Create a simple generic GraphViz renderer for DiGraph (#1034)

* Create a simple generic graphviz renderer for DiGraph
There are three basic kinds
- A simple default renderer
- A ranked renderer that places nodes in columns based on depth from sources
- A sub-graph render for graphs that contain a loop
  - Renders just nodes that are part of first loop found
  - Plus the neighbors of the loop
  - Loop edges are shown in red.

* Create a simple generic graphviz renderer for DiGraph
- Moved the graph loop finder into DiGraph
- Fixed scala doc per Edward's comments

1 files changed, 24 insertions, 0 deletions

diff --git a/src/main/scala/firrtl/graph/DiGraph.scala b/src/main/scala/firrtl/graph/DiGraph.scala
index 9cfcad52..1cac34b5 100644
--- a/src/main/scala/firrtl/graph/DiGraph.scala
+++ b/src/main/scala/firrtl/graph/DiGraph.scala
@@ -109,6 +109,30 @@ class DiGraph[T] private[graph] (private[graph] val edges: LinkedHashMap[T, Link
     order.reverse.toSeq
   }
 
+  /**
+    * Finds a Seq of Nodes that form a loop
+    * @param node Node to start loop path search from.
+    * @return     The found Seq, the Seq is empty if there is no loop
+    */
+  def findLoopAtNode(node: T): Seq[T] = {
+    var foundPath = Seq.empty[T]
+    getEdges(node).exists { vertex =>
+      try {
+        foundPath = path(vertex, node, blacklist = Set.empty)
+        true
+      }
+      catch {
+        case _: PathNotFoundException =>
+          foundPath = Seq.empty[T]
+          false
+        case t: Throwable =>
+          throw t
+
+      }
+    }
+    foundPath
+  }
+
   /** Performs breadth-first search on the directed graph
     *
     * @param root the start node