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    use std::borrow::Borrow;
    use std::collections::BTreeSet;
    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()
        }

        /// 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.
        ///
        /// 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, mut pkg: &'a N) -> Vec<(&'a N, Option<&'a E>)> {
            let mut result = vec![(pkg, None)];
            while let Some(p) = self.nodes.get(pkg).and_then(|p| {
                p.iter()
                    // Note that we can have "cycles" introduced through dev-dependency
                    // edges, so make sure we don't loop infinitely.
                    .find(|&(node, _)| result.iter().all(|p| p.0 != node))
                    .map(|(node, edge)| (node, Some(edge)))
            }) {
                result.push(p);
                pkg = p.0;
            }
            result
        }

        /// Resolves one of the paths from the given dependent package up to
        /// the root.
        ///
        /// 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, mut pkg: &'a N) -> Vec<(&'a N, Option<&'a E>)> {
            // 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!
            let mut result = vec![(pkg, None)];
            let first_pkg_depending_on = |pkg, res: &[(&N, Option<&E>)]| {
                self.nodes
                    .iter()
                    .filter(|(_, adjacent)| adjacent.contains_key(pkg))
                    // Note that we can have "cycles" introduced through dev-dependency
                    // edges, so make sure we don't loop infinitely.
                    .find(|&(node, _)| !res.iter().any(|p| p.0 == node))
                    .map(|(p, adjacent)| (p, adjacent.get(pkg)))
            };
            while let Some(p) = first_pkg_depending_on(pkg, &result) {
                result.push(p);
                pkg = p.0;
            }
            result
        }
    }

    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: 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(),
            }
        }
    }