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```
``````use super::BigInt;
use super::Sign::{self, Minus, Plus};

use crate::BigUint;

use num_integer::Integer;
use num_traits::{Pow, Signed, Zero};

/// Help function for pow
///
/// Computes the effect of the exponent on the sign.
#[inline]
fn powsign<T: Integer>(sign: Sign, other: &T) -> Sign {
if other.is_zero() {
Plus
} else if sign != Minus || other.is_odd() {
sign
} else {
-sign
}
}

macro_rules! pow_impl {
(\$T:ty) => {
impl Pow<\$T> for BigInt {
type Output = BigInt;

#[inline]
fn pow(self, rhs: \$T) -> BigInt {
BigInt::from_biguint(powsign(self.sign, &rhs), self.data.pow(rhs))
}
}

impl<'b> Pow<&'b \$T> for BigInt {
type Output = BigInt;

#[inline]
fn pow(self, rhs: &\$T) -> BigInt {
BigInt::from_biguint(powsign(self.sign, rhs), self.data.pow(rhs))
}
}

impl<'a> Pow<\$T> for &'a BigInt {
type Output = BigInt;

#[inline]
fn pow(self, rhs: \$T) -> BigInt {
BigInt::from_biguint(powsign(self.sign, &rhs), Pow::pow(&self.data, rhs))
}
}

impl<'a, 'b> Pow<&'b \$T> for &'a BigInt {
type Output = BigInt;

#[inline]
fn pow(self, rhs: &\$T) -> BigInt {
BigInt::from_biguint(powsign(self.sign, rhs), Pow::pow(&self.data, rhs))
}
}
};
}

pow_impl!(u8);
pow_impl!(u16);
pow_impl!(u32);
pow_impl!(u64);
pow_impl!(usize);
pow_impl!(u128);
pow_impl!(BigUint);

pub(super) fn modpow(x: &BigInt, exponent: &BigInt, modulus: &BigInt) -> BigInt {
assert!(
!exponent.is_negative(),
"negative exponentiation is not supported!"
);
assert!(
!modulus.is_zero(),
"attempt to calculate with zero modulus!"
);

let result = x.data.modpow(&exponent.data, &modulus.data);
if result.is_zero() {
return BigInt::zero();
}

// The sign of the result follows the modulus, like `mod_floor`.
let (sign, mag) = match (x.is_negative() && exponent.is_odd(), modulus.is_negative()) {
(false, false) => (Plus, result),
(true, false) => (Plus, &modulus.data - result),
(false, true) => (Minus, &modulus.data - result),
(true, true) => (Minus, result),
};
BigInt::from_biguint(sign, mag)
}
``````