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// Copyright © 2026 Mikhail Hogrefe
//
// Uses code adopted from the GNU MP Library.
//
// Copyright © 2001, 2002 Free Software Foundation, Inc.
//
// 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::natural::InnerNatural::{Large, Small};
use crate::natural::{Natural, bit_to_limb_count_floor};
use crate::platform::Limb;
use malachite_base::num::arithmetic::traits::DivisibleByPowerOf2;
use malachite_base::num::basic::integers::PrimitiveInt;
use malachite_base::slices::slice_test_zero;
// Interpreting a slice of `Limb`s as the limbs of a `Natural` in ascending order, determines
// whether that `Natural` is divisible by 2 raised to a given power.
//
// This function assumes that `xs` is nonempty and does not only contain zeros.
//
// # Worst-case complexity
// $T(n) = O(n)$
//
// $M(n) = O(1)$
//
// where $T$ is time, $M$ is additional memory, and $n$ is `min(pow, xs.len())`.
//
// This is equivalent to `mpz_divisible_2exp_p` from `mpz/divis_2exp.c`, GMP 6.2.1, where `a` is
// non-negative.
pub_crate_test! {limbs_divisible_by_power_of_2(xs: &[Limb], pow: u64) -> bool {
assert!(!xs.is_empty());
let zeros = bit_to_limb_count_floor(pow);
zeros < xs.len()
&& slice_test_zero(&xs[..zeros])
&& xs[zeros].divisible_by_power_of_2(pow & Limb::WIDTH_MASK)
}}
impl DivisibleByPowerOf2 for &Natural {
/// Returns whether a [`Natural`] is divisible by $2^k$.
///
/// $f(x, k) = (2^k|x)$.
///
/// $f(x, k) = (\exists n \in \N : \ x = n2^k)$.
///
/// If `self` is 0, the result is always true; otherwise, it is equivalent to
/// `self.trailing_zeros().unwrap() <= pow`, but more efficient.
///
/// # Worst-case complexity
/// $T(n) = O(n)$
///
/// $M(n) = O(1)$
///
/// where $T$ is time, $M$ is additional memory, and $n$ is `min(pow, self.significant_bits())`.
///
/// # Examples
/// ```
/// use malachite_base::num::arithmetic::traits::{DivisibleByPowerOf2, Pow};
/// use malachite_base::num::basic::traits::Zero;
/// use malachite_nz::natural::Natural;
///
/// assert_eq!(Natural::ZERO.divisible_by_power_of_2(100), true);
/// assert_eq!(Natural::from(100u32).divisible_by_power_of_2(2), true);
/// assert_eq!(Natural::from(100u32).divisible_by_power_of_2(3), false);
/// assert_eq!(
/// Natural::from(10u32).pow(12).divisible_by_power_of_2(12),
/// true
/// );
/// assert_eq!(
/// Natural::from(10u32).pow(12).divisible_by_power_of_2(13),
/// false
/// );
/// ```
fn divisible_by_power_of_2(self, pow: u64) -> bool {
match (self, pow) {
(_, 0) => true,
(&Natural(Small(small)), pow) => small.divisible_by_power_of_2(pow),
(&Natural(Large(ref limbs)), pow) => limbs_divisible_by_power_of_2(limbs, pow),
}
}
}