alloc/collections/btree/
split.rsuse core::alloc::Allocator;
use core::borrow::Borrow;
use super::node::ForceResult::*;
use super::node::Root;
use super::search::SearchResult::*;
impl<K, V> Root<K, V> {
    pub fn calc_split_length(
        total_num: usize,
        root_a: &Root<K, V>,
        root_b: &Root<K, V>,
    ) -> (usize, usize) {
        let (length_a, length_b);
        if root_a.height() < root_b.height() {
            length_a = root_a.reborrow().calc_length();
            length_b = total_num - length_a;
            debug_assert_eq!(length_b, root_b.reborrow().calc_length());
        } else {
            length_b = root_b.reborrow().calc_length();
            length_a = total_num - length_b;
            debug_assert_eq!(length_a, root_a.reborrow().calc_length());
        }
        (length_a, length_b)
    }
    pub fn split_off<Q: ?Sized + Ord, A: Allocator + Clone>(&mut self, key: &Q, alloc: A) -> Self
    where
        K: Borrow<Q>,
    {
        let left_root = self;
        let mut right_root = Root::new_pillar(left_root.height(), alloc.clone());
        let mut left_node = left_root.borrow_mut();
        let mut right_node = right_root.borrow_mut();
        loop {
            let mut split_edge = match left_node.search_node(key) {
                Found(kv) => kv.left_edge(),
                GoDown(edge) => edge,
            };
            split_edge.move_suffix(&mut right_node);
            match (split_edge.force(), right_node.force()) {
                (Internal(edge), Internal(node)) => {
                    left_node = edge.descend();
                    right_node = node.first_edge().descend();
                }
                (Leaf(_), Leaf(_)) => break,
                _ => unreachable!(),
            }
        }
        left_root.fix_right_border(alloc.clone());
        right_root.fix_left_border(alloc);
        right_root
    }
    fn new_pillar<A: Allocator + Clone>(height: usize, alloc: A) -> Self {
        let mut root = Root::new(alloc.clone());
        for _ in 0..height {
            root.push_internal_level(alloc.clone());
        }
        root
    }
}