docs: public int prevSetBit(final int i) {
      int x = i >> 6; // signed i / 64
      long w = bitmap.get(x);
      w <<= 64 - i - 1;
      if (w != 0) {
        return i - Long.numberOfLeadingZeros(w);
      }
      for (--x; x >= 0; --x) {
        long X = bitmap.get(x);
        if (X != 0) {
          return x * 64 + 63 - Long.numberOfLeadingZeros(X);
        }
      }
      return -1;
    }