diff --git a/cmd/internal/graph/common.go b/cmd/internal/graph/common.go
new file mode 100644
index 0000000..92259cf
--- /dev/null
+++ b/cmd/internal/graph/common.go
@@ -0,0 +1,68 @@
+/*
+Copyright © 2025 filifa
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program. If not, see <http://www.gnu.org/licenses/>.
+*/
+package graph
+
+import (
+	"gonum.org/v1/gonum/graph"
+	"gonum.org/v1/gonum/mat"
+)
+
+type WeightedGraph interface {
+	graph.Weighted
+
+	WeightedEdges() graph.WeightedEdges
+
+	AdjacencyMatrix() *mat.Dense
+}
+
+type WeightedMatrix interface {
+	graph.Weighted
+
+	SetWeightedEdge(e graph.WeightedEdge)
+	Matrix() mat.Matrix
+}
+
+func toAdjMatrix(g WeightedGraph, adj WeightedMatrix) *mat.Dense {
+	copyEdges(g, adj)
+	matrix := mat.DenseCopyOf(adj.Matrix())
+	addSelfEdges(g, matrix)
+
+	return matrix
+}
+
+func copyEdges(g WeightedGraph, adj WeightedMatrix) {
+	for edges := g.WeightedEdges(); edges.Next(); {
+		e := edges.WeightedEdge()
+		if e.From() == e.To() {
+			continue
+		}
+
+		adj.SetWeightedEdge(e)
+	}
+}
+
+func addSelfEdges(g WeightedGraph, matrix mat.Mutable) {
+	nodes := g.Nodes()
+	for i := 0; nodes.Next(); i++ {
+		u := nodes.Node()
+
+		e := g.WeightedEdge(u.ID(), u.ID())
+		if e != nil {
+			matrix.Set(i, i, e.Weight())
+		}
+	}
+}
diff --git a/cmd/internal/graph/graph.go b/cmd/internal/graph/directed.go
similarity index 77%
rename from cmd/internal/graph/graph.go
rename to cmd/internal/graph/directed.go
index 7ab1372..8a71b06 100644
--- a/cmd/internal/graph/graph.go
+++ b/cmd/internal/graph/directed.go
@@ -36,27 +36,6 @@ func NewDirectedGraph() *DirectedGraph {
 // AdjacencyMatrix returns the graph's adjacency matrix.
 func (g *DirectedGraph) AdjacencyMatrix() *mat.Dense {
 	adj := simple.NewDirectedMatrix(g.Nodes().Len(), 0, 0, 0)
-	for edges := g.WeightedEdges(); edges.Next(); {
-		e := edges.WeightedEdge()
-		if e.From() == e.To() {
-			continue
-		}
-
-		adj.SetWeightedEdge(e)
-	}
-
-	a := mat.DenseCopyOf(adj.Matrix())
-
-	nodes := adj.Nodes()
-	for i := 0; nodes.Next(); i++ {
-		id := nodes.Node().ID()
-		u := g.Node(id)
-
-		e := g.WeightedEdge(u.ID(), u.ID())
-		if e != nil {
-			a.Set(i, i, e.Weight())
-		}
-	}
-
-	return a
+	matrix := toAdjMatrix(g, adj)
+	return matrix
 }
diff --git a/cmd/internal/graph/undirected.go b/cmd/internal/graph/undirected.go
new file mode 100644
index 0000000..a4ef291
--- /dev/null
+++ b/cmd/internal/graph/undirected.go
@@ -0,0 +1,41 @@
+/*
+Copyright © 2025 filifa
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program. If not, see <http://www.gnu.org/licenses/>.
+*/
+package graph
+
+import (
+	"gonum.org/v1/gonum/graph/multi"
+	"gonum.org/v1/gonum/graph/simple"
+	"gonum.org/v1/gonum/mat"
+)
+
+// UndirectedGraph embeds a multi.WeightedUndirectedGraph (as opposed to
+// simple.WeightedUndirectedGraph) to handle self loops.
+type UndirectedGraph struct {
+	*multi.WeightedUndirectedGraph
+}
+
+// NewUndirectedGraph returns a graph with no nodes or edges.
+func NewUndirectedGraph() *UndirectedGraph {
+	return &UndirectedGraph{WeightedUndirectedGraph: multi.NewWeightedUndirectedGraph()}
+}
+
+// AdjacencyMatrix returns the graph's adjacency matrix.
+func (g *UndirectedGraph) AdjacencyMatrix() *mat.Dense {
+	adj := simple.NewUndirectedMatrix(g.Nodes().Len(), 0, 0, 0)
+	matrix := toAdjMatrix(g, adj)
+	return matrix
+}