ExperienceOrb merging/stacking API

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

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

paper-api/src/main/java/com/destroystokyo/paper/event/entity/ExperienceOrbMergeEvent.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2017 Daniel Ennis (Aikar) MIT License
+ *
+ *  Permission is hereby granted, free of charge, to any person obtaining
+ *  a copy of this software and associated documentation files (the
+ *  "Software"), to deal in the Software without restriction, including
+ *  without limitation the rights to use, copy, modify, merge, publish,
+ *  distribute, sublicense, and/or sell copies of the Software, and to
+ *  permit persons to whom the Software is furnished to do so, subject to
+ *  the following conditions:
+ *
+ *  The above copyright notice and this permission notice shall be
+ *  included in all copies or substantial portions of the Software.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ *  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ *  MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ *  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ *  LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ *  OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ *  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package com.destroystokyo.paper.event.entity;
+
+import org.bukkit.entity.ExperienceOrb;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.HandlerList;
+import org.bukkit.event.entity.EntityEvent;
+import org.jetbrains.annotations.ApiStatus;
+import org.jspecify.annotations.NullMarked;
+
+/**
+ * Fired anytime the server is about to merge 2 experience orbs into one
+ */
+@NullMarked
+public class ExperienceOrbMergeEvent extends EntityEvent implements Cancellable {
+
+    private static final HandlerList HANDLER_LIST = new HandlerList();
+
+    private final ExperienceOrb mergeTarget;
+    private final ExperienceOrb mergeSource;
+
+    private boolean cancelled;
+
+    @ApiStatus.Internal
+    public ExperienceOrbMergeEvent(final ExperienceOrb mergeTarget, final ExperienceOrb mergeSource) {
+        super(mergeTarget);
+        this.mergeTarget = mergeTarget;
+        this.mergeSource = mergeSource;
+    }
+
+    /**
+     * @return The orb that will absorb the other experience orb
+     */
+    public ExperienceOrb getMergeTarget() {
+        return this.mergeTarget;
+    }
+
+    /**
+     * @return The orb that is subject to being removed and merged into the target orb
+     */
+    public ExperienceOrb getMergeSource() {
+        return this.mergeSource;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return this.cancelled;
+    }
+
+    /**
+     * @param cancel {@code true} if you wish to cancel this event, and prevent the orbs from merging
+     */
+    @Override
+    public void setCancelled(final boolean cancel) {
+        this.cancelled = cancel;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return HANDLER_LIST;
+    }
+
+    public static HandlerList getHandlerList() {
+        return HANDLER_LIST;
+    }
+}

paper-api/src/main/java/org/bukkit/entity/ExperienceOrb.java

@@ -22,6 +22,22 @@ public interface ExperienceOrb extends Entity {
      */
     public void setExperience(int value);
     
+    // Paper start - expose count
+    /**
+     * Get the stacked count for this experience orb.
+     *
+     * @return the count
+     */
+    int getCount();
+
+    /**
+     * Sets the stacked count for this experience orb.
+     *
+     * @param count the new count
+     */
+    void setCount(int count);
+    // Paper end
+
     // Paper start
     /**
      * Check if this orb was spawned from a {@link ThrownExpBottle}