Undo stack walking

scripts/GameField.gd

@@ -122,7 +122,7 @@ func _draw() -> void:
 	# Undo Stack
 	var u = len(undo_stack)
 	for i in u:
-		draw_string(font, grid_corner(i, cols+1), str(undo_stack[u-i-1]), Color.black)
+		draw_string(font, grid_corner(i, cols+1), str(undo_stack[u-i-1]), Color.black if i>=undo_preview_pos else Color.gray)
 #		print(action)
 
 func _process(delta: float) -> void:
@@ -272,8 +272,38 @@ func _input(event: InputEvent) -> void:
 		match event.scancode:
 			KEY_Z:
 				undo()
+			KEY_DOWN:
+				self.undo_preview_pos += 1
+			KEY_UP:
+				self.undo_preview_pos -= 1
 
 var undo_stack = []
+var undo_preview_pos := 0 setget set_undo_preview_pos
+
+func do_action(action):
+	action.activate()
+	undo_stack.append(action)
+
+func undo():
+	if !undo_stack.empty():
+		var action = undo_stack.pop_back()
+		action.undo()
+
+func set_undo_preview_pos(value):
+	if value < 0:  # Negative is past the end of the stack
+		value = 0
+	elif value > len(undo_stack):
+		value = len(undo_stack)
+
+	var old_pos = undo_preview_pos
+	undo_preview_pos = value
+	var delta = value - old_pos
+	if delta > 0:  # We are undoing
+		for i in delta:
+			undo_stack[-old_pos-i].undo()
+	elif delta < 0:  # We are redoing
+		for i in -delta:
+			undo_stack[-old_pos+1+i].activate()
 
 class UndoAction:
 	var action_type
@@ -354,15 +384,6 @@ class UndoActionCell extends UndoAction:
 			UndoActions.SET_COLOR:
 				cell_colors.set_cell(cell_row, cell_col, old_state)
 
-func do_action(action):
-	action.activate()
-	undo_stack.append(action)
-
-func undo():
-	if !undo_stack.empty():
-		var action = undo_stack.pop_back()
-		action.undo()
-
 func set_cell_color(row, col, new_color):
 	if new_color != cell_colors.get_cell(row, col):
 		do_action(UndoActionCell.new(UndoActions.SET_COLOR, row, col, new_color, cell_colors))