1 files changed, 63 insertions, 0 deletions
diff --git a/src/internal/dag/alg.go b/src/internal/dag/alg.go
new file mode 100644
index 0000000000..88002797c0
--- /dev/null
+++ b/src/internal/dag/alg.go
@@ -0,0 +1,63 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dag
+
+// Transpose reverses all edges in g.
+func (g *Graph) Transpose() {
+	old := g.edges
+
+	g.edges = make(map[string]map[string]bool)
+	for _, n := range g.Nodes {
+		g.edges[n] = make(map[string]bool)
+	}
+
+	for from, tos := range old {
+		for to := range tos {
+			g.edges[to][from] = true
+		}
+	}
+}
+
+// Topo returns a topological sort of g. This function is deterministic.
+func (g *Graph) Topo() []string {
+	topo := make([]string, 0, len(g.Nodes))
+	marks := make(map[string]bool)
+
+	var visit func(n string)
+	visit = func(n string) {
+		if marks[n] {
+			return
+		}
+		for _, to := range g.Edges(n) {
+			visit(to)
+		}
+		marks[n] = true
+		topo = append(topo, n)
+	}
+	for _, root := range g.Nodes {
+		visit(root)
+	}
+	for i, j := 0, len(topo)-1; i < j; i, j = i+1, j-1 {
+		topo[i], topo[j] = topo[j], topo[i]
+	}
+	return topo
+}
+
+// TransitiveReduction removes edges from g that are transitively
+// reachable. g must be transitively closed.
+func (g *Graph) TransitiveReduction() {
+	// For i -> j -> k, if i -> k exists, delete it.
+	for _, i := range g.Nodes {
+		for _, j := range g.Nodes {
+			if g.HasEdge(i, j) {
+				for _, k := range g.Nodes {
+					if g.HasEdge(j, k) {
+						g.DelEdge(i, k)
+					}
+				}
+			}
+		}
+	}
+}