public override string Scramble(string validNumber)
{
Contract.Requires(IsValid(validNumber));
string rot5number = Rot.Rot5(validNumber);
int lastDigitPosition = rot5number.LastIndexOfAny(Chars.Digits);
// aaaaaaa1bb
// 0123456789
// length = 10
// lastDigitPosition = 7
// suffixLength = 2
// prefixLength = 7
string incompleteNumber = rot5number.Substring(0, lastDigitPosition);
string result;
try
{
result = Complete(incompleteNumber);
}
catch (Exception)
{
// checkDigit was 10, so force it to be 8:
// we alter the 2nd digit from the end; the modulus was 10, the total will be 1*20+10 resulting in a modulus of 8
int alterDigitPosition = incompleteNumber.LastIndexOfAny(Chars.Digits);
char alterDigitChar = incompleteNumber[alterDigitPosition];
int alterDigit = int.Parse(alterDigitChar.ToString());
int alteredDigit = (alterDigit + 1) % 10;
string alteredIncompleteNumber = incompleteNumber.Substring(0, alterDigitPosition) + alteredDigit + incompleteNumber.Substring(alterDigitPosition + 1);
result = Complete(alteredIncompleteNumber);
}
result += rot5number.Substring(lastDigitPosition + 1); // make sure non digits get at the end
return(result);
}