Partition graph into groups of neighbours having the same class

Question

How can I use JGraphT to divide a graph into sub-graphs where each sub-graph is a connected group of vertices with the same "class" (indicated by colors below)?

Example (the desired groups are in red):

I think this is a rather simple demand and yet I can't find a (built-in) way to do this. I notice there is a PartitioningImpl class, that one constructs using a List<Set<V>> classes, yet I don't see a way to use this to partition a graph.

Ideally, I'd provide something with my graph and vertex classes (a map of V-->Integer for instance) and it would return something like a List<Set<V>> of partitioned vertex groups.

Answer 1

This is a fairly simple approach using JGraphT:

• Remove edges between adjacent vertices that are in distinct classes.
• Then, apply ConnectivityInspector to the resulting graph to identify the connected components, which will represent the groups.

SimpleGraph<V, E> graph; // given by user
Map<V, Integer> classes; // given by user

List<E> toRemove = new ArrayList<>();
graph.edgeSet().forEach(e -> {
    V a = graph.getEdgeSource(e);
    V b = graph.getEdgeTarget(e);
    if (!classes.get(a).equals(classes.get(b))) {
        toRemove.add(e);
    }
});

graph.removeAllEdges(toRemove);
ConnectivityInspector<V, E> ci = new ConnectivityInspector<>(graph);
List<Set<V>> groups = ci.connectedSets();

Answer 2

Sometimes you just cannot avoid writing some code

LOOP over classes
   LOOP over nodes that are in class
       Copy node to new graph
   LOOP over edges that connect nodes in class
       Copy edge to new graph
   LOOP over connected components in new graph
       Save component as a group graph for class