SXSSFWorkbook possible performance overhead #434

[tony1223]

As title and issue explained.

[tony1223]

About the firstRowNum issue, reproduce code:

var start = DateTime.UtcNow;
var rowAccessWindowSize = 10000;

var workbook = new SXSSFWorkbook(null, rowAccessWindowSize);

ISheet worksheet = workbook.CreateSheet("Sheet 1");
var maxRow = 10000;
for (int rownum = 0; rownum < maxRow; rownum++)
{
    IRow row = ((SXSSFSheet) worksheet).CreateRow(rownum);
    for (int celnum = 0; celnum < 34; celnum++)
    {
        ICell Cell = row.CreateCell(celnum);
        if (celnum %3 != 0)
        {
            Cell.SetCellValue("Cell: Row-" + rownum + ";CellNo:" + celnum);
        }

        // Console.WriteLine("  ExcelGeneratePerformanceTest: Cell Content :"+rownum+"/"+maxRow +" , "+row.FirstCellNum +","+row.LastCellNum+" "+(DateTime.UtcNow - start).TotalMilliseconds+" ms");
    }

    if (rownum != 0 && rownum % 1000 == 0 )
    {
        ((SXSSFSheet) worksheet).FlushRows();
         Console.WriteLine("  ExcelGeneratePerformanceTest: Content :"+rownum+"/"+maxRow +" , "+worksheet.FirstRowNum+","+worksheet.LastRowNum+" "+(DateTime.UtcNow - start).TotalMilliseconds+" ms");
    }
}

the log result:
ExcelGeneratePerformanceTest: Content :1000/10000 , 1000,0 72.6543 ms |
ExcelGeneratePerformanceTest: Content :2000/10000 , 1000,0 127.5498 ms |
ExcelGeneratePerformanceTest: Content :3000/10000 , 1000,0 192.3763 ms |
ExcelGeneratePerformanceTest: Content :4000/10000 , 1000,0 251.1753 ms |
ExcelGeneratePerformanceTest: Content :5000/10000 , 1000,0 316.0018 ms |
ExcelGeneratePerformanceTest: Content :6000/10000 , 1000,0 379.8311 ms |
ExcelGeneratePerformanceTest: Content :7000/10000 , 1000,0 436.6791 ms |
ExcelGeneratePerformanceTest: Content :8000/10000 , 1000,0 499.5923 ms |
ExcelGeneratePerformanceTest: Content :9000/10000 , 1000,0 562.4246 ms |

The FirstRowNum is locked after first FlushRows() function call,
but LastRowNum not considering the flushedRows (it's zero).

It should be clarify that the two numbers should be in logical (in the view of sheet) or physically (in the view of _rows variable).

The result should be two possible result:
(A)
ExcelGeneratePerformanceTest: Content :1000/10000 , 0,1000
ExcelGeneratePerformanceTest: Content :2000/10000 , 0,2000
ExcelGeneratePerformanceTest: Content :3000/10000 , 0,3000
ExcelGeneratePerformanceTest: Content :4000/10000 , 0,4000
ExcelGeneratePerformanceTest: Content :5000/10000 , 0,5000
ExcelGeneratePerformanceTest: Content :6000/10000 , 0,6000
ExcelGeneratePerformanceTest: Content :7000/10000 , 0,7000
ExcelGeneratePerformanceTest: Content :8000/10000 , 0,8000
ExcelGeneratePerformanceTest: Content :9000/10000 , 0,9000

or
(B)
ExcelGeneratePerformanceTest: Content :1000/10000 , 0,0
ExcelGeneratePerformanceTest: Content :2000/10000 , 0,0
ExcelGeneratePerformanceTest: Content :3000/10000 , 0,0
ExcelGeneratePerformanceTest: Content :4000/10000 , 0,0
ExcelGeneratePerformanceTest: Content :5000/10000 , 0,0
ExcelGeneratePerformanceTest: Content :6000/10000 , 0,0
ExcelGeneratePerformanceTest: Content :7000/10000 , 0,0
ExcelGeneratePerformanceTest: Content :8000/10000 , 0,0
ExcelGeneratePerformanceTest: Content :9000/10000 , 0,0

In the PR it's implement as (A).

[tonyqus]

This is a valid fix