Traverse into zip files when walking the file system

func WalkDirZip(fsys fs.FS, root string, fn WalkDirFunc) error {
    return fs.WalkDir(fsys, root, func(path string, d fs.DirEntry, err error) error {
        if d.IsDir() || !strings.EqualFold(filepath.Ext(path), ".zip") {
            return fn(fsys, path, d, err)
        } else if f, err := fsys.Open(path); err != nil {
            return err
        } else if s, err := f.Stat(); err != nil {
            return err
        } else if zfs, err := zip.NewReader(f.(io.ReaderAt), s.Size()); err != nil {
            return err
        } else {
            return fs.WalkDir(zfs, ".", func(path string, d fs.DirEntry, err error) error {
                return fn(zfs, path, d, err)
            })
        }
    })
}

type WalkDirFunc func(fsys fs.FS, path string, d fs.DirEntry, err error) error