diff --git a/internal/ceres/graph_test.cc b/internal/ceres/graph_test.cc
new file mode 100644
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+++ b/internal/ceres/graph_test.cc
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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2015 Google Inc. All rights reserved.
+// http://ceres-solver.org/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// * Redistributions of source code must retain the above copyright notice,
+//   this list of conditions and the following disclaimer.
+// * Redistributions in binary form must reproduce the above copyright notice,
+//   this list of conditions and the following disclaimer in the documentation
+//   and/or other materials provided with the distribution.
+// * Neither the name of Google Inc. nor the names of its contributors may be
+//   used to endorse or promote products derived from this software without
+//   specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+// POSSIBILITY OF SUCH DAMAGE.
+//
+// Author: sameeragarwal@google.com (Sameer Agarwal)
+
+#include "ceres/graph.h"
+
+#include <unordered_set>
+#include "gtest/gtest.h"
+
+namespace ceres {
+namespace internal {
+
+TEST(Graph, EmptyGraph) {
+  Graph<int> graph;
+  EXPECT_EQ(graph.vertices().size(), 0);
+}
+
+TEST(Graph, AddVertexAndEdge) {
+  Graph<int> graph;
+  graph.AddVertex(0);
+  graph.AddVertex(1);
+  graph.AddEdge(0, 1);
+
+  const std::unordered_set<int>& vertices = graph.vertices();
+  EXPECT_EQ(vertices.size(), 2);
+  EXPECT_EQ(graph.Neighbors(0).size(), 1);
+  EXPECT_EQ(graph.Neighbors(1).size(), 1);
+}
+
+TEST(Graph, AddVertexIdempotence) {
+  Graph<int> graph;
+  graph.AddVertex(0);
+  graph.AddVertex(1);
+  graph.AddEdge(0, 1);
+
+  const std::unordered_set<int>& vertices = graph.vertices();
+
+  EXPECT_EQ(vertices.size(), 2);
+
+  // Try adding the vertex again with a new weight.
+  graph.AddVertex(0);
+  EXPECT_EQ(vertices.size(), 2);
+
+  // Rest of the graph remains the same.
+  EXPECT_EQ(graph.Neighbors(0).size(), 1);
+  EXPECT_EQ(graph.Neighbors(1).size(), 1);
+}
+
+TEST(Graph, DieOnNonExistentVertex) {
+  Graph<int> graph;
+  graph.AddVertex(0);
+  graph.AddVertex(1);
+  graph.AddEdge(0, 1);
+
+  EXPECT_DEATH_IF_SUPPORTED(graph.Neighbors(2), "key not found");
+}
+
+TEST(WeightedGraph, EmptyGraph) {
+  WeightedGraph<int> graph;
+  EXPECT_EQ(graph.vertices().size(), 0);
+}
+
+TEST(WeightedGraph, AddVertexAndEdge) {
+  WeightedGraph<int> graph;
+  graph.AddVertex(0, 1.0);
+  graph.AddVertex(1, 2.0);
+  graph.AddEdge(0, 1, 0.5);
+
+  const std::unordered_set<int>& vertices = graph.vertices();
+  EXPECT_EQ(vertices.size(), 2);
+  EXPECT_EQ(graph.VertexWeight(0), 1.0);
+  EXPECT_EQ(graph.VertexWeight(1), 2.0);
+  EXPECT_EQ(graph.Neighbors(0).size(), 1);
+  EXPECT_EQ(graph.Neighbors(1).size(), 1);
+  EXPECT_EQ(graph.EdgeWeight(0, 1), 0.5);
+  EXPECT_EQ(graph.EdgeWeight(1, 0), 0.5);
+}
+
+TEST(WeightedGraph, AddVertexIdempotence) {
+  WeightedGraph<int> graph;
+  graph.AddVertex(0, 1.0);
+  graph.AddVertex(1, 2.0);
+  graph.AddEdge(0, 1, 0.5);
+
+  const std::unordered_set<int>& vertices = graph.vertices();
+
+  EXPECT_EQ(vertices.size(), 2);
+
+  // Try adding the vertex again with a new weight.
+  graph.AddVertex(0, 3.0);
+  EXPECT_EQ(vertices.size(), 2);
+
+  // The vertex weight is reset.
+  EXPECT_EQ(graph.VertexWeight(0), 3.0);
+
+  // Rest of the graph remains the same.
+  EXPECT_EQ(graph.VertexWeight(1), 2.0);
+  EXPECT_EQ(graph.Neighbors(0).size(), 1);
+  EXPECT_EQ(graph.Neighbors(1).size(), 1);
+  EXPECT_EQ(graph.EdgeWeight(0, 1), 0.5);
+  EXPECT_EQ(graph.EdgeWeight(1, 0), 0.5);
+}
+
+TEST(WeightedGraph, DieOnNonExistentVertex) {
+  WeightedGraph<int> graph;
+  graph.AddVertex(0, 1.0);
+  graph.AddVertex(1, 2.0);
+  graph.AddEdge(0, 1, 0.5);
+
+  EXPECT_DEATH_IF_SUPPORTED(graph.VertexWeight(2), "key not found");
+  EXPECT_DEATH_IF_SUPPORTED(graph.Neighbors(2), "key not found");
+}
+
+TEST(WeightedGraph, NonExistentEdge) {
+  WeightedGraph<int> graph;
+  graph.AddVertex(0, 1.0);
+  graph.AddVertex(1, 2.0);
+  graph.AddEdge(0, 1, 0.5);
+
+  // Default value for non-existent edges is 0.
+  EXPECT_EQ(graph.EdgeWeight(2, 3), 0);
+}
+
+}  // namespace internal
+}  // namespace ceres