Number of Digits

Implement the function

val num_digits : int -> int

so that num_digits n is the number of digits in n. You may only use arithmetic operations. In particular, you may not convert n into a string.

let _ = assert (num_digits 12345 = 5)
let _ = assert (num_digits (-10) = 2)
let _ = assert (num_digits 0 = 1)