cmd/digraph: add "to dot" to emit Graphviz dot

This CL adds "digraph to dot", which prints the graph in Graphviz dot
format, which can then be piped into dot.

Change-Id: I8209d9038bb4e1df1ca2ced427ce14f6df8051db
Reviewed-on: https://go-review.googlesource.com/c/tools/+/503437
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
gopls-CI: kokoro <[email protected]>
Run-TryBot: Austin Clements <[email protected]>

Author: aclements

cmd/digraph/digraph.go

@@ -37,6 +37,8 @@ The support commands are:
 		the set of nodes strongly connected to the specified one
 	focus <node>
 		the subgraph containing all directed paths that pass through the specified node
+	to dot
+		print the graph in Graphviz dot format (other formats may be supported in the future)
 
 Input format:
 
@@ -81,6 +83,11 @@ Show which packages in x/tools depend, perhaps indirectly, on the callgraph pack
 	$ go list -f '{{.ImportPath}} {{join .Imports " "}}' -deps golang.org/x/tools/... |
 		digraph reverse golang.org/x/tools/go/callgraph
 
+Visualize the package dependency graph of the current package:
+
+	$ go list -f '{{.ImportPath}} {{join .Imports " "}}' -deps |
+		digraph to dot | dot -Tpng -o x.png
+
 Using a module graph produced by go mod, show all dependencies of the current module:
 
 	$ go mod graph | digraph forward $(go list -m)
@@ -137,6 +144,8 @@ The support commands are:
 		the set of nodes nodes strongly connected to the specified one
 	focus <node>
 		the subgraph containing all directed paths that pass through the specified node
+	to dot
+		print the graph in Graphviz dot format (other formats may be supported in the future)
 `)
 	os.Exit(2)
 }
@@ -207,6 +216,14 @@ func (g graph) addEdges(from string, to ...string) {
 	}
 }
 
+func (g graph) nodelist() nodelist {
+	nodes := make(nodeset)
+	for node := range g {
+		nodes[node] = true
+	}
+	return nodes.sort()
+}
+
 func (g graph) reachableFrom(roots nodeset) nodeset {
 	seen := make(nodeset)
 	var visit func(node string)
@@ -356,6 +373,20 @@ func (g graph) somepath(from, to string) error {
 	return nil
 }
 
+func (g graph) toDot(w *bytes.Buffer) {
+	fmt.Fprintln(w, "digraph {")
+	for _, src := range g.nodelist() {
+		for _, dst := range g[src].sort() {
+			// Dot's quoting rules appear to align with Go's for escString,
+			// which is the syntax of node IDs. Labels require significantly
+			// more quoting, but that appears not to be necessary if the node ID
+			// is implicitly used as the label.
+			fmt.Fprintf(w, "\t%q -> %q;\n", src, dst)
+		}
+	}
+	fmt.Fprintln(w, "}")
+}
+
 func parse(rd io.Reader) (graph, error) {
 	g := make(graph)
 
@@ -404,11 +435,7 @@ func digraph(cmd string, args []string) error {
 		if len(args) != 0 {
 			return fmt.Errorf("usage: digraph nodes")
 		}
-		nodes := make(nodeset)
-		for node := range g {
-			nodes[node] = true
-		}
-		nodes.sort().println("\n")
+		g.nodelist().println("\n")
 
 	case "degree":
 		if len(args) != 0 {
@@ -563,6 +590,14 @@ func digraph(cmd string, args []string) error {
 		sort.Strings(edgesSorted)
 		fmt.Fprintln(stdout, strings.Join(edgesSorted, "\n"))
 
+	case "to":
+		if len(args) != 1 || args[0] != "dot" {
+			return fmt.Errorf("usage: digraph to dot")
+		}
+		var b bytes.Buffer
+		g.toDot(&b)
+		stdout.Write(b.Bytes())
+
 	default:
 		return fmt.Errorf("no such command %q", cmd)
 	}

cmd/digraph/digraph_test.go

@@ -6,6 +6,7 @@ package main
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"reflect"
 	"sort"
 	"strings"
@@ -346,3 +347,27 @@ func TestFocus(t *testing.T) {
 		})
 	}
 }
+
+func TestToDot(t *testing.T) {
+	in := `a b c
+b "d\"\\d"
+c "d\"\\d"`
+	want := `digraph {
+	"a" -> "b";
+	"a" -> "c";
+	"b" -> "d\"\\d";
+	"c" -> "d\"\\d";
+}
+`
+	defer func(in io.Reader, out io.Writer) { stdin, stdout = in, out }(stdin, stdout)
+	stdin = strings.NewReader(in)
+	stdout = new(bytes.Buffer)
+	if err := digraph("to", []string{"dot"}); err != nil {
+		t.Fatal(err)
+	}
+	got := stdout.(fmt.Stringer).String()
+	if got != want {
+		t.Errorf("digraph(to, dot) = got %q, want %q", got, want)
+	}
+
+}