/*! Retroactively revert a previous operation.
* \param t The time point to revert.
*/
void revert(const time_point &t)
{
if (t.operation() == queue::push)
{
// This element was never pushed.
--size_;
// If t occurs before front, then that means we've previously popped an
// element that no longer exists, so it should have popped the element
// after it. Hence, move the front to its successor.
if (t.it->second)
{
move_front_succ();
}
else if (front_ == t.it)
{
// We're removing the front, so move it right to ensure validity.
move_front_succ();
}
// Remove the element corresponding to the push.
data_.erase(t.it);
}
else
{
move_front_pred();
++size_; // One less pop means the size increases by 1
}
}
