chore: add vendor directory

1 files changed, 2411 insertions, 0 deletions

diff --git a/vendor/petgraph/src/graph_impl/mod.rs b/vendor/petgraph/src/graph_impl/mod.rs
new file mode 100644
index 00000000..3a123f72
--- /dev/null
+++ b/vendor/petgraph/src/graph_impl/mod.rs
@@ -0,0 +1,2411 @@
+use std::cmp;
+use std::fmt;
+use std::hash::Hash;
+use std::iter;
+use std::marker::PhantomData;
+use std::mem::size_of;
+use std::ops::{Index, IndexMut, Range};
+use std::slice;
+
+use fixedbitset::FixedBitSet;
+
+use crate::{Directed, Direction, EdgeType, Incoming, IntoWeightedEdge, Outgoing, Undirected};
+
+use crate::iter_format::{DebugMap, IterFormatExt, NoPretty};
+
+use crate::util::enumerate;
+use crate::visit;
+
+#[cfg(feature = "serde-1")]
+mod serialization;
+
+/// The default integer type for graph indices.
+/// `u32` is the default to reduce the size of the graph's data and improve
+/// performance in the common case.
+///
+/// Used for node and edge indices in `Graph` and `StableGraph`, used
+/// for node indices in `Csr`.
+pub type DefaultIx = u32;
+
+/// Trait for the unsigned integer type used for node and edge indices.
+///
+/// # Safety
+///
+/// Marked `unsafe` because: the trait must faithfully preserve
+/// and convert index values.
+pub unsafe trait IndexType: Copy + Default + Hash + Ord + fmt::Debug + 'static {
+    fn new(x: usize) -> Self;
+    fn index(&self) -> usize;
+    fn max() -> Self;
+}
+
+unsafe impl IndexType for usize {
+    #[inline(always)]
+    fn new(x: usize) -> Self {
+        x
+    }
+    #[inline(always)]
+    fn index(&self) -> Self {
+        *self
+    }
+    #[inline(always)]
+    fn max() -> Self {
+        ::std::usize::MAX
+    }
+}
+
+unsafe impl IndexType for u32 {
+    #[inline(always)]
+    fn new(x: usize) -> Self {
+        x as u32
+    }
+    #[inline(always)]
+    fn index(&self) -> usize {
+        *self as usize
+    }
+    #[inline(always)]
+    fn max() -> Self {
+        ::std::u32::MAX
+    }
+}
+
+unsafe impl IndexType for u16 {
+    #[inline(always)]
+    fn new(x: usize) -> Self {
+        x as u16
+    }
+    #[inline(always)]
+    fn index(&self) -> usize {
+        *self as usize
+    }
+    #[inline(always)]
+    fn max() -> Self {
+        ::std::u16::MAX
+    }
+}
+
+unsafe impl IndexType for u8 {
+    #[inline(always)]
+    fn new(x: usize) -> Self {
+        x as u8
+    }
+    #[inline(always)]
+    fn index(&self) -> usize {
+        *self as usize
+    }
+    #[inline(always)]
+    fn max() -> Self {
+        ::std::u8::MAX
+    }
+}
+
+/// Node identifier.
+#[derive(Copy, Clone, Default, PartialEq, PartialOrd, Eq, Ord, Hash)]
+pub struct NodeIndex<Ix = DefaultIx>(Ix);
+
+impl<Ix: IndexType> NodeIndex<Ix> {
+    #[inline]
+    pub fn new(x: usize) -> Self {
+        NodeIndex(IndexType::new(x))
+    }
+
+    #[inline]
+    pub fn index(self) -> usize {
+        self.0.index()
+    }
+
+    #[inline]
+    pub fn end() -> Self {
+        NodeIndex(IndexType::max())
+    }
+
+    fn _into_edge(self) -> EdgeIndex<Ix> {
+        EdgeIndex(self.0)
+    }
+}
+
+unsafe impl<Ix: IndexType> IndexType for NodeIndex<Ix> {
+    fn index(&self) -> usize {
+        self.0.index()
+    }
+    fn new(x: usize) -> Self {
+        NodeIndex::new(x)
+    }
+    fn max() -> Self {
+        NodeIndex(<Ix as IndexType>::max())
+    }
+}
+
+impl<Ix: IndexType> From<Ix> for NodeIndex<Ix> {
+    fn from(ix: Ix) -> Self {
+        NodeIndex(ix)
+    }
+}
+
+impl<Ix: fmt::Debug> fmt::Debug for NodeIndex<Ix> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "NodeIndex({:?})", self.0)
+    }
+}
+
+/// Short version of `NodeIndex::new`
+pub fn node_index<Ix: IndexType>(index: usize) -> NodeIndex<Ix> {
+    NodeIndex::new(index)
+}
+
+/// Short version of `EdgeIndex::new`
+pub fn edge_index<Ix: IndexType>(index: usize) -> EdgeIndex<Ix> {
+    EdgeIndex::new(index)
+}
+
+/// Edge identifier.
+#[derive(Copy, Clone, Default, PartialEq, PartialOrd, Eq, Ord, Hash)]
+pub struct EdgeIndex<Ix = DefaultIx>(Ix);
+
+impl<Ix: IndexType> EdgeIndex<Ix> {
+    #[inline]
+    pub fn new(x: usize) -> Self {
+        EdgeIndex(IndexType::new(x))
+    }
+
+    #[inline]
+    pub fn index(self) -> usize {
+        self.0.index()
+    }
+
+    /// An invalid `EdgeIndex` used to denote absence of an edge, for example
+    /// to end an adjacency list.
+    #[inline]
+    pub fn end() -> Self {
+        EdgeIndex(IndexType::max())
+    }
+
+    fn _into_node(self) -> NodeIndex<Ix> {
+        NodeIndex(self.0)
+    }
+}
+
+impl<Ix: IndexType> From<Ix> for EdgeIndex<Ix> {
+    fn from(ix: Ix) -> Self {
+        EdgeIndex(ix)
+    }
+}
+
+impl<Ix: fmt::Debug> fmt::Debug for EdgeIndex<Ix> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "EdgeIndex({:?})", self.0)
+    }
+}
+/*
+ * FIXME: Use this impl again, when we don't need to add so many bounds
+impl<Ix: IndexType> fmt::Debug for EdgeIndex<Ix>
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        try!(write!(f, "EdgeIndex("));
+        if *self == EdgeIndex::end() {
+            try!(write!(f, "End"));
+        } else {
+            try!(write!(f, "{}", self.index()));
+        }
+        write!(f, ")")
+    }
+}
+*/
+
+const DIRECTIONS: [Direction; 2] = [Outgoing, Incoming];
+
+/// The graph's node type.
+#[derive(Debug)]
+pub struct Node<N, Ix = DefaultIx> {
+    /// Associated node data.
+    pub weight: N,
+    /// Next edge in outgoing and incoming edge lists.
+    next: [EdgeIndex<Ix>; 2],
+}
+
+impl<E, Ix> Clone for Node<E, Ix>
+where
+    E: Clone,
+    Ix: Copy,
+{
+    clone_fields!(Node, weight, next,);
+}
+
+impl<N, Ix: IndexType> Node<N, Ix> {
+    /// Accessor for data structure internals: the first edge in the given direction.
+    pub fn next_edge(&self, dir: Direction) -> EdgeIndex<Ix> {
+        self.next[dir.index()]
+    }
+}
+
+/// The graph's edge type.
+#[derive(Debug)]
+pub struct Edge<E, Ix = DefaultIx> {
+    /// Associated edge data.
+    pub weight: E,
+    /// Next edge in outgoing and incoming edge lists.
+    next: [EdgeIndex<Ix>; 2],
+    /// Start and End node index
+    node: [NodeIndex<Ix>; 2],
+}
+
+impl<E, Ix> Clone for Edge<E, Ix>
+where
+    E: Clone,
+    Ix: Copy,
+{
+    clone_fields!(Edge, weight, next, node,);
+}
+
+impl<E, Ix: IndexType> Edge<E, Ix> {
+    /// Accessor for data structure internals: the next edge for the given direction.
+    pub fn next_edge(&self, dir: Direction) -> EdgeIndex<Ix> {
+        self.next[dir.index()]
+    }
+
+    /// Return the source node index.
+    pub fn source(&self) -> NodeIndex<Ix> {
+        self.node[0]
+    }
+
+    /// Return the target node index.
+    pub fn target(&self) -> NodeIndex<Ix> {
+        self.node[1]
+    }
+}
+
+/// `Graph<N, E, Ty, Ix>` is a graph datastructure using an adjacency list representation.
+///
+/// `Graph` is parameterized over:
+///
+/// - Associated data `N` for nodes and `E` for edges, called *weights*.
+///   The associated data can be of arbitrary type.
+/// - Edge type `Ty` that determines whether the graph edges are directed or undirected.
+/// - Index type `Ix`, which determines the maximum size of the graph.
+///
+/// The `Graph` is a regular Rust collection and is `Send` and `Sync` (as long
+/// as associated data `N` and `E` are).
+///
+/// The graph uses **O(|V| + |E|)** space, and allows fast node and edge insert,
+/// efficient graph search and graph algorithms.
+/// It implements **O(e')** edge lookup and edge and node removals, where **e'**
+/// is some local measure of edge count.
+/// Based on the graph datastructure used in rustc.
+///
+/// Here's an example of building a graph with directed edges, and below
+/// an illustration of how it could be rendered with graphviz (see
+/// [`Dot`](../dot/struct.Dot.html)):
+///
+/// ```
+/// use petgraph::Graph;
+///
+/// let mut deps = Graph::<&str, &str>::new();
+/// let pg = deps.add_node("petgraph");
+/// let fb = deps.add_node("fixedbitset");
+/// let qc = deps.add_node("quickcheck");
+/// let rand = deps.add_node("rand");
+/// let libc = deps.add_node("libc");
+/// deps.extend_with_edges(&[
+///     (pg, fb), (pg, qc),
+///     (qc, rand), (rand, libc), (qc, libc),
+/// ]);
+/// ```
+///
+/// ![graph-example](https://bluss.github.io/ndarray/images/graph-example.svg)
+///
+/// ### Graph Indices
+///
+/// The graph maintains indices for nodes and edges, and node and edge
+/// weights may be accessed mutably. Indices range in a compact interval, for
+/// example for *n* nodes indices are 0 to *n* - 1 inclusive.
+///
+/// `NodeIndex` and `EdgeIndex` are types that act as references to nodes and edges,
+/// but these are only stable across certain operations:
+///
+/// * **Removing nodes or edges may shift other indices.** Removing a node will
+///     force the last node to shift its index to take its place. Similarly,
+///     removing an edge shifts the index of the last edge.
+/// * Adding nodes or edges keeps indices stable.
+///
+/// The `Ix` parameter is `u32` by default. The goal is that you can ignore this parameter
+/// completely unless you need a very big graph -- then you can use `usize`.
+///
+/// * The fact that the node and edge indices in the graph each are numbered in compact
+///     intervals (from 0 to *n* - 1 for *n* nodes) simplifies some graph algorithms.
+///
+/// * You can select graph index integer type after the size of the graph. A smaller
+///     size may have better performance.
+///
+/// * Using indices allows mutation while traversing the graph, see `Dfs`,
+///     and `.neighbors(a).detach()`.
+///
+/// * You can create several graphs using the equal node indices but with
+///     differing weights or differing edges.
+///
+/// * Indices don't allow as much compile time checking as references.
+///
+pub struct Graph<N, E, Ty = Directed, Ix = DefaultIx> {
+    nodes: Vec<Node<N, Ix>>,
+    edges: Vec<Edge<E, Ix>>,
+    ty: PhantomData<Ty>,
+}
+
+/// A `Graph` with directed edges.
+///
+/// For example, an edge from *1* to *2* is distinct from an edge from *2* to
+/// *1*.
+pub type DiGraph<N, E, Ix = DefaultIx> = Graph<N, E, Directed, Ix>;
+
+/// A `Graph` with undirected edges.
+///
+/// For example, an edge between *1* and *2* is equivalent to an edge between
+/// *2* and *1*.
+pub type UnGraph<N, E, Ix = DefaultIx> = Graph<N, E, Undirected, Ix>;
+
+/// The resulting cloned graph has the same graph indices as `self`.
+impl<N, E, Ty, Ix: IndexType> Clone for Graph<N, E, Ty, Ix>
+where
+    N: Clone,
+    E: Clone,
+{
+    fn clone(&self) -> Self {
+        Graph {
+            nodes: self.nodes.clone(),
+            edges: self.edges.clone(),
+            ty: self.ty,
+        }
+    }
+
+    fn clone_from(&mut self, rhs: &Self) {
+        self.nodes.clone_from(&rhs.nodes);
+        self.edges.clone_from(&rhs.edges);
+        self.ty = rhs.ty;
+    }
+}
+
+impl<N, E, Ty, Ix> fmt::Debug for Graph<N, E, Ty, Ix>
+where
+    N: fmt::Debug,
+    E: fmt::Debug,
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let etype = if self.is_directed() {
+            "Directed"
+        } else {
+            "Undirected"
+        };
+        let mut fmt_struct = f.debug_struct("Graph");
+        fmt_struct.field("Ty", &etype);
+        fmt_struct.field("node_count", &self.node_count());
+        fmt_struct.field("edge_count", &self.edge_count());
+        if self.edge_count() > 0 {
+            fmt_struct.field(
+                "edges",
+                &self
+                    .edges
+                    .iter()
+                    .map(|e| NoPretty((e.source().index(), e.target().index())))
+                    .format(", "),
+            );
+        }
+        // skip weights if they are ZST!
+        if size_of::<N>() != 0 {
+            fmt_struct.field(
+                "node weights",
+                &DebugMap(|| self.nodes.iter().map(|n| &n.weight).enumerate()),
+            );
+        }
+        if size_of::<E>() != 0 {
+            fmt_struct.field(
+                "edge weights",
+                &DebugMap(|| self.edges.iter().map(|n| &n.weight).enumerate()),
+            );
+        }
+        fmt_struct.finish()
+    }
+}
+
+enum Pair<T> {
+    Both(T, T),
+    One(T),
+    None,
+}
+
+use std::cmp::max;
+
+/// Get mutable references at index `a` and `b`.
+fn index_twice<T>(slc: &mut [T], a: usize, b: usize) -> Pair<&mut T> {
+    if max(a, b) >= slc.len() {
+        Pair::None
+    } else if a == b {
+        Pair::One(&mut slc[max(a, b)])
+    } else {
+        // safe because a, b are in bounds and distinct
+        unsafe {
+            let ptr = slc.as_mut_ptr();
+            let ar = &mut *ptr.add(a);
+            let br = &mut *ptr.add(b);
+            Pair::Both(ar, br)
+        }
+    }
+}
+
+impl<N, E> Graph<N, E, Directed> {
+    /// Create a new `Graph` with directed edges.
+    ///
+    /// This is a convenience method. Use `Graph::with_capacity` or `Graph::default` for
+    /// a constructor that is generic in all the type parameters of `Graph`.
+    pub fn new() -> Self {
+        Graph {
+            nodes: Vec::new(),
+            edges: Vec::new(),
+            ty: PhantomData,
+        }
+    }
+}
+
+impl<N, E> Graph<N, E, Undirected> {
+    /// Create a new `Graph` with undirected edges.
+    ///
+    /// This is a convenience method. Use `Graph::with_capacity` or `Graph::default` for
+    /// a constructor that is generic in all the type parameters of `Graph`.
+    pub fn new_undirected() -> Self {
+        Graph {
+            nodes: Vec::new(),
+            edges: Vec::new(),
+            ty: PhantomData,
+        }
+    }
+}
+
+impl<N, E, Ty, Ix> Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    /// Create a new `Graph` with estimated capacity.
+    pub fn with_capacity(nodes: usize, edges: usize) -> Self {
+        Graph {
+            nodes: Vec::with_capacity(nodes),
+            edges: Vec::with_capacity(edges),
+            ty: PhantomData,
+        }
+    }
+
+    /// Return the number of nodes (vertices) in the graph.
+    ///
+    /// Computes in **O(1)** time.
+    pub fn node_count(&self) -> usize {
+        self.nodes.len()
+    }
+
+    /// Return the number of edges in the graph.
+    ///
+    /// Computes in **O(1)** time.
+    pub fn edge_count(&self) -> usize {
+        self.edges.len()
+    }
+
+    /// Whether the graph has directed edges or not.
+    #[inline]
+    pub fn is_directed(&self) -> bool {
+        Ty::is_directed()
+    }
+
+    /// Add a node (also called vertex) with associated data `weight` to the graph.
+    ///
+    /// Computes in **O(1)** time.
+    ///
+    /// Return the index of the new node.
+    ///
+    /// **Panics** if the Graph is at the maximum number of nodes for its index
+    /// type (N/A if usize).
+    pub fn add_node(&mut self, weight: N) -> NodeIndex<Ix> {
+        let node = Node {
+            weight,
+            next: [EdgeIndex::end(), EdgeIndex::end()],
+        };
+        let node_idx = NodeIndex::new(self.nodes.len());
+        // check for max capacity, except if we use usize
+        assert!(<Ix as IndexType>::max().index() == !0 || NodeIndex::end() != node_idx);
+        self.nodes.push(node);
+        node_idx
+    }
+
+    /// Access the weight for node `a`.
+    ///
+    /// If node `a` doesn't exist in the graph, return `None`.
+    /// Also available with indexing syntax: `&graph[a]`.
+    pub fn node_weight(&self, a: NodeIndex<Ix>) -> Option<&N> {
+        self.nodes.get(a.index()).map(|n| &n.weight)
+    }
+
+    /// Access the weight for node `a`, mutably.
+    ///
+    /// If node `a` doesn't exist in the graph, return `None`.
+    /// Also available with indexing syntax: `&mut graph[a]`.
+    pub fn node_weight_mut(&mut self, a: NodeIndex<Ix>) -> Option<&mut N> {
+        self.nodes.get_mut(a.index()).map(|n| &mut n.weight)
+    }
+
+    /// Add an edge from `a` to `b` to the graph, with its associated
+    /// data `weight`.
+    ///
+    /// Return the index of the new edge.
+    ///
+    /// Computes in **O(1)** time.
+    ///
+    /// **Panics** if any of the nodes don't exist.<br>
+    /// **Panics** if the Graph is at the maximum number of edges for its index
+    /// type (N/A if usize).
+    ///
+    /// **Note:** `Graph` allows adding parallel (“duplicate”) edges. If you want
+    /// to avoid this, use [`.update_edge(a, b, weight)`](#method.update_edge) instead.
+    pub fn add_edge(&mut self, a: NodeIndex<Ix>, b: NodeIndex<Ix>, weight: E) -> EdgeIndex<Ix> {
+        let edge_idx = EdgeIndex::new(self.edges.len());
+        assert!(<Ix as IndexType>::max().index() == !0 || EdgeIndex::end() != edge_idx);
+        let mut edge = Edge {
+            weight,
+            node: [a, b],
+            next: [EdgeIndex::end(); 2],
+        };
+        match index_twice(&mut self.nodes, a.index(), b.index()) {
+            Pair::None => panic!("Graph::add_edge: node indices out of bounds"),
+            Pair::One(an) => {
+                edge.next = an.next;
+                an.next[0] = edge_idx;
+                an.next[1] = edge_idx;
+            }
+            Pair::Both(an, bn) => {
+                // a and b are different indices
+                edge.next = [an.next[0], bn.next[1]];
+                an.next[0] = edge_idx;
+                bn.next[1] = edge_idx;
+            }
+        }
+        self.edges.push(edge);
+        edge_idx
+    }
+
+    /// Add or update an edge from `a` to `b`.
+    /// If the edge already exists, its weight is updated.
+    ///
+    /// Return the index of the affected edge.
+    ///
+    /// Computes in **O(e')** time, where **e'** is the number of edges
+    /// connected to `a` (and `b`, if the graph edges are undirected).
+    ///
+    /// **Panics** if any of the nodes doesn't exist.
+    pub fn update_edge(&mut self, a: NodeIndex<Ix>, b: NodeIndex<Ix>, weight: E) -> EdgeIndex<Ix> {
+        if let Some(ix) = self.find_edge(a, b) {
+            if let Some(ed) = self.edge_weight_mut(ix) {
+                *ed = weight;
+                return ix;
+            }
+        }
+        self.add_edge(a, b, weight)
+    }
+
+    /// Access the weight for edge `e`.
+    ///
+    /// If edge `e` doesn't exist in the graph, return `None`.
+    /// Also available with indexing syntax: `&graph[e]`.
+    pub fn edge_weight(&self, e: EdgeIndex<Ix>) -> Option<&E> {
+        self.edges.get(e.index()).map(|ed| &ed.weight)
+    }
+
+    /// Access the weight for edge `e`, mutably.
+    ///
+    /// If edge `e` doesn't exist in the graph, return `None`.
+    /// Also available with indexing syntax: `&mut graph[e]`.
+    pub fn edge_weight_mut(&mut self, e: EdgeIndex<Ix>) -> Option<&mut E> {
+        self.edges.get_mut(e.index()).map(|ed| &mut ed.weight)
+    }
+
+    /// Access the source and target nodes for `e`.
+    ///
+    /// If edge `e` doesn't exist in the graph, return `None`.
+    pub fn edge_endpoints(&self, e: EdgeIndex<Ix>) -> Option<(NodeIndex<Ix>, NodeIndex<Ix>)> {
+        self.edges
+            .get(e.index())
+            .map(|ed| (ed.source(), ed.target()))
+    }
+
+    /// Remove `a` from the graph if it exists, and return its weight.
+    /// If it doesn't exist in the graph, return `None`.
+    ///
+    /// Apart from `a`, this invalidates the last node index in the graph
+    /// (that node will adopt the removed node index). Edge indices are
+    /// invalidated as they would be following the removal of each edge
+    /// with an endpoint in `a`.
+    ///
+    /// Computes in **O(e')** time, where **e'** is the number of affected
+    /// edges, including *n* calls to `.remove_edge()` where *n* is the number
+    /// of edges with an endpoint in `a`, and including the edges with an
+    /// endpoint in the displaced node.
+    pub fn remove_node(&mut self, a: NodeIndex<Ix>) -> Option<N> {
+        self.nodes.get(a.index())?;
+        for d in &DIRECTIONS {
+            let k = d.index();
+
+            // Remove all edges from and to this node.
+            loop {
+                let next = self.nodes[a.index()].next[k];
+                if next == EdgeIndex::end() {
+                    break;
+                }
+                let ret = self.remove_edge(next);
+                debug_assert!(ret.is_some());
+                let _ = ret;
+            }
+        }
+
+        // Use swap_remove -- only the swapped-in node is going to change
+        // NodeIndex<Ix>, so we only have to walk its edges and update them.
+
+        let node = self.nodes.swap_remove(a.index());
+
+        // Find the edge lists of the node that had to relocate.
+        // It may be that no node had to relocate, then we are done already.
+        let swap_edges = match self.nodes.get(a.index()) {
+            None => return Some(node.weight),
+            Some(ed) => ed.next,
+        };
+
+        // The swapped element's old index
+        let old_index = NodeIndex::new(self.nodes.len());
+        let new_index = a;
+
+        // Adjust the starts of the out edges, and ends of the in edges.
+        for &d in &DIRECTIONS {
+            let k = d.index();
+            let mut edges = edges_walker_mut(&mut self.edges, swap_edges[k], d);
+            while let Some(curedge) = edges.next_edge() {
+                debug_assert!(curedge.node[k] == old_index);
+                curedge.node[k] = new_index;
+            }
+        }
+        Some(node.weight)
+    }
+
+    /// For edge `e` with endpoints `edge_node`, replace links to it,
+    /// with links to `edge_next`.
+    fn change_edge_links(
+        &mut self,
+        edge_node: [NodeIndex<Ix>; 2],
+        e: EdgeIndex<Ix>,
+        edge_next: [EdgeIndex<Ix>; 2],
+    ) {
+        for &d in &DIRECTIONS {
+            let k = d.index();
+            let node = match self.nodes.get_mut(edge_node[k].index()) {
+                Some(r) => r,
+                None => {
+                    debug_assert!(
+                        false,
+                        "Edge's endpoint dir={:?} index={:?} not found",
+                        d, edge_node[k]
+                    );
+                    return;
+                }
+            };
+            let fst = node.next[k];
+            if fst == e {
+                //println!("Updating first edge 0 for node {}, set to {}", edge_node[0], edge_next[0]);
+                node.next[k] = edge_next[k];
+            } else {
+                let mut edges = edges_walker_mut(&mut self.edges, fst, d);
+                while let Some(curedge) = edges.next_edge() {
+                    if curedge.next[k] == e {
+                        curedge.next[k] = edge_next[k];
+                        break; // the edge can only be present once in the list.
+                    }
+                }
+            }
+        }
+    }
+
+    /// Remove an edge and return its edge weight, or `None` if it didn't exist.
+    ///
+    /// Apart from `e`, this invalidates the last edge index in the graph
+    /// (that edge will adopt the removed edge index).
+    ///
+    /// Computes in **O(e')** time, where **e'** is the size of four particular edge lists, for
+    /// the vertices of `e` and the vertices of another affected edge.
+    pub fn remove_edge(&mut self, e: EdgeIndex<Ix>) -> Option<E> {
+        // every edge is part of two lists,
+        // outgoing and incoming edges.
+        // Remove it from both
+        let (edge_node, edge_next) = match self.edges.get(e.index()) {
+            None => return None,
+            Some(x) => (x.node, x.next),
+        };
+        // Remove the edge from its in and out lists by replacing it with
+        // a link to the next in the list.
+        self.change_edge_links(edge_node, e, edge_next);
+        self.remove_edge_adjust_indices(e)
+    }
+
+    fn remove_edge_adjust_indices(&mut self, e: EdgeIndex<Ix>) -> Option<E> {
+        // swap_remove the edge -- only the removed edge
+        // and the edge swapped into place are affected and need updating
+        // indices.
+        let edge = self.edges.swap_remove(e.index());
+        let swap = match self.edges.get(e.index()) {
+            // no elment needed to be swapped.
+            None => return Some(edge.weight),
+            Some(ed) => ed.node,
+        };
+        let swapped_e = EdgeIndex::new(self.edges.len());
+
+        // Update the edge lists by replacing links to the old index by references to the new
+        // edge index.
+        self.change_edge_links(swap, swapped_e, [e, e]);
+        Some(edge.weight)
+    }
+
+    /// Return an iterator of all nodes with an edge starting from `a`.
+    ///
+    /// - `Directed`: Outgoing edges from `a`.
+    /// - `Undirected`: All edges from or to `a`.
+    ///
+    /// Produces an empty iterator if the node doesn't exist.<br>
+    /// Iterator element type is `NodeIndex<Ix>`.
+    ///
+    /// Use [`.neighbors(a).detach()`][1] to get a neighbor walker that does
+    /// not borrow from the graph.
+    ///
+    /// [1]: struct.Neighbors.html#method.detach
+    pub fn neighbors(&self, a: NodeIndex<Ix>) -> Neighbors<E, Ix> {
+        self.neighbors_directed(a, Outgoing)
+    }
+
+    /// Return an iterator of all neighbors that have an edge between them and
+    /// `a`, in the specified direction.
+    /// If the graph's edges are undirected, this is equivalent to *.neighbors(a)*.
+    ///
+    /// - `Directed`, `Outgoing`: All edges from `a`.
+    /// - `Directed`, `Incoming`: All edges to `a`.
+    /// - `Undirected`: All edges from or to `a`.
+    ///
+    /// Produces an empty iterator if the node doesn't exist.<br>
+    /// Iterator element type is `NodeIndex<Ix>`.
+    ///
+    /// For a `Directed` graph, neighbors are listed in reverse order of their
+    /// addition to the graph, so the most recently added edge's neighbor is
+    /// listed first. The order in an `Undirected` graph is arbitrary.
+    ///
+    /// Use [`.neighbors_directed(a, dir).detach()`][1] to get a neighbor walker that does
+    /// not borrow from the graph.
+    ///
+    /// [1]: struct.Neighbors.html#method.detach
+    pub fn neighbors_directed(&self, a: NodeIndex<Ix>, dir: Direction) -> Neighbors<E, Ix> {
+        let mut iter = self.neighbors_undirected(a);
+        if self.is_directed() {
+            let k = dir.index();
+            iter.next[1 - k] = EdgeIndex::end();
+            iter.skip_start = NodeIndex::end();
+        }
+        iter
+    }
+
+    /// Return an iterator of all neighbors that have an edge between them and
+    /// `a`, in either direction.
+    /// If the graph's edges are undirected, this is equivalent to *.neighbors(a)*.
+    ///
+    /// - `Directed` and `Undirected`: All edges from or to `a`.
+    ///
+    /// Produces an empty iterator if the node doesn't exist.<br>
+    /// Iterator element type is `NodeIndex<Ix>`.
+    ///
+    /// Use [`.neighbors_undirected(a).detach()`][1] to get a neighbor walker that does
+    /// not borrow from the graph.
+    ///
+    /// [1]: struct.Neighbors.html#method.detach
+    ///
+    pub fn neighbors_undirected(&self, a: NodeIndex<Ix>) -> Neighbors<E, Ix> {
+        Neighbors {
+            skip_start: a,
+            edges: &self.edges,
+            next: match self.nodes.get(a.index()) {
+                None => [EdgeIndex::end(), EdgeIndex::end()],
+                Some(n) => n.next,
+            },
+        }
+    }
+
+    /// Return an iterator of all edges of `a`.
+    ///
+    /// - `Directed`: Outgoing edges from `a`.
+    /// - `Undirected`: All edges connected to `a`.
+    ///
+    /// Produces an empty iterator if the node doesn't exist.<br>
+    /// Iterator element type is `EdgeReference<E, Ix>`.
+    pub fn edges(&self, a: NodeIndex<Ix>) -> Edges<E, Ty, Ix> {
+        self.edges_directed(a, Outgoing)
+    }
+
+    /// Return an iterator of all edges of `a`, in the specified direction.
+    ///
+    /// - `Directed`, `Outgoing`: All edges from `a`.
+    /// - `Directed`, `Incoming`: All edges to `a`.
+    /// - `Undirected`, `Outgoing`: All edges connected to `a`, with `a` being the source of each
+    ///   edge.
+    /// - `Undirected`, `Incoming`: All edges connected to `a`, with `a` being the target of each
+    ///   edge.
+    ///
+    /// Produces an empty iterator if the node `a` doesn't exist.<br>
+    /// Iterator element type is `EdgeReference<E, Ix>`.
+    pub fn edges_directed(&self, a: NodeIndex<Ix>, dir: Direction) -> Edges<E, Ty, Ix> {
+        Edges {
+            skip_start: a,
+            edges: &self.edges,
+            direction: dir,
+            next: match self.nodes.get(a.index()) {
+                None => [EdgeIndex::end(), EdgeIndex::end()],
+                Some(n) => n.next,
+            },
+            ty: PhantomData,
+        }
+    }
+
+    /// Return an iterator over all the edges connecting `a` and `b`.
+    ///
+    /// - `Directed`: Outgoing edges from `a`.
+    /// - `Undirected`: All edges connected to `a`.
+    ///
+    /// Iterator element type is `EdgeReference<E, Ix>`.
+    pub fn edges_connecting(
+        &self,
+        a: NodeIndex<Ix>,
+        b: NodeIndex<Ix>,
+    ) -> EdgesConnecting<E, Ty, Ix> {
+        EdgesConnecting {
+            target_node: b,
+            edges: self.edges_directed(a, Direction::Outgoing),
+            ty: PhantomData,
+        }
+    }
+
+    /// Lookup if there is an edge from `a` to `b`.
+    ///
+    /// Computes in **O(e')** time, where **e'** is the number of edges
+    /// connected to `a` (and `b`, if the graph edges are undirected).
+    pub fn contains_edge(&self, a: NodeIndex<Ix>, b: NodeIndex<Ix>) -> bool {
+        self.find_edge(a, b).is_some()
+    }
+
+    /// Lookup an edge from `a` to `b`.
+    ///
+    /// Computes in **O(e')** time, where **e'** is the number of edges
+    /// connected to `a` (and `b`, if the graph edges are undirected).
+    pub fn find_edge(&self, a: NodeIndex<Ix>, b: NodeIndex<Ix>) -> Option<EdgeIndex<Ix>> {
+        if !self.is_directed() {
+            self.find_edge_undirected(a, b).map(|(ix, _)| ix)
+        } else {
+            match self.nodes.get(a.index()) {
+                None => None,
+                Some(node) => self.find_edge_directed_from_node(node, b),
+            }
+        }
+    }
+
+    fn find_edge_directed_from_node(
+        &self,
+        node: &Node<N, Ix>,
+        b: NodeIndex<Ix>,
+    ) -> Option<EdgeIndex<Ix>> {
+        let mut edix = node.next[0];
+        while let Some(edge) = self.edges.get(edix.index()) {
+            if edge.node[1] == b {
+                return Some(edix);
+            }
+            edix = edge.next[0];
+        }
+        None
+    }
+
+    /// Lookup an edge between `a` and `b`, in either direction.
+    ///
+    /// If the graph is undirected, then this is equivalent to `.find_edge()`.
+    ///
+    /// Return the edge index and its directionality, with `Outgoing` meaning
+    /// from `a` to `b` and `Incoming` the reverse,
+    /// or `None` if the edge does not exist.
+    pub fn find_edge_undirected(
+        &self,
+        a: NodeIndex<Ix>,
+        b: NodeIndex<Ix>,
+    ) -> Option<(EdgeIndex<Ix>, Direction)> {
+        match self.nodes.get(a.index()) {
+            None => None,
+            Some(node) => self.find_edge_undirected_from_node(node, b),
+        }
+    }
+
+    fn find_edge_undirected_from_node(
+        &self,
+        node: &Node<N, Ix>,
+        b: NodeIndex<Ix>,
+    ) -> Option<(EdgeIndex<Ix>, Direction)> {
+        for &d in &DIRECTIONS {
+            let k = d.index();
+            let mut edix = node.next[k];
+            while let Some(edge) = self.edges.get(edix.index()) {
+                if edge.node[1 - k] == b {
+                    return Some((edix, d));
+                }
+                edix = edge.next[k];
+            }
+        }
+        None
+    }
+
+    /// Return an iterator over either the nodes without edges to them
+    /// (`Incoming`) or from them (`Outgoing`).
+    ///
+    /// An *internal* node has both incoming and outgoing edges.
+    /// The nodes in `.externals(Incoming)` are the source nodes and
+    /// `.externals(Outgoing)` are the sinks of the graph.
+    ///
+    /// For a graph with undirected edges, both the sinks and the sources are
+    /// just the nodes without edges.
+    ///
+    /// The whole iteration computes in **O(|V|)** time.
+    pub fn externals(&self, dir: Direction) -> Externals<N, Ty, Ix> {
+        Externals {
+            iter: self.nodes.iter().enumerate(),
+            dir,
+            ty: PhantomData,
+        }
+    }
+
+    /// Return an iterator over the node indices of the graph.
+    ///
+    /// For example, in a rare case where a graph algorithm were not applicable,
+    /// the following code will iterate through all nodes to find a
+    /// specific index:
+    ///
+    /// ```
+    /// # use petgraph::Graph;
+    /// # let mut g = Graph::<&str, i32>::new();
+    /// # g.add_node("book");
+    /// let index = g.node_indices().find(|i| g[*i] == "book").unwrap();
+    /// ```
+    pub fn node_indices(&self) -> NodeIndices<Ix> {
+        NodeIndices {
+            r: 0..self.node_count(),
+            ty: PhantomData,
+        }
+    }
+
+    /// Return an iterator yielding mutable access to all node weights.
+    ///
+    /// The order in which weights are yielded matches the order of their
+    /// node indices.
+    pub fn node_weights_mut(&mut self) -> NodeWeightsMut<N, Ix> {
+        NodeWeightsMut {
+            nodes: self.nodes.iter_mut(),
+        }
+    }
+
+    /// Return an iterator yielding immutable access to all node weights.
+    ///
+    /// The order in which weights are yielded matches the order of their
+    /// node indices.
+    pub fn node_weights(&self) -> NodeWeights<N, Ix> {
+        NodeWeights {
+            nodes: self.nodes.iter(),
+        }
+    }
+
+    /// Return an iterator over the edge indices of the graph
+    pub fn edge_indices(&self) -> EdgeIndices<Ix> {
+        EdgeIndices {
+            r: 0..self.edge_count(),
+            ty: PhantomData,
+        }
+    }
+
+    /// Create an iterator over all edges, in indexed order.
+    ///
+    /// Iterator element type is `EdgeReference<E, Ix>`.
+    pub fn edge_references(&self) -> EdgeReferences<E, Ix> {
+        EdgeReferences {
+            iter: self.edges.iter().enumerate(),
+        }
+    }
+
+    /// Return an iterator yielding immutable access to all edge weights.
+    ///
+    /// The order in which weights are yielded matches the order of their
+    /// edge indices.
+    pub fn edge_weights(&self) -> EdgeWeights<E, Ix> {
+        EdgeWeights {
+            edges: self.edges.iter(),
+        }
+    }
+    /// Return an iterator yielding mutable access to all edge weights.
+    ///
+    /// The order in which weights are yielded matches the order of their
+    /// edge indices.
+    pub fn edge_weights_mut(&mut self) -> EdgeWeightsMut<E, Ix> {
+        EdgeWeightsMut {
+            edges: self.edges.iter_mut(),
+        }
+    }
+
+    // Remaining methods are of the more internal flavour, read-only access to
+    // the data structure's internals.
+
+    /// Access the internal node array.
+    pub fn raw_nodes(&self) -> &[Node<N, Ix>] {
+        &self.nodes
+    }
+
+    /// Access the internal edge array.
+    pub fn raw_edges(&self) -> &[Edge<E, Ix>] {
+        &self.edges
+    }
+
+    #[allow(clippy::type_complexity)]
+    /// Convert the graph into a vector of Nodes and a vector of Edges
+    pub fn into_nodes_edges(self) -> (Vec<Node<N, Ix>>, Vec<Edge<E, Ix>>) {
+        (self.nodes, self.edges)
+    }
+
+    /// Accessor for data structure internals: the first edge in the given direction.
+    pub fn first_edge(&self, a: NodeIndex<Ix>, dir: Direction) -> Option<EdgeIndex<Ix>> {
+        match self.nodes.get(a.index()) {
+            None => None,
+            Some(node) => {
+                let edix = node.next[dir.index()];
+                if edix == EdgeIndex::end() {
+                    None
+                } else {
+                    Some(edix)
+                }
+            }
+        }
+    }
+
+    /// Accessor for data structure internals: the next edge for the given direction.
+    pub fn next_edge(&self, e: EdgeIndex<Ix>, dir: Direction) -> Option<EdgeIndex<Ix>> {
+        match self.edges.get(e.index()) {
+            None => None,
+            Some(node) => {
+                let edix = node.next[dir.index()];
+                if edix == EdgeIndex::end() {
+                    None
+                } else {
+                    Some(edix)
+                }
+            }
+        }
+    }
+
+    /// Index the `Graph` by two indices, any combination of
+    /// node or edge indices is fine.
+    ///
+    /// **Panics** if the indices are equal or if they are out of bounds.
+    ///
+    /// ```
+    /// use petgraph::{Graph, Incoming};
+    /// use petgraph::visit::Dfs;
+    ///
+    /// let mut gr = Graph::new();
+    /// let a = gr.add_node(0.);
+    /// let b = gr.add_node(0.);
+    /// let c = gr.add_node(0.);
+    /// gr.add_edge(a, b, 3.);
+    /// gr.add_edge(b, c, 2.);
+    /// gr.add_edge(c, b, 1.);
+    ///
+    /// // walk the graph and sum incoming edges into the node weight
+    /// let mut dfs = Dfs::new(&gr, a);
+    /// while let Some(node) = dfs.next(&gr) {
+    ///     // use a walker -- a detached neighbors iterator
+    ///     let mut edges = gr.neighbors_directed(node, Incoming).detach();
+    ///     while let Some(edge) = edges.next_edge(&gr) {
+    ///         let (nw, ew) = gr.index_twice_mut(node, edge);
+    ///         *nw += *ew;
+    ///     }
+    /// }
+    ///
+    /// // check the result
+    /// assert_eq!(gr[a], 0.);
+    /// assert_eq!(gr[b], 4.);
+    /// assert_eq!(gr[c], 2.);
+    /// ```
+    pub fn index_twice_mut<T, U>(
+        &mut self,
+        i: T,
+        j: U,
+    ) -> (
+        &mut <Self as Index<T>>::Output,
+        &mut <Self as Index<U>>::Output,
+    )
+    where
+        Self: IndexMut<T> + IndexMut<U>,
+        T: GraphIndex,
+        U: GraphIndex,
+    {
+        assert!(T::is_node_index() != U::is_node_index() || i.index() != j.index());
+
+        // Allow two mutable indexes here -- they are nonoverlapping
+        unsafe {
+            let self_mut = self as *mut _;
+            (
+                <Self as IndexMut<T>>::index_mut(&mut *self_mut, i),
+                <Self as IndexMut<U>>::index_mut(&mut *self_mut, j),
+            )
+        }
+    }
+
+    /// Reverse the direction of all edges
+    pub fn reverse(&mut self) {
+        // swap edge endpoints,
+        // edge incoming / outgoing lists,
+        // node incoming / outgoing lists
+        for edge in &mut self.edges {
+            edge.node.swap(0, 1);
+            edge.next.swap(0, 1);
+        }
+        for node in &mut self.nodes {
+            node.next.swap(0, 1);
+        }
+    }
+
+    /// Remove all nodes and edges
+    pub fn clear(&mut self) {
+        self.nodes.clear();
+        self.edges.clear();
+    }
+
+    /// Remove all edges
+    pub fn clear_edges(&mut self) {
+        self.edges.clear();
+        for node in &mut self.nodes {
+            node.next = [EdgeIndex::end(), EdgeIndex::end()];
+        }
+    }
+
+    /// Return the current node and edge capacity of the graph.
+    pub fn capacity(&self) -> (usize, usize) {
+        (self.nodes.capacity(), self.edges.capacity())
+    }
+
+    /// Reserves capacity for at least `additional` more nodes to be inserted in
+    /// the graph. Graph may reserve more space to avoid frequent reallocations.
+    ///
+    /// **Panics** if the new capacity overflows `usize`.
+    pub fn reserve_nodes(&mut self, additional: usize) {
+        self.nodes.reserve(additional);
+    }
+
+    /// Reserves capacity for at least `additional` more edges to be inserted in
+    /// the graph. Graph may reserve more space to avoid frequent reallocations.
+    ///
+    /// **Panics** if the new capacity overflows `usize`.
+    pub fn reserve_edges(&mut self, additional: usize) {
+        self.edges.reserve(additional);
+    }
+
+    /// Reserves the minimum capacity for exactly `additional` more nodes to be
+    /// inserted in the graph. Does nothing if the capacity is already
+    /// sufficient.
+    ///
+    /// Prefer `reserve_nodes` if future insertions are expected.
+    ///
+    /// **Panics** if the new capacity overflows `usize`.
+    pub fn reserve_exact_nodes(&mut self, additional: usize) {
+        self.nodes.reserve_exact(additional);
+    }
+
+    /// Reserves the minimum capacity for exactly `additional` more edges to be
+    /// inserted in the graph.
+    /// Does nothing if the capacity is already sufficient.
+    ///
+    /// Prefer `reserve_edges` if future insertions are expected.
+    ///
+    /// **Panics** if the new capacity overflows `usize`.
+    pub fn reserve_exact_edges(&mut self, additional: usize) {
+        self.edges.reserve_exact(additional);
+    }
+
+    /// Shrinks the capacity of the underlying nodes collection as much as possible.
+    pub fn shrink_to_fit_nodes(&mut self) {
+        self.nodes.shrink_to_fit();
+    }
+
+    /// Shrinks the capacity of the underlying edges collection as much as possible.
+    pub fn shrink_to_fit_edges(&mut self) {
+        self.edges.shrink_to_fit();
+    }
+
+    /// Shrinks the capacity of the graph as much as possible.
+    pub fn shrink_to_fit(&mut self) {
+        self.nodes.shrink_to_fit();
+        self.edges.shrink_to_fit();
+    }
+
+    /// Keep all nodes that return `true` from the `visit` closure,
+    /// remove the others.
+    ///
+    /// `visit` is provided a proxy reference to the graph, so that
+    /// the graph can be walked and associated data modified.
+    ///
+    /// The order nodes are visited is not specified.
+    pub fn retain_nodes<F>(&mut self, mut visit: F)
+    where
+        F: FnMut(Frozen<Self>, NodeIndex<Ix>) -> bool,
+    {
+        for index in self.node_indices().rev() {
+            if !visit(Frozen(self), index) {
+                let ret = self.remove_node(index);
+                debug_assert!(ret.is_some());
+                let _ = ret;
+            }
+        }
+    }
+
+    /// Keep all edges that return `true` from the `visit` closure,
+    /// remove the others.
+    ///
+    /// `visit` is provided a proxy reference to the graph, so that
+    /// the graph can be walked and associated data modified.
+    ///
+    /// The order edges are visited is not specified.
+    pub fn retain_edges<F>(&mut self, mut visit: F)
+    where
+        F: FnMut(Frozen<Self>, EdgeIndex<Ix>) -> bool,
+    {
+        for index in self.edge_indices().rev() {
+            if !visit(Frozen(self), index) {
+                let ret = self.remove_edge(index);
+                debug_assert!(ret.is_some());
+                let _ = ret;
+            }
+        }
+    }
+
+    /// Create a new `Graph` from an iterable of edges.
+    ///
+    /// Node weights `N` are set to default values.
+    /// Edge weights `E` may either be specified in the list,
+    /// or they are filled with default values.
+    ///
+    /// Nodes are inserted automatically to match the edges.
+    ///
+    /// ```
+    /// use petgraph::Graph;
+    ///
+    /// let gr = Graph::<(), i32>::from_edges(&[
+    ///     (0, 1), (0, 2), (0, 3),
+    ///     (1, 2), (1, 3),
+    ///     (2, 3),
+    /// ]);
+    /// ```
+    pub fn from_edges<I>(iterable: I) -> Self
+    where
+        I: IntoIterator,
+        I::Item: IntoWeightedEdge<E>,
+        <I::Item as IntoWeightedEdge<E>>::NodeId: Into<NodeIndex<Ix>>,
+        N: Default,
+    {
+        let mut g = Self::with_capacity(0, 0);
+        g.extend_with_edges(iterable);
+        g
+    }
+
+    /// Extend the graph from an iterable of edges.
+    ///
+    /// Node weights `N` are set to default values.
+    /// Edge weights `E` may either be specified in the list,
+    /// or they are filled with default values.
+    ///
+    /// Nodes are inserted automatically to match the edges.
+    pub fn extend_with_edges<I>(&mut self, iterable: I)
+    where
+        I: IntoIterator,
+        I::Item: IntoWeightedEdge<E>,
+        <I::Item as IntoWeightedEdge<E>>::NodeId: Into<NodeIndex<Ix>>,
+        N: Default,
+    {
+        let iter = iterable.into_iter();
+        let (low, _) = iter.size_hint();
+        self.edges.reserve(low);
+
+        for elt in iter {
+            let (source, target, weight) = elt.into_weighted_edge();
+            let (source, target) = (source.into(), target.into());
+            let nx = cmp::max(source, target);
+            while nx.index() >= self.node_count() {
+                self.add_node(N::default());
+            }
+            self.add_edge(source, target, weight);
+        }
+    }
+
+    /// Create a new `Graph` by mapping node and
+    /// edge weights to new values.
+    ///
+    /// The resulting graph has the same structure and the same
+    /// graph indices as `self`.
+    pub fn map<'a, F, G, N2, E2>(
+        &'a self,
+        mut node_map: F,
+        mut edge_map: G,
+    ) -> Graph<N2, E2, Ty, Ix>
+    where
+        F: FnMut(NodeIndex<Ix>, &'a N) -> N2,
+        G: FnMut(EdgeIndex<Ix>, &'a E) -> E2,
+    {
+        let mut g = Graph::with_capacity(self.node_count(), self.edge_count());
+        g.nodes.extend(enumerate(&self.nodes).map(|(i, node)| Node {
+            weight: node_map(NodeIndex::new(i), &node.weight),
+            next: node.next,
+        }));
+        g.edges.extend(enumerate(&self.edges).map(|(i, edge)| Edge {
+            weight: edge_map(EdgeIndex::new(i), &edge.weight),
+            next: edge.next,
+            node: edge.node,
+        }));
+        g
+    }
+
+    /// Create a new `Graph` by mapping nodes and edges.
+    /// A node or edge may be mapped to `None` to exclude it from
+    /// the resulting graph.
+    ///
+    /// Nodes are mapped first with the `node_map` closure, then
+    /// `edge_map` is called for the edges that have not had any endpoint
+    /// removed.
+    ///
+    /// The resulting graph has the structure of a subgraph of the original graph.
+    /// If no nodes are removed, the resulting graph has compatible node
+    /// indices; if neither nodes nor edges are removed, the result has
+    /// the same graph indices as `self`.
+    pub fn filter_map<'a, F, G, N2, E2>(
+        &'a self,
+        mut node_map: F,
+        mut edge_map: G,
+    ) -> Graph<N2, E2, Ty, Ix>
+    where
+        F: FnMut(NodeIndex<Ix>, &'a N) -> Option<N2>,
+        G: FnMut(EdgeIndex<Ix>, &'a E) -> Option<E2>,
+    {
+        let mut g = Graph::with_capacity(0, 0);
+        // mapping from old node index to new node index, end represents removed.
+        let mut node_index_map = vec![NodeIndex::end(); self.node_count()];
+        for (i, node) in enumerate(&self.nodes) {
+            if let Some(nw) = node_map(NodeIndex::new(i), &node.weight) {
+                node_index_map[i] = g.add_node(nw);
+            }
+        }
+        for (i, edge) in enumerate(&self.edges) {
+            // skip edge if any endpoint was removed
+            let source = node_index_map[edge.source().index()];
+            let target = node_index_map[edge.target().index()];
+            if source != NodeIndex::end() && target != NodeIndex::end() {
+                if let Some(ew) = edge_map(EdgeIndex::new(i), &edge.weight) {
+                    g.add_edge(source, target, ew);
+                }
+            }
+        }
+        g
+    }
+
+    /// Convert the graph into either undirected or directed. No edge adjustments
+    /// are done, so you may want to go over the result to remove or add edges.
+    ///
+    /// Computes in **O(1)** time.
+    pub fn into_edge_type<NewTy>(self) -> Graph<N, E, NewTy, Ix>
+    where
+        NewTy: EdgeType,
+    {
+        Graph {
+            nodes: self.nodes,
+            edges: self.edges,
+            ty: PhantomData,
+        }
+    }
+
+    //
+    // internal methods
+    //
+    #[cfg(feature = "serde-1")]
+    /// Fix up node and edge links after deserialization
+    fn link_edges(&mut self) -> Result<(), NodeIndex<Ix>> {
+        for (edge_index, edge) in enumerate(&mut self.edges) {
+            let a = edge.source();
+            let b = edge.target();
+            let edge_idx = EdgeIndex::new(edge_index);
+            match index_twice(&mut self.nodes, a.index(), b.index()) {
+                Pair::None => return Err(if a > b { a } else { b }),
+                Pair::One(an) => {
+                    edge.next = an.next;
+                    an.next[0] = edge_idx;
+                    an.next[1] = edge_idx;
+                }
+                Pair::Both(an, bn) => {
+                    // a and b are different indices
+                    edge.next = [an.next[0], bn.next[1]];
+                    an.next[0] = edge_idx;
+                    bn.next[1] = edge_idx;
+                }
+            }
+        }
+        Ok(())
+    }
+}
+
+/// An iterator over either the nodes without edges to them or from them.
+#[derive(Debug, Clone)]
+pub struct Externals<'a, N: 'a, Ty, Ix: IndexType = DefaultIx> {
+    iter: iter::Enumerate<slice::Iter<'a, Node<N, Ix>>>,
+    dir: Direction,
+    ty: PhantomData<Ty>,
+}
+
+impl<'a, N: 'a, Ty, Ix> Iterator for Externals<'a, N, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type Item = NodeIndex<Ix>;
+    fn next(&mut self) -> Option<NodeIndex<Ix>> {
+        let k = self.dir.index();
+        loop {
+            match self.iter.next() {
+                None => return None,
+                Some((index, node)) => {
+                    if node.next[k] == EdgeIndex::end()
+                        && (Ty::is_directed() || node.next[1 - k] == EdgeIndex::end())
+                    {
+                        return Some(NodeIndex::new(index));
+                    } else {
+                        continue;
+                    }
+                }
+            }
+        }
+    }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (_, upper) = self.iter.size_hint();
+        (0, upper)
+    }
+}
+
+/// Iterator over the neighbors of a node.
+///
+/// Iterator element type is `NodeIndex<Ix>`.
+///
+/// Created with [`.neighbors()`][1], [`.neighbors_directed()`][2] or
+/// [`.neighbors_undirected()`][3].
+///
+/// [1]: struct.Graph.html#method.neighbors
+/// [2]: struct.Graph.html#method.neighbors_directed
+/// [3]: struct.Graph.html#method.neighbors_undirected
+#[derive(Debug)]
+pub struct Neighbors<'a, E: 'a, Ix: 'a = DefaultIx> {
+    /// starting node to skip over
+    skip_start: NodeIndex<Ix>,
+    edges: &'a [Edge<E, Ix>],
+    next: [EdgeIndex<Ix>; 2],
+}
+
+impl<E, Ix> Iterator for Neighbors<'_, E, Ix>
+where
+    Ix: IndexType,
+{
+    type Item = NodeIndex<Ix>;
+
+    fn next(&mut self) -> Option<NodeIndex<Ix>> {
+        // First any outgoing edges
+        match self.edges.get(self.next[0].index()) {
+            None => {}
+            Some(edge) => {
+                self.next[0] = edge.next[0];
+                return Some(edge.node[1]);
+            }
+        }
+        // Then incoming edges
+        // For an "undirected" iterator (traverse both incoming
+        // and outgoing edge lists), make sure we don't double
+        // count selfloops by skipping them in the incoming list.
+        while let Some(edge) = self.edges.get(self.next[1].index()) {
+            self.next[1] = edge.next[1];
+            if edge.node[0] != self.skip_start {
+                return Some(edge.node[0]);
+            }
+        }
+        None
+    }
+}
+
+impl<E, Ix> Clone for Neighbors<'_, E, Ix>
+where
+    Ix: IndexType,
+{
+    clone_fields!(Neighbors, skip_start, edges, next,);
+}
+
+impl<E, Ix> Neighbors<'_, E, Ix>
+where
+    Ix: IndexType,
+{
+    /// Return a “walker” object that can be used to step through the
+    /// neighbors and edges from the origin node.
+    ///
+    /// Note: The walker does not borrow from the graph, this is to allow mixing
+    /// edge walking with mutating the graph's weights.
+    pub fn detach(&self) -> WalkNeighbors<Ix> {
+        WalkNeighbors {
+            skip_start: self.skip_start,
+            next: self.next,
+        }
+    }
+}
+
+struct EdgesWalkerMut<'a, E: 'a, Ix: IndexType = DefaultIx> {
+    edges: &'a mut [Edge<E, Ix>],
+    next: EdgeIndex<Ix>,
+    dir: Direction,
+}
+
+fn edges_walker_mut<E, Ix>(
+    edges: &mut [Edge<E, Ix>],
+    next: EdgeIndex<Ix>,
+    dir: Direction,
+) -> EdgesWalkerMut<E, Ix>
+where
+    Ix: IndexType,
+{
+    EdgesWalkerMut { edges, next, dir }
+}
+
+impl<E, Ix> EdgesWalkerMut<'_, E, Ix>
+where
+    Ix: IndexType,
+{
+    fn next_edge(&mut self) -> Option<&mut Edge<E, Ix>> {
+        self.next().map(|t| t.1)
+    }
+
+    fn next(&mut self) -> Option<(EdgeIndex<Ix>, &mut Edge<E, Ix>)> {
+        let this_index = self.next;
+        let k = self.dir.index();
+        match self.edges.get_mut(self.next.index()) {
+            None => None,
+            Some(edge) => {
+                self.next = edge.next[k];
+                Some((this_index, edge))
+            }
+        }
+    }
+}
+
+impl<'a, N, E, Ty, Ix> visit::IntoEdges for &'a Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type Edges = Edges<'a, E, Ty, Ix>;
+    fn edges(self, a: Self::NodeId) -> Self::Edges {
+        self.edges(a)
+    }
+}
+
+impl<'a, N, E, Ty, Ix> visit::IntoEdgesDirected for &'a Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type EdgesDirected = Edges<'a, E, Ty, Ix>;
+    fn edges_directed(self, a: Self::NodeId, dir: Direction) -> Self::EdgesDirected {
+        self.edges_directed(a, dir)
+    }
+}
+
+/// Iterator over the edges of from or to a node
+#[derive(Debug)]
+pub struct Edges<'a, E: 'a, Ty, Ix: 'a = DefaultIx>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    /// starting node to skip over
+    skip_start: NodeIndex<Ix>,
+    edges: &'a [Edge<E, Ix>],
+
+    /// Next edge to visit.
+    next: [EdgeIndex<Ix>; 2],
+
+    /// For directed graphs: the direction to iterate in
+    /// For undirected graphs: the direction of edges
+    direction: Direction,
+    ty: PhantomData<Ty>,
+}
+
+impl<'a, E, Ty, Ix> Iterator for Edges<'a, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type Item = EdgeReference<'a, E, Ix>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        //      type        direction    |    iterate over    reverse
+        //                               |
+        //    Directed      Outgoing     |      outgoing        no
+        //    Directed      Incoming     |      incoming        no
+        //   Undirected     Outgoing     |        both       incoming
+        //   Undirected     Incoming     |        both       outgoing
+
+        // For iterate_over, "both" is represented as None.
+        // For reverse, "no" is represented as None.
+        let (iterate_over, reverse) = if Ty::is_directed() {
+            (Some(self.direction), None)
+        } else {
+            (None, Some(self.direction.opposite()))
+        };
+
+        if iterate_over.unwrap_or(Outgoing) == Outgoing {
+            let i = self.next[0].index();
+            if let Some(Edge { node, weight, next }) = self.edges.get(i) {
+                self.next[0] = next[0];
+                return Some(EdgeReference {
+                    index: edge_index(i),
+                    node: if reverse == Some(Outgoing) {
+                        swap_pair(*node)
+                    } else {
+                        *node
+                    },
+                    weight,
+                });
+            }
+        }
+
+        if iterate_over.unwrap_or(Incoming) == Incoming {
+            while let Some(Edge { node, weight, next }) = self.edges.get(self.next[1].index()) {
+                let edge_index = self.next[1];
+                self.next[1] = next[1];
+                // In any of the "both" situations, self-loops would be iterated over twice.
+                // Skip them here.
+                if iterate_over.is_none() && node[0] == self.skip_start {
+                    continue;
+                }
+
+                return Some(EdgeReference {
+                    index: edge_index,
+                    node: if reverse == Some(Incoming) {
+                        swap_pair(*node)
+                    } else {
+                        *node
+                    },
+                    weight,
+                });
+            }
+        }
+
+        None
+    }
+}
+
+/// Iterator over the multiple directed edges connecting a source node to a target node
+#[derive(Debug, Clone)]
+pub struct EdgesConnecting<'a, E: 'a, Ty, Ix: 'a = DefaultIx>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    target_node: NodeIndex<Ix>,
+    edges: Edges<'a, E, Ty, Ix>,
+    ty: PhantomData<Ty>,
+}
+
+impl<'a, E, Ty, Ix> Iterator for EdgesConnecting<'a, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type Item = EdgeReference<'a, E, Ix>;
+
+    fn next(&mut self) -> Option<EdgeReference<'a, E, Ix>> {
+        let target_node = self.target_node;
+        self.edges
+            .by_ref()
+            .find(|&edge| edge.node[1] == target_node)
+    }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (_, upper) = self.edges.size_hint();
+        (0, upper)
+    }
+}
+
+fn swap_pair<T>(mut x: [T; 2]) -> [T; 2] {
+    x.swap(0, 1);
+    x
+}
+
+impl<E, Ty, Ix> Clone for Edges<'_, E, Ty, Ix>
+where
+    Ix: IndexType,
+    Ty: EdgeType,
+{
+    fn clone(&self) -> Self {
+        Edges {
+            skip_start: self.skip_start,
+            edges: self.edges,
+            next: self.next,
+            direction: self.direction,
+            ty: self.ty,
+        }
+    }
+}
+
+/// Iterator yielding immutable access to all node weights.
+pub struct NodeWeights<'a, N: 'a, Ix: IndexType = DefaultIx> {
+    nodes: ::std::slice::Iter<'a, Node<N, Ix>>,
+}
+impl<'a, N, Ix> Iterator for NodeWeights<'a, N, Ix>
+where
+    Ix: IndexType,
+{
+    type Item = &'a N;
+
+    fn next(&mut self) -> Option<&'a N> {
+        self.nodes.next().map(|node| &node.weight)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.nodes.size_hint()
+    }
+}
+/// Iterator yielding mutable access to all node weights.
+#[derive(Debug)]
+pub struct NodeWeightsMut<'a, N: 'a, Ix: IndexType = DefaultIx> {
+    nodes: ::std::slice::IterMut<'a, Node<N, Ix>>, // TODO: change type to something that implements Clone?
+}
+
+impl<'a, N, Ix> Iterator for NodeWeightsMut<'a, N, Ix>
+where
+    Ix: IndexType,
+{
+    type Item = &'a mut N;
+
+    fn next(&mut self) -> Option<&'a mut N> {
+        self.nodes.next().map(|node| &mut node.weight)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.nodes.size_hint()
+    }
+}
+
+/// Iterator yielding immutable access to all edge weights.
+pub struct EdgeWeights<'a, E: 'a, Ix: IndexType = DefaultIx> {
+    edges: ::std::slice::Iter<'a, Edge<E, Ix>>,
+}
+
+impl<'a, E, Ix> Iterator for EdgeWeights<'a, E, Ix>
+where
+    Ix: IndexType,
+{
+    type Item = &'a E;
+
+    fn next(&mut self) -> Option<&'a E> {
+        self.edges.next().map(|edge| &edge.weight)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.edges.size_hint()
+    }
+}
+
+/// Iterator yielding mutable access to all edge weights.
+#[derive(Debug)]
+pub struct EdgeWeightsMut<'a, E: 'a, Ix: IndexType = DefaultIx> {
+    edges: ::std::slice::IterMut<'a, Edge<E, Ix>>, // TODO: change type to something that implements Clone?
+}
+
+impl<'a, E, Ix> Iterator for EdgeWeightsMut<'a, E, Ix>
+where
+    Ix: IndexType,
+{
+    type Item = &'a mut E;
+
+    fn next(&mut self) -> Option<&'a mut E> {
+        self.edges.next().map(|edge| &mut edge.weight)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.edges.size_hint()
+    }
+}
+
+/// Index the `Graph` by `NodeIndex` to access node weights.
+///
+/// **Panics** if the node doesn't exist.
+impl<N, E, Ty, Ix> Index<NodeIndex<Ix>> for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type Output = N;
+    fn index(&self, index: NodeIndex<Ix>) -> &N {
+        &self.nodes[index.index()].weight
+    }
+}
+
+/// Index the `Graph` by `NodeIndex` to access node weights.
+///
+/// **Panics** if the node doesn't exist.
+impl<N, E, Ty, Ix> IndexMut<NodeIndex<Ix>> for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    fn index_mut(&mut self, index: NodeIndex<Ix>) -> &mut N {
+        &mut self.nodes[index.index()].weight
+    }
+}
+
+/// Index the `Graph` by `EdgeIndex` to access edge weights.
+///
+/// **Panics** if the edge doesn't exist.
+impl<N, E, Ty, Ix> Index<EdgeIndex<Ix>> for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type Output = E;
+    fn index(&self, index: EdgeIndex<Ix>) -> &E {
+        &self.edges[index.index()].weight
+    }
+}
+
+/// Index the `Graph` by `EdgeIndex` to access edge weights.
+///
+/// **Panics** if the edge doesn't exist.
+impl<N, E, Ty, Ix> IndexMut<EdgeIndex<Ix>> for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    fn index_mut(&mut self, index: EdgeIndex<Ix>) -> &mut E {
+        &mut self.edges[index.index()].weight
+    }
+}
+
+/// Create a new empty `Graph`.
+impl<N, E, Ty, Ix> Default for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    fn default() -> Self {
+        Self::with_capacity(0, 0)
+    }
+}
+
+/// A `GraphIndex` is a node or edge index.
+pub trait GraphIndex: Copy {
+    #[doc(hidden)]
+    fn index(&self) -> usize;
+    #[doc(hidden)]
+    fn is_node_index() -> bool;
+}
+
+impl<Ix: IndexType> GraphIndex for NodeIndex<Ix> {
+    #[inline]
+    #[doc(hidden)]
+    fn index(&self) -> usize {
+        NodeIndex::index(*self)
+    }
+    #[inline]
+    #[doc(hidden)]
+    fn is_node_index() -> bool {
+        true
+    }
+}
+
+impl<Ix: IndexType> GraphIndex for EdgeIndex<Ix> {
+    #[inline]
+    #[doc(hidden)]
+    fn index(&self) -> usize {
+        EdgeIndex::index(*self)
+    }
+    #[inline]
+    #[doc(hidden)]
+    fn is_node_index() -> bool {
+        false
+    }
+}
+
+/// A “walker” object that can be used to step through the edge list of a node.
+///
+/// Created with [`.detach()`](struct.Neighbors.html#method.detach).
+///
+/// The walker does not borrow from the graph, so it lets you step through
+/// neighbors or incident edges while also mutating graph weights, as
+/// in the following example:
+///
+/// ```
+/// use petgraph::{Graph, Incoming};
+/// use petgraph::visit::Dfs;
+///
+/// let mut gr = Graph::new();
+/// let a = gr.add_node(0.);
+/// let b = gr.add_node(0.);
+/// let c = gr.add_node(0.);
+/// gr.add_edge(a, b, 3.);
+/// gr.add_edge(b, c, 2.);
+/// gr.add_edge(c, b, 1.);
+///
+/// // step through the graph and sum incoming edges into the node weight
+/// let mut dfs = Dfs::new(&gr, a);
+/// while let Some(node) = dfs.next(&gr) {
+///     // use a detached neighbors walker
+///     let mut edges = gr.neighbors_directed(node, Incoming).detach();
+///     while let Some(edge) = edges.next_edge(&gr) {
+///         gr[node] += gr[edge];
+///     }
+/// }
+///
+/// // check the result
+/// assert_eq!(gr[a], 0.);
+/// assert_eq!(gr[b], 4.);
+/// assert_eq!(gr[c], 2.);
+/// ```
+pub struct WalkNeighbors<Ix> {
+    skip_start: NodeIndex<Ix>,
+    next: [EdgeIndex<Ix>; 2],
+}
+
+impl<Ix> Clone for WalkNeighbors<Ix>
+where
+    Ix: IndexType,
+{
+    fn clone(&self) -> Self {
+        WalkNeighbors {
+            skip_start: self.skip_start,
+            next: self.next,
+        }
+    }
+}
+
+impl<Ix: IndexType> WalkNeighbors<Ix> {
+    /// Step to the next edge and its endpoint node in the walk for graph `g`.
+    ///
+    /// The next node indices are always the others than the starting point
+    /// where the `WalkNeighbors` value was created.
+    /// For an `Outgoing` walk, the target nodes,
+    /// for an `Incoming` walk, the source nodes of the edge.
+    pub fn next<N, E, Ty: EdgeType>(
+        &mut self,
+        g: &Graph<N, E, Ty, Ix>,
+    ) -> Option<(EdgeIndex<Ix>, NodeIndex<Ix>)> {
+        // First any outgoing edges
+        match g.edges.get(self.next[0].index()) {
+            None => {}
+            Some(edge) => {
+                let ed = self.next[0];
+                self.next[0] = edge.next[0];
+                return Some((ed, edge.node[1]));
+            }
+        }
+        // Then incoming edges
+        // For an "undirected" iterator (traverse both incoming
+        // and outgoing edge lists), make sure we don't double
+        // count selfloops by skipping them in the incoming list.
+        while let Some(edge) = g.edges.get(self.next[1].index()) {
+            let ed = self.next[1];
+            self.next[1] = edge.next[1];
+            if edge.node[0] != self.skip_start {
+                return Some((ed, edge.node[0]));
+            }
+        }
+        None
+    }
+
+    pub fn next_node<N, E, Ty: EdgeType>(
+        &mut self,
+        g: &Graph<N, E, Ty, Ix>,
+    ) -> Option<NodeIndex<Ix>> {
+        self.next(g).map(|t| t.1)
+    }
+
+    pub fn next_edge<N, E, Ty: EdgeType>(
+        &mut self,
+        g: &Graph<N, E, Ty, Ix>,
+    ) -> Option<EdgeIndex<Ix>> {
+        self.next(g).map(|t| t.0)
+    }
+}
+
+/// Iterator over the node indices of a graph.
+#[derive(Clone, Debug)]
+pub struct NodeIndices<Ix = DefaultIx> {
+    r: Range<usize>,
+    ty: PhantomData<fn() -> Ix>,
+}
+
+impl<Ix: IndexType> Iterator for NodeIndices<Ix> {
+    type Item = NodeIndex<Ix>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.r.next().map(node_index)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.r.size_hint()
+    }
+}
+
+impl<Ix: IndexType> DoubleEndedIterator for NodeIndices<Ix> {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.r.next_back().map(node_index)
+    }
+}
+
+impl<Ix: IndexType> ExactSizeIterator for NodeIndices<Ix> {}
+
+/// Iterator over the edge indices of a graph.
+#[derive(Clone, Debug)]
+pub struct EdgeIndices<Ix = DefaultIx> {
+    r: Range<usize>,
+    ty: PhantomData<fn() -> Ix>,
+}
+
+impl<Ix: IndexType> Iterator for EdgeIndices<Ix> {
+    type Item = EdgeIndex<Ix>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.r.next().map(edge_index)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.r.size_hint()
+    }
+}
+
+impl<Ix: IndexType> DoubleEndedIterator for EdgeIndices<Ix> {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.r.next_back().map(edge_index)
+    }
+}
+
+impl<Ix: IndexType> ExactSizeIterator for EdgeIndices<Ix> {}
+
+/// Reference to a `Graph` edge.
+#[derive(Debug)]
+pub struct EdgeReference<'a, E: 'a, Ix = DefaultIx> {
+    index: EdgeIndex<Ix>,
+    node: [NodeIndex<Ix>; 2],
+    weight: &'a E,
+}
+
+impl<E, Ix: IndexType> Clone for EdgeReference<'_, E, Ix> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<E, Ix: IndexType> Copy for EdgeReference<'_, E, Ix> {}
+
+impl<E, Ix: IndexType> PartialEq for EdgeReference<'_, E, Ix>
+where
+    E: PartialEq,
+{
+    fn eq(&self, rhs: &Self) -> bool {
+        self.index == rhs.index && self.weight == rhs.weight
+    }
+}
+
+impl<N, E, Ty, Ix> visit::GraphBase for Graph<N, E, Ty, Ix>
+where
+    Ix: IndexType,
+{
+    type NodeId = NodeIndex<Ix>;
+    type EdgeId = EdgeIndex<Ix>;
+}
+
+impl<N, E, Ty, Ix> visit::Visitable for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type Map = FixedBitSet;
+    fn visit_map(&self) -> FixedBitSet {
+        FixedBitSet::with_capacity(self.node_count())
+    }
+
+    fn reset_map(&self, map: &mut Self::Map) {
+        map.clear();
+        map.grow(self.node_count());
+    }
+}
+
+impl<N, E, Ty, Ix> visit::GraphProp for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type EdgeType = Ty;
+}
+
+impl<'a, N, E: 'a, Ty, Ix> visit::IntoNodeIdentifiers for &'a Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type NodeIdentifiers = NodeIndices<Ix>;
+    fn node_identifiers(self) -> NodeIndices<Ix> {
+        Graph::node_indices(self)
+    }
+}
+
+impl<N, E, Ty, Ix> visit::NodeCount for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    fn node_count(&self) -> usize {
+        self.node_count()
+    }
+}
+
+impl<N, E, Ty, Ix> visit::NodeIndexable for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    #[inline]
+    fn node_bound(&self) -> usize {
+        self.node_count()
+    }
+    #[inline]
+    fn to_index(&self, ix: NodeIndex<Ix>) -> usize {
+        ix.index()
+    }
+    #[inline]
+    fn from_index(&self, ix: usize) -> Self::NodeId {
+        NodeIndex::new(ix)
+    }
+}
+
+impl<N, E, Ty, Ix> visit::NodeCompactIndexable for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+}
+
+impl<'a, N, E: 'a, Ty, Ix> visit::IntoNeighbors for &'a Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type Neighbors = Neighbors<'a, E, Ix>;
+    fn neighbors(self, n: NodeIndex<Ix>) -> Neighbors<'a, E, Ix> {
+        Graph::neighbors(self, n)
+    }
+}
+
+impl<'a, N, E: 'a, Ty, Ix> visit::IntoNeighborsDirected for &'a Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type NeighborsDirected = Neighbors<'a, E, Ix>;
+    fn neighbors_directed(self, n: NodeIndex<Ix>, d: Direction) -> Neighbors<'a, E, Ix> {
+        Graph::neighbors_directed(self, n, d)
+    }
+}
+
+impl<'a, N: 'a, E: 'a, Ty, Ix> visit::IntoEdgeReferences for &'a Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type EdgeRef = EdgeReference<'a, E, Ix>;
+    type EdgeReferences = EdgeReferences<'a, E, Ix>;
+    fn edge_references(self) -> Self::EdgeReferences {
+        (*self).edge_references()
+    }
+}
+
+impl<N, E, Ty, Ix> visit::EdgeCount for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    #[inline]
+    fn edge_count(&self) -> usize {
+        self.edge_count()
+    }
+}
+
+impl<'a, N, E, Ty, Ix> visit::IntoNodeReferences for &'a Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    type NodeRef = (NodeIndex<Ix>, &'a N);
+    type NodeReferences = NodeReferences<'a, N, Ix>;
+    fn node_references(self) -> Self::NodeReferences {
+        NodeReferences {
+            iter: self.nodes.iter().enumerate(),
+        }
+    }
+}
+
+/// Iterator over all nodes of a graph.
+#[derive(Debug, Clone)]
+pub struct NodeReferences<'a, N: 'a, Ix: IndexType = DefaultIx> {
+    iter: iter::Enumerate<slice::Iter<'a, Node<N, Ix>>>,
+}
+
+impl<'a, N, Ix> Iterator for NodeReferences<'a, N, Ix>
+where
+    Ix: IndexType,
+{
+    type Item = (NodeIndex<Ix>, &'a N);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.iter
+            .next()
+            .map(|(i, node)| (node_index(i), &node.weight))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
+
+impl<N, Ix> DoubleEndedIterator for NodeReferences<'_, N, Ix>
+where
+    Ix: IndexType,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.iter
+            .next_back()
+            .map(|(i, node)| (node_index(i), &node.weight))
+    }
+}
+
+impl<N, Ix> ExactSizeIterator for NodeReferences<'_, N, Ix> where Ix: IndexType {}
+
+impl<'a, Ix, E> EdgeReference<'a, E, Ix>
+where
+    Ix: IndexType,
+{
+    /// Access the edge’s weight.
+    ///
+    /// **NOTE** that this method offers a longer lifetime
+    /// than the trait (unfortunately they don't match yet).
+    pub fn weight(&self) -> &'a E {
+        self.weight
+    }
+}
+
+impl<Ix, E> visit::EdgeRef for EdgeReference<'_, E, Ix>
+where
+    Ix: IndexType,
+{
+    type NodeId = NodeIndex<Ix>;
+    type EdgeId = EdgeIndex<Ix>;
+    type Weight = E;
+
+    fn source(&self) -> Self::NodeId {
+        self.node[0]
+    }
+    fn target(&self) -> Self::NodeId {
+        self.node[1]
+    }
+    fn weight(&self) -> &E {
+        self.weight
+    }
+    fn id(&self) -> Self::EdgeId {
+        self.index
+    }
+}
+
+/// Iterator over all edges of a graph.
+#[derive(Debug, Clone)]
+pub struct EdgeReferences<'a, E: 'a, Ix: IndexType = DefaultIx> {
+    iter: iter::Enumerate<slice::Iter<'a, Edge<E, Ix>>>,
+}
+
+impl<'a, E, Ix> Iterator for EdgeReferences<'a, E, Ix>
+where
+    Ix: IndexType,
+{
+    type Item = EdgeReference<'a, E, Ix>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.iter.next().map(|(i, edge)| EdgeReference {
+            index: edge_index(i),
+            node: edge.node,
+            weight: &edge.weight,
+        })
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
+
+impl<E, Ix> DoubleEndedIterator for EdgeReferences<'_, E, Ix>
+where
+    Ix: IndexType,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.iter.next_back().map(|(i, edge)| EdgeReference {
+            index: edge_index(i),
+            node: edge.node,
+            weight: &edge.weight,
+        })
+    }
+}
+
+impl<E, Ix> ExactSizeIterator for EdgeReferences<'_, E, Ix> where Ix: IndexType {}
+
+impl<N, E, Ty, Ix> visit::EdgeIndexable for Graph<N, E, Ty, Ix>
+where
+    Ty: EdgeType,
+    Ix: IndexType,
+{
+    fn edge_bound(&self) -> usize {
+        self.edge_count()
+    }
+
+    fn to_index(&self, ix: EdgeIndex<Ix>) -> usize {
+        ix.index()
+    }
+
+    fn from_index(&self, ix: usize) -> Self::EdgeId {
+        EdgeIndex::new(ix)
+    }
+}
+
+mod frozen;
+#[cfg(feature = "stable_graph")]
+pub mod stable_graph;
+
+/// `Frozen` is a graph wrapper.
+///
+/// The `Frozen` only allows shared access (read-only) to the
+/// underlying graph `G`, but it allows mutable access to its
+/// node and edge weights.
+///
+/// This is used to ensure immutability of the graph's structure
+/// while permitting weights to be both read and written.
+///
+/// See indexing implementations and the traits `Data` and `DataMap`
+/// for read-write access to the graph's weights.
+pub struct Frozen<'a, G: 'a>(&'a mut G);