>
/// Compute the connected components in a set of nodes and fills these in the list connectedComponents
/// </summary>
/// <param name="nodes">All nodes in the entire tree</param>
/// <param name="ancestors">The list of ancestors for the current subtree</param>
/// <param name="selectedNode">The node that is the root of the current subtree</param>
/// <param name="left">The INCLUSIVE index in nodes where the current subtree starts</param>
/// <param name="right">The EXCLUSIVE index in nodes where the current subtree ends</param>
private void ComputeConnectedComponents(Node[] nodes, HashSet <Node> ancestors, Node selectedNode, int left, int right)
{
ancestors.Add(selectedNode);
beenList.Clear();
beenList.UnionWith(ancestors);
connectedComponents.Clear();
// For all nodes in the current subtree, do a DFS to find the connected components
for (int i = left; i < right; i++)
{
if (beenList.Contains(nodes[i]))
{
continue;
}
List <Node> connectedNodes = DFS.All(nodes[i], node => { return(true); }, beenList);
connectedComponents.Add(connectedNodes);
}
}
/// <summary>
/// Recursive method for calculating the best heuristic tree
/// </summary>
/// <param name="bestFoundSolution">The optimal solution thus far from this level of the tree, used for early stopping</param>
/// <param name="nodes">A list of all nodes in this subtree</param>
/// <param name="ancestors">The list of all ancestors of the current subtree</param>
/// <param name="checkedSubsets">A dictionary with checked subsets, used for memoization</param>
/// <returns>The optimal exact tree for the list of input nodes</returns>
private RecursiveTree <Node> RecGetBestTree(int bestFoundSolution, List <Node> nodes, HashSet <Node> ancestors, Dictionary <string, RecursiveTree <Node> > checkedSubsets)
{
// If the currently best found solution is smaller than the list of ancestors here, we cannot possibly improve it. Thus, we return an empty tree
if (bestFoundSolution < ancestors.Count + 1)
{
return(null);
}
// Check if we have already computed a subtree for this set of nodes, if so, return that tree
string asBits = NodeSubsetRepresentation(nodes);
if (checkedSubsets.ContainsKey(asBits) && checkedSubsets[asBits] != null)
{
RecursiveTree <Node> computedTree = new RecursiveTree <Node>(checkedSubsets[asBits]);
return(computedTree);
}
// Sort the nodes on their remaining degree value (descending) and recursively compute the best trees
HashSet <Node> nodesAsHash = new HashSet <Node>(nodes);
nodes = nodes.OrderByDescending(n => n.RemainingDegree(nodesAsHash)).ToList();
RecursiveTree <Node> bestTree = null;
foreach (Node selectedNode in nodes)
{
RecursiveTree <Node> newTree = new RecursiveTree <Node>(selectedNode);
HashSet <Node> beenList = new HashSet <Node>(ancestors)
{
selectedNode
};
bool broken = false;
foreach (Node n in nodes)
{
// Find the connected component this node is in
if (beenList.Contains(n))
{
continue;
}
List <Node> connectedNodes = DFS.All(n, (nn) => { return(true); }, beenList);
HashSet <Node> newHash = new HashSet <Node>(ancestors)
{
selectedNode
};
// Compute the best possible subtree for this connected component
RecursiveTree <Node> ChildTree = RecGetBestTree(bestFoundSolution, connectedNodes, newHash, checkedSubsets);
if (ChildTree == null)
{
// If the resulting tree is null, it is not a viable solution
broken = true;
break;
}
ChildTree.Parent = newTree;
newTree.AddChild(ChildTree);
}
// If we found a viable solution, update the best found solution thus far
if (!broken)
{
int newDepth = newTree.Depth;
if (newDepth + ancestors.Count < bestFoundSolution)
{
bestFoundSolution = newDepth + ancestors.Count;
bestTree = newTree;
}
}
}
// Save the tree in the memoization dictionary and return it
checkedSubsets[asBits] = bestTree;
return(bestTree);
}
