doodle/pkg/drawtool/history.go

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package drawtool
// History manages a history of Strokes added to a drawing.
type History struct {
limit int
head *HistoryElement // oldest history element, top of linked list
tail *HistoryElement // newest element added to history
}
// HistoryElement is a doubly linked list of stroke history.
type HistoryElement struct {
stroke *Stroke
next *HistoryElement
previous *HistoryElement
}
// NewHistory initializes a History list.
func NewHistory(limit int) *History {
return &History{
limit: limit,
}
}
// Reset clears the history.
func (h *History) Reset() {
h.head = nil
h.tail = nil
}
// Size returns the current size of the history list.
func (h *History) Size() int {
var (
size int
node = h.head
)
for node != nil {
size++
node = node.next
}
return size
}
// Latest returns the tail of the history (the most recent stroke). If you had
// recently called Undo, the latest stroke may still have a 'next' stroke.
// Returns nil if there was no stroke in history.
func (h *History) Latest() *Stroke {
if h.tail == nil {
return nil
}
return h.tail.stroke
}
// Oldest returns the head of the history (the earliest stroke added). If the
// history size limit had been reached, the oldest stroke will creep along
// forward and not necessarily be the FIRST EVER stroke added.
func (h *History) Oldest() *Stroke {
if h.head == nil {
return nil
}
return h.head.stroke
}
// AddStroke adds a stroke to the history, becoming the new tail at the end
// of the history data.
func (h *History) AddStroke(s *Stroke) {
var (
elem = &HistoryElement{
stroke: s,
}
tail = h.tail
)
// Make the current tail point to this one.
if tail != nil {
tail.next = elem
elem.previous = tail
}
// First stroke of the history? Make it the head of the linked list.
if h.head == nil {
h.head = elem
}
h.tail = elem
// Have we reached the history storage limit?
var size = h.Size()
if size > h.limit {
var node = h.tail
for i := 0; i < h.limit-1; i++ {
if node.previous == nil {
break
}
node = node.previous
}
h.head = node
h.head.previous = nil
}
}
// Undo steps back a step in the history. This sets the current tail to point
// to the "tail - 1" element, but doesn't change the link of that element to
// its future value yet; so that you can Redo it. But if you add a new stroke
// from this state, it will overwrite the tail.next and invalidate the old
// history that came after, starting a new branch of history from that point on.
//
// Returns false if the undo failed (no earlier node to move to).
func (h *History) Undo() bool {
if h.tail == nil {
return false
}
// if h.tail.previous == nil {
// return false
// }
h.tail = h.tail.previous
return true
}
// Redo advances forwards after a recent Undo. Note that if you added new strokes
// after an Undo, the new tail has no next node to move to and Redo returns false.
func (h *History) Redo() bool {
if h.tail == nil || h.tail.next == nil {
return false
}
h.tail = h.tail.next
return true
}