type Element interface {
    Accept(Visitor)
}

type File struct {
    Name string
    Size int
}

func (f *File) Accept(v Visitor) {
    v.VisitFile(f)
}

type Directory struct {
    Name     string
    Elements []Element
}

func (d *Directory) Accept(v Visitor) {
    v.VisitDirectory(d)
}

type Visitor interface {
    VisitFile(*File)
    VisitDirectory(*Directory)
}

// 打印信息的访问者
type PrintVisitor struct{}

func (pv *PrintVisitor) VisitFile(f *File) {
    println("File:", f.Name, "(size:", f.Size, "bytes)")
}

func (pv *PrintVisitor) VisitDirectory(d *Directory) {
    println("Directory:", d.Name)
    for _, e := range d.Elements {
        e.Accept(pv)
    }
}

// 统计总大小的访问er
type SizeVisitor struct {
    Total int
}

func (sv *SizeVisitor) VisitFile(f *File) {
    sv.Total += f.Size
}

func (sv *SizeVisitor) VisitDirectory(d *Directory) {
    for _, e := range d.Elements {
        e.Accept(sv)
    }
}

func main() {
    root := &Directory{
        Name: "root",
        Elements: []Element{
            &File{Name: "a.txt", Size: 100},
            &File{Name: "b.go", Size: 200},
            &Directory{
                Name: "sub",
                Elements: []Element{
                    &File{Name: "c.txt", Size: 50},
                },
            },
        },
    }

    // 打印结构
    root.Accept(&PrintVisitor{})

    // 计算总大小
    sv := &SizeVisitor{}
    root.Accept(sv)
    println("Total size:", sv.Total) // 输出 350
}