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extern crate num;
use std::ops::Mul;
use num::traits::{Zero, One};
pub use num::bigint::{ToBigInt, BigInt};
use num::iter::range_step;
pub fn modexp<A>(b: A, e: A, m: A) -> BigInt
where A: ToBigInt
{
modexp_fast_internal(b.to_bigint().expect("Could not create bigint from base."),
e.to_bigint().expect("Could not create bigint from exponent."),
m.to_bigint().expect("Could not create bigint from modulus."))
}
pub fn modexp_slow<A>(b: A, e: A, m: A) -> BigInt
where A: ToBigInt
{
modexp_internal(b.to_bigint().expect("Could not create bigint from base."),
e.to_bigint().expect("Could not create bigint from exponent."),
m.to_bigint().expect("Could not create bigint from modulus."))
}
fn modexp_internal(b: BigInt, e: BigInt, m: BigInt) -> BigInt {
if m == BigInt::one() {
return BigInt::zero();
}
let mut c: BigInt = BigInt::one();
for _ in range_step(BigInt::one(), e + BigInt::one(), BigInt::one()) {
c = (c.mul(b.clone())) % m.clone();
}
c
}
fn modexp_fast_internal(mut b: BigInt, mut e: BigInt, m: BigInt) -> BigInt {
let mut res = BigInt::one();
let two = 2.to_bigint().unwrap();
while e > BigInt::zero() {
if e.clone() % two.clone() == BigInt::one() {
res = (res.clone() * b.clone()) % m.clone();
}
b = (b.clone() * b.clone()) % m.clone();
e = e.clone() / two.clone();
}
res % m
}
#[test]
fn test() {
assert!(modexp_slow(500, 2000, 50323) == 12847.to_bigint().unwrap());
}
#[test]
fn test_fast() {
let a = modexp(500, 2000, 50323);
assert!(a == 12847.to_bigint().unwrap());
}