Obtaining a powerset of a set in Java

Question

The powerset of {1, 2, 3} is:

{{}, {2}, {3}, {2, 3}, {1, 2}, {1, 3}, {1, 2, 3}, {1}}

Let\'s say I have a Set in Java:<

Answer 1

Yet another solution - with java8+ streaming api It is lazy and ordered so it returns correct subsets when it is used with "limit()".

 public long bitRangeMin(int size, int bitCount){
    BitSet bs = new BitSet(size);
    bs.set(0, bitCount);
    return bs.toLongArray()[0];
}

public long bitRangeMax(int size, int bitCount){
    BitSet bs = BitSet.valueOf(new long[]{0});
    bs.set(size - bitCount, size);
    return bs.toLongArray()[0];
}

public  Stream> powerSet(Collection data)
{
    List list = new LinkedHashSet<>(data).stream().collect(Collectors.toList());
    Stream head = LongStream.of(0).mapToObj( i -> BitSet.valueOf(new long[]{i}));
    Stream tail = IntStream.rangeClosed(1, list.size())
            .boxed()
            .flatMap( v1 -> LongStream.rangeClosed( bitRangeMin(list.size(), v1), bitRangeMax(list.size(), v1))
                    .mapToObj(v2 -> BitSet.valueOf(new long[]{v2}))
                    .filter( bs -> bs.cardinality() == v1));

    return Stream.concat(head, tail)
            .map( bs -> bs
                    .stream()
                    .mapToObj(list::get)
                    .collect(Collectors.toList()));
}

And the client code is

@Test
public void testPowerSetOfGivenCollection(){
    List data = new LinkedList<>();
    for(char i = 'a'; i < 'a'+5; i++ ){
        data.add(i);
    }
    powerSet(data)
            .limit(9)
            .forEach(System.out::print);

}

/* Prints : [][a][b][c][d][e][a, b][a, c][b, c] */