cmd/digraph: only print non-trivial sccs

In 'digraph sccs', do not print trivial single-node components.
This is more useful because it distinguishes nodes with self-loops (printed)
from nodes without (not printed).

Also remove an unnecessary TODO about map[string]bool vs map[string]struct{}.
The savings is at most 5% and if we really cared about storage we would probably
not use a map at all.

Change-Id: I6049b3c0f99a913c65f08c6c40e77ae99d1ba8c4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/396834
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

cmd/digraph/digraph.go

@@ -122,7 +122,8 @@ The support commands are:
 	allpaths <node> <node>
 		the set of nodes on all paths from the first node to the second
 	sccs
-		all strongly connected components (one per line)
+		all non-trivial strongly connected components, one per line
+		(single-node components are only printed for nodes with self-loops)
 	scc <node>
 		the set of nodes nodes strongly connected to the specified one
 	focus <node>
@@ -158,7 +159,7 @@ func (l nodelist) println(sep string) {
 	fmt.Fprintln(stdout)
 }
 
-type nodeset map[string]bool // TODO(deklerk): change bool to struct to reduce memory footprint
+type nodeset map[string]bool
 
 func (s nodeset) sort() nodelist {
 	nodes := make(nodelist, len(s))
@@ -266,6 +267,9 @@ func (g graph) sccs() []nodeset {
 		if !seen[top] {
 			scc = make(nodeset)
 			rvisit(top)
+			if len(scc) == 1 && !g[top][top] {
+				continue
+			}
 			sccs = append(sccs, scc)
 		}
 	}
@@ -365,7 +369,7 @@ func parse(rd io.Reader) (graph, error) {
 	return g, nil
 }
 
-// Overridable for testing purposes.
+// Overridable for redirection.
 var stdin io.Reader = os.Stdin
 var stdout io.Writer = os.Stdout
 
@@ -485,9 +489,16 @@ func digraph(cmd string, args []string) error {
 		if len(args) != 0 {
 			return fmt.Errorf("usage: digraph sccs")
 		}
+		buf := new(bytes.Buffer)
+		oldStdout := stdout
+		stdout = buf
 		for _, scc := range g.sccs() {
 			scc.sort().println(" ")
 		}
+		lines := strings.SplitAfter(buf.String(), "\n")
+		sort.Strings(lines)
+		stdout = oldStdout
+		io.WriteString(stdout, strings.Join(lines, ""))
 
 	case "scc":
 		if len(args) != 1 {

cmd/digraph/digraph_test.go

@@ -27,6 +27,7 @@ a b c
 b d
 c d
 d c
+e e
 `
 
 	for _, test := range []struct {
@@ -41,7 +42,7 @@ d c
 		{"transpose", g1, "transpose", nil, "belt pants\njacket sweater\npants shorts\nshoes pants\nshoes socks\nsweater shirt\ntie shirt\n"},
 		{"forward", g1, "forward", []string{"socks"}, "shoes\nsocks\n"},
 		{"forward multiple args", g1, "forward", []string{"socks", "sweater"}, "jacket\nshoes\nsocks\nsweater\n"},
-		{"scss", g2, "sccs", nil, "a\nb\nc d\n"},
+		{"scss", g2, "sccs", nil, "c d\ne\n"},
 		{"scc", g2, "scc", []string{"d"}, "c\nd\n"},
 		{"succs", g2, "succs", []string{"a"}, "b\nc\n"},
 		{"preds", g2, "preds", []string{"c"}, "a\nd\n"},