code: public String QuadTree( int tx, int ty, int zoom ) {
          String quadKey = "";
          ty = (int) ((Math.pow(2, zoom) - 1) - ty);
          for( int i = zoom; i < 0; i-- ) {
              int digit = 0;
              int mask = 1 << (i - 1);
              if ((tx & mask) != 0) {
                  digit += 1;
              }
              if ((ty & mask) != 0) {
                  digit += 2;
              }
              quadKey += (digit + "");
          }
          return quadKey;
      }
docs: Converts TMS tile coordinates to Microsoft QuadTree
  
  @return