private static int trailingZeroBits(int x) {
        assert wordlen == 32;
        if (x == 0) return 32;
        int n = 1;
        if ((x & 0xffff) == 0) { n += 16; x >>>= 16; }
        if ((x & 0x00ff) == 0) { n +=  8; x >>>=  8; }
        if ((x & 0x000f) == 0) { n +=  4; x >>>=  4; }
        if ((x & 0x0003) == 0) { n +=  2; x >>>=  2; }
        return n - (x&1);
    }