From 24b08a332d7c5ae2bf6b1debfd89bdc92395de0d Mon Sep 17 00:00:00 2001
From: James Shubin <james@shubin.ca>
Date: Tue, 12 Jun 2018 17:41:32 -0400
Subject: [PATCH] pgraph: Handle empty graphs when merging two

In case we choose to add an empty (nil) graph, handle it safely. This
could allow us to return nil in a lang/structs Graph method without
issue.
---
 pgraph/subgraph.go | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/pgraph/subgraph.go b/pgraph/subgraph.go
index 917f16b4..df1df587 100644
--- a/pgraph/subgraph.go
+++ b/pgraph/subgraph.go
@@ -66,6 +66,9 @@ func (g *Graph) AddEdgeGraphVertexLight(graph *Graph, vertex Vertex, edgeGenFn f
 // or from the vertices with an indegree or outdegree equal to zero depending on
 // if we specified reverse or not.
 func (g *Graph) addEdgeVertexGraphHelper(vertex Vertex, graph *Graph, edgeGenFn func(v1, v2 Vertex) Edge, reverse, light bool) {
+	if graph == nil {
+		return // if the graph is empty, there's nothing to do!
+	}
 
 	var degree map[Vertex]int // compute all of the in/outdegree's if needed
 	if light && reverse {