Build Graph given a SuccessorsFunction and a set of nodes - guava

Question

I would like to build a Guava ImmutableGraph given a set of nodes (starting points) and a SuccessorsFunction. The graph would contain all the nodes reachable from any of the starting node and all the edges seen on the way thanks to the SuccessorsFunction. (E.g., given starting node {a} and successors a → b and b → c, the resulting graph should be {(a, b), (b, c)}.)
I see how I can obtain a Traverser to explore the reachable nodes in a certain order, given starting nodes and a SuccessorsFunction, but it does not meet my needs as I want to obtain a graph, not just the nodes.
It is not very hard to define an algorithm that does this, but it’s subtle enough to deserve trying to re-use an existing solution. I would be surprised if it didn’t exist already in the library. Does it? Or is this requirement not sensible?
I didn’t find this in the related wiki page.

Answer 1

Guava doesn't have this feature built in, so you'll need a custom solution that does some sort of graph traversal (like breadth-first traversal), like the following code snippet.
public static <N> ImmutableGraph<N> buildGraphWithBreadthFirstTraversal(
Iterable<N> startingNodes, SuccessorsFunction<N> successorsFunction) {
MutableGraph<N> result = GraphBuilder.directed().allowsSelfLoops(true).build();
startingNodes.forEach(result::addNode);
Queue<N> nodesRemaining = Queues.newArrayDeque(startingNodes);
while (!nodesRemaining.isEmpty()) {
N next = nodesRemaining.remove();
for (N successor : successorsFunction.successors(next)) {
if (!result.edges().contains(EndpointPair.ordered(next, successor))) {
nodesRemaining.add(successor);
result.putEdge(next, successor);
}
}
}
return ImmutableGraph.copyOf(result);
}
Here is a basic JUnit 5 unit test that confirms the code works when given a starting node and a successorsFunction that together form a cycle of 1 -> 2 -> 4 -> 1.
#Test
void succeedsOnTraversalWithCycle() {
var result =
MoreGraphs.buildGraphWithBreadthFirstTraversal(
ImmutableList.of(1),
node -> {
int nextNode = node * 2;
return nextNode <= 4 ? ImmutableList.of(nextNode) : ImmutableList.of(1);
});
assertThat(result)
.isEqualTo(
GraphBuilder.directed()
.allowsSelfLoops(true)
.immutable()
.putEdge(1, 2)
.putEdge(2, 4)
.putEdge(4, 1)
.build());
}