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

@@ -32,7 +32,7 @@ final class Adjacency(val neighbors: Vector[Vector[Vertex]]):
     if vertex.ordinal >= neighbors.length then Vector.empty
     else
       neighbors.zipWithIndex
-        .filterNot {
+        .filter {
           // ignore the neighbors of the input vertex
           case (_, index) => index != vertex.ordinal
         }

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

@@ -0,0 +1,27 @@
+package gs.graph.v0
+
+class AdjacencyTests extends munit.FunSuite:
+
+  test("should provide incoming") {
+    val N  = Size(7)
+    val vs = (0 until N.value).map(Vertex(_)).toArray
+    val E  = List(
+      Edge(vs(0), vs(1)),
+      Edge(vs(0), vs(2)),
+      Edge(vs(0), vs(3)),
+      Edge(vs(1), vs(4)),
+      Edge(vs(2), vs(4)),
+      Edge(vs(3), vs(4)),
+      Edge(vs(3), vs(5)),
+      Edge(vs(4), vs(6))
+    )
+    val A = Adjacency.fromDirectedEdges(N, E)
+
+    assertEquals(A.incoming(vs(0)), Vector.empty)
+    assertEquals(A.incoming(vs(1)), Vector(vs(0)))
+    assertEquals(A.incoming(vs(2)), Vector(vs(0)))
+    assertEquals(A.incoming(vs(3)), Vector(vs(0)))
+    assertEquals(A.incoming(vs(4)), Vector(vs(1), vs(2), vs(3)))
+    assertEquals(A.incoming(vs(5)), Vector(vs(3)))
+    assertEquals(A.incoming(vs(6)), Vector(vs(4)))
+  }