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    use std::borrow::Borrow;
    use std::collections::{BTreeMap, BTreeSet, VecDeque};
    use std::fmt;

    pub struct Graph<N: Clone, E: Clone> {
        nodes: im_rc::OrdMap<N, im_rc::OrdMap<N, E>>,
    }

    impl<N: Eq + Ord + Clone, E: Default + Clone> Graph<N, E> {
        pub fn new() -> Graph<N, E> {
            Graph {
                nodes: im_rc::OrdMap::new(),
            }
        }

        pub fn add(&mut self, node: N) {
            self.nodes.entry(node).or_insert_with(im_rc::OrdMap::new);
        }

        pub fn link(&mut self, node: N, child: N) -> &mut E {
            self.nodes
                .entry(node)
                .or_insert_with(im_rc::OrdMap::new)
                .entry(child)
                .or_insert_with(Default::default)
        }

        pub fn contains<Q: ?Sized>(&self, k: &Q) -> bool
        where
            N: Borrow<Q>,
            Q: Ord + Eq,
        {
            self.nodes.contains_key(k)
        }

        pub fn edge(&self, from: &N, to: &N) -> Option<&E> {
            self.nodes.get(from)?.get(to)
        }

        pub fn edges(&self, from: &N) -> impl Iterator<Item = (&N, &E)> {
            self.nodes.get(from).into_iter().flat_map(|x| x.iter())
        }

        /// A topological sort of the `Graph`
        pub fn sort(&self) -> Vec<N> {
            let mut ret = Vec::new();
            let mut marks = BTreeSet::new();

            for node in self.nodes.keys() {
                self.sort_inner_visit(node, &mut ret, &mut marks);
            }

            ret
        }

        fn sort_inner_visit(&self, node: &N, dst: &mut Vec<N>, marks: &mut BTreeSet<N>) {
            if !marks.insert(node.clone()) {
                return;
            }

            for child in self.nodes[node].keys() {
                self.sort_inner_visit(child, dst, marks);
            }

            dst.push(node.clone());
        }

        pub fn iter(&self) -> impl Iterator<Item = &N> {
            self.nodes.keys()
        }

        pub fn len(&self) -> usize {
            self.nodes.len()
        }

        /// Checks if there is a path from `from` to `to`.
        pub fn is_path_from_to<'a>(&'a self, from: &'a N, to: &'a N) -> bool {
            let mut stack = vec![from];
            let mut seen = BTreeSet::new();
            seen.insert(from);
            while let Some(iter) = stack.pop().and_then(|p| self.nodes.get(p)) {
                for p in iter.keys() {
                    if p == to {
                        return true;
                    }
                    if seen.insert(p) {
                        stack.push(p);
                    }
                }
            }
            false
        }

        /// Resolves one of the paths from the given dependent package down to a leaf.
        ///
        /// The path return will be the shortest path, or more accurately one of the paths with the shortest length.
        ///
        /// Each element contains a node along with an edge except the first one.
        /// The representation would look like:
        ///
        /// (Node0,) -> (Node1, Edge01) -> (Node2, Edge12)...
        pub fn path_to_bottom<'a>(&'a self, pkg: &'a N) -> Vec<(&'a N, Option<&'a E>)> {
            self.path_to(pkg, |s, p| s.edges(p))
        }

        /// Resolves one of the paths from the given dependent package up to the root.
        ///
        /// The path return will be the shortest path, or more accurately one of the paths with the shortest length.
        ///
        /// Each element contains a node along with an edge except the first one.
        /// The representation would look like:
        ///
        /// (Node0,) -> (Node1, Edge01) -> (Node2, Edge12)...
        pub fn path_to_top<'a>(&'a self, pkg: &'a N) -> Vec<(&'a N, Option<&'a E>)> {
            self.path_to(pkg, |s, pk| {
                // Note that this implementation isn't the most robust per se, we'll
                // likely have to tweak this over time. For now though it works for what
                // it's used for!
                s.nodes
                    .iter()
                    .filter_map(|(p, adjacent)| adjacent.get(pk).map(|e| (p, e)))
            })
        }
    }

    impl<'s, N: Eq + Ord + Clone + 's, E: Default + Clone + 's> Graph<N, E> {
        fn path_to<'a, F, I>(&'s self, pkg: &'a N, fn_edge: F) -> Vec<(&'a N, Option<&'a E>)>
        where
            I: Iterator<Item = (&'a N, &'a E)>,
            F: Fn(&'s Self, &'a N) -> I,
            'a: 's,
        {
            let mut back_link = BTreeMap::new();
            let mut queue = VecDeque::from([pkg]);
            let mut last = pkg;

            while let Some(p) = queue.pop_front() {
                last = p;
                let mut out_edges = true;
                for (child, edge) in fn_edge(&self, p) {
                    out_edges = false;
                    back_link.entry(child).or_insert_with(|| {
                        queue.push_back(child);
                        (p, edge)
                    });
                }
                if out_edges {
                    break;
                }
            }

            let mut result = Vec::new();
            let mut next = last;
            while let Some((p, e)) = back_link.remove(&next) {
                result.push((next, Some(e)));
                next = p;
            }
            if result.iter().all(|(n, _)| n != &next) {
                result.push((next, None));
            }
            result.reverse();
            #[cfg(debug_assertions)]
            {
                for x in result.windows(2) {
                    let [(n2, _), (n1, Some(e12))] = x else {
                        unreachable!()
                    };
                    assert!(std::ptr::eq(
                        self.edge(n1, n2).or(self.edge(n2, n1)).unwrap(),
                        *e12
                    ));
                }
                let last = result.last().unwrap().0;
                let set: Vec<_> = result.iter().map(|(k, _)| k).collect();
                if !fn_edge(&self, last)
                    .filter(|(e, _)| !set.contains(&e))
                    .next()
                    .is_none()
                {
                    self.print_for_test();
                    unreachable!("The last element in the path should not have outgoing edges");
                }
            }
            result
        }
    }

    #[test]
    fn path_to_case() {
        let mut new = Graph::new();
        new.link(0, 3);
        new.link(1, 0);
        new.link(2, 0);
        new.link(2, 1);
        assert_eq!(
            new.path_to_bottom(&2),
            vec![(&2, None), (&0, Some(&())), (&3, Some(&()))]
        );
    }

    #[test]
    fn path_to_self() {
        // Extracted from #12941
        let mut new: Graph<i32, ()> = Graph::new();
        new.link(0, 0);
        assert_eq!(new.path_to_bottom(&0), vec![(&0, Some(&()))]);
    }

    impl<N: Eq + Ord + Clone, E: Default + Clone> Default for Graph<N, E> {
        fn default() -> Graph<N, E> {
            Graph::new()
        }
    }

    impl<N: fmt::Display + Eq + Ord + Clone, E: Clone> fmt::Debug for Graph<N, E> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            writeln!(fmt, "Graph {{")?;

            for (n, e) in &self.nodes {
                writeln!(fmt, "  - {}", n)?;

                for n in e.keys() {
                    writeln!(fmt, "    - {}", n)?;
                }
            }

            write!(fmt, "}}")?;

            Ok(())
        }
    }

    impl<N: Eq + Ord + Clone, E: Clone> Graph<N, E> {
        /// Prints the graph for constructing unit tests.
        ///
        /// For purposes of graph traversal algorithms the edge values do not matter,
        /// and the only value of the node we care about is the order it gets compared in.
        /// This constructs a graph with the same topology but with integer keys and unit edges.
        #[cfg(debug_assertions)]
        #[allow(clippy::print_stderr)]
        fn print_for_test(&self) {
            // Isolate and print a test case.
            let names = self
                .nodes
                .keys()
                .chain(self.nodes.values().flat_map(|vs| vs.keys()))
                .collect::<BTreeSet<_>>()
                .into_iter()
                .collect::<Vec<_>>();
            let mut new = Graph::new();
            for n1 in self.nodes.keys() {
                let name1 = names.binary_search(&n1).unwrap();
                new.add(name1);
                for n2 in self.nodes[n1].keys() {
                    let name2 = names.binary_search(&n2).unwrap();
                    *new.link(name1, name2) = ();
                }
            }
            eprintln!("Graph for tests = {new:#?}");
        }
    }

    impl<N: Eq + Ord + Clone, E: Eq + Clone> PartialEq for Graph<N, E> {
        fn eq(&self, other: &Graph<N, E>) -> bool {
            self.nodes.eq(&other.nodes)
        }
    }
    impl<N: Eq + Ord + Clone, E: Eq + Clone> Eq for Graph<N, E> {}

    impl<N: Eq + Ord + Clone, E: Clone> Clone for Graph<N, E> {
        fn clone(&self) -> Graph<N, E> {
            Graph {
                nodes: self.nodes.clone(),
            }
        }
    }