How to find all subsets of a set in Dart? - dart

Question

I need to get all possible subsets of an array.
Say I have this:
<int>[1, 2, 3]
How do I get this?
[], [1], [2], [3],[1, 2], [2, 3], [1, 3], [1, 2, 3]
I am interested in all subsets. For subsets of specific length, refer to the following questions:
How to find all subsets of a set in JavaScript?

Answer 1

Here is my take on it, with only native function as in your link:
List getAllSubsets(List l) => l.fold<List>([[]], (subLists, element) {
return subLists
.map((subList) => [
subList,
subList + [element]
])
.expand((element) => element)
.toList();
});
If you want a specific size:
List getSizedSubsets(List l, int size) =>
getAllSubsets(l).where((element) => element.length == size).toList();

Answer 2

I'd probably go with something simple like:
Iterable<Set<E>> subsets<E>(Set<E> elements) sync* {
if (elements.length >= 32) {
// Otherwise there'll be more than 2^32 subsets. And bitops will overflow in JS.
throw ArgumentError.value(elements, "elements", "must have less than 32 elements");
}
var list = [...elements];
var subsetCount = 1 << list.length;
for (var i = 0; i < subsetCount; i++) {
yield {
for (var j = 0, bit = 1; j < elements.length; j++, bit <<= 1)
if (i & bit != 0) list[j]
};
}
}
Another approach is to only have one set, and then update it iteratively to contain different elements. It's possible to go through all the sets doing only single-element changes on each step (using Gray-code):
/// Iterates a set through all combinations of its elements.
///
/// Adds and removes elements from [set] to make it iterate through all
/// possible combinations of its initial elements.
/// The current value of the iterator is always [set].
/// If iterated through to the end, the [set] ends up with all its original elements.
Iterable<Set<E>> subsets<E>(Set<E> set) sync* {
if (set.length >= 32) {
throw ArgumentError.value(set, "set", "must have less than 32 elements");
}
var list = [...set];
var prev = 0;
var counter = 0;
do {
yield set;
var next = ++counter ^ (counter >> 1);
var bit = prev ^ next; // One bit set.
var index = bit.bitLength - 1;
if (index >= list.length) index = 0;
var element = list[index];
if (next & bit == 0) {
set.add(element);
} else {
set.remove(element);
}
prev = next;
} while (set.length < list.length);
}