public static void optimiseCellStyles(HSSFWorkbook workbook) {
		// Where each style has ended up, and if we need to
		//  delete the record for it. Start off with no change
		short[] newPos = 
			new short[workbook.getWorkbook().getNumExFormats()];
		boolean[] isUsed = new boolean[newPos.length];
		boolean[] zapRecords = new boolean[newPos.length];
		for(int i=0; i<newPos.length; i++) {
         isUsed[i] = false;
			newPos[i] = (short)i;
			zapRecords[i] = false;
		}
		
		// Get each style record, so we can do deletes
		//  without getting confused
		ExtendedFormatRecord[] xfrs = new ExtendedFormatRecord[newPos.length];
		InternalWorkbook wb = InternalWorkbook.createWorkbook();
		
		for(int i=0; i<newPos.length; i++) {
				
			xfrs[i] = workbook.getWorkbook().getExFormatAt(i);
			
		}
		
		// Loop over each style, seeing if it is the same
		//  as an earlier one. If it is, point users of the
		//  later duplicate copy to the earlier one, and 
		//  mark the later one as needing deleting
		// Only work on user added ones, which come after 20
		for(int i=21; i<newPos.length; i++) {
			
			// Check this one for being a duplicate
			//  of an earlier one
			int earlierDuplicate = -1;
			for(int j=0; j<i && earlierDuplicate == -1; j++) {
				ExtendedFormatRecord xfCheck = workbook.getWorkbook().getExFormatAt(j);
				if(xfCheck.equals(xfrs[i])) {
						earlierDuplicate = j;
					}
				}
			
			// If we got a duplicate, mark it as such
			if(earlierDuplicate != -1) {
				newPos[i] = (short)earlierDuplicate;
				zapRecords[i] = true;
			}
		}
		
		// Loop over all the cells in the file, and identify any user defined
		//  styles aren't actually being used (don't touch built-in ones)
      for(int sheetNum=0; sheetNum<workbook.getNumberOfSheets(); sheetNum++) {
         HSSFSheet s = workbook.getSheetAt(sheetNum);
         for (Row row : s) {
            for (Cell cellI : row) {
               HSSFCell cell = (HSSFCell)cellI;
               short oldXf = cell.getCellValueRecord().getXFIndex();
               
               if( isUsed[ oldXf ] == false ) {
            	   isUsed[ newPos[ oldXf ] ] = true;
               }
            }
         }
      }//
      // Mark any that aren't used as needing zapping
    	for (int i=21; i<isUsed.length; i++) {
    		if ( ! isUsed[ i ] ) {
    			if( zapRecords[i] == false )
    			{
    				zapRecords[i] = true;
    				newPos[i] = 0;
    			}
    		}
   	    }
		// Update the new positions based on
		//  deletes that have occurred between
		//  the start and them
		// Only work on user added ones, which come after 20
		for(int i=21; i<newPos.length; i++) {
			// Find the number deleted to that
			//  point, and adjust
			short preDeletePos = newPos[i];
			short newPosition = preDeletePos;
			for(int j=0; j<preDeletePos; j++) {
				if(zapRecords[j]) newPosition--;
			}
			
			// Update the new position
			newPos[i] = newPosition;
		}


		// Zap the un-needed user style records
       // removing by index, because removing by object may delete
       // styles we did not intend to (the ones that _were_ duplicated and not the duplicates)
       int max = newPos.length;
       int removed = 0; // to adjust index after deletion
       for(int i=21; i<max; i++) {
           if(zapRecords[i + removed]) {
               workbook.getWorkbook().removeExFormatRecord(i);
               i--;
               max--;
               removed++;
           }
       }
		
		// Finally, update the cells to point at their new extended format records
		for(int sheetNum=0; sheetNum<workbook.getNumberOfSheets(); sheetNum++) {
			HSSFSheet s = workbook.getSheetAt(sheetNum);
			for (Row row : s) {
			   for (Cell cellI : row) {
					HSSFCell cell = (HSSFCell)cellI;
					short oldXf = cell.getCellValueRecord().getXFIndex();
					
						HSSFCellStyle newStyle = workbook.getCellStyleAt(newPos[oldXf]);
                                                cell.setCellStyle(newStyle);

				}
			}
		}