Add item pickup functionality

plugin/src/main/java/me/filoghost/holographicdisplays/plugin/hologram/tracking/CollisionHelper.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) filoghost and contributors
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+package me.filoghost.holographicdisplays.plugin.hologram.tracking;
+
+import org.bukkit.Location;
+import org.bukkit.entity.Player;
+
+public class CollisionHelper {
+
+    private static final double PLAYER_WIDTH_RADIUS = 0.6 / 2;
+    private static final double PLAYER_HEIGHT_RADIUS = 1.8 / 2;
+
+    private static final double ITEM_WIDTH_RADIUS = 0.25 / 2;
+    private static final double ITEM_HEIGHT_RADIUS = 0.25 / 2;
+
+    // The server increases the player's bounding box in all directions before checking if it intersects with an item
+    private static final double PLAYER_GROW_WIDTH = 1;
+    private static final double PLAYER_GROW_HEIGHT = 0.5;
+
+    private static final double REQUIRED_DISTANCE_X = PLAYER_WIDTH_RADIUS + PLAYER_GROW_WIDTH + ITEM_WIDTH_RADIUS;
+    private static final double REQUIRED_DISTANCE_Z = REQUIRED_DISTANCE_X;
+    private static final double REQUIRED_DISTANCE_Y = PLAYER_HEIGHT_RADIUS + PLAYER_GROW_HEIGHT + ITEM_HEIGHT_RADIUS;
+
+    public static boolean isInPickupRange(Player player, double itemX, double itemY, double itemZ) {
+        /*
+         * Normally, the server determines if a player is picking up a dropped item
+         * by checking if the bounding boxes of the player and the item intersect.
+         *
+         * This can be simplified: for each direction (X, Y, Z), check if the distance between the centers of the bounding boxes
+         * is lower than the sum of the radius (half the length) of each box.
+         *
+         * If the condition is satisfied for all directions, the bounding boxes intersect. Visual example:
+         *
+         *     Box #1
+         * ._____________.
+         * |             |
+         * |             |   Box #2
+         * |      *  .___|_______________.
+         * |         |   |               |
+         * |_________|___|     *         |
+         *           |                   |
+         *           |___________________|
+         *
+         *        *------>
+         *        Radius #1
+         *
+         *           <---------*
+         *             Radius #2
+         *
+         *        *------------*
+         *   Distance between centers
+         *
+         *
+         * In the example above, the distance between the centers is lower than the sum of the two radii,
+         * so the boxes collide in that direction.
+         */
+        Location playerLocation = player.getLocation();
+
+        // The Y position is on lowest point of the bounding box, not in the center
+        double playerCenterY = playerLocation.getY() + PLAYER_HEIGHT_RADIUS;
+        double itemCenterY = itemY + ITEM_HEIGHT_RADIUS;
+
+        double xDiff = Math.abs(playerLocation.getX() - itemX);
+        double zDiff = Math.abs(playerLocation.getZ() - itemZ);
+        double yDiff = Math.abs(playerCenterY - itemCenterY);
+
+        return xDiff <= REQUIRED_DISTANCE_X
+                && yDiff <= REQUIRED_DISTANCE_Y
+                && zDiff <= REQUIRED_DISTANCE_Z;
+    }
+
+}

plugin/src/main/java/me/filoghost/holographicdisplays/plugin/hologram/tracking/ItemLineTracker.java

@@ -9,9 +9,11 @@ import me.filoghost.holographicdisplays.common.nms.EntityID;
 import me.filoghost.holographicdisplays.common.nms.NMSManager;
 import me.filoghost.holographicdisplays.common.nms.NMSPacketList;
 import me.filoghost.holographicdisplays.plugin.hologram.base.BaseItemLine;
+import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
 import org.jetbrains.annotations.MustBeInvokedByOverriders;
 
+import java.util.Collection;
 import java.util.Objects;
 
 public class ItemLineTracker extends ClickableLineTracker<BaseItemLine> {
@@ -31,6 +33,20 @@ public class ItemLineTracker extends ClickableLineTracker<BaseItemLine> {
         this.itemEntityID = nmsManager.newEntityID();
     }
 
+    @MustBeInvokedByOverriders
+    @Override
+    protected void update(Collection<? extends Player> onlinePlayers) {
+        super.update(onlinePlayers);
+
+        if (spawnItemEntities && hasTrackedPlayers()) {
+            for (Player trackedPlayer : getTrackedPlayers()) {
+                if (CollisionHelper.isInPickupRange(trackedPlayer, locationX, locationY, locationZ)) {
+                    line.onPickup(trackedPlayer);
+                }
+            }
+        }
+    }
+
     @Override
     protected boolean updatePlaceholders() {
         return false;

plugin/src/main/java/me/filoghost/holographicdisplays/plugin/hologram/tracking/LineTracker.java

@@ -46,7 +46,8 @@ public abstract class LineTracker<T extends BaseHologramLine> {
         lineChanged = true;
     }
 
-    protected final void update(Collection<? extends Player> onlinePlayers) {
+    @MustBeInvokedByOverriders
+    protected void update(Collection<? extends Player> onlinePlayers) {
         boolean sendChangesPackets = false;
 
         // First, detect the changes if the flag is on and set it off
@@ -115,10 +116,14 @@ public abstract class LineTracker<T extends BaseHologramLine> {
 
     protected abstract boolean shouldTrackPlayer(Player player);
 
-    private boolean hasTrackedPlayers() {
+    protected final boolean hasTrackedPlayers() {
         return !trackedPlayers.isEmpty();
     }
 
+    protected final Set<Player> getTrackedPlayers() {
+        return trackedPlayers;
+    }
+
     public final boolean isTrackedPlayer(Player player) {
         return trackedPlayers.contains(player);
     }