Feedback arc set

Author: matth2k

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "safety-net"
-version = "0.5.1"
+version = "0.5.2"
 edition = "2024"
 license = "MIT OR Apache-2.0"
 

src/graph.rs

@@ -327,6 +327,16 @@ where
     pub fn get_graph(&self) -> &DiGraph<Node<I, String>, Edge<I, Net>> {
         &self.graph
     }
+
+    /// Iterates through a [greedy feedback arc set](https://doi.org/10.1016/0020-0190(93)90079-O) for the graph.
+    pub fn greedy_feedback_arcs(&self) -> impl Iterator<Item = Connection<I>> {
+        petgraph::algo::feedback_arc_set::greedy_feedback_arc_set(&self.graph)
+            .map(|e| match e.weight() {
+                Edge::Connection(c) => c,
+                _ => unreachable!("Outputs should be sinks"),
+            })
+            .cloned()
+    }
 }
 
 #[cfg(feature = "graph")]

tests/analysis.rs

@@ -142,6 +142,39 @@ fn test_petgraph() {
     // Outputs are a pseudo node
     assert_eq!(graph.node_count(), 4);
     assert_eq!(graph.edge_count(), 3);
+
+    // Ayclic graph should have no feedback arcs.
+    let mut arcs = petgraph.greedy_feedback_arcs();
+    assert!(arcs.next().is_none());
+}
+
+#[cfg(feature = "graph")]
+#[test]
+fn test_feedback_arcs() {
+    use safety_net::MultiDiGraph;
+
+    let netlist = divider_netlist();
+
+    let petgraph = netlist.get_analysis::<MultiDiGraph<_>>();
+    assert!(petgraph.is_ok());
+    let petgraph = petgraph.unwrap();
+
+    // Has exactly one arc
+    let mut arcs = petgraph.greedy_feedback_arcs();
+    let arc = arcs.next();
+    assert!(arc.is_some());
+    assert!(arcs.next().is_none());
+
+    // Disconnect the arc
+    let arc = arc.unwrap();
+    arc.target().disconnect();
+
+    // Should be acyclic now
+    let petgraph = netlist.get_analysis::<MultiDiGraph<_>>();
+    assert!(petgraph.is_ok());
+    let petgraph = petgraph.unwrap();
+    let mut arcs = petgraph.greedy_feedback_arcs();
+    assert!(arcs.next().is_none());
 }
 
 #[test]