1 files changed, 42 insertions, 0 deletions
diff --git a/internal/model/undo.go b/internal/model/undo.go
new file mode 100644
index 0000000..5b0297b
--- /dev/null
+++ b/internal/model/undo.go
@@ -0,0 +1,42 @@
+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 }