WIP: Adding streaming traversal.

modules/core/src/main/scala/gs/graph/v0/Adjacency.scala

@@ -81,6 +81,11 @@ object Adjacency:
     */
   final val Empty: Adjacency = new Adjacency(Vector.empty)
 
+  /** @return
+    *   The single-vertex [[Adjacency]] list.
+    */
+  final val Single: Adjacency = new Adjacency(Vector(Vector.empty))
+
   /** Calculate an [[Adjacency]] from some collection of [[Edge]].
     *
     * @param numberOfVertices

modules/core/src/main/scala/gs/graph/v0/UndirectedGraph.scala

@@ -27,6 +27,13 @@ object UndirectedGraph:
   /** @return
     *   An empty [[UndirectedGraph]].
     */
-  def empty(): UndirectedGraph = new UndirectedGraph(Size.Zero, Adjacency.Empty)
+  final val Empty: UndirectedGraph =
+    new UndirectedGraph(Size.Zero, Adjacency.Empty)
+
+  /** @return
+    *   An [[UndirectedGraph]] with a single vertex.
+    */
+  final val Single: UndirectedGraph =
+    new UndirectedGraph(Size.One, Adjacency.Single)
 
 end UndirectedGraph

modules/core/src/main/scala/gs/graph/v0/directed/Digraph.scala

@@ -46,13 +46,14 @@ object Digraph:
   /** @return
     *   An empty [[Digraph]].
     */
-  def empty(): Digraph = new Digraph(Size.Zero, Adjacency.Empty, Vector.empty)
+  final val Empty: Digraph =
+    new Digraph(Size.Zero, Adjacency.Empty, Vector.empty)
 
   /** @return
-    *   A [[Digraph]] with a single [[Vertex]] and no edges.
+    *   A [[Digraph]] with one vertex.
     */
-  def single(): Digraph =
-    new Digraph(Size.One, Adjacency(Vector(Vector.empty)), Vector(Vertex.Zero))
+  final val Single: Digraph =
+    new Digraph(Size.One, Adjacency.Single, Vector(Vertex.Zero))
 
   def fromEdges(
     numberOfVertices: Size,

modules/core/src/test/scala/gs/graph/v0/directed/DagTests.scala

@@ -9,14 +9,14 @@ import munit.*
 class DagTests extends FunSuite:
 
   test("should validate an empty graph") {
-    Dag.validate(Digraph.empty()) match
-      case Right(dag) => assertEquals(dag, Digraph.empty())
+    Dag.validate(Digraph.Empty) match
+      case Right(dag) => assertEquals(dag, Digraph.Empty)
       case _ => fail("Expected the empty graph to be validated as a DAG.")
   }
 
   test("should validate a graph with one node") {
-    Dag.validate(Digraph.single()) match
-      case Right(dag) => assertEquals(dag, Digraph.single())
+    Dag.validate(Digraph.Single) match
+      case Right(dag) => assertEquals(dag, Digraph.Single)
       case _          =>
         fail("Expected a graph with one vertex to be validated as a DAG.")
   }

modules/fs2/src/main/scala/gs/graph/v0/fs2/GraphTraversalFs2.scala

@@ -14,6 +14,8 @@ object GraphTraversalFs2:
     graph: Graph,
     visit: Vertex => F[Out]
   ): Stream[F, Out] =
+    if graph.isEmpty then Stream.empty
+    else
       val state = new DfsState(graph.numberOfVertices)
       graph
         .selectRoots()

modules/fs2/src/test/scala/gs/graph/v0/fs2/Fs2DfsTests.scala

@@ -0,0 +1,43 @@
+package gs.graph.v0.fs2
+
+import cats.effect.IO
+import cats.effect.unsafe.IORuntime
+import gs.graph.v0.UndirectedGraph
+import gs.graph.v0.Vertex
+import gs.graph.v0.directed.Digraph
+import munit.*
+
+class Fs2DfsTests extends FunSuite:
+  given IORuntime = IORuntime.global
+
+  private def iotest(
+    name: String
+  )(
+    body: => IO[Unit]
+  )(
+    using
+    Location
+  ): Unit =
+    test(name)(body.unsafeRunSync())
+
+  iotest("(DFS) should return an empty stream for an empty graph") {
+    val s = GraphTraversalFs2.dfs(
+      UndirectedGraph.Empty,
+      _ => IO.raiseError(IllegalStateException("Should not reach this point."))
+    )
+    s.compile.last.map(result => assertEquals(result, None))
+  }
+
+  iotest(
+    "(DFS) should return a stream of one for a graph with a single vertex"
+  ) {
+    val s1 = GraphTraversalFs2.dfs(UndirectedGraph.Single, v => IO(v))
+    val s2 = GraphTraversalFs2.dfs(Digraph.Single, v => IO(v))
+
+    for
+      r1 <- s1.compile.toList
+      r2 <- s2.compile.toList
+    yield
+      assertEquals(r1, List(Vertex.Zero))
+      assertEquals(r2, List(Vertex.Zero))
+  }