go/packages/internal/nodecount: count ast.Node frequency

This internal tool counts node frequencies, which may be useful
when choosing type switch case order. It is also a nice
illustration of packages.Load and ast.Inspect.

Change-Id: I88c08987b6fe7b10e15f00844cd71fc850fc70c3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/480915
Reviewed-by: Robert Findley <[email protected]>
Run-TryBot: Alan Donovan <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

go/packages/internal/nodecount/nodecount.go

@@ -0,0 +1,71 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The nodecount program illustrates the use of packages.Load to print
+// the frequency of occurrence of each type of syntax node among the
+// selected packages.
+//
+// Example usage:
+//
+//	$ nodecount golang.org/x/tools/... std
+//
+// A typical distribution is 40% identifiers, 10% literals, 8%
+// selectors, and 6% calls; around 3% each of BinaryExpr, BlockStmt,
+// AssignStmt, Field, and Comment; and the rest accounting for 20%.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"go/ast"
+	"log"
+	"reflect"
+	"sort"
+
+	"golang.org/x/tools/go/packages"
+)
+
+func main() {
+	flag.Parse()
+
+	// Parse specified packages.
+	config := packages.Config{
+		Mode:  packages.NeedSyntax | packages.NeedFiles,
+		Tests: true,
+	}
+	pkgs, err := packages.Load(&config, flag.Args()...)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Count each type of syntax node.
+	var (
+		byType = make(map[reflect.Type]int)
+		total  int
+	)
+	packages.Visit(pkgs, nil, func(p *packages.Package) {
+		for _, f := range p.Syntax {
+			ast.Inspect(f, func(n ast.Node) bool {
+				if n != nil {
+					byType[reflect.TypeOf(n)]++
+					total++
+				}
+				return true
+			})
+		}
+	})
+
+	// Print results (percent, count, type) in descending order.
+	var types []reflect.Type
+	for t := range byType {
+		types = append(types, t)
+	}
+	sort.Slice(types, func(i, j int) bool {
+		return byType[types[i]] > byType[types[j]]
+	})
+	for _, t := range types {
+		percent := 100 * float64(byType[t]) / float64(total)
+		fmt.Printf("%6.2f%%\t%8d\t%s\n", percent, byType[t], t)
+	}
+}