build.sbt

@@ -1,4 +1,4 @@
-val scala3: String = "3.8.3"
+val scala3: String = "3.8.2"
 
 ThisBuild / scalaVersion  := scala3
 ThisBuild / versionScheme := Some("semver-spec")

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

@@ -6,51 +6,8 @@ import scala.collection.mutable.ListBuffer
 import scala.collection.mutable.Stack
 
 /** Represents a _Strongly Connected Component_ (SCC).
-  *
-  * Each SCC is guaranteed to be non-empty.
   */
-final class SCC private (val value: Set[Vertex]):
-
-  /** @inheritDocs
-    */
-  override def equals(obj: Any): Boolean =
-    obj match
-      case other: SCC => value.equals(other.value)
-      case _          => false
-
-  /** @inheritDocs
-    */
-  override def hashCode(): Int = value.hashCode()
-
-  /** @inheritDocs
-    */
-  override def toString(): String =
-    val sb = StringBuilder()
-    sb.append("[")
-    sb.append(value.mkString(","))
-    sb.append("]")
-    sb.toString()
-
-  /** @return
-    *   The number of vertices contained by this SCC.
-    */
-  def size: Int = value.size
-
-  /** @return
-    *   True if this SCC contains a single vertex.
-    */
-  def isSingle: Boolean = value.size == 1
-
-  /** @return
-    *   The set of vertices in the SCC.
-    */
-  def vertices: Set[Vertex] = value
-
-  /** Retrieve an arbitrary SCC member as the root vertex.
-    */
-  lazy val root: Vertex = value.toList.head
-
-end SCC
+opaque type SCC = Vector[Vertex]
 
 object SCC:
 
@@ -63,38 +20,32 @@ object SCC:
     * @return
     *   The new SCC.
     */
-  def apply(value: Vertex*): SCC =
-    if value.isEmpty then
-      throw new IllegalArgumentException(
-        "Empty strongly connected components (SCC) are not allowed."
-      )
-    else new SCC(value.toSet)
-
-  def of(value: Int*): SCC =
-    if value.isEmpty then
-      throw new IllegalArgumentException(
-        "Empty strongly connected components (SCC) are not allowed."
-      )
-    else new SCC(value.map(Vertex.apply).toSet)
-
-  /** Instantiate a new SCC that contains a single [[Vertex]].
-    *
-    * @param value
-    *   The input vertex.
-    * @return
-    *   The new SCC.
-    */
-  def single(value: Vertex): SCC = new SCC(Set(value))
+  def apply(value: Seq[Vertex]): SCC = value.distinct.toVector
 
   given CanEqual[SCC, SCC] = CanEqual.derived
 
+  extension (scc: SCC)
+    /** @return
+      *   The underlying value of this SCC.
+      */
+    def unwrap(): Vector[Vertex] = scc
+
+    /** @return
+      *   The vertices contained by this SCC.
+      */
+    def vertices: Vector[Vertex] = scc
+
+    /** @return
+      *   The number of vertices contained by this SCC.
+      */
+    def size: Int = scc.size
+
+    /** @return
+      *   True if this SCC contains a single vertex.
+      */
+    def isSingle: Boolean = scc.size == 1
+
   /** Alias for `findAll`.
-    *
-    * Implementation of Tarjan's Algorithm for finding all strongly connected
-    * components for some graph.
-    *
-    * This algorithm captures directed cycles. Any vertex not part of some
-    * directed cycle (e.g. every node in a DAG) forms an SCC by itself.
     *
     * @param g
     *   The graph to analyze.
@@ -106,9 +57,6 @@ object SCC:
   /** Implementation of Tarjan's Algorithm for finding all strongly connected
     * components for some graph.
     *
-    * This algorithm captures directed cycles. Any vertex not part of some
-    * directed cycle (e.g. every node in a DAG) forms an SCC by itself.
-    *
     * @param g
     *   The graph to analyze.
     * @return
@@ -195,22 +143,14 @@ object SCC:
         sccVertex = s.pop()
 
       scc.addOne(sccVertex)
-      if !scc.isEmpty then output.addOne(new SCC(scc.toSet))
-      else println("ERROR")
+      output.addOne(SCC(scc.toVector))
     else ()
 
-  /** Mutable state wrapper around an integer.
-    *
-    * @param i
-    *   The integer value.
-    */
-  final private class Counter(var i: Int):
+  final private class Counter(var i: Int = 0):
 
     def increment(): Unit =
       i += 1
 
     def get(): Int = i
 
-  end Counter
-
 end SCC

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

@@ -34,7 +34,7 @@ class UndirectedGraph(
     SCC.findAll(this)
 
   private def oneRootFromEachSCC(sccs: List[SCC]): List[Vertex] =
-    sccs.map(_.root)
+    sccs.map(_.vertices.apply(0))
 
 end UndirectedGraph
 

modules/core/src/test/scala/gs/graph/v0/TarjansTests.scala

@@ -1,102 +0,0 @@
-package gs.graph.v0
-
-import gs.graph.v0.directed.Digraph
-import munit.*
-
-class TarjansTests extends FunSuite:
-
-  test("should provide basic SCC representation") {
-    val v0 = Vertex.Zero
-    val v1 = Vertex(1)
-    val v2 = Vertex(2)
-    val s1 = SCC(v0)
-    val s2 = SCC(v0, v1, v2)
-    val s3 = SCC(v0)
-
-    assertEquals(s1, s3)
-    assertNotEquals(s1, s2)
-    assertEquals(s2.value, Set(v0, v1, v2))
-    assertEquals(s2.vertices, Set(v0, v1, v2))
-    assertEquals(s2.size, 3)
-    assertEquals(s2.isSingle, false)
-    assertEquals(s1.isSingle, true)
-  }
-
-  test("should produce an empty list of SCCs for an empty graph") {
-    val g    = UndirectedGraph.Empty
-    val sccs = SCC.tarjan(g)
-    val alt  = SCC.findAll(g)
-    assertEquals(sccs, Nil)
-    assertEquals(alt, Nil)
-  }
-
-  test(
-    "should produce a list of once SCC with one vertex for a graph with one vertex"
-  ) {
-    val g        = Graph.undirected(Size.One)
-    val sccs     = SCC.findAll(g)
-    val expected = SCC.single(Vertex.Zero)
-    assertEquals(sccs, List(expected))
-  }
-
-  /** Each vertex of a DAG is an SCC.
-    */
-  test("should identify a single SCC for a single 'line' of vertices") {
-    val n = Size(3)
-
-    // 0 -> 1 -> 2
-    val g = Digraph.fromAdjacency(
-      Adjacency.fromEdges(
-        n,
-        List(
-          Edge(0, 1),
-          Edge(1, 2)
-        )
-      )
-    )
-
-    val sccs = SCC.findAll(g)
-    assertEquals(g.roots, Vector(Vertex.Zero))
-    assertEquals(sccs.toSet, Set(SCC.of(0), SCC.of(1), SCC.of(2)))
-  }
-
-  test("should handle a graph with multiple SCCs") {
-    val n = Size(9)
-
-    // Establish discrete cycles
-    // 0 -> 1 -> 2 -> 0
-    // 3 -> 4 -> 5 -> 3
-    // 6 -> 7 -> 8 -> 6
-    val g = Digraph.fromAdjacency(
-      Adjacency.fromEdges(
-        n,
-        List(
-          // Cycle 1
-          Edge(0, 1),
-          Edge(1, 2),
-          Edge(2, 0),
-          // Cycle 2
-          Edge(3, 4),
-          Edge(4, 5),
-          Edge(5, 3),
-          // Cycle 3
-          Edge(6, 7),
-          Edge(7, 8),
-          Edge(8, 6)
-        )
-      )
-    )
-
-    val sccs = SCC.findAll(g)
-    assertEquals(g.roots, Vector.empty)
-    assertEquals(
-      sccs.toSet,
-      Set(
-        SCC.of(0, 1, 2),
-        SCC.of(3, 4, 5),
-        SCC.of(6, 7, 8)
-      )
-    )
-  }
-
-end TarjansTests

project/build.properties

@@ -1 +1 @@
-sbt.version=1.12.9
+sbt.version=1.12.8