Expand PotionMeta Api to allow getting effective potion colour and effects (#12390)

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaPotion.java

@@ -1,6 +1,7 @@
 package org.bukkit.craftbukkit.inventory;
 
 import com.google.common.base.Preconditions;
+import com.google.common.collect.Collections2;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap.Builder;
 import java.util.ArrayList;
@@ -25,6 +26,7 @@ import org.bukkit.potion.PotionData;
 import org.bukkit.potion.PotionEffect;
 import org.bukkit.potion.PotionEffectType;
 import org.bukkit.potion.PotionType;
+import org.jetbrains.annotations.NotNull;
 
 @DelegateDeserialization(SerializableMeta.class)
 class CraftMetaPotion extends CraftMetaItem implements PotionMeta {
@@ -202,6 +204,19 @@ class CraftMetaPotion extends CraftMetaItem implements PotionMeta {
         return ImmutableList.of();
     }
 
+    @Override
+    @NotNull
+    public List<PotionEffect> getAllEffects() {
+        final ImmutableList.Builder<PotionEffect> builder = ImmutableList.builder();
+        if (this.hasBasePotionType()) {
+            builder.addAll(this.getBasePotionType().getPotionEffects());
+        }
+        if (this.hasCustomEffects()) {
+            builder.addAll(this.customEffects);
+        }
+        return builder.build();
+    }
+
     @Override
     public boolean addCustomEffect(PotionEffect effect, boolean overwrite) {
         Preconditions.checkArgument(effect != null, "Potion effect cannot be null");
@@ -305,6 +320,17 @@ class CraftMetaPotion extends CraftMetaItem implements PotionMeta {
         this.color = color == null ? null : color.asRGB();
     }
 
+    @Override
+    @NotNull
+    public Color computeEffectiveColor() {
+        if (hasColor()) return getColor();
+
+        return Color.fromRGB(
+            PotionContents.getColorOptional(Collections2.transform(getAllEffects(), CraftPotionUtil::fromBukkit))
+                .orElse(PotionContents.BASE_POTION_COLOR) & 0xFFFFFF
+        );
+    }
+
     @Override
     public boolean hasCustomPotionName() {
         return this.customName != null;