You have to try this.....

[Punkingal]

It is amazing.

http://digicc.com/fido/

[Commie Pinko Dirtbag]

1. Math geek fun-spoiler, that's me:

Every number subtracted from a permutation of itself is necessarily a multiple of 9. (The same digit in place A minus the same digit in place B will be always of the form 99999.....99000...00)

Multiples of 9 have the property of having the sum of their digits also a multiple of 9.

Therefore, all the software has to do is pick the number the user typed, sum its digits until it's lower than 9 and subtracting 9 from that. This is the answer.

[ScreamingMeemie]

2. Well pooh on you. ;) Me, I like thinking that it's aliens or something.

:P

[Punkingal]

3. Gee....

I KNEW there had to be a reasonable explanation, but I didn't know the first poster would jump in and EXPLAIN it immediately.