// 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;
}
public IEnumerable<IReductionRuleCommand> Find(Graph graph)
{
foreach (int vertex in graph.Vertices)
{
if (graph.Degree(vertex) == 0)
{
graph.RemoveVertex(vertex);
yield return new Command(vertex);
}
}
}
public IEnumerable<IReductionRuleCommand> Find(Graph graph)
{
foreach (int vertex in graph.Vertices)
{
// TODO: Count is not O(1)
if (graph.Degree(vertex) == 1 && graph.Vertices.Count > 1)
{
BitSet connection = graph.OpenNeighborhood(vertex);
graph.RemoveVertex(vertex);
yield return new Command(vertex, connection);
}
}
}
