Index: cmd/sockdrawer/cluster.go
===================================================================
new file mode 100644
--- /dev/null
+++ b/cmd/sockdrawer/cluster.go
@@ -0,0 +1,162 @@
+package main
+
+// This file defines the cluster graph.
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+type cluster struct {
+	id int
+	importPath string // declared name, e.g. "runtime/internal/core"
+	name string // short import name, e.g. "_core"
+	nodes map[*node]bool
+	scope map[string]*node // maps package-level names to decls
+	outputFiles map[string]*outputFile // output file data, keyed by file base name
+}
+
+func (c *cluster) finish() {
+	// mark applies n's cluster to all nodes reachable from it that
+	// don't have a cluster assignment yet.
+	var mark func(n *node)
+	mark = func(n *node) {
+		for s := range n.succs {
+			if s.cluster == nil {
+				s.cluster = n.cluster
+				n.cluster.nodes[s] = true
+				if debug {
+					fmt.Printf("\t%-50s (indirect)\n", s)
+				}
+				mark(s)
+			}
+		}
+	}
+
+	var first, prev *node
+	for n := range c.nodes {
+		if first == nil {
+			first = n
+		}
+		if prev != nil {
+			mark(n)
+		}
+		prev = n
+	}
+	if prev != nil {
+		mark(first)
+	}
+
+	c.outputFiles = make(map[string]*outputFile)
+}
+
+func loadClusterFile(filename string, nodes []*node) ([]*cluster, error) {
+	clusterNames := map[string]bool{"residue": true}
+
+	byName := make(map[string]*node)
+	for _, n := range nodes {
+		byName[n.name] = n
+	}
+
+	f, err := os.Open(*clusterFile)
+	if err != nil {
+		return nil, err
+	}
+	in := bufio.NewScanner(f)
+	var linenum int
+	var c *cluster
+	var clusters []*cluster
+	for in.Scan() {
+		linenum++
+		line := strings.TrimSpace(in.Text())
+		if i := strings.IndexByte(line, '#'); i>= 0 {
+			line = strings.TrimSpace(line[:i]) // strip comments
+		}
+		if line == "" {
+			continue // skip blanks
+		}
+		if strings.HasPrefix(line, "= ") {
+			if c != nil {
+				c.finish()
+			}
+
+			c = &cluster{
+				id: len(clusters),
+				importPath: line[2:],
+				nodes: make(map[*node]bool),
+			}
+			if clusterNames[c.importPath] {
+				fmt.Fprintf(os.Stderr,
+					"%s:%d: warning: duplicate cluster name: %s; ignoring\n",
+					*clusterFile, linenum, c.importPath)
+				continue
+			}
+			clusters = append(clusters, c)
+			if debug {
+				fmt.Printf("\n# cluster %s\n", c.importPath)
+			}
+			continue
+		}
+		if c == nil {
+			fmt.Fprintf(os.Stderr,
+				"%s:%d: warning: node before '= cluster' marker; ignoring\n",
+				*clusterFile, linenum)
+			continue
+		}
+
+		n := byName[line]
+		if n == nil {
+			fmt.Fprintf(os.Stderr,
+				"%s:%d: warning: can't find node %q; ignoring\n",
+				*clusterFile, linenum, line)
+		} else if n.cluster != nil {
+			fmt.Fprintf(os.Stderr,
+				"%s:%d: warning: node %q appears in clusters %q and %q; ignoring\n",
+				*clusterFile, linenum, line, n.cluster.importPath, c.importPath)
+		} else {
+			n.cluster = c
+			if debug {
+				fmt.Printf("\t%s\n", n)
+			}
+			c.nodes[n] = true
+		}
+	}
+	if c != nil {
+		c.finish()
+	}
+
+	f.Close()
+	if err := in.Err(); err != nil {
+		return nil, err
+	}
+
+	return clusters, nil
+}
+
+func addResidualCluster(nodes []*node, clusters []*cluster) []*cluster {
+	// The final cluster, residue, includes all other nodes.
+	c := &cluster{
+		id: len(clusters),
+		importPath: "residue",
+		nodes: make(map[*node]bool),
+	}
+	if debug {
+		fmt.Printf("\n# cluster %s\n", c.importPath)
+	}
+	for _, n := range nodes {
+		if n.cluster == nil {
+			n.cluster = c
+			if debug {
+				fmt.Printf("\t%-50s\n", n)
+			}
+			c.nodes[n] = true
+		}
+	}
+	c.finish()
+	if len(c.nodes)> 0 {
+		clusters = append(clusters, c)
+	}
+	return clusters
+}
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