- Implemented basic "spell magnet" AI (currently used by Spellskite).

- Made the AI able to pay mana costs consisting of only phyrexian mana symbols in absence of relevant colored mana sources (e.g. Mutagenic Growth, Spellskite activated ability).
- Fixed the AI killing itself when paying phyrexian mana while being at 2 life (we might want to consider actually implementing a certain "safe limit" above 2 life, maybe 4 or 5 life, at least outside the range of a typical burn spell).

forge-ai/src/main/java/forge/ai/ComputerUtil.java

@@ -2076,6 +2076,32 @@ public class ComputerUtil {
         return ComputerUtilCard.getBestCreatureAI(killables);
     }
     
+    public static int predictDamageFromSpell(final SpellAbility sa, final Player targetPlayer) {
+        int damage = -1; // returns -1 if the spell does not deal damage
+        final Card card = sa.getHostCard();
+    
+        SpellAbility ab = sa;
+        while (ab != null) {
+            if (ab.getApi() == ApiType.DealDamage) {
+                if (damage == -1) { damage = 0; } // found a damage-dealing spell
+                if (!ab.hasParam("NumDmg")) {
+                    continue;
+                }
+                damage += ComputerUtilCombat.predictDamageTo(targetPlayer,
+                        AbilityUtils.calculateAmount(card, ab.getParam("NumDmg"), ab), card, false);
+            } else if (ab.getApi() == ApiType.LoseLife) {
+                if (damage == -1) { damage = 0; } // found a damage-dealing spell
+                if (!ab.hasParam("LifeAmount")) {
+                    continue;
+                }
+                damage += AbilityUtils.calculateAmount(card, ab.getParam("LifeAmount"), ab);
+            }
+            ab = ab.getSubAbility();
+        }
+        
+        return damage;
+    }
+    
     public static int getDamageForPlaying(final Player player, final SpellAbility sa) {
         
         // check for bad spell cast triggers

forge-ai/src/main/java/forge/ai/ComputerUtilAbility.java

@@ -15,7 +15,9 @@ import forge.game.card.CardLists;
 import forge.game.card.CardPredicates.Presets;
 import forge.game.player.Player;
 import forge.game.spellability.SpellAbility;
+import forge.game.spellability.SpellAbilityStackInstance;
 import forge.game.zone.ZoneType;
+import java.util.Iterator;
 
 public class ComputerUtilAbility {
     public static CardCollection getAvailableLandsToPlay(final Game game, final Player player) {
@@ -106,4 +108,22 @@ public class ComputerUtilAbility {
         }
         return result;
     }
+
+    public static SpellAbility getTopSpellAbilityOnStack(Game game, SpellAbility sa) {
+        Iterator<SpellAbilityStackInstance> it = game.getStack().iterator();
+
+        if (!it.hasNext()) {
+            return null;
+        }
+
+        SpellAbility tgtSA = it.next().getSpellAbility(true);
+        // Grab the topmost spellability that isn't this SA and use that for comparisons
+        if (sa.equals(tgtSA) && game.getStack().size() > 1) {
+            if (!it.hasNext()) {
+                return null;
+            }
+            tgtSA = it.next().getSpellAbility(true);
+        }
+        return tgtSA;
+    }
 }

forge-ai/src/main/java/forge/ai/ComputerUtilMana.java

@@ -314,6 +314,7 @@ public class ComputerUtilMana {
     private static boolean payManaCost(final ManaCostBeingPaid cost, final SpellAbility sa, final Player ai, final boolean test, boolean checkPlayable) {
         adjustManaCostToAvoidNegEffects(cost, sa.getHostCard(), ai);
         List<Mana> manaSpentToPay = test ? new ArrayList<Mana>() : sa.getPayingMana();
+        boolean purePhyrexian = cost.containsOnlyPhyrexianMana();
 
         List<SpellAbility> paymentList = Lists.newArrayList();
 
@@ -324,7 +325,7 @@ public class ComputerUtilMana {
         boolean hasConverge = sa.getHostCard().hasConverge();
         ListMultimap<ManaCostShard, SpellAbility> sourcesForShards = getSourcesForShards(cost, sa, ai, test,
 				checkPlayable, manaSpentToPay, hasConverge);
-        if (sourcesForShards == null) {
+        if (sourcesForShards == null && !purePhyrexian) {
         	return false;	// no mana abilities to use for paying
         }
 
@@ -353,7 +354,11 @@ public class ComputerUtilMana {
             		hasConverge = false;
             	}
             } else {
+                if (!(sourcesForShards == null && purePhyrexian)) {
                     saList = sourcesForShards.get(toPay);
+                } else {
+                    saList = Lists.newArrayList(); // Phyrexian mana only: no valid mana sources, but can still pay life
+                }
             }
             if (saList == null) {
                 break;
@@ -361,7 +366,7 @@ public class ComputerUtilMana {
 
             saList.removeAll(saExcludeList);
 
-            SpellAbility saPayment = chooseManaAbility(cost, sa, ai, toPay, saList, checkPlayable || !test);
+            SpellAbility saPayment = saList.isEmpty() ? null : chooseManaAbility(cost, sa, ai, toPay, saList, checkPlayable || !test);
 
             if (saPayment != null && saPayment.hasParam("AILogic")) {
                 boolean consider = false;
@@ -375,7 +380,7 @@ public class ComputerUtilMana {
             }
 
             if (saPayment == null) {
-                if (!toPay.isPhyrexian() || !ai.canPayLife(2)) {
+                if (!toPay.isPhyrexian() || !ai.canPayLife(2) || (ai.getLife() <= 2 && !ai.cantLoseForZeroOrLessLife())) {
                     break; // cannot pay
                 }
 

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

@@ -1,6 +1,13 @@
 package forge.ai.ability;
 
+import forge.ai.ComputerUtil;
+import forge.ai.ComputerUtilAbility;
+import forge.ai.ComputerUtilMana;
 import forge.ai.SpellAbilityAi;
+import forge.card.mana.ManaCost;
+import forge.card.mana.ManaCostParser;
+import forge.game.Game;
+import forge.game.mana.ManaCostBeingPaid;
 import forge.game.player.Player;
 import forge.game.spellability.SpellAbility;
 
@@ -16,8 +23,48 @@ public class ChangeTargetsAi extends SpellAbilityAi {
      */
     @Override
     protected boolean canPlayAI(Player aiPlayer, SpellAbility sa) {
+        final Game game = sa.getHostCard().getGame();
+        final SpellAbility topSa = game.getStack().isEmpty() ? null : ComputerUtilAbility.getTopSpellAbilityOnStack(game, sa);
+
+        if (sa.hasParam("AILogic")) {
+            if ("SpellMagnet".equals(sa.getParam("AILogic"))) {
+                // Cards like Spellskite that retarget spells to itself
+
+                if (topSa == null) {
+                    // nothing on stack, so nothing to target
                     return false;
                 }
+                if (!topSa.usesTargeting() || topSa.getTargets().getTargetCards().contains(sa.getHostCard())) {
+                    // if this does not target at all or already targets host, no need to redirect it again
+                    return false;
+                }
+                if (topSa.getHostCard() != null && !topSa.getHostCard().getController().isOpponentOf(aiPlayer)) {
+                    // make sure not to redirect our own abilities
+                    return false;
+                }
+                if (!topSa.canTarget(sa.getHostCard())) {
+                    // don't try targeting it if we can't legally target Spellskite with it in the first place
+                    return false;
+                }
+
+                if ("Spellskite".equals(sa.getHostCard().getName())) {
+                    int potentialDmg = ComputerUtil.predictDamageFromSpell(topSa, aiPlayer);
+                    boolean canPayBlue = ComputerUtilMana.canPayManaCost(new ManaCostBeingPaid(new ManaCost(new ManaCostParser("U"))), sa, aiPlayer);
+                    if (potentialDmg != -1 && potentialDmg <= 2 && !canPayBlue && topSa.getTargets().getTargets().contains(aiPlayer)) {
+                        // do not pay Phyrexian mana if the spell is a damaging one but it deals less damage or the same damage as we'll pay life
+                        return false;
+                    }
+                }
+
+                sa.resetTargets();
+                sa.getTargets().add(topSa);
+                return true;
+            }
+        }
+
+        // The AI can't otherwise play this ability, but should at least not miss mandatory activations (e.g. triggers).
+        return sa.isMandatory();
+    }
 
     @Override
     protected boolean doTriggerAINoCost(Player aiPlayer, SpellAbility sa, boolean mandatory) {

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

@@ -1,6 +1,7 @@
 package forge.ai.ability;
 
 import forge.ai.AiProps;
+import forge.ai.ComputerUtilAbility;
 import java.util.Iterator;
 
 import forge.ai.ComputerUtilCost;
@@ -54,7 +55,7 @@ public class CounterAi extends SpellAbilityAi {
         final TargetRestrictions tgt = sa.getTargetRestrictions();
         if (tgt != null) {
 
-            final SpellAbility topSA = findTopSpellAbility(game, sa);
+            final SpellAbility topSA = ComputerUtilAbility.getTopSpellAbilityOnStack(game, sa);
             if (!CardFactoryUtil.isCounterableBy(topSA.getHostCard(), sa) || topSA.getActivatingPlayer() == ai
                     || ai.getAllies().contains(topSA.getActivatingPlayer())) {
                 // might as well check for player's friendliness
@@ -257,17 +258,4 @@ public class CounterAi extends SpellAbilityAi {
 
         return new ImmutablePair<>(bestOption != null ? bestOption : leastBadOption, bestOption != null);
     }
-
-    public SpellAbility findTopSpellAbility(Game game, SpellAbility sa) {
-        Iterator<SpellAbilityStackInstance> it = game.getStack().iterator();
-        SpellAbility tgtSA = it.next().getSpellAbility(true);
-        // Grab the topmost spellability that isn't this SA and use that for comparisons
-        if (sa.equals(tgtSA) && game.getStack().size() > 1) {
-            if (!it.hasNext()) {
-                return null;
-            }
-            tgtSA = it.next().getSpellAbility(true);
-        }
-        return tgtSA;
-    }
 }

forge-game/src/main/java/forge/game/mana/ManaCostBeingPaid.java

@@ -170,6 +170,15 @@ public class ManaCostBeingPaid {
         return false;
     }
 
+    public final boolean containsOnlyPhyrexianMana() {
+        for (ManaCostShard shard : unpaidShards.keySet()) {
+            if (!shard.isPhyrexian()) {
+                return false;
+            }
+        }
+        return true;
+    }
+
     public final boolean payPhyrexian() {
         ManaCostShard phy = null;
         for (ManaCostShard mcs : unpaidShards.keySet()) {

forge-gui/res/cardsfolder/s/spellskite.txt

@@ -2,6 +2,6 @@ Name:Spellskite
 ManaCost:2
 Types:Artifact Creature Horror
 PT:0/4
-A:AB$ ChangeTargets | Cost$ PU | TargetType$ Spell,Activated,Triggered | ValidTgts$ Card | DefinedMagnet$ Self | ChangeSingleTarget$ True | SpellDescription$ Change a target of target spell or ability to CARDNAME.
+A:AB$ ChangeTargets | Cost$ PU | TargetType$ Spell,Activated,Triggered | ValidTgts$ Card | DefinedMagnet$ Self | ChangeSingleTarget$ True | AILogic$ SpellMagnet | SpellDescription$ Change a target of target spell or ability to CARDNAME.
 SVar:Picture:http://www.wizards.com/global/images/magic/general/spellskite.jpg
 Oracle:{P/U}: Change a target of target spell or ability to Spellskite. ({P/U} can be paid with either {U} or 2 life.)