code: private int unFilledSpacesInHeaderGroup(int header) {
          //If mNumColumns is equal to zero we will have a divide by 0 exception
          if(mNumColumns == 0){
              return 0;
          }
  
          int remainder = mDelegate.getCountForHeader(header) % mNumColumns;
          return remainder == 0 ? 0 : mNumColumns - remainder;
      }
docs: Counts the number of items that would be need to fill out the last row in
  the group of items with the given header.
  
  @param header Header set of items are grouped by.
  @return The count of unfilled spaces in the last row.