code: public static int previousClearBit(long v, int start) {
      if(start < 0) {
        return -1;
      }
      start = start < Long.SIZE ? start : Long.SIZE - 1;
      long cur = ~v & (LONG_ALL_BITS >>> -(start + 1));
      return cur == 0 ? -1 : 63 - Long.numberOfLeadingZeros(cur);
    }
docs: Find the previous clear bit.
  
  @param v Values to process
  @param start Start position (inclusive)
  @return Position of previous clear bit, or -1.