java - powerset of an array

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import java.lang.reflect.Array; import java.util.Comparator; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; public class Toast { /** * https://en.wikipedia.org/wiki/Power_set * @param set * @return */ @SuppressWarnings("unchecked") public static <E> List<E[]> powerSet(E[] set) { return Stream.of(binaryCounter(set.length)) .map(is -> { E[] ar = (E[])Array.newInstance(set.getClass().getComponentType(), is.length); for (int i = 0; i < is.length; i++) { if(is[i] != 0) ar[i] = set[i]; } return Stream.of(ar).filter(a -> a != null).toArray(size -> (E[])Array.newInstance(set.getClass().getComponentType(), size)); }).sorted(Comparator.comparing(ar -> ar.length)).collect(Collectors.toList()); } static int[][] binaryCounter(int length) { int size = (int) Math.pow(2, length); int[][] indices = new int[size][length]; int half = size/2; for (int i = 0; i < length && half > 0; i++) { int num = 1; for (int j = 0; j < indices.length; j++) { if(j%half == 0) num = num == 1 ? 0 : 1; indices[j][i] = num; } half /= 2; } return indices; } }

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