Fix bug in DiGraph.reverse on an graph with one vertex, no edges (#628)

1 files changed, 4 insertions, 1 deletions

diff --git a/src/main/scala/firrtl/graph/DiGraph.scala b/src/main/scala/firrtl/graph/DiGraph.scala
index e28e53e5..ee00789e 100644
--- a/src/main/scala/firrtl/graph/DiGraph.scala
+++ b/src/main/scala/firrtl/graph/DiGraph.scala
@@ -295,7 +295,10 @@ class DiGraph[T] (val edges: Map[T, Set[T]]) extends DiGraphLike[T] {
   /** Returns a graph with all edges reversed */
   def reverse: DiGraph[T] = {
     val mdg = new MutableDiGraph[T]
-    edges foreach { case (u,edges) => edges.foreach({ v => mdg.addEdge(v,u) }) }
+    edges.foreach { case (u, edges) =>
+      mdg.addVertex(u)
+      edges.foreach(v => mdg.addEdge(v,u))
+    }
     DiGraph(mdg)
   }