cleanup despawn logic and add explanation (#565)

Author: HaHaWTH

leaf-server/minecraft-patches/features/0304-Rewrite-entity-despawn-time.patch

@@ -3,6 +3,19 @@ From: HaHaWTH <[email protected]>
 Date: Tue, 9 Nov 2077 00:00:00 +0800
 Subject: [PATCH] Rewrite entity despawn time
 
+HOW IT WORKS:
+In the default implementation, totalEntityAge only increments when an entity is actively ticked.
+Consequently, if an entity resides in a weak-loaded chunk, its aging process effectively pauses, allowing it to exist far beyond the configured despawn time.
+
+This patch addresses this discrepancy by synchronizing entity age with absolute server time:
+Introduces lastTickTime to record the MinecraftServer.currentTick when an entity was last ticked.
+Once the entity is ticked again or unloaded, the system calculates the elapsed time (currentTick - lastTickTime), and adds it to the totalEntityAge.
+Also, it moves the despawn logic of projectiles to their own tick method (see ThrowableProjectile#tick), making it faster to despawn massive stuck projectiles.
+
+Benchmarks (60,000 snowballs):
+Before: Around 120 seconds, server thread is struggling to process projectile physics.
+After: 1.5 seconds
+
 Brings the ability to despawn weak-loaded entities once they are ticked,
 a solution for https://github.com/PaperMC/Paper/issues/12986
 
@@ -27,14 +40,14 @@ index f58b91b277ba85fa7c0e7ad10157ecbf10023065..b7f9f22abf60c75bc09126d9168fda9a
              for (Entity entity : passengerEntity.getPassengers()) {
                  this.tickPassenger(passengerEntity, entity, isActive); // Paper - EAR 2
 diff --git a/net/minecraft/world/entity/Entity.java b/net/minecraft/world/entity/Entity.java
-index f6d619709d4e5b0e6d1b943226579d7388835cdb..fefd0df22b32bb6f933bbf9530584c2c9616bfb6 100644
+index f6d619709d4e5b0e6d1b943226579d7388835cdb..137ec39c1458e6adc854df24807f0e26770d95da 100644
 --- a/net/minecraft/world/entity/Entity.java
 +++ b/net/minecraft/world/entity/Entity.java
 @@ -373,6 +373,7 @@ public abstract class Entity implements SyncedDataHolder, Nameable, EntityAccess
      public boolean fixedPose = false; // Paper - Expand Pose API
      private final int despawnTime; // Paper - entity despawn time limit
      public int totalEntityAge; // Paper - age-like counter for all entities
-+    private int lastTickTicks; // Leaf - Rewrite entity despawn time
++    private int lastTickTime; // Leaf - Rewrite entity despawn time
      public boolean activatedPriorityReset = false; // Pufferfish - DAB
      public int activatedPriority = org.dreeam.leaf.config.modules.opt.DynamicActivationofBrain.maximumActivationPrio; // Pufferfish - DAB (golf score)
      public final io.papermc.paper.entity.activation.ActivationType activationType = io.papermc.paper.entity.activation.ActivationType.activationTypeFor(this); // Paper - EAR 2/tracking ranges
@@ -54,7 +67,7 @@ index f6d619709d4e5b0e6d1b943226579d7388835cdb..fefd0df22b32bb6f933bbf9530584c2c
      }
 
      public boolean isColliding(BlockPos pos, BlockState state) {
-@@ -887,15 +890,50 @@ public abstract class Entity implements SyncedDataHolder, Nameable, EntityAccess
+@@ -887,15 +890,46 @@ public abstract class Entity implements SyncedDataHolder, Nameable, EntityAccess
      }
 
      public void tick() {
@@ -79,33 +92,29 @@ index f6d619709d4e5b0e6d1b943226579d7388835cdb..fefd0df22b32bb6f933bbf9530584c2c
 +        if (this.despawnTime >= 0) {
 +            int missedTicks = this.calculateMissedTicks();
 +            if (missedTicks > 1) {
-+                if ((this.totalEntityAge += missedTicks) >= this.despawnTime) {
-+                    this.discard(org.bukkit.event.entity.EntityRemoveEvent.Cause.DESPAWN);
-+                    return true;
-+                }
-+            } else {
-+                if (this.totalEntityAge >= this.despawnTime) {
-+                    this.discard(org.bukkit.event.entity.EntityRemoveEvent.Cause.DESPAWN);
-+                    return true;
-+                }
++                this.totalEntityAge += missedTicks;
++            }
++            if (this.totalEntityAge >= this.despawnTime) {
++                this.discard(org.bukkit.event.entity.EntityRemoveEvent.Cause.DESPAWN);
++                return true;
 +            }
 +        }
 +        return false;
 +    }
 +
 +    private int calculateMissedTicks() {
-+        return net.minecraft.server.MinecraftServer.currentTick - this.lastTickTicks;
++        return net.minecraft.server.MinecraftServer.currentTick - this.lastTickTime;
 +    }
 +
 +    public void updateLastTick() {
-+        this.lastTickTicks = net.minecraft.server.MinecraftServer.currentTick;
++        this.lastTickTime = net.minecraft.server.MinecraftServer.currentTick;
 +    }
 +    // Leaf end - Rewrite entity despawn time
 +
      // CraftBukkit start
      public void postTick() {
          // No clean way to break out of ticking once the entity has been copied to a new world, so instead we move the portalling later in the tick cycle
-@@ -2564,6 +2602,12 @@ public abstract class Entity implements SyncedDataHolder, Nameable, EntityAccess
+@@ -2564,6 +2598,12 @@ public abstract class Entity implements SyncedDataHolder, Nameable, EntityAccess
              if (this.maxAirTicks != this.getDefaultMaxAirSupply()) {
                  output.putInt("Bukkit.MaxAirSupply", this.getMaxAirSupply());
              }