Optimise EntityScheduler ticking

The vast majority of the time, there are no tasks scheduled to
the EntityScheduler. We can avoid iterating the entire entity list
by tracking which schedulers have any tasks scheduled.

paper-server/src/main/java/io/papermc/paper/threadedregions/EntityScheduler.java

@@ -1,6 +1,7 @@
 package io.papermc.paper.threadedregions;
 
 import ca.spottedleaf.concurrentutil.util.Validate;
+import ca.spottedleaf.moonrise.common.list.ReferenceList;
 import ca.spottedleaf.moonrise.common.util.TickThread;
 import it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap;
 import net.minecraft.world.entity.Entity;
@@ -9,6 +10,8 @@ import org.bukkit.craftbukkit.entity.CraftEntity;
 import java.util.ArrayDeque;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.locks.StampedLock;
 import java.util.function.Consumer;
 
 /**
@@ -43,6 +46,8 @@ public final class EntityScheduler {
     private static final long RETIRED_TICK_COUNT = -1L;
     private final Object stateLock = new Object();
     private final Long2ObjectOpenHashMap<List<ScheduledTask>> oneTimeDelayed = new Long2ObjectOpenHashMap<>();
+    private EntitySchedulerTickList scheduledList;
+    private boolean insideScheduledList;
 
     private final ArrayDeque<ScheduledTask> currentlyExecuting = new ArrayDeque<>();
 
@@ -50,6 +55,51 @@ public final class EntityScheduler {
         this.entity = Validate.notNull(entity);
     }
 
+    // must own state lock
+    private boolean hasTasks() {
+        return !this.currentlyExecuting.isEmpty() || !this.oneTimeDelayed.isEmpty();
+    }
+
+    public void registerTo(final EntitySchedulerTickList newTickList) {
+        synchronized (this.stateLock) {
+            final EntitySchedulerTickList prevList = this.scheduledList;
+            if (prevList == newTickList) {
+                return;
+            }
+            this.scheduledList = newTickList;
+
+            // make sure tasks scheduled before registration can be ticked
+            if (prevList == null && this.hasTasks()) {
+                this.insideScheduledList = true;
+            }
+
+            // transfer to new list
+            if (this.insideScheduledList) {
+                if (prevList != null) {
+                    prevList.remove(this);
+                }
+                if (newTickList != null) {
+                    newTickList.add(this);
+                } else {
+                    // retired
+                    this.insideScheduledList = false;
+                }
+            }
+        }
+    }
+
+    /**
+     * Returns whether this scheduler is retired.
+     *
+     * <p>
+     * Note: This should only be invoked on the owning thread for the entity.
+     * </p>
+     * @return whether this scheduler is retired.
+     */
+    public boolean isRetired() {
+        return this.tickCount == RETIRED_TICK_COUNT;
+    }
+
     /**
      * Retires the scheduler, preventing new tasks from being scheduled and invoking the retired callback
      * on all currently scheduled tasks.
@@ -66,6 +116,7 @@ public final class EntityScheduler {
                 throw new IllegalStateException("Already retired");
             }
             this.tickCount = RETIRED_TICK_COUNT;
+            this.registerTo(null);
         }
 
         final Entity thisEntity = this.entity.getHandleRaw();
@@ -127,6 +178,11 @@ public final class EntityScheduler {
             this.oneTimeDelayed.computeIfAbsent(this.tickCount + Math.max(1L, delay), (final long keyInMap) -> {
                 return new ArrayList<>();
             }).add(task);
+
+            if (!this.insideScheduledList && this.scheduledList != null) {
+                this.scheduledList.add(this);
+                this.insideScheduledList = true;
+            }
         }
 
         return true;
@@ -147,6 +203,12 @@ public final class EntityScheduler {
                 throw new IllegalStateException("Ticking retired scheduler");
             }
             ++this.tickCount;
+
+            if (this.scheduledList != null && !this.hasTasks()) {
+                this.scheduledList.remove(this);
+                this.insideScheduledList = false;
+            }
+
             if (this.oneTimeDelayed.isEmpty()) {
                 toRun = null;
             } else {
@@ -178,4 +240,34 @@ public final class EntityScheduler {
             }
         }
     }
+
+    public static final class EntitySchedulerTickList {
+
+        private static final EntityScheduler[] ENTITY_SCHEDULER_ARRAY = new EntityScheduler[0];
+
+        private final ReferenceList<EntityScheduler> entitySchedulers = new ReferenceList<>(ENTITY_SCHEDULER_ARRAY);
+
+        public boolean add(final EntityScheduler scheduler) {
+            synchronized (this) {
+                return this.entitySchedulers.add(scheduler);
+            }
+        }
+
+        public void remove(final EntityScheduler scheduler) {
+            synchronized (this) {
+                this.entitySchedulers.remove(scheduler);
+            }
+        }
+
+        public EntityScheduler[] getAllSchedulers() {
+            EntityScheduler[] ret = new EntityScheduler[this.entitySchedulers.size()];
+            synchronized (this) {
+                if (ret.length != this.entitySchedulers.size()) {
+                    ret = new EntityScheduler[this.entitySchedulers.size()];
+                }
+                System.arraycopy(this.entitySchedulers.getRawDataUnchecked(), 0, ret, 0, this.entitySchedulers.size());
+                return ret;
+            }
+        }
+    }
 }