cmd/digraph: implement graph.Graph interface

Change-Id: I2050c2cebd6ea0c9d6412e5219ea690bf7234cb9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/747363
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

cmd/digraph/digraph.go

diff --git a/cmd/digraph/digraph.go b/cmd/digraph/digraph.go
index bf5f285..8a96baf 100644
--- a/cmd/digraph/digraph.go
+++ b/cmd/digraph/digraph.go
@@ -18,12 +18,17 @@
 	"flag"
 	"fmt"
 	"io"
+	"iter"
+	"maps"
 	"os"
+	"slices"
 	"sort"
 	"strconv"
 	"strings"
 	"unicode"
 	"unicode/utf8"
+
+	"golang.org/x/tools/internal/graph"
 )
 
 func usage() {
@@ -90,6 +95,21 @@
 // A digraph maps nodes to the non-nil set of their immediate successors.
 type digraph map[string]nodeset
 
+func (g digraph) Nodes() iter.Seq[string] {
+	return slices.Values(slices.Sorted(maps.Keys(g)))
+}
+
+func (g digraph) NumNodes() int {
+	return len(g)
+}
+
+func (g digraph) Out(node string) iter.Seq[string] {
+	// Out must be deterministic.
+	return slices.Values(slices.Sorted(maps.Keys(g[node])))
+}
+
+var _ graph.Graph[string] = digraph{}
+
 func (g digraph) addNode(node string) nodeset {
 	edges := g[node]
 	if edges == nil {
@@ -108,11 +128,7 @@
 }
 
 func (g digraph) nodelist() nodelist {
-	nodes := make(nodeset)
-	for node := range g {
-		nodes[node] = true
-	}
-	return nodes.sort()
+	return nodelist(slices.Collect(g.Nodes()))
 }
 
 func (g digraph) reachableFrom(roots nodeset) nodeset {