- AI: Avoid infinitely activating AF Untap on another permanent that will then be used to untap the first one (e.g. 2x Kiora's Follower)

forge-ai/src/main/java/forge/ai/ability/UntapAi.java

@@ -13,6 +13,8 @@ import forge.game.card.CardCollection;
 import forge.game.card.CardLists;
 import forge.game.card.CardPredicates.Presets;
 import forge.game.cost.Cost;
+import forge.game.cost.CostTap;
+import forge.game.cost.CostTapType;
 import forge.game.phase.PhaseType;
 import forge.game.player.Player;
 import forge.game.player.PlayerCollection;
@@ -148,6 +150,25 @@ public class UntapAi extends SpellAbilityAi {
         final String[] tappablePermanents = {"Creature", "Land", "Artifact"};
         untapList = CardLists.getValidCards(untapList, tappablePermanents, source.getController(), source, sa);
 
+        // Try to avoid potential infinite recursion,
+        // e.g. Kiora's Follower untapping another Kiora's Follower and repeating infinitely
+        if (sa.getPayCosts() != null && sa.getPayCosts().hasOnlySpecificCostType(CostTap.class)) {
+            CardCollection toRemove = new CardCollection();
+            for (Card c : untapList) {
+                for (SpellAbility ab : c.getAllSpellAbilities()) {
+                    if (ab.getApi() == ApiType.Untap
+                            && ab.getPayCosts() != null
+                            && ab.getPayCosts().hasOnlySpecificCostType(CostTap.class)
+                            && ab.canTarget(source)) {
+                        System.out.println("Found a recursive untap target: " + c);
+                        toRemove.add(c);
+                        break;
+                    }
+                }
+            }
+            untapList.removeAll(toRemove);
+        }
+
         sa.resetTargets();
         while (sa.getTargets().getNumTargeted() < tgt.getMaxTargets(sa.getHostCard(), sa)) {
             Card choice = null;

forge-game/src/main/java/forge/game/cost/Cost.java

@@ -73,6 +73,15 @@ public class Cost {
         return false;
     }
 
+    public final boolean hasOnlySpecificCostType(Class<? extends CostPart> costType) {
+        for (CostPart p : getCostParts()) {
+            if (!costType.isInstance(p)) {
+                return false;
+            }
+        }
+        return true;
+    }
+
     /**
      * Gets the cost parts.
      *