flipByMod.java

/**
 * Flips all bits for which index % mod == 0 such that
 * the bits (i % mod >= 0 && i % mod < mod) are 0. The
 * invariant here is that, for all ranges of i such
 * that (i % mod >= 0 && i % mod < mod), there is *at least*
 * one value of i such that get(i) == true.
 */
public void flipByMod(int mod) {
    long flipper = 1L;
    long mask = (1L << mod) - 1;  // bitmask which covers everything >= 0 and < mod
    for (int i = 0; i < _length; i++) {
        do {
            if ((bits[i] & mask) == 0L) {
                bits[i] ^= flipper;
            }
            mask = Long.rotateLeft(mask, mod);
            flipper = Long.rotateLeft(flipper, mod);
        } while (!(flipper >= 0 && flipper < mod));
    }
}