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// Copyright © 2026 Mikhail Hogrefe
//
// This file is part of Malachite.
//
// Malachite is free software: you can redistribute it and/or modify it under the terms of the GNU
// Lesser General Public License (LGPL) as published by the Free Software Foundation; either version
// 3 of the License, or (at your option) any later version. See <https://www.gnu.org/licenses/>.
use crate::num::arithmetic::traits::{ModPowerOf2Pow, ModPowerOf2PowAssign};
use crate::num::basic::unsigneds::PrimitiveUnsigned;
use crate::num::logic::traits::BitIterable;
fn mod_power_of_2_pow<T: PrimitiveUnsigned>(x: T, exp: u64, pow: u64) -> T {
assert!(pow <= T::WIDTH);
assert!(
x.significant_bits() <= pow,
"x must be reduced mod 2^pow, but {x} >= 2^{pow}"
);
if pow == 0 {
return T::ZERO;
}
let mut out = T::ONE;
for bit in exp.bits().rev() {
out.mod_power_of_2_mul_assign(out, pow);
if bit {
out.mod_power_of_2_mul_assign(x, pow);
}
}
out
}
macro_rules! impl_mod_power_of_2_pow {
($t:ident) => {
impl ModPowerOf2Pow<u64> for $t {
type Output = $t;
/// Raises a number to a power modulo another number $2^k$. The base must be already
/// reduced modulo $2^k$.
///
/// $f(x, n, k) = y$, where $x, y < 2^k$ and $x^n \equiv y \mod 2^k$.
///
/// # Worst-case complexity
/// $T(n) = O(n)$
///
/// $M(n) = O(1)$
///
/// where $T$ is time, $M$ is additional memory, and $n$ is `exp.significant_bits()`.
///
/// # Panics
/// Panics if `pow` is greater than `Self::WIDTH` or if `self` is greater than or equal
/// to $2^k$.
///
/// # Examples
/// See [here](super::mod_power_of_2_pow#mod_power_of_2_pow).
#[inline]
fn mod_power_of_2_pow(self, exp: u64, pow: u64) -> $t {
mod_power_of_2_pow(self, exp, pow)
}
}
impl ModPowerOf2PowAssign<u64> for $t {
/// Raises a number to a power modulo another number $2^k$, in place. The base must be
/// already reduced modulo $2^k$.
///
/// $x \gets y$, where $x, y < 2^k$ and $x^n \equiv y \mod 2^k$.
///
/// # Worst-case complexity
/// $T(n) = O(n)$
///
/// $M(n) = O(1)$
///
/// where $T$ is time, $M$ is additional memory, and $n$ is `exp.significant_bits()`.
///
/// # Panics
/// Panics if `pow` is greater than `Self::WIDTH` or if `self` is greater than or equal
/// to $2^k$.
///
/// # Examples
/// See [here](super::mod_power_of_2_pow#mod_power_of_2_pow_assign).
#[inline]
fn mod_power_of_2_pow_assign(&mut self, exp: u64, pow: u64) {
*self = self.mod_power_of_2_pow(exp, pow);
}
}
};
}
apply_to_unsigneds!(impl_mod_power_of_2_pow);