If you would want to optimize it a bit, i would first get rid of NextChar() function. You could put all valid characters in a string constant, something like
Then matter of changing character is simply increasing character index by 1.Code:const validstr = 'abcd....ABCD....XYZ0123456789';
Code:index:=1; validstr[index] is 'a' inc(index); validstr[index] is now 'b'
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