Is there a function for finding all subgroups from one main group with filtering some of the subgroups?

Question

I am coding with java so if you can share code with java it would be nice :)

Lets say I have a group of (1,2,3,4,5) and I want to create all subgroups of this group with maximum size of a given natural number (for example 3).

I have found already a code that returns all subgroups however, in my project my group size can reach up to 40 Therefore I it takes too much time to calculate and its very problematic. I am also preferring it to be a function rather than an object. Efficiency is important as well, I can't create all possible groups and then filter them out.

public static <T> Set<Set<T>> powerSet(Set<T> originalSet) {
        Set<Set<T>> sets = new HashSet<>();
        if (originalSet.isEmpty()) {
            sets.add(new HashSet<>());
            return sets;
        }
        List<T> list = new ArrayList<>(originalSet);
        T head = list.get(0);
        Set<T> rest = new HashSet<>(list.subList(1, list.size()));
        for (Set<T> set : powerSet(rest)) {
            Set<T> newSet = new HashSet<>();
            newSet.add(head);
            newSet.addAll(set);
            sets.add(newSet);
            sets.add(set);
        }
        return sets;
    }

I have found it here: Obtaining a powerset of a set in Java.

Answer 1

I saw your code and adjusted it a little.

You now enter the maximum size of the inner set and it will be the size of the biggest Set.

    public static <T> Set<Set<T>> powerSet(Set<T> originalSet, int size) {
        Set<Set<T>> sets = new HashSet<>();
        if (originalSet.isEmpty()) {
            sets.add(new HashSet<>());
            return sets;
        }
        List<T> list = new ArrayList<>(originalSet);
        T head = list.get(0);
        Set<T> rest = new HashSet<>(list.subList(1, list.size()));
        for (Set<T> set : powerSet(rest, size)) {
            if(set.size() <= size-1 ){
                Set<T> newSet = new HashSet<>();
                newSet.add(head);
                newSet.addAll(set);
                sets.add(newSet);
                sets.add(set);
            }

        }

        return sets;
    }