If there isn't a primary ability, use the passive ability instead

src/main/java/com/minelittlepony/unicopia/ability/Abilities.java

@@ -2,16 +2,27 @@ package com.minelittlepony.unicopia.ability;
 
 import java.util.EnumMap;
 import java.util.LinkedHashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.function.BiFunction;
+
+import com.minelittlepony.unicopia.Race;
 import com.minelittlepony.unicopia.util.Registries;
 
 import net.minecraft.util.Identifier;
+import net.minecraft.util.Util;
 import net.minecraft.util.registry.Registry;
 
 public interface Abilities {
-    Map<AbilitySlot, Set<Ability<?>>> BY_SLOT = new EnumMap<>(AbilitySlot.class);
     Registry<Ability<?>> REGISTRY = Registries.createSimple(new Identifier("unicopia", "abilities"));
+    Map<AbilitySlot, Set<Ability<?>>> BY_SLOT = new EnumMap<>(AbilitySlot.class);
+    BiFunction<AbilitySlot, Race, List<Ability<?>>> BY_SLOT_AND_RACE = Util.memoize((slot, race) -> {
+        return BY_SLOT.computeIfAbsent(slot, s -> new LinkedHashSet<>())
+                .stream()
+                .filter(a -> a.canUse(race))
+                .toList();
+    });
 
     // unicorn / alicorn
     Ability<?> CAST = register(new UnicornCastingAbility(), "cast", AbilitySlot.PRIMARY);
@@ -45,4 +56,5 @@ public interface Abilities {
         BY_SLOT.computeIfAbsent(slot, s -> new LinkedHashSet<>()).add(power);
         return Registry.register(REGISTRY, id, power);
     }
+
 }

src/main/java/com/minelittlepony/unicopia/ability/AbilityDispatcher.java

@@ -1,12 +1,10 @@
 package com.minelittlepony.unicopia.ability;
 
 import java.util.Collection;
-import java.util.Collections;
 import java.util.EnumMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
-import java.util.stream.Collectors;
 
 import org.jetbrains.annotations.Nullable;
 
@@ -70,6 +68,10 @@ public class AbilityDispatcher implements Tickable, NbtSerialisable {
         return stats.computeIfAbsent(slot, Stat::new);
     }
 
+    public boolean isFilled(AbilitySlot slot) {
+        return getStat(slot).getMaxPage() > 0;
+    }
+
     public long getMaxPage() {
         if (maxPage < 0 || prevRace != player.getSpecies()) {
             prevRace = player.getSpecies();
@@ -221,25 +223,16 @@ public class AbilityDispatcher implements Tickable, NbtSerialisable {
         }
 
         public Optional<Ability<?>> getAbility(long page) {
-            Race race = player.getSpecies();
-            List<Ability<?>> found = Abilities.BY_SLOT.computeIfAbsent(slot, c -> Collections.emptySet())
-                    .stream()
-                    .filter(a -> a.canUse(race))
-                    .collect(Collectors.toList());
+            List<Ability<?>> found = Abilities.BY_SLOT_AND_RACE.apply(slot, player.getSpecies());
             if (found.isEmpty()) {
                 return Optional.empty();
             }
-            page = Math.min(found.size() - 1, page);
 
-            return Optional.ofNullable(found.get((int)page));
+            return Optional.ofNullable(found.get((int)Math.min(found.size() - 1, page)));
         }
 
         public long getMaxPage() {
-            Race race = player.getSpecies();
-            return Abilities.BY_SLOT.computeIfAbsent(slot, c -> Collections.emptySet())
-                    .stream()
-                    .filter(a -> a.canUse(race))
-                    .count();
+            return Abilities.BY_SLOT_AND_RACE.apply(slot, player.getSpecies()).size();
         }
 
         protected synchronized Optional<Ability<?>> setActiveAbility(@Nullable Ability<?> power) {

src/main/java/com/minelittlepony/unicopia/client/KeyBindingsHandler.java

@@ -78,7 +78,10 @@ public class KeyBindingsHandler {
         } else {
             for (Binding i : keys.keySet()) {
                 AbilitySlot slot = keys.get(i);
-                if (slot == AbilitySlot.PRIMARY && client.options.keySneak.isPressed()) {
+                if (slot == AbilitySlot.PRIMARY && client.options.keySneak.isPressed() && abilities.isFilled(AbilitySlot.PASSIVE)) {
+                    slot = AbilitySlot.PASSIVE;
+                }
+                if (slot == AbilitySlot.PRIMARY && !abilities.isFilled(slot)) {
                     slot = AbilitySlot.PASSIVE;
                 }
 

src/main/java/com/minelittlepony/unicopia/client/gui/Slot.java

@@ -75,18 +75,27 @@ class Slot {
     }
 
     void renderBackground(MatrixStack matrices, AbilityDispatcher abilities, boolean bSwap, float tickDelta) {
+
+        if (aSlot != bSlot) {
+            bSwap |= !abilities.isFilled(aSlot);
+            bSwap &= abilities.isFilled(bSlot);
+        }
+
         RenderSystem.setShaderColor(1, 1, 1, 1);
         RenderSystem.enableBlend();
         matrices.push();
         matrices.translate(getX(), getY(), 0);
 
-        float cooldown = abilities.getStat(bSwap ? bSlot : aSlot).getFillProgress();
-
         // background
         UHud.drawTexture(matrices, 0, 0, backgroundU, backgroundV, size, size, 128, 128);
 
+        AbilityDispatcher.Stat stat = abilities.getStat(bSwap ? bSlot : aSlot);
+
+
         int sz = iconSize - slotPadding;
-        uHud.renderAbilityIcon(matrices, abilities.getStat(bSwap ? bSlot : aSlot), slotPadding, slotPadding, sz, sz, sz, sz);
+        uHud.renderAbilityIcon(matrices, stat, slotPadding, slotPadding, sz, sz, sz, sz);
+
+        float cooldown = stat.getFillProgress();
 
         if (cooldown > 0 && cooldown <= 1) {
             float lerpCooldown = MathHelper.lerp(tickDelta, cooldown, lastCooldown);