/*
	UndoIntSet是IntSet的功能扩展，可以进行undo操作
	通过委托和反转控制将IntSet的逻辑和Undo的逻辑分开
	undo的功能由委托给functions，这样可以实现IntSet依赖functions，
*/
type undo []func()

func (u *undo) Add(f func())  {
	*u = append(*u,f)
}

func (u *undo) Undo() error{
	functions := *u
	if len(functions)==0{
		return errors.New("do not undo")
	}
	index := len(functions)-1
	if f := functions[index];f!=nil{
		f()
		functions[index]=nil
	}
	*u = functions[:index]
	return nil
}
type UndoIntSet struct {
	data map[int]bool
	functions undo
}

func (s *UndoIntSet) Add(i int) {
	if s.data[i]{
		return
	}
	s.data[i]=true
	s.functions = append(s.functions, func() {
		delete(s.data,i)
	})
}

func (s *UndoIntSet) remove(i int)  {
	if !s.data[i]{
		return
	}
	delete(s.data,i)
	s.functions = append(s.functions, func() {
		s.data[i]=true
	})
}

func (s *UndoIntSet) undo()  {
	s.functions.Undo()
}