It turns out that often when a number comes back as ERR or ILL it is because the scanner has failed to pick up on one pipe or underscore for one of the figures. For example
_ _ _ _ _ _ _
|_||_|| || ||_ | | ||_
| _||_||_||_| | | | _|
The 9 could be an 8 if the scanner had missed one |. Or the 0 could be an 8. Or the 1 could be a 7. The 5 could be a 9 or 6. So your next task is to look at numbers that have come back as ERR or ILL, and try to guess what they should be, by adding or removing just one pipe or underscore. If there is only one possible number with a valid checksum, then use that. If there are several options, the status should be AMB. If you still can’t work out what it should be, the status should be reported ILL.
- see also: codingdojo.org/kata/BankOCR