From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: BillyGalbreath <Blake.Galbreath@GMail.com>
Date: Fri, 10 Nov 2017 23:03:12 -0500
Subject: [PATCH] ExperienceOrb merging/stacking API and fixes

Adds an option for maximum exp value when merging orbs

Adds ExperienceOrbMergeEvent
Fired when the server is about to merge 2 experience orbs
as entities. Plugins can cancel it if they want to ensure experience orbs do not lose important
metadata such as spawn reason, or conditionally move data from source to target.

Fixes an issue where the stacked count was not taking into account
for mending repairs and when merging with spigot's merge-on-spawn
logic

== AT ==
public net.minecraft.world.entity.ExperienceOrb count

Co-authored-by: Aikar <aikar@aikar.co>
Co-authored-by: Jake Potrebic <jake.m.potrebic@gmail.com>

diff --git a/src/main/java/net/minecraft/world/entity/ExperienceOrb.java b/src/main/java/net/minecraft/world/entity/ExperienceOrb.java
index 0916e24271d07ad5db51c5bc68791722b0f69c2b..a758b2456acac23095fe4619ae10300a034cb460 100644
--- a/src/main/java/net/minecraft/world/entity/ExperienceOrb.java
+++ b/src/main/java/net/minecraft/world/entity/ExperienceOrb.java
@@ -244,6 +244,7 @@ public class ExperienceOrb extends Entity {
     }
 
     private static boolean tryMergeToExisting(ServerLevel world, Vec3 pos, int amount) {
+        // Paper - TODO some other event for this kind of merge
         AABB axisalignedbb = AABB.ofSize(pos, 1.0D, 1.0D, 1.0D);
         int j = world.getRandom().nextInt(40);
         List<ExperienceOrb> list = world.getEntities(EntityTypeTest.forClass(ExperienceOrb.class), axisalignedbb, (entityexperienceorb) -> {
@@ -270,6 +271,11 @@ public class ExperienceOrb extends Entity {
     }
 
     private void merge(ExperienceOrb other) {
+        // Paper start - call orb merge event
+        if (!new com.destroystokyo.paper.event.entity.ExperienceOrbMergeEvent((org.bukkit.entity.ExperienceOrb) this.getBukkitEntity(), (org.bukkit.entity.ExperienceOrb) other.getBukkitEntity()).callEvent()) {
+            return;
+        }
+        // Paper end - call orb merge event
         this.count += other.count;
         this.age = Math.min(this.age, other.age);
         other.discard(EntityRemoveEvent.Cause.MERGE); // CraftBukkit - add Bukkit remove cause
@@ -360,7 +366,7 @@ public class ExperienceOrb extends Entity {
                 int l = amount - k * amount / j;
 
                 if (l > 0) {
-                    this.value = l; // CraftBukkit - update exp value of orb for PlayerItemMendEvent calls
+                    // this.value = l; // CraftBukkit - update exp value of orb for PlayerItemMendEvent calls // Paper - the value field should not be mutated here because it doesn't take "count" into account
                     return this.repairPlayerItems(player, l);
                 }
             }
diff --git a/src/main/java/org/bukkit/craftbukkit/entity/CraftExperienceOrb.java b/src/main/java/org/bukkit/craftbukkit/entity/CraftExperienceOrb.java
index 5a7d314ec0562e472f5dc45924a7b24841cff126..650e4a01cecc4cc08e7ff9ebcc4c367084351f21 100644
--- a/src/main/java/org/bukkit/craftbukkit/entity/CraftExperienceOrb.java
+++ b/src/main/java/org/bukkit/craftbukkit/entity/CraftExperienceOrb.java
@@ -18,6 +18,18 @@ public class CraftExperienceOrb extends CraftEntity implements ExperienceOrb {
         this.getHandle().value = value;
     }
 
+    // Paper start - expose count
+    @Override
+    public int getCount() {
+        return this.getHandle().count;
+    }
+
+    @Override
+    public void setCount(final int count) {
+        this.getHandle().count = count;
+    }
+    // Paper end
+
     // Paper start
     public java.util.UUID getTriggerEntityId() {
         return getHandle().triggerEntityId;
diff --git a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
index 720ea811407b4288b9923cf7db0ac44ad1a086e0..82f7a36e71ed17d44ad0e5be99f8f6fc450d0732 100644
--- a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
+++ b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
@@ -704,15 +704,29 @@ public class CraftEventFactory {
         if (entity instanceof net.minecraft.world.entity.ExperienceOrb xp) {
             double radius = world.spigotConfig.expMerge;
             if (radius > 0) {
+                // Paper start - Maximum exp value when merging; Whole section has been tweaked, see comments for specifics
+                final long maxValue = world.paperConfig().entities.behavior.experienceMergeMaxValue;
+                final boolean mergeUnconditionally = maxValue <= 0;
+                if (mergeUnconditionally || xp.value < maxValue) { // Paper - Skip iteration if unnecessary
+
                 List<Entity> entities = world.getEntities(entity, entity.getBoundingBox().inflate(radius, radius, radius));
                 for (Entity e : entities) {
                     if (e instanceof net.minecraft.world.entity.ExperienceOrb loopItem) {
-                        if (!loopItem.isRemoved()) {
+                        // Paper start
+                        if (!loopItem.isRemoved() && xp.count == loopItem.count && (mergeUnconditionally || loopItem.value < maxValue) && new com.destroystokyo.paper.event.entity.ExperienceOrbMergeEvent((org.bukkit.entity.ExperienceOrb) entity.getBukkitEntity(), (org.bukkit.entity.ExperienceOrb) loopItem.getBukkitEntity()).callEvent()) { // Paper - ExperienceOrbMergeEvent
+                            long newTotal = (long)xp.value + (long)loopItem.value;
+                            if ((int) newTotal < 0) continue; // Overflow
+                            if (!mergeUnconditionally && newTotal > maxValue) {
+                                loopItem.value = (int) (newTotal - maxValue);
+                                xp.value = (int) maxValue;
+                            } else {
                             xp.value += loopItem.value;
                             loopItem.discard(null); // Add Bukkit remove cause
+                            } // Paper end - Maximum exp value when merging
                         }
                     }
                 }
+                } // Paper end - End iteration skip check - All tweaking ends here
             }
         }
         // Spigot end
