I will explain in sudo code for now.. But if you need assistance I will help you out.. But what I would do, (Which is not always the best thing), is use a loop with a function.. use a loop to go through the length of the word.. if the letter is an A than return an F (for example).. and repeat the if then statement until all the letters you want to use are complete.. then make another function to convert it back to normal.. If you want to use numbers.. Id use 2 digits.. For example: A = 15..
> Thats called hashing.
Hashing is not guaranteed to produce unique codes except in special circumstances. The answer to this question really depends on what the code is used for. The needs of the program determine how the code is generated. For example, if the code is unique for each user, the code needs to be salted with identifying information about the user to get around two users typing the same string.