package model

type UndoStack struct {
	states []Entry
	pos    int
}

type Entry struct {
	Target *Map
	State  *Map
}

func (u *UndoStack) Push(target *Map) {
	keep := u.pos + 1
	if keep > len(u.states) {
		keep = len(u.states)
	}
	u.states = append(u.states[:keep], Entry{Target: target, State: target.Clone()})
	u.pos = len(u.states) - 1
	if len(u.states) > 100 {
		u.states = u.states[1:]
		u.pos--
	}
}

func (u *UndoStack) Undo() *Entry {
	if u.pos <= 0 {
		return nil
	}
	u.pos--
	return &u.states[u.pos]
}

func (u *UndoStack) Redo() *Entry {
	if u.pos >= len(u.states)-1 {
		return nil
	}
	u.pos++
	return &u.states[u.pos]
}

func (u *UndoStack) Pos() int { return u.pos }