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    use core::borrow::Borrow;
    use core::cmp::Ordering;
    use core::ops::{Bound, RangeBounds};

    use super::node::{marker, ForceResult::*, Handle, NodeRef};

    use SearchBound::*;
    use SearchResult::*;

    pub enum SearchBound<T> {
        /// An inclusive bound to look for, just like `Bound::Included(T)`.
        Included(T),
        /// An exclusive bound to look for, just like `Bound::Excluded(T)`.
        Excluded(T),
        /// An unconditional inclusive bound, just like `Bound::Unbounded`.
        AllIncluded,
        /// An unconditional exclusive bound.
        AllExcluded,
    }

    impl<T> SearchBound<T> {
        pub fn from_range(range_bound: Bound<T>) -> Self {
            match range_bound {
                Bound::Included(t) => Included(t),
                Bound::Excluded(t) => Excluded(t),
                Bound::Unbounded => AllIncluded,
            }
        }
    }

    pub enum SearchResult<BorrowType, K, V, FoundType, GoDownType> {
        Found(Handle<NodeRef<BorrowType, K, V, FoundType>, marker::KV>),
        GoDown(Handle<NodeRef<BorrowType, K, V, GoDownType>, marker::Edge>),
    }

    pub enum IndexResult {
        KV(usize),
        Edge(usize),
    }

    impl<BorrowType: marker::BorrowType, K, V> NodeRef<BorrowType, K, V, marker::LeafOrInternal> {
        /// Looks up a given key in a (sub)tree headed by the node, recursively.
        /// Returns a `Found` with the handle of the matching KV, if any. Otherwise,
        /// returns a `GoDown` with the handle of the leaf edge where the key belongs.
        ///
        /// The result is meaningful only if the tree is ordered by key, like the tree
        /// in a `BTreeMap` is.
        pub fn search_tree<Q: ?Sized>(
            mut self,
            key: &Q,
        ) -> SearchResult<BorrowType, K, V, marker::LeafOrInternal, marker::Leaf>
        where
            Q: Ord,
            K: Borrow<Q>,
        {
            loop {
                self = match self.search_node(key) {
                    Found(handle) => return Found(handle),
                    GoDown(handle) => match handle.force() {
                        Leaf(leaf) => return GoDown(leaf),
                        Internal(internal) => internal.descend(),
                    },
                }
            }
        }

        /// Descends to the nearest node where the edge matching the lower bound
        /// of the range is different from the edge matching the upper bound, i.e.,
        /// the nearest node that has at least one key contained in the range.
        ///
        /// If found, returns an `Ok` with that node, the pair of edge indices in it
        /// delimiting the range, and the corresponding pair of bounds for
        /// continuing the search in the child nodes, in case the node is internal.
        ///
        /// If not found, returns an `Err` with the leaf edge matching the entire
        /// range.
        ///
        /// The result is meaningful only if the tree is ordered by key.
        pub fn search_tree_for_bifurcation<'r, Q: ?Sized, R>(
            mut self,
            range: &'r R,
        ) -> Result<
            (
                NodeRef<BorrowType, K, V, marker::LeafOrInternal>,
                usize,
                usize,
                SearchBound<&'r Q>,
                SearchBound<&'r Q>,
            ),
            Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge>,
        >
        where
            Q: Ord,
            K: Borrow<Q>,
            R: RangeBounds<Q>,
        {
            // Inlining these variables should be avoided. We assume the bounds reported by `range`
            // remain the same, but an adversarial implementation could change between calls (#81138).
            let (start, end) = (range.start_bound(), range.end_bound());
            match (start, end) {
                (Bound::Excluded(s), Bound::Excluded(e)) if s == e => {
                    panic!("range start and end are equal and excluded in BTreeMap")
                }
                (Bound::Included(s) | Bound::Excluded(s), Bound::Included(e) | Bound::Excluded(e))
                    if s > e =>
                {
                    panic!("range start is greater than range end in BTreeMap")
                }
                _ => {}
            }
            let mut lower_bound = SearchBound::from_range(start);
            let mut upper_bound = SearchBound::from_range(end);
            loop {
                let (lower_edge_idx, lower_child_bound) = self.find_lower_bound_index(lower_bound);
                let (upper_edge_idx, upper_child_bound) = self.find_upper_bound_index(upper_bound);
                if lower_edge_idx > upper_edge_idx {
                    panic!("Ord is ill-defined in BTreeMap range")
                }
                if lower_edge_idx < upper_edge_idx {
                    return Ok((
                        self,
                        lower_edge_idx,
                        upper_edge_idx,
                        lower_child_bound,
                        upper_child_bound,
                    ));
                }
                let common_edge = unsafe { Handle::new_edge(self, lower_edge_idx) };
                match common_edge.force() {
                    Leaf(common_edge) => return Err(common_edge),
                    Internal(common_edge) => {
                        self = common_edge.descend();
                        lower_bound = lower_child_bound;
                        upper_bound = upper_child_bound;
                    }
                }
            }
        }

        /// Finds an edge in the node delimiting the lower bound of a range.
        /// Also returns the lower bound to be used for continuing the search in
        /// the matching child node, if `self` is an internal node.
        ///
        /// The result is meaningful only if the tree is ordered by key.
        pub fn find_lower_bound_edge<'r, Q>(
            self,
            bound: SearchBound<&'r Q>,
        ) -> (Handle<Self, marker::Edge>, SearchBound<&'r Q>)
        where
            Q: ?Sized + Ord,
            K: Borrow<Q>,
        {
            let (edge_idx, bound) = self.find_lower_bound_index(bound);
            let edge = unsafe { Handle::new_edge(self, edge_idx) };
            (edge, bound)
        }

        /// Clone of `find_lower_bound_edge` for the upper bound.
        pub fn find_upper_bound_edge<'r, Q>(
            self,
            bound: SearchBound<&'r Q>,
        ) -> (Handle<Self, marker::Edge>, SearchBound<&'r Q>)
        where
            Q: ?Sized + Ord,
            K: Borrow<Q>,
        {
            let (edge_idx, bound) = self.find_upper_bound_index(bound);
            let edge = unsafe { Handle::new_edge(self, edge_idx) };
            (edge, bound)
        }
    }

    impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
        /// Looks up a given key in the node, without recursion.
        /// Returns a `Found` with the handle of the matching KV, if any. Otherwise,
        /// returns a `GoDown` with the handle of the edge where the key might be found
        /// (if the node is internal) or where the key can be inserted.
        ///
        /// The result is meaningful only if the tree is ordered by key, like the tree
        /// in a `BTreeMap` is.
        pub fn search_node<Q: ?Sized>(self, key: &Q) -> SearchResult<BorrowType, K, V, Type, Type>
        where
            Q: Ord,
            K: Borrow<Q>,
        {
            match self.find_key_index(key) {
                IndexResult::KV(idx) => Found(unsafe { Handle::new_kv(self, idx) }),
                IndexResult::Edge(idx) => GoDown(unsafe { Handle::new_edge(self, idx) }),
            }
        }

        /// Returns either the KV index in the node at which the key (or an equivalent)
        /// exists, or the edge index where the key belongs.
        ///
        /// The result is meaningful only if the tree is ordered by key, like the tree
        /// in a `BTreeMap` is.
        fn find_key_index<Q: ?Sized>(&self, key: &Q) -> IndexResult
        where
            Q: Ord,
            K: Borrow<Q>,
        {
            let node = self.reborrow();
            let keys = node.keys();
            for (i, k) in keys.iter().enumerate() {
                match key.cmp(k.borrow()) {
                    Ordering::Greater => {}
                    Ordering::Equal => return IndexResult::KV(i),
                    Ordering::Less => return IndexResult::Edge(i),
                }
            }
            IndexResult::Edge(keys.len())
        }

        /// Finds an edge index in the node delimiting the lower bound of a range.
        /// Also returns the lower bound to be used for continuing the search in
        /// the matching child node, if `self` is an internal node.
        ///
        /// The result is meaningful only if the tree is ordered by key.
        fn find_lower_bound_index<'r, Q>(
            &self,
            bound: SearchBound<&'r Q>,
        ) -> (usize, SearchBound<&'r Q>)
        where
            Q: ?Sized + Ord,
            K: Borrow<Q>,
        {
            match bound {
                Included(key) => match self.find_key_index(key) {
                    IndexResult::KV(idx) => (idx, AllExcluded),
                    IndexResult::Edge(idx) => (idx, bound),
                },
                Excluded(key) => match self.find_key_index(key) {
                    IndexResult::KV(idx) => (idx + 1, AllIncluded),
                    IndexResult::Edge(idx) => (idx, bound),
                },
                AllIncluded => (0, AllIncluded),
                AllExcluded => (self.len(), AllExcluded),
            }
        }

        /// Clone of `find_lower_bound_index` for the upper bound.
        fn find_upper_bound_index<'r, Q>(
            &self,
            bound: SearchBound<&'r Q>,
        ) -> (usize, SearchBound<&'r Q>)
        where
            Q: ?Sized + Ord,
            K: Borrow<Q>,
        {
            match bound {
                Included(key) => match self.find_key_index(key) {
                    IndexResult::KV(idx) => (idx + 1, AllExcluded),
                    IndexResult::Edge(idx) => (idx, bound),
                },
                Excluded(key) => match self.find_key_index(key) {
                    IndexResult::KV(idx) => (idx, AllIncluded),
                    IndexResult::Edge(idx) => (idx, bound),
                },
                AllIncluded => (self.len(), AllIncluded),
                AllExcluded => (0, AllExcluded),
            }
        }
    }