// Counts the number of independent sets in a graph, such that:
// - The vertices in P are legal choices for our IS, initially set to all vertices in the graph
// - None of the vertices in X are used, initially empty
// The performDFS boolean is used to check if we should perform a check for connectedness on this level of the recursion
private static long Compute(Graph graph, BitSet P, BitSet X, bool performDFS)
{
// Base case, when P and X are both empty we cannot expand the IS
if (P.IsEmpty && X.IsEmpty)
return 1;
// Base case, if a vertex w in X has no neighbor in P, then it means that this IS will never get maximal
// since we could always include w. Thus, the IS will not be valid and we return 0.
foreach (int w in X)
if ((graph.OpenNeighborhood(w) * P).IsEmpty)
return 0;
long count = 0;
// If a DFS is needed we check if the graph induced by (P + X) is still connected.
// If the graph is disconnected, in components c1,...,cn then we can simply count the IS of all these components
// after which we simply multiply these numbers.
if (performDFS)
{
if (!DepthFirstSearch.Connected(graph, P + X))
{
count = 1;
foreach (BitSet component in DepthFirstSearch.ConnectedComponents(graph, P + X))
count *= Compute(graph, component * P, component * X, false);
return count;
}
}
// Select a pivot in P to branch on
// In this case we pick the vertex with the largest degree
int maxDegree = -1; ;
int pivot = -1;
foreach (int u in P)
{
int deg = graph.Degree(u);
if (deg > maxDegree)
{
maxDegree = deg;
pivot = u;
}
}
// There should always be a pivot after the selection procedure
if (pivot == -1)
throw new Exception("Pivot has not been selected");
// We branch on the pivot, one branch we include the pivot in the IS.
// This might possibly disconnect the subgraph G(P + X), thus we set the performDFS boolean to true.
count = Compute(graph, P - graph.ClosedNeighborhood(pivot), X - graph.OpenNeighborhood(pivot), true);
// One branch we exclude the pivot of the IS. This will not cause the graph to get possibly disconnected
count += Compute(graph, P - pivot, X + pivot, false);
return count;
}
// Counts the number of independent sets in a graph, such that:
// - All vertices in R are included in the independent set, initially empty
// - Some of the vertices in P are included in the independent set, initially all vertices of the graph
// - None of the vertices in X are included in the independent set, initially empty
private static long Compute(Graph graph, BitSet R, BitSet P, BitSet X)
{
// Base case, when P and X are both empty we cannot expand the IS
if (P.IsEmpty && X.IsEmpty)
return 1;
long count = 0;
int pivot = -1;
int min = int.MaxValue;
// Procedure to find a pivot
// The idea is that in any maximal IS, either vertex u or a neighbor of u is included (else we could expand by adding u to the IS)
// We find the u with the smallest neighborhood, so that we will keep the number of branches low
foreach (int u in (P + X))
{
int size = (P * graph.OpenNeighborhood(u)).Count;
if (size < min)
{
min = size;
pivot = u;
}
}
// There should always be a pivot after the selection procedure, else P and X should both have been empty
if (pivot == -1)
throw new Exception("Pivot has not been selected");
// Foreach vertex v in the set containing the legal choices of the the closed neighborhood of the pivot,
// we include each choice in the IS and compute how many maximal IS will include v by going into recursion
foreach (int v in (P * graph.ClosedNeighborhood(pivot)))
{
count += Compute(graph, R + v, P - graph.ClosedNeighborhood(v), X - graph.OpenNeighborhood(v));
P.Remove(v);
X.Add(v);
}
return count;
}
// Counts the number of independent sets in a graph, such that:
// - All vertices in R are included in the independent set, initially empty
// - Some of the vertices in P are included in the independent set, initially all vertices of the graph
// - None of the vertices in X are included in the independent set, initially empty
private static int Compute(Graph graph, BitSet R, BitSet P, BitSet X)
{
// Base case, when P and X are both empty we cannot expand the IS
if (P.IsEmpty && X.IsEmpty)
return 1;
int count = 0;
BitSet copy = P.Copy();
// Foreach vertex v in P we include it in the IS and compute how many maximal IS will include v by going into recursion.
foreach (int v in copy)
{
count += Compute(graph, R + v, P - graph.ClosedNeighborhood(v), X - graph.OpenNeighborhood(v));
P.Remove(v);
X.Add(v);
}
return count;
}