Fix 3917 jump vehicles with motive damage

megamek/src/megamek/client/ui/swing/MovementDisplay.java

@@ -570,7 +570,7 @@ public void performAction() {
                         final Entity ce = ce();
                         boolean isAero = ce.isAero();
                         // first check if jumping is available at all
-                        if (!isAero && !ce.isImmobile() && (ce.getJumpMP() > 0)
+                        if (!isAero && !ce.isImmobileForJump() && (ce.getJumpMP() > 0)
                                 && !(ce.isStuck() && !ce.canUnstickByJumping())) {
                             if (gear != MovementDisplay.GEAR_JUMP) {
                                 if (!((cmd.getLastStep() != null)
@@ -813,7 +813,7 @@ private void updateButtons() {
 
         setWalkEnabled(!ce.isImmobile() && ((ce.getWalkMP() > 0) || (ce.getRunMP() > 0))
                 && !ce.isStuck());
-        setJumpEnabled(!isAero && !ce.isImmobile() && !ce.isProne()
+        setJumpEnabled(!isAero && !ce.isImmobileForJump() && !ce.isProne()
                 // Conventional infantry also uses jump MP for VTOL and UMU MP
                 && ((ce.getJumpMP() > 0) && (!ce.isConventionalInfantry() || ce.getMovementMode().isJumpInfantry()))
                 && !(ce.isStuck() && !ce.canUnstickByJumping()));

megamek/src/megamek/common/Compute.java

@@ -1072,7 +1072,13 @@ public static ToHitData getImmobileMod(Targetable target, int aimingAt, AimingMo
             if ((target instanceof Mech) && (aimingAt == Mech.LOC_HEAD) && aimingMode.isImmobile()) {
                 return new ToHitData(3, "aiming at head");
             }
-            return new ToHitData(-4, "target immobile");
+            ToHitData immobileTHD = new ToHitData(-4, "target immobile");
+            if(target instanceof Tank) {
+                // An "immobilized" but jumping CV is not actually immobile for targeting purposes
+                // (See issue #3917)
+                return ((Tank)target).moved == EntityMovementType.MOVE_JUMP ? null : immobileTHD;
+            }
+            return immobileTHD;
         }
         return null;
     }

megamek/src/megamek/common/Entity.java

@@ -1751,6 +1751,10 @@ public boolean isImmobile() {
         return isImmobile(true);
     }
 
+    public boolean isImmobileForJump() {
+        return isImmobile();
+    }
+
     /**
      * Is this entity shut down, or if applicable is the crew unconscious?
      * @param checkCrew If true, consider the fitness of the crew when determining

megamek/src/megamek/common/MoveStep.java

@@ -3250,6 +3250,11 @@ public boolean isMovementPossible(Game game, Coords src, int srcEl, CachedEntity
             return true;
         }
 
+        // Motive hit has immobilized CV, but it still wants to (and can) jump: okay!
+        if (movementType == EntityMovementType.MOVE_JUMP && (entity instanceof Tank) && !entity.isImmobileForJump()) {
+            return true;
+        }
+
         // super-easy, but not any more
         if (entity.isImmobile() && !entity.isBracing()) {
             return false;

megamek/src/megamek/common/Tank.java

@@ -567,6 +567,23 @@ public boolean isPermanentlyImmobilized(boolean checkCrew) {
         return super.isPermanentlyImmobilized(checkCrew) || isMovementHit();
     }
 
+    /**
+     * Per https://bg.battletech.com/forums/index.php/topic,78336.msg1869386.html#msg1869386
+     * CVs with working engines and Jump Jets should still have the option to jump during the movement
+     * phase, even if reduced to 0 MP by motive hits, or rolling 12 on the Motive System Damage table.
+     */
+    @Override
+    public boolean isImmobileForJump() {
+        // *Can* jump unless 0 Jump MP, or 1+ Jump MP but engine is critted, or crew unconscious/dead.
+        boolean jumpImmobile = (
+                super.isImmobile(true) ||
+                super.isPermanentlyImmobilized(true) ||
+                (getJumpMP() == 0) ||
+                (isEngineHit())
+        );
+        return jumpImmobile;
+    }
+
     @Override
     public boolean hasCommandConsoleBonus() {
         if (!hasWorkingMisc(MiscType.F_COMMAND_CONSOLE) || isCommanderHit() || isUsingConsoleCommander()) {

megamek/src/megamek/server/GameManager.java

@@ -33657,7 +33657,7 @@ private Vector<Report> vehicleMotiveDamage(Tank te, int modifier, boolean noRoll
             default:
                 break;
         }
-        // Apply vehicle effectiveness...except for jumps.
+        // Apply vehicle effectiveness...except for hits from jumps.
         if (game.getOptions().booleanOption(OptionsConstants.ADVCOMBAT_TACOPS_VEHICLE_EFFECTIVE)
                 && !jumpDamage) {
             modifier = Math.max(modifier - 1, 0);
@@ -33722,7 +33722,7 @@ private Vector<Report> vehicleMotiveDamage(Tank te, int modifier, boolean noRoll
         // unsure how to *report* any outcomes from there. Note that these treat
         // being reduced to 0 MP and being actually immobilized as the same thing,
         // which for these particular purposes may or may not be the intent of
-        // the rules in all cases.
+        // the rules in all cases (for instance, motive-immobilized CVs can still jump).
         // Immobile hovercraft on water sink...
         if (!te.isOffBoard() && (te.getMovementMode() == EntityMovementMode.HOVER
                 && (te.isMovementHitPending() || (te.getWalkMP() <= 0))

megamek/unittests/megamek/common/EntityTest.java

@@ -26,8 +26,7 @@
 import java.io.File;
 import java.util.ArrayList;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.fail;
+import static org.junit.jupiter.api.Assertions.*;
 import static org.mockito.ArgumentMatchers.*;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
@@ -73,14 +72,14 @@ public void testCalculateBattleValue() {
         actual = testEntity.calculateBattleValue(true, true);
         assertEquals(expected, actual);
     }
-    
+
     @Test
     public void testCalculateWeight() {
-        File f; 
+        File f;
         MechFileParser mfp;
         Entity e;
         int expectedWeight, computedWeight;
-        
+
         // Test 1/1
         try {
             f = new File("data/mechfiles/mechs/3050U/Exterminator EXT-4A.mtf");
@@ -93,4 +92,65 @@ public void testCalculateWeight() {
             fail(ex.getMessage());
         }
     }
+
+    /**
+     * Verify new Tank method .isImmobilizedForJump() returns correct values in
+     * various states.  Note: vehicles cannot lose individual Jump Jets via crits,
+     * so this is not tested.
+     */
+    @Test
+    public void testIsImmobilizedForJump() {
+        File f;
+        MechFileParser mfp;
+        Entity e;
+
+        // Test 1/1
+        try {
+            f = new File("data/mechfiles/vehicles/3050U/Kanga Medium Hovertank.blk");
+            mfp  = new MechFileParser(f);
+            e = mfp.getEntity();
+            Tank t = (Tank) e;
+            Crew c = t.getCrew();
+
+            // 1 Crew condition
+            // 1.a Killed crew should prevent jumping; live crew should allow jumping
+            c.setDead(true);
+            assertTrue(t.isImmobileForJump());
+            c.resetGameState();
+            assertFalse(t.isImmobileForJump());
+
+            // 1.b Unconscious crew should prevent jumping; conscious crew should allow jumping
+            c.setUnconscious(true);
+            assertTrue(t.isImmobileForJump());
+            c.resetGameState();
+            assertFalse(t.isImmobileForJump());
+
+            // 1.c Stunned crew should _not_ prevent jumping
+            t.setStunnedTurns(1);
+            assertFalse(t.isImmobileForJump());
+            t.setStunnedTurns(0);
+
+            // 2. Engine condition
+            // 2.a Engine hit should prevent jumping; fixing engine should enable jumping
+            t.engineHit();
+            assertTrue(t.isImmobileForJump());
+            t.engineFix();
+            assertFalse(t.isImmobileForJump());
+
+            // 2.b Shutdown should prevent jumping; restarting should enable jumping
+            t.setShutDown(true);
+            assertTrue(t.isImmobileForJump());
+            t.setShutDown(false);
+            assertFalse(t.isImmobileForJump());
+
+            // 3. Immobilization due to massive damage motive hit / reducing MP to 0 should
+            //    _not_ prevent jumping
+            t.setMotiveDamage(t.getOriginalWalkMP());
+            assertFalse(t.isImmobileForJump());
+            t.setMotiveDamage(0);
+
+        } catch (Exception ex) {
+            fail(ex.getMessage());
+        }
+    }
 }