Add tests for some trivial graphs

src/models.rs

@@ -117,12 +117,31 @@ impl Graph {
 mod tests {
     use super::*;
 
+    #[test]
+    fn add_vertex() {
+        let mut graph = Graph::new();
+        let v = graph.add_vertex();
+        assert_ne!(graph.add_vertex(), v, "unexpected duplicate vertex");
+    }
+
+    #[test]
+    fn vertex_count_empty() {
+        let graph = Graph::new();
+        assert_eq!(graph.vertex_count(), 0, "unexpected vertex count");
+    }
+
     #[test]
     fn vertex_count() {
         let (graph, _) = make_test_graph();
         assert_eq!(graph.vertex_count(), 10, "unexpected vertex count");
     }
 
+    #[test]
+    fn edge_count_empty() {
+        let graph = Graph::new();
+        assert_eq!(graph.edge_count(), 0, "unexpected edge count");
+    }
+
     #[test]
     fn edge_count() {
         let (graph, _) = make_test_graph();
@@ -130,7 +149,14 @@ mod tests {
     }
 
     #[test]
-    fn degrees() {
+    fn degree_zero() {
+        let mut graph = Graph::new();
+        let v = graph.add_vertex();
+        assert_eq!(graph.degree(v), 0, "unexpected non-zero degree");
+    }
+
+    #[test]
+    fn degree() {
         let (graph, vertices) = make_test_graph();
         let expected_degrees = [1, 4, 4, 2, 4, 2, 3, 3, 2, 3];
         for i in 0..graph.vertex_count() {
@@ -144,7 +170,19 @@ mod tests {
     }
 
     #[test]
-    fn adjacencies() {
+    fn are_adjacent_single_edge() {
+        let mut graph = Graph::new();
+        let v1 = graph.add_vertex();
+        let v2 = graph.add_vertex();
+        assert!(!graph.are_adjacent(v1, v2), "should not be adjacent");
+        assert!(!graph.are_adjacent(v2, v1), "should not be adjacent");
+        graph.add_edge(v1, v2);
+        assert!(graph.are_adjacent(v1, v2), "should be adjacent");
+        assert!(graph.are_adjacent(v2, v1), "should be adjacent");
+    }
+
+    #[test]
+    fn are_adjacent() {
         let (graph, vertices) = make_test_graph();
         assert!(
             graph.are_adjacent(vertices[0], vertices[1]),