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// Copyright © 2024 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::natural::InnerNatural::{Large, Small};
use crate::natural::Natural;
use crate::platform::Limb;
use malachite_base::num::arithmetic::traits::IsPowerOf2;
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 an integer power of 2.
//
// This function assumes that `xs` is nonempty and the last (most significant) limb is nonzero.
//
// # Worst-case complexity
// $T(n) = O(n)$
//
// $M(n) = O(1)$
//
// where $T$ is time, $M$ is additional memory, and $n$ is `xs.len()`.
//
// # Panics
// Panics if `xs` is empty.
pub_crate_test! {limbs_is_power_of_2(xs: &[Limb]) -> bool {
let (xs_last, xs_init) = xs.split_last().unwrap();
slice_test_zero(xs_init) && xs_last.is_power_of_2()
}}
impl IsPowerOf2 for Natural {
/// Determines whether a [`Natural`] is an integer power of 2.
///
/// $f(x) = (\exists n \in \Z : 2^n = x)$.
///
/// # Worst-case complexity
/// $T(n) = O(n)$
///
/// $M(n) = O(1)$
///
/// where $T$ is time, $M$ is additional memory, and $n$ is `self.significant_bits()`.
///
/// # Examples
/// ```
/// use malachite_base::num::arithmetic::traits::{IsPowerOf2, Pow};
/// use malachite_base::num::basic::traits::Zero;
/// use malachite_nz::natural::Natural;
/// use core::str::FromStr;
///
/// assert_eq!(Natural::ZERO.is_power_of_2(), false);
/// assert_eq!(Natural::from(123u32).is_power_of_2(), false);
/// assert_eq!(Natural::from(0x80u32).is_power_of_2(), true);
/// assert_eq!(Natural::from(10u32).pow(12).is_power_of_2(), false);
/// assert_eq!(Natural::from_str("1099511627776").unwrap().is_power_of_2(), true);
/// ```
fn is_power_of_2(&self) -> bool {
match *self {
Natural(Small(small)) => small.is_power_of_2(),
Natural(Large(ref limbs)) => limbs_is_power_of_2(limbs),
}
}
}