215 = 32768 and the sum of its digits is 3 + 2 + 7 + 6 + 8 = 26.
What is the sum of the digits of the number 21000?
This looks like a job for rug. There are probably more clever ways,
but that seems like the right tool for the job.
...
So that took way too long, and it was all me fighting with the borrow checker.
Though, to be fair, given the way the borrow checker exists the type
of the method rug::Integer::div_rem makes no sense to me - I can't
imagine a situation where you'd want to call that instead of
rug::Integer::div_rem_ref - the latter has exactly the semantics of
the former, except that it doesn't take ownership of the divisor. Why
would you want to pass ownership of the divisor away? That can't do
anything.
Example fighting with the borrow checker:
error[E0382]: use of moved value: `ten`
--> src/main.rs:11:43
|
9 | let ten = Integer::from(10);
| --- move occurs because `ten` has type `rug::integer::big::Integer`, which does not implement the `Copy` trait
10 | while longpow.cmp0() == Ordering::Greater {
11 | let (quot, rem) = longpow.div_rem(ten);
| ^^^ value moved here, in previous iteration of loop