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Datastructures and utility classes for non-negative integers
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/*
 * Copyright (C) 2017 Tobias Meggendorfer
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */

package de.tum.in.naturals.set;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * This iterator yields all elements of the power set of the given {@code base}. More specifically
 * it yields all boolean arrays of length {@code base.length} which are a subset of {@code base}.
 * The iteration always returns the elements in the order and always starts with the empty array.
 *
 * Warning: For performance, the returned array is modified in place.
 */
public class PowerSetIterator implements Iterator {
    private final boolean[] base;
    private final boolean[] current;
    private final int domainSize;
    private boolean first = true;

    public PowerSetIterator(boolean[] base) {
        this.base = base.clone();
        this.current = new boolean[base.length];
        int domainSize = 0;
        for (boolean value : this.base) {
            if (value) {
                domainSize += 1;
            }
        }
        this.domainSize = domainSize;
    }

    public int currentIndex() {
        if (domainSize > Integer.SIZE) {
            throw new IllegalStateException();
        }

        int index = 0;
        for (int i = 0; i < base.length; i++) {
            if (current[i]) {
                index |= 1 << i;
            }
        }
        return index;
    }

    public long currentIndexLong() {
        if (domainSize > Long.SIZE) {
            throw new IllegalStateException();
        }

        long index = 0L;
        for (int i = 0; i < base.length; i++) {
            if (current[i]) {
                index |= 1L << i;
            }
        }
        return index;
    }

    @Override
    public boolean hasNext() {
        if (first) {
            return true;
        }
        for (int i = 0; i < base.length; i++) {
            if (base[i] && !current[i]) {
                return true;
            }
        }
        return false;
    }

    @SuppressWarnings("AssignmentOrReturnOfFieldWithMutableType")
    @Override
    public boolean[] next() {
        if (first) {
            first = false;
            return current;
        }

        for (int i = 0; i < base.length; i++) {
            if (!base[i]) {
                continue;
            }
            if (current[i]) {
                current[i] = false;
            } else {
                current[i] = true;
                return current;
            }
        }

        throw new NoSuchElementException("No next element");
    }
}