chain: a prototype is added.

I have an ostensibly working prototype now.

Further tests are needed to make sure that the algorithm meets the
time complexity requirement, though.

1 files changed, 7 insertions, 3 deletions

diff --git a/graph/src/lib.rs b/graph/src/lib.rs
index 6af7889..2a0c50d 100644
--- a/graph/src/lib.rs
+++ b/graph/src/lib.rs
@@ -251,10 +251,14 @@ pub trait ParentsGraph: Graph {
 
     /// Return an iterator of parents for a node.
     fn parents_of(&self, node_id: usize) -> Result<<Self as ParentsGraph>::Iter<'_>, Error>;
-}
 
-// TODO: Design a trait of graphs which can "replace" a certain child
-// by another child.  To re-direct children, so to speak.
+    /// We need to be able to retrieve the `n`-the child, for the edge
+    /// index to be of any use.
+    fn nth_child(&self, node_id: usize, n: usize) -> Result<usize, Error>;
+
+    /// Convert an edge to a `Parent` construct.
+    fn edge_to_parent(&self, source: usize, target: usize) -> Result<Option<Parent>, Error>;
+}
 
 /// An /exended/ graph in the sense that it offers the ability to
 /// "redirect" children of a node to another node.