Fix an Issue with C# Desktop Application

A sequence or index number only increase once a group more than 1 rows. However, the group doesn't increase a number once there is only 1 row. For an example, group 1 has 5 rows. the next group is group 2 once there is 3 rows. Then, the 3rd group is group 0 (it must be group 3 instead) because there is only 1 row. Consequently, the next following group must be group 4th once there is more than 2 rows instead for showing group 3. Fixing matching long string and improve process speed (currently run 2 days 24 hours a day). C# expert is preferred for this complication project and need accurate output report.