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    use super::map::MIN_LEN;
    use super::node::{ForceResult::*, Root};
    use super::search::SearchResult::*;
    use core::borrow::Borrow;

    impl<K, V> Root<K, V> {
        pub fn split_off<Q: ?Sized + Ord>(&mut self, right_root: &mut Self, key: &Q)
        where
            K: Borrow<Q>,
        {
            debug_assert!(right_root.height() == 0);
            debug_assert!(right_root.len() == 0);

            let left_root = self;
            for _ in 0..left_root.height() {
                right_root.push_internal_level();
            }

            {
                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) {
                        // key is going to the right tree
                        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();
            right_root.fix_left_border();
        }

        /// Removes empty levels on the top, but keeps an empty leaf if the entire tree is empty.
        fn fix_top(&mut self) {
            while self.height() > 0 && self.len() == 0 {
                self.pop_internal_level();
            }
        }

        /// Stock up or merge away any underfull nodes on the right border of the
        /// tree. The other nodes, those that are not the root nor a rightmost edge,
        /// must already have at least MIN_LEN elements.
        fn fix_right_border(&mut self) {
            self.fix_top();

            {
                let mut cur_node = self.borrow_mut();

                while let Internal(node) = cur_node.force() {
                    let mut last_kv = node.last_kv().consider_for_balancing();

                    if last_kv.can_merge() {
                        cur_node = last_kv.merge_tracking_child();
                    } else {
                        let right_len = last_kv.right_child_len();
                        // `MIN_LEN + 1` to avoid readjust if merge happens on the next level.
                        if right_len < MIN_LEN + 1 {
                            last_kv.bulk_steal_left(MIN_LEN + 1 - right_len);
                        }
                        cur_node = last_kv.into_right_child();
                    }
                    debug_assert!(cur_node.len() > MIN_LEN);
                }
            }

            self.fix_top();
        }

        /// The symmetric clone of `fix_right_border`.
        fn fix_left_border(&mut self) {
            self.fix_top();

            {
                let mut cur_node = self.borrow_mut();

                while let Internal(node) = cur_node.force() {
                    let mut first_kv = node.first_kv().consider_for_balancing();

                    if first_kv.can_merge() {
                        cur_node = first_kv.merge_tracking_child();
                    } else {
                        let left_len = first_kv.left_child_len();
                        // `MIN_LEN + 1` to avoid readjust if merge happens on the next level.
                        if left_len < MIN_LEN + 1 {
                            first_kv.bulk_steal_right(MIN_LEN + 1 - left_len);
                        }
                        cur_node = first_kv.into_left_child();
                    }
                    debug_assert!(cur_node.len() > MIN_LEN);
                }
            }

            self.fix_top();
        }
    }