Use a dynamic pool size for the iterator cache

Many codepaths only end up with one iterator being used at a time and
most of the rest only get up to two being used so using a static pool of
three is wasteful. This also allows us to efficiently handle cases that
exceed 3 iterators in use. Overall this dramatically increases the hit rate
and results in less iterators being created.

src/main/java/net/minecraft/server/PathfinderGoalSelector.java

@@ -111,11 +111,13 @@ public class PathfinderGoalSelector {
                     // CraftBukkit - switch order
                     if (!this.a(pathfindergoalselectoritem, pathfindergoalselectoritem1) && this.b.contains(pathfindergoalselectoritem1)) {
                         // this.c.b(); // CraftBukkit - not in production code
+                        ((UnsafeList.Itr) iterator).valid = false; // CraftBukkit - mark iterator for reuse
                         return false;
                     }
                 // CraftBukkit - switch order
                 } else if (!pathfindergoalselectoritem1.a.g() && this.b.contains(pathfindergoalselectoritem1)) {
                     // this.c.b(); // CraftBukkit - not in production code
+                    ((UnsafeList.Itr) iterator).valid = false; // CraftBukkit - mark iterator for reuse
                     return false;
                 }
             }

src/main/java/org/bukkit/craftbukkit/util/UnsafeList.java

@@ -14,25 +14,28 @@ import java.util.RandomAccess;
 // implementation of an ArrayList that offers a getter without range checks
 @SuppressWarnings("unchecked")
 public class UnsafeList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Serializable {
-    private static final long serialVersionUID = 8683452581112892190L;
+    private static final long serialVersionUID = 8683452581112892191L;
 
     private transient Object[] data;
     private int size;
     private int initialCapacity;
 
-    private Iterator[] iterPool = new Iterator[3];
+    private Iterator[] iterPool = new Iterator[1];
+    private int maxPool;
     private int poolCounter;
 
-    public UnsafeList(int capacity) {
+    public UnsafeList(int capacity, int maxIterPool) {
         super();
         if (capacity < 0) capacity = 32;
         int rounded = Integer.highestOneBit(capacity - 1) << 1;
         data = new Object[rounded];
         initialCapacity = rounded;
-
-        for (int i = 0; i < iterPool.length; i++) {
-            iterPool[i] = new Itr();
+        maxPool = maxIterPool;
+        iterPool[0] = new Itr();
     }
+
+    public UnsafeList(int capacity) {
+        this(capacity, 5);
     }
 
     public UnsafeList() {
@@ -143,32 +146,39 @@ public class UnsafeList<E> extends AbstractList<E> implements List<E>, RandomAcc
         copy.data = Java15Compat.Arrays_copyOf(data, size);
         copy.size = size;
         copy.initialCapacity = initialCapacity;
-        copy.iterPool = new Iterator[iterPool.length];
+        copy.iterPool = new Iterator[1];
+        copy.iterPool[0] = new Itr();
+        copy.maxPool = maxPool;
+        copy.poolCounter = 0;
         return copy;
     }
 
     public Iterator<E> iterator() {
-        Itr iterator = null;
-        poolCounter = poolCounter++ % iterPool.length;
-
         // Try to find an iterator that isn't in use
         for (Iterator iter : iterPool) {
             if (!((Itr) iter).valid) {
-                iterator = (Itr) iter;
-                break;
-            }
-        }
-
-        // Couldn't find a free one, round robin replace one with a new iterator
-        // This is done in the hope that the new one finishes so can be reused
-        if (iterator == null) {
-            iterPool[poolCounter] = new Itr();
-            iterator = (Itr) iterPool[poolCounter];
-        }
-
+                Itr iterator = (Itr) iter;
                 iterator.reset();
                 return iterator;
             }
+        }
+
+        // Couldn't find one, see if we can grow our pool size
+        if (iterPool.length < maxPool) {
+            Iterator[] newPool = new Iterator[iterPool.length + 1];
+            System.arraycopy(iterPool, 0, newPool, 0, iterPool.length);
+            iterPool = newPool;
+
+            iterPool[iterPool.length - 1] = new Itr();
+            return iterPool[iterPool.length - 1];
+        }
+
+        // Still couldn't find a free one, round robin replace one with a new iterator
+        // This is done in the hope that the new one finishes so can be reused
+        poolCounter = ++poolCounter % iterPool.length;
+        iterPool[poolCounter] = new Itr();
+        return iterPool[poolCounter];
+    }
 
     private void rangeCheck(int index) {
         if (index >= size || index < 0) {
@@ -192,6 +202,7 @@ public class UnsafeList<E> extends AbstractList<E> implements List<E>, RandomAcc
         for (int i = 0; i < size; i++) {
             os.writeObject(data[i]);
         }
+        os.writeInt(maxPool);
     }
 
     private void readObject(ObjectInputStream is) throws IOException, ClassNotFoundException {
@@ -203,13 +214,16 @@ public class UnsafeList<E> extends AbstractList<E> implements List<E>, RandomAcc
         for (int i = 0; i < size; i++) {
             data[i] = is.readObject();
         }
+        maxPool = is.readInt();
+        iterPool = new Iterator[1];
+        iterPool[0] = new Itr();
     }
 
-    private class Itr implements Iterator<E> {
+    public class Itr implements Iterator<E> {
         int index;
         int lastRet = -1;
         int expectedModCount = modCount;
-        boolean valid = true;
+        public boolean valid = true;
 
         public void reset() {
             index = 0;