#![feature(core_intrinsics)]
use std::intrinsics;

fn main() {
    for i in 0..=16 {
        println!("{} -> {}", i, prev_power_of_two(i));
    }
}

fn prev_power_of_two(x: u32) -> u32 {
    if x <= 1 {
        return x;
    }
    let z = unsafe { intrinsics::ctlz_nonzero(x) };

    (u32::max_value() >> z + 1) + 1
}