/**
* Compute the transitive closure of the universe, then breadth-first search from the argument
* towards the universe while staying within the transitive closure.
*/
@Override
public <T> Set<T> eval(QueryEnvironment<T> env, QueryExpression expression, List<Argument> args)
throws QueryException {
Set<T> universeValue = args.get(0).getExpression().eval(env);
Set<T> argumentValue = args.get(1).getExpression().eval(env);
int depthBound = args.size() > 2 ? args.get(2).getInteger() : Integer.MAX_VALUE;
env.buildTransitiveClosure(expression, universeValue, Integer.MAX_VALUE);
Set<T> visited = new LinkedHashSet<>();
Set<T> reachableFromUniverse = env.getTransitiveClosure(universeValue);
Collection<T> current = argumentValue;
// We need to iterate depthBound + 1 times.
for (int i = 0; i <= depthBound; i++) {
List<T> next = new ArrayList<>();
for (T node : current) {
if (!reachableFromUniverse.contains(node)) {
// Traversed outside the transitive closure of the universe.
continue;
}
if (!visited.add(node)) {
// Already visited; if we see a node in a later round, then we don't need to visit it
// again, because the depth at which we see it at must be greater than or equal to the
// last visit.
continue;
}
next.addAll(env.getReverseDeps(node));
}
if (next.isEmpty()) {
// Exit when there are no more nodes to visit.
break;
}
current = next;
}
return visited;
}
