Fix cascade: use mana value of spell, not card (#14693)

* Use spell mana value instead of card mana value for cascade

Cascade should use the mana value of the spell rather than the mana
value of the card.  (Only?) relevant for X-cost spells.

No need to document rule 202.3e in `CascadeAbility` since it is already
documented in `Spell.getManaValue()`.

Fixes #14687

* And also have cascade look back at LKI

---------

Co-authored-by: matoro <matoro@users.noreply.github.com>

Author: matoro

Mage.Tests/src/test/java/org/mage/test/cards/abilities/keywords/CascadeTest.java

@@ -4,6 +4,7 @@
 import mage.constants.Zone;
 import org.junit.Assert;
 import org.junit.Test;
+import org.mage.test.player.TestPlayer;
 import org.mage.test.serverside.base.CardTestPlayerBase;
 
 /**
@@ -236,4 +237,56 @@ public void testWithSplitSpell() {
         assertGraveyardCount(playerA, "Breaking // Entering", 0);
         assertLibraryCount(playerA, "Breaking // Entering", 1);
     }
+
+    // https://github.com/magefree/mage/issues/14687
+    @Test
+    public void testWithXCost() {
+        skipInitShuffling();
+        playerA.getLibrary().clear();
+        addCard(Zone.LIBRARY, playerA, "Lotus Petal");
+        addCard(Zone.LIBRARY, playerA, "Dwarven Trader");
+        addCard(Zone.HAND, playerA, "Fireball");
+        addCard(Zone.BATTLEFIELD, playerA, "Mountain", 5);
+        addCard(Zone.BATTLEFIELD, playerA, "Maelstrom Nexus");
+
+        castSpell(1, PhaseStep.PRECOMBAT_MAIN, playerA, "Fireball");
+        setChoice(playerA, "X=4");
+        addTarget(playerA, playerB);
+        addTarget(playerA, TestPlayer.TARGET_SKIP);
+        setChoice(playerA, true);
+
+        setStrictChooseMode(true);
+        setStopAt(1, PhaseStep.PRECOMBAT_MAIN);
+        execute();
+
+        assertPermanentCount(playerA, "Dwarven Trader", 1);
+        assertPermanentCount(playerA, "Lotus Petal", 0);
+        assertGraveyardCount(playerA, "Fireball", 1);
+        assertLife(playerB, 16);
+    }
+
+    @Test
+    public void testRemovedFromStack() {
+        skipInitShuffling();
+        playerA.getLibrary().clear();
+        addCard(Zone.LIBRARY, playerA, "Dwarven Trader");
+        addCard(Zone.HAND, playerA, "Balduvian Bears");
+        addCard(Zone.BATTLEFIELD, playerA, "Forest", 2);
+        addCard(Zone.HAND, playerB, "Unsubstantiate");
+        addCard(Zone.BATTLEFIELD, playerB, "Island", 2);
+        addCard(Zone.BATTLEFIELD, playerA, "Maelstrom Nexus");
+
+        castSpell(1, PhaseStep.PRECOMBAT_MAIN, playerA, "Balduvian Bears");
+        setChoice(playerA, true);
+        castSpell(1, PhaseStep.PRECOMBAT_MAIN, playerB, "Unsubstantiate");
+        addTarget(playerB, "Balduvian Bears");
+
+        setStrictChooseMode(true);
+        setStopAt(1, PhaseStep.PRECOMBAT_MAIN);
+        execute();
+
+        assertPermanentCount(playerA, "Dwarven Trader", 1);
+        assertPermanentCount(playerA, "Balduvian Bears", 0);
+        assertHandCount(playerA, "Balduvian Bears", 1);
+    }
 }

Mage/src/main/java/mage/abilities/keyword/CascadeAbility.java

@@ -17,6 +17,7 @@
 import mage.game.stack.Spell;
 import mage.players.Player;
 import mage.target.common.TargetCardInExile;
+import mage.target.targetpointer.FixedTarget;
 import mage.util.CardUtil;
 
 /**
@@ -79,6 +80,7 @@ public boolean checkEventType(GameEvent event, Game game) {
     @Override
     public boolean checkTrigger(GameEvent event, Game game) {
         Spell spell = game.getStack().getSpell(event.getTargetId());
+        this.getEffects().setTargetPointer(new FixedTarget(event.getTargetId()));
         return spell != null
                 && spell.getSourceId().equals(this.getSourceId());
     }
@@ -111,14 +113,14 @@ public boolean apply(Game game, Ability source) {
         if (controller == null) {
             return false;
         }
-        Card sourceCard = game.getCard(source.getSourceId());
-        if (sourceCard == null) {
+        Spell sourceSpell = game.getSpellOrLKIStack(this.getTargetPointer().getFirst(game, source));
+        if (sourceSpell == null) {
             return false;
         }
 
         // exile cards from the top of your library until you exile a nonland card whose converted mana cost is less than this spell's converted mana cost
         Cards cardsToExile = new CardsImpl();
-        int sourceCost = sourceCard.getManaValue();
+        int sourceCost = sourceSpell.getManaValue();
         Card cardToCast = null;
         for (Card card : controller.getLibrary().getCards(game)) {
             cardsToExile.add(card);