docs: public int prevClearBit(final int i) {
      int x = i >> 6; // i / 64 with sign extension
      long w = ~bitmap.get(x);
      w <<= 64 - i - 1;
      if (w != 0L) {
        return i - Long.numberOfLeadingZeros(w);
      }
      for (--x; x >= 0; --x) {
        long map = ~bitmap.get(x);
        if (map != 0L) {
          return x * 64 + 63 - Long.numberOfLeadingZeros(map);
        }
      }
      return -1;
    }