1 files changed, 25 insertions, 2 deletions

diff --git a/src/swarm/topology/graph.rs b/src/swarm/topology/graph.rs
index 5401506..92240e9 100644
--- a/src/swarm/topology/graph.rs
+++ b/src/swarm/topology/graph.rs
@@ -35,6 +35,7 @@ pub(crate) fn schemes(addresses: &[String]) -> BTreeSet<String> {
 /// `local_transports`: on each, its neighbours among the members on it
 /// (`neighbours`), all together.
 pub(crate) fn neighbours_across(
+    topology: &dyn Topology,
     members: &BTreeMap<String, BTreeSet<String>>,
     local: &str,
     local_transports: &BTreeSet<String>,
@@ -49,7 +50,7 @@ pub(crate) fn neighbours_across(
             .filter(|(member, transports)| *member != local && transports.contains(transport))
             .map(|(member, _)| member.clone())
             .collect();
-        all.extend(neighbours(&on_it, local, k, unreachable, stable));
+        all.extend(neighbours(topology, &on_it, local, k, unreachable, stable));
     }
     all
 }
@@ -57,6 +58,7 @@ pub(crate) fn neighbours_across(
 /// The neighbours of `local` among `members` (which may or may not contain `local`),
 /// skipping members for which `unreachable` holds.
 pub(crate) fn neighbours(
+    topology: &dyn Topology,
     members: &[String],
     local: &str,
     k: usize,
@@ -65,11 +67,32 @@ pub(crate) fn neighbours(
 ) -> BTreeSet<String> {
     let mut all: Vec<String> = members.to_vec();
     all.push(local.to_owned());
-    graph(&all, k, &|m: &str| m != local && unreachable(m), stable)
+    topology
+        .graph(&all, k, &|m: &str| m != local && unreachable(m), stable)
         .remove(local)
         .unwrap_or_default()
 }
 
+/// The graph described above (`graph`): a ring by key hash, plus each member's
+/// lowest-scoring pairs, stable members preferred.
+pub(crate) struct RingAndPairs;
+
+impl Topology for RingAndPairs {
+    fn name(&self) -> &'static str {
+        "ring-and-pairs"
+    }
+
+    fn graph(
+        &self,
+        members: &[String],
+        k: usize,
+        unreachable: &dyn Fn(&str) -> bool,
+        stable: &dyn Fn(&str) -> bool,
+    ) -> BTreeMap<String, BTreeSet<String>> {
+        graph(members, k, unreachable, stable)
+    }
+}
+
 /// Every reachable member's neighbours. Hashes each member once and scores pairs by
 /// mixing the two hashes (`pair_score`), so the N² scores are a few nanoseconds each:
 /// tens of milliseconds for thousands of members. `stable` members (`swarm::stability`)