depgraph: add String method to Graph for testing/debugging

depgraph/graph.go

@@ -6,7 +6,9 @@
 package depgraph
 
 import (
+	"bytes"
 	"fmt"
+	"strings"
 
 	"github.com/hashicorp/terraform/digraph"
 )
@@ -104,6 +106,44 @@ func (g *Graph) CheckConstraints() error {
 	return nil
 }
 
+// String generates a little ASCII string of the graph, useful in
+// debugging output.
+func (g *Graph) String() string {
+	var buf bytes.Buffer
+
+	cb := func(n *Noun, depth int) {
+		buf.WriteString(fmt.Sprintf(
+			"%s%s\n",
+			strings.Repeat("  ", depth),
+			n.Name))
+	}
+
+	type listItem struct {
+		n *Noun
+		d int
+	}
+	nodes := []listItem{{g.Root, 0}}
+	for len(nodes) > 0 {
+		// Pop current node
+		n := len(nodes)
+		current := nodes[n-1]
+		nodes = nodes[:n-1]
+
+		// Visit
+		cb(current.n, current.d)
+
+		// Traverse
+		for _, dep := range current.n.Deps {
+			nodes = append(nodes, listItem{
+				n: dep.Target,
+				d: current.d + 1,
+			})
+		}
+	}
+
+	return buf.String()
+}
+
 // Validate is used to ensure that a few properties of the graph are not violated:
 // 1) There must be a single "root", or source on which nothing depends.
 // 2) All nouns in the graph must be reachable from the root

depgraph/graph_test.go

@@ -53,6 +53,38 @@ func NounMapToList(m map[string]*Noun) []*Noun {
 	return list
 }
 
+func TestGraph_String(t *testing.T) {
+	nodes := ParseNouns(`a -> b
+a -> c
+b -> d
+b -> e
+c -> d
+c -> e`)
+
+	g := &Graph{
+		Name: "Test",
+		Nouns: NounMapToList(nodes),
+		Root: nodes["a"],
+	}
+	actual := g.String()
+
+	expected := `
+a
+  c
+    e
+    d
+  b
+    e
+    d
+`
+
+	actual = strings.TrimSpace(actual)
+	expected = strings.TrimSpace(expected)
+	if actual != expected {
+		t.Fatalf("bad:\n%s\n!=\n%s", actual, expected)
+	}
+}
+
 func TestGraph_Validate(t *testing.T) {
 	nodes := ParseNouns(`a -> b
 a -> c