pub trait Math {
fn log_2(self) -> usize;
}
impl Math for usize {
fn log_2(self) -> usize {
assert_ne!(self, 0);
if self.is_power_of_two() {
(1usize.leading_zeros() - self.leading_zeros()) as usize
} else {
(0usize.leading_zeros() - self.leading_zeros()) as usize
}
}
}