Shuffle cards back by default.

If no After function is provided for a SwitchingCard, it uses ShuffleIntoBottomHalf.

This also modifies Then to be callable with a nil `o`, which it recognizes as "nothing happened but we need cleanup anyway", in preparation for the incoming "make cards sometimes not drawable" feature.

koboldsim/cardtypes.go

@@ -96,18 +96,24 @@ func (s *SwitchingCard) Options(player *Player) ([]CardOption, error) {
 
 // Then implements Card.
 func (s *SwitchingCard) Then(p *Player, o CardOption) error {
-	newPolicy := o.(Policy)
 	var errs cardsim.ErrorCollector
-	if s.lastPolicy != nil && !newPolicy.Is(s.lastPolicy) {
-		err := s.lastPolicy.Unenact(p)
-		if cardsim.IsSeriousError(err) {
-			return err
+	if o != nil {
+		newPolicy := o.(Policy)
+		if s.lastPolicy != nil && !newPolicy.Is(s.lastPolicy) {
+			err := s.lastPolicy.Unenact(p)
+			if cardsim.IsSeriousError(err) {
+				return err
+			}
+			errs.Add(err)
 		}
-		errs.Add(err)
+		s.lastPolicy = o.(Policy)
 	}
-	s.lastPolicy = o.(Policy)
 	if s.After != nil {
 		errs.Add(s.After(s, p, o))
+	} else {
+		// Fallback: Shuffle the card back into the bottom half of the deck.
+		errs.Add(ShuffleIntoBottomHalf(s, p, o))
+
 	}
 	return errs.Emit()
 }