Support undirected graphs.

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

@@ -55,6 +55,8 @@ end Graph
 
 object Graph:
 
+  /** The empty graph. Contains no vertices and no edges.
+    */
   object Empty extends Graph:
 
     override def numberOfVertices: Size = Size.Zero

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

@@ -0,0 +1,26 @@
+package gs.graph.v0
+
+/** Undirected implementation of [[Graph]].
+  *
+  * @param numberOfVertices
+  *   The number of vertices present in this graph.
+  * @param adjacency
+  *   The [[Adjacency]] which describes this graph.
+  */
+class UndirectedGraph(
+  val numberOfVertices: Size,
+  val adjacency: Adjacency
+) extends Graph:
+  /** @inheritDocs
+    */
+  final override val disposition: GraphDisposition = GraphDisposition.Undirected
+end UndirectedGraph
+
+object UndirectedGraph:
+
+  /** @return
+    *   An empty [[UndirectedGraph]].
+    */
+  def empty(): UndirectedGraph = new UndirectedGraph(Size.Zero, Adjacency.Empty)
+
+end UndirectedGraph

modules/core/src/main/scala/gs/graph/v0/data/AnyGraphWithData.scala

@@ -0,0 +1,56 @@
+package gs.graph.v0.data
+
+import gs.graph.v0.Adjacency
+import gs.graph.v0.GraphDisposition
+import gs.graph.v0.GraphException
+import gs.graph.v0.Size
+import gs.graph.v0.Vertex
+
+/** Trait that describes _any_ graph (undirected, directed, or other
+  * specializations of those such as DAGs) that is correlated with data.
+  */
+trait AnyGraphWithData[A]:
+  /** @return
+    *   The data stored by each [[gs.graph.v0.Vertex]] in this graph.
+    */
+  def data: Vector[A]
+
+  /** @return
+    *   The number of vertices present in this graph.
+    */
+  def numberOfVertices: Size
+
+  /** @return
+    *   The [[Adjacency]] that describes this graph.
+    */
+  def adjacency: Adjacency
+
+  /** @return
+    *   The underlying disposition of this graph.
+    */
+  def disposition: GraphDisposition
+
+  /** Retrieve the data associated with some [[Vertex]].
+    *
+    * This implementation throws an exception if the input [[Vertex]] is out of
+    * range for this graph.
+    *
+    * @param vertex
+    *   The input [[Vertex]].
+    * @return
+    *   The data associated with the input [[Vertex]].
+    */
+  def dataAtUnsafe(vertex: Vertex): A =
+    if vertex < numberOfVertices then data.apply(vertex.ordinal)
+    else throw GraphException.VertexExceedsGraphBounds(vertex, numberOfVertices)
+
+  /** Retrieve the data associated with some [[Vertex]].
+    *
+    * @param vertex
+    *   The input [[Vertex]].
+    * @return
+    *   The data associated with the input [[Vertex]], or `None` if the
+    *   [[Vertex]] is out of range for this graph.
+    */
+  def dataAt(vertex: Vertex): Option[A] =
+    if vertex < numberOfVertices then Some(data.apply(vertex.ordinal)) else None

modules/core/src/main/scala/gs/graph/v0/data/DataDag.scala

@@ -13,32 +13,8 @@ final class DataDag[A] private (
   n: Size,
   a: Adjacency,
   r: Vector[Vertex]
-) extends Dag(n, a, r):
-
-  /** Retrieve the data associated with some [[Vertex]].
-    *
-    * This implementation throws an exception if the input [[Vertex]] is out of
-    * range for this graph.
-    *
-    * @param vertex
-    *   The input [[Vertex]].
-    * @return
-    *   The data associated with the input [[Vertex]].
-    */
-  def dataAtUnsafe(vertex: Vertex): A =
-    if vertex < n then data.apply(vertex.ordinal)
-    else throw GraphException.VertexExceedsGraphBounds(vertex, n)
-
-  /** Retrieve the data associated with some [[Vertex]].
-    *
-    * @param vertex
-    *   The input [[Vertex]].
-    * @return
-    *   The data associated with the input [[Vertex]], or `None` if the
-    *   [[Vertex]] is out of range for this graph.
-    */
-  def dataAt(vertex: Vertex): Option[A] =
-    if vertex < n then Some(data.apply(vertex.ordinal)) else None
+) extends Dag(n, a, r)
+      with AnyGraphWithData[A]
 
 object DataDag:
 

modules/core/src/main/scala/gs/graph/v0/data/DataDigraph.scala

@@ -13,32 +13,8 @@ final class DataDigraph[A] private (
   n: Size,
   a: Adjacency,
   r: Vector[Vertex]
-) extends Digraph(n, a, r):
-
-  /** Retrieve the data associated with some [[Vertex]].
-    *
-    * This implementation throws an exception if the input [[Vertex]] is out of
-    * range for this graph.
-    *
-    * @param vertex
-    *   The input [[Vertex]].
-    * @return
-    *   The data associated with the input [[Vertex]].
-    */
-  def dataAtUnsafe(vertex: Vertex): A =
-    if vertex < n then data.apply(vertex.ordinal)
-    else throw GraphException.VertexExceedsGraphBounds(vertex, n)
-
-  /** Retrieve the data associated with some [[Vertex]].
-    *
-    * @param vertex
-    *   The input [[Vertex]].
-    * @return
-    *   The data associated with the input [[Vertex]], or `None` if the
-    *   [[Vertex]] is out of range for this graph.
-    */
-  def dataAt(vertex: Vertex): Option[A] =
-    if vertex < n then Some(data.apply(vertex.ordinal)) else None
+) extends Digraph(n, a, r)
+      with AnyGraphWithData[A]
 
 object DataDigraph:
 

modules/core/src/main/scala/gs/graph/v0/data/UndirectedDataGraph.scala

@@ -0,0 +1,57 @@
+package gs.graph.v0.data
+
+import gs.graph.v0.Adjacency
+import gs.graph.v0.GraphException
+import gs.graph.v0.Size
+import gs.graph.v0.UndirectedGraph
+
+/** Specialization of [[UndirectedGraph]] that associates _data_ with _each
+  * [[Vertex]]_.
+  */
+final class UndirectedDataGraph[A] private (
+  val data: Vector[A],
+  n: Size,
+  a: Adjacency
+) extends UndirectedGraph(n, a)
+      with AnyGraphWithData[A]
+
+object UndirectedDataGraph:
+
+  /** @return
+    *   An empty [[UndirectedGraph]] with no data.
+    */
+  def empty[A]: UndirectedDataGraph[A] =
+    new UndirectedDataGraph[A](
+      Vector.empty,
+      Size.Zero,
+      Adjacency.Empty
+    )
+
+  /** Instantiate a new [[UndirectedDataGraph]] given an input
+    * [[UndirectedGraph]] and some data.
+    *
+    * Throws an exception if the input data vector does not exactly match the
+    * number of nodes in the graph -- each node MUST have a data element.
+    *
+    * @param graph
+    *   The input [[UndirectedGraph]].
+    * @param data
+    *   The data to associate with the [[UndirectedGraph]].
+    * @return
+    *   The new [[UndirectedDataGraph]] instance.
+    */
+  def fromUndirectedGraph[A](
+    graph: UndirectedGraph,
+    data: Vector[A]
+  ): UndirectedDataGraph[A] =
+    if graph.numberOfVertices != Size.fromVector(data) then
+      throw GraphException.DataGraphMismatch(
+        graph.numberOfVertices,
+        data.length
+      )
+    else
+      new UndirectedDataGraph(
+        data,
+        graph.numberOfVertices,
+        graph.adjacency
+      )