From 3d0eb1adbc568bf3666a68179daea856901d59f8 Mon Sep 17 00:00:00 2001
From: Shaw Vrana <shaw@vranix.com>
Date: Tue, 11 Jun 2019 09:25:32 -0700
Subject: [PATCH] DeleteEdge: remove tail from parent list of head vertex

Makes DeleteEdge the inverse of AddEdge.
---
 dag.go      |  5 +++++
 dag_test.go | 10 ++++++++++
 2 files changed, 15 insertions(+)

diff --git a/dag.go b/dag.go
index 6682772..c7c027a 100644
--- a/dag.go
+++ b/dag.go
@@ -118,6 +118,11 @@ func (d *DAG) DeleteEdge(tailVertex *Vertex, headVertex *Vertex) error {
 			tailVertex.Children.Remove(childVertex)
 		}
 	}
+	for _, parentVertex := range headVertex.Parents.Values() {
+		if parentVertex == tailVertex {
+			headVertex.Parents.Remove(parentVertex)
+		}
+	}
 
 	return nil
 }
diff --git a/dag_test.go b/dag_test.go
index f5233d6..9f8ebac 100644
--- a/dag_test.go
+++ b/dag_test.go
@@ -183,6 +183,16 @@ func TestDAG_DeleteEdge(t *testing.T) {
 	if size != 0 {
 		t.Fatalf("Dag expected to have 0 edges but got %d", size)
 	}
+
+	outDegree := vertex1.OutDegree()
+	if outDegree != 0 {
+		t.Fatalf("Vertex %s expected to have 0 outgoing edges but got %d", vertex1.ID, outDegree)
+	}
+	inDegree := vertex2.InDegree()
+	if inDegree != 0 {
+		t.Fatalf("Vertex %s expected to have 0 incoming edges but got %d", vertex2.ID, inDegree)
+	}
+
 }
 
 func TestDAG_GetVertex(t *testing.T) {