From 76367bd946a39c89580f52d0f07b242a8bcd0ac6 Mon Sep 17 00:00:00 2001
From: Chris Beall <chrisbeall@gmail.com>
Date: Thu, 10 Oct 2019 10:34:07 -0700
Subject: [PATCH] Fix Metis ordering for empty graph and single node

---
 gtsam/inference/Ordering.cpp           | 12 +++++++++++-
 gtsam/inference/Ordering.h             |  5 ++++-
 gtsam/inference/tests/testOrdering.cpp | 22 ++++++++++++++++++++++
 3 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/gtsam/inference/Ordering.cpp b/gtsam/inference/Ordering.cpp
index da61ca57e..0875755aa 100644
--- a/gtsam/inference/Ordering.cpp
+++ b/gtsam/inference/Ordering.cpp
@@ -213,11 +213,21 @@ Ordering Ordering::Metis(const MetisIndex& met) {
 #ifdef GTSAM_SUPPORT_NESTED_DISSECTION
   gttic(Ordering_METIS);
 
+  idx_t size = met.nValues();
+  if (size == 0)
+  {
+    return Ordering();
+  }
+
+  if (size == 1)
+  {
+    return Ordering(KeyVector(1, met.intToKey(0)));
+  }
+
   vector<idx_t> xadj = met.xadj();
   vector<idx_t> adj = met.adj();
   vector<idx_t> perm, iperm;
 
-  idx_t size = met.nValues();
   for (idx_t i = 0; i < size; i++) {
     perm.push_back(0);
     iperm.push_back(0);
diff --git a/gtsam/inference/Ordering.h b/gtsam/inference/Ordering.h
index f770c7da1..a2d165792 100644
--- a/gtsam/inference/Ordering.h
+++ b/gtsam/inference/Ordering.h
@@ -191,7 +191,10 @@ public:
 
   template<class FACTOR_GRAPH>
   static Ordering Metis(const FACTOR_GRAPH& graph) {
-    return Metis(MetisIndex(graph));
+    if (graph.empty())
+      return Ordering();
+    else
+      return Metis(MetisIndex(graph));
   }
 
   /// @}
diff --git a/gtsam/inference/tests/testOrdering.cpp b/gtsam/inference/tests/testOrdering.cpp
index 0d23d9655..0305218af 100644
--- a/gtsam/inference/tests/testOrdering.cpp
+++ b/gtsam/inference/tests/testOrdering.cpp
@@ -294,6 +294,28 @@ TEST(Ordering, MetisLoop) {
 }
 #endif
 /* ************************************************************************* */
+#ifdef GTSAM_SUPPORT_NESTED_DISSECTION
+TEST(Ordering, MetisEmptyGraph) {
+  SymbolicFactorGraph symbolicGraph;
+
+  Ordering actual = Ordering::Create(Ordering::METIS, symbolicGraph);
+  Ordering expected;
+  EXPECT(assert_equal(expected, actual));
+}
+#endif
+/* ************************************************************************* */
+#ifdef GTSAM_SUPPORT_NESTED_DISSECTION
+TEST(Ordering, MetisSingleNode) {
+  // create graph with a single node
+  SymbolicFactorGraph symbolicGraph;
+  symbolicGraph.push_factor(7);
+
+  Ordering actual = Ordering::Create(Ordering::METIS, symbolicGraph);
+  Ordering expected = Ordering(list_of(7));
+  EXPECT(assert_equal(expected, actual));
+}
+#endif
+/* ************************************************************************* */
 TEST(Ordering, Create) {
 
   // create chain graph