func (d fileInfoDirs) len() int          { return len(d) }
func (d fileInfoDirs) isDir(i int) bool  { return d[i].IsDir() }
func (d fileInfoDirs) name(i int) string { return d[i].Name() }

type dirEntryDirs []fs.DirEntry

func (d dirEntryDirs) len() int          { return len(d) }
func (d dirEntryDirs) isDir(i int) bool  { return d[i].IsDir() }
func (d dirEntryDirs) name(i int) string { return d[i].Name() }

	rem(1)
	return res
}

func sortedMarkdownFilenames(fsys fs.FS) ([]string, error) {
	var filenames []string
	err := fs.WalkDir(fsys, ".", func(path string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if !d.IsDir() && strings.HasSuffix(path, ".md") {
			filenames = append(filenames, path)
		}
		return nil
	})
	if err != nil {

func makeDirsReadOnly(dir string) {
	type pathMode struct {
		path string
		mode fs.FileMode
	}
	var dirs []pathMode // in lexical order
	filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
		if err == nil && d.IsDir() {
			info, err := d.Info()
			if err == nil && info.Mode()&0222 != 0 {
				dirs = append(dirs, pathMode{path, info.Mode()})
			}
		}
		return nil
	})

		var err error
		data, err = os.ReadFile("../../testdata/Isaac.Newton-Opticks.txt")
		if err != nil {
			return nil, err
		}
	case "go":
		err := filepath.WalkDir("../..", func(path string, info fs.DirEntry, err error) error {
			if err == nil && strings.HasSuffix(path, ".go") && !info.IsDir() {
				file, err := os.ReadFile(path)
				if err != nil {
					return err
				}
				data = append(data, file...)
			}

		return d.info, nil
	}
	return lstat(d.parent + "/" + d.name)
}

func (d *unixDirent) String() string {
	return fs.FormatDirEntry(d)
}

func newUnixDirent(parent, name string, typ FileMode) (DirEntry, error) {
	ude := &unixDirent{
		parent: parent,
		name:   name,
		typ:    typ,
	}
	if typ != ^FileMode(0) && !testingForceReadDirLstat {
		return ude, nil
	}

// Readdirnames.
type WalkFunc func(path string, info fs.FileInfo, err error) error

var lstat = os.Lstat // for testing

// walkDir recursively descends path, calling walkDirFn.
func walkDir(path string, d fs.DirEntry, walkDirFn fs.WalkDirFunc) error {
	if err := walkDirFn(path, d, nil); err != nil || !d.IsDir() {
		if err == SkipDir && d.IsDir() {
			// Successfully skipped directory.
			err = nil
		}
		return err
	}

	// explicitly setting that. Otherwise it might get out of sync with FindFirstFile. See golang.org/issues/42637.
	if err := filepath.WalkDir(path, func(path string, d fs.DirEntry, err error) error {
		if err != nil {
			t.Fatal(err)
		}
		info, err := d.Info()
		if err != nil {
			t.Fatal(err)
		}

func listStdPkgs(goroot string) ([]string, error) {
	// Based on cmd/go's matchPackages function.
	var pkgs []string

	src := filepath.Join(goroot, "src") + string(filepath.Separator)
	walkFn := func(path string, d fs.DirEntry, err error) error {
		if err != nil || !d.IsDir() || path == src {
			return nil
		}

		base := filepath.Base(path)
		if strings.HasPrefix(base, ".") || strings.HasPrefix(base, "_") || base == "testdata" {

		root := testenv.GOROOT(t)
		if !os.IsPathSeparator(root[len(root)-1]) {
			root += string(filepath.Separator)
		}
		goroot.err = filepath.WalkDir(root, func(path string, info fs.DirEntry, err error) error {
			if err != nil {
				return err
			}
			if info.IsDir() && path != root && (info.Name() == "vendor" || info.Name() == "testdata") {
				return filepath.SkipDir
			}

}

// readDir calls ctxt.ReadDir (if not nil) or else os.ReadDir.
func (ctxt *Context) readDir(path string) ([]fs.DirEntry, error) {
	// TODO: add a fs.DirEntry version of Context.ReadDir
	if f := ctxt.ReadDir; f != nil {
		fis, err := f(path)
		if err != nil {
			return nil, err
		}
		des := make([]fs.DirEntry, len(fis))
		for i, fi := range fis {
			des[i] = fs.FileInfoToDirEntry(fi)
		}
		return des, nil
	}