Merge pull request #41 from mitchmindtree/feature/transitive-reduction

Author: azriel91

CHANGELOG.md

@@ -0,0 +1,7 @@
+# Changelog
+
+## unreleased
+
+* Add `Dag::transitive_reduce` ([#36][#36]).
+
+[#36]: https://github.com/mitchmindtree/daggy/pull/36

src/lib.rs

@@ -641,6 +641,40 @@ where
     {
         walker::Recursive::new(start, recursive_fn)
     }
+
+    fn transitive_reduce_iter(
+        &mut self,
+        curr_node: NodeIndex<Ix>,
+        ancestors: &mut Vec<NodeIndex<Ix>>,
+    ) {
+        let mut redundant_edges = Vec::new();
+        for (_, child) in self.children(curr_node).iter(self) {
+            for (edge, coparent) in self.parents(child).iter(self) {
+                if ancestors.contains(&coparent) {
+                    redundant_edges.push(edge);
+                }
+            }
+        }
+        for edge in redundant_edges {
+            self.remove_edge(edge);
+        }
+
+        ancestors.push(curr_node);
+
+        let mut children = self.children(curr_node);
+        while let Some((_, child)) = children.walk_next(self) {
+            self.transitive_reduce_iter(child, ancestors)
+        }
+
+        ancestors.pop();
+    }
+
+    /// Mutates the DAG into its [transitive reduction](https://en.wikipedia.org/wiki/Directed_acyclic_graph#Transitive_closure_and_transitive_reduction)
+    pub fn transitive_reduce(&mut self, roots: Vec<NodeIndex<Ix>>) {
+        for root in roots {
+            self.transitive_reduce_iter(root, &mut Vec::new())
+        }
+    }
 }
 
 /// After adding a new edge to the graph, we use this function immediately after to check whether

tests/transitive_reduce.rs

@@ -0,0 +1,40 @@
+extern crate daggy;
+
+use daggy::Dag;
+
+#[test]
+fn transitive_reduce() {
+    let mut dag = Dag::<&str, &str>::new();
+
+    // construct example DAG from wikipedia
+
+    let a = dag.add_node("a");
+
+    let (_, b) = dag.add_child(a, "a->b", "b");
+    let (_, c) = dag.add_child(a, "a->c", "c");
+    let (_, d) = dag.add_child(a, "a->d", "d");
+    let (_, e) = dag.add_child(a, "a->e", "e");
+
+    dag.add_edge(b, d, "b->d").unwrap();
+
+    dag.add_edge(c, d, "c->d").unwrap();
+    dag.add_edge(c, e, "c->e").unwrap();
+
+    dag.add_edge(d, e, "d->e").unwrap();
+
+    assert_eq!(dag.edge_count(), 8);
+
+    dag.transitive_reduce(vec![a]);
+
+    assert_eq!(dag.edge_count(), 5);
+
+    // test case where the alternate route from the parent is more than one node long
+
+    dag.add_edge(a, e, "a->e").unwrap();
+
+    assert_eq!(dag.edge_count(), 6);
+
+    dag.transitive_reduce(vec![a]);
+
+    assert_eq!(dag.edge_count(), 5);
+}