Note you only need to do that if calculating the check digit, (when it
matters that you get 0 not 10) if you are just checking that a given
checksum is right then you don't need that complication as using 10 or 0
is the same in a (..) mod 10 = 0 test.

If you a_re_ calculating the digit, rather than write
(10 - (foo mod 10)) mod 10
it's probably more natural just to use
-(-foo mod 10)