Implement StableGraph::edges_connecting (#521)

src/graph_impl/stable_graph/mod.rs

@@ -574,6 +574,24 @@ where
         }
     }
 
+    /// 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
@@ -1439,6 +1457,37 @@ where
     }
 }
 
+/// 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

tests/stable_graph.rs

@@ -5,6 +5,8 @@ extern crate petgraph;
 #[macro_use]
 extern crate defmac;
 
+use std::collections::HashSet;
+
 use itertools::assert_equal;
 use petgraph::algo::{kosaraju_scc, min_spanning_tree, tarjan_scc};
 use petgraph::dot::Dot;
@@ -312,6 +314,70 @@ fn iterators_undir() {
     itertools::assert_equal(g.neighbors(c), vec![]);
 }
 
+#[test]
+fn iter_multi_edges() {
+    let mut gr = StableGraph::new();
+    let a = gr.add_node("a");
+    let b = gr.add_node("b");
+    let c = gr.add_node("c");
+
+    let mut connecting_edges = HashSet::new();
+
+    gr.add_edge(a, a, ());
+    connecting_edges.insert(gr.add_edge(a, b, ()));
+    gr.add_edge(a, c, ());
+    gr.add_edge(c, b, ());
+    connecting_edges.insert(gr.add_edge(a, b, ()));
+    gr.add_edge(b, a, ());
+
+    let mut iter = gr.edges_connecting(a, b);
+
+    let edge_id = iter.next().unwrap().id();
+    assert!(connecting_edges.contains(&edge_id));
+    connecting_edges.remove(&edge_id);
+
+    let edge_id = iter.next().unwrap().id();
+    assert!(connecting_edges.contains(&edge_id));
+    connecting_edges.remove(&edge_id);
+
+    assert_eq!(None, iter.next());
+    assert!(connecting_edges.is_empty());
+}
+
+#[test]
+fn iter_multi_undirected_edges() {
+    let mut gr: StableUnGraph<_, _> = Default::default();
+    let a = gr.add_node("a");
+    let b = gr.add_node("b");
+    let c = gr.add_node("c");
+
+    let mut connecting_edges = HashSet::new();
+
+    gr.add_edge(a, a, ());
+    connecting_edges.insert(gr.add_edge(a, b, ()));
+    gr.add_edge(a, c, ());
+    gr.add_edge(c, b, ());
+    connecting_edges.insert(gr.add_edge(a, b, ()));
+    connecting_edges.insert(gr.add_edge(b, a, ()));
+
+    let mut iter = gr.edges_connecting(a, b);
+
+    let edge_id = iter.next().unwrap().id();
+    assert!(connecting_edges.contains(&edge_id));
+    connecting_edges.remove(&edge_id);
+
+    let edge_id = iter.next().unwrap().id();
+    assert!(connecting_edges.contains(&edge_id));
+    connecting_edges.remove(&edge_id);
+
+    let edge_id = iter.next().unwrap().id();
+    assert!(connecting_edges.contains(&edge_id));
+    connecting_edges.remove(&edge_id);
+
+    assert_eq!(None, iter.next());
+    assert!(connecting_edges.is_empty());
+}
+
 #[test]
 fn dot() {
     let mut gr = StableGraph::new();