private void undo() {
if (steps.isEmpty()) {
logger.warn("There is nothing to undo.");
return;
}
Step step = steps.pop();
Set<Integer> groupIds = board.undo(step);
// update all affected group sum view
for (int groupId : groupIds) {
updateGroupSumView(groupId);
}
// update all cell view
for (CellSnapshot snapshot : step.snapshots) {
Cell cell = board.getCell(snapshot.row, snapshot.col);
boardView.updateCellView(cell);
}
}
