/// From https://docs.rs/mod_exp/1.0.1/mod_exp, specialised to i128.
pub fn mod_exp(base: i128, exponent: i128, modulus: i128) -> i128 {
const ONE: i128 = 1;
const TWO: i128 = 2;
const ZERO: i128 = 0;
const MAX: i128 = i128::MAX;
assert!((modulus - ONE) < (MAX / (modulus - ONE)));
let mut result = ONE;
let mut base = base % modulus;
let mut exponent = exponent;
loop {
if exponent <= ZERO {
break;
}
if exponent % TWO == ONE {
result = (result * base) % modulus;
}
exponent >>= ONE;
base = (base * base) % modulus;
}
result
}