null-out removed elements from SlimBag<T> and ActiveList<T>

src/Jitter2/Collision/DynamicTree.cs

@@ -37,7 +37,7 @@ namespace Jitter2.Collision;
 /// <typeparam name="T">The type of elements stored in the dynamic tree.</typeparam>
 public class DynamicTree<T> where T : class, IDynamicTreeProxy, IListIndex
 {
-    private SlimBag<T>[] lists = Array.Empty<SlimBag<T>>();
+    private volatile SlimBag<T>[] lists = Array.Empty<SlimBag<T>>();
 
     private readonly ActiveList<T> activeList;
 
@@ -466,6 +466,11 @@ private void ScanForMovedProxies(Parallel.Batch batch)
 
             // else proxy is well contained within the nodes expanded Box:
         }
+
+        // Make sure we do not hold too many dangling references
+        // in the internal array of the SlimBag<T> data structure which might
+        // prevent GC. But do only free them one-by-one to prevent overhead.
+        list.NullOutOne();
     }
 
     private void ScanForOverlaps(int fraction, bool add)

src/Jitter2/DataStructures/ActiveList.cs

@@ -129,7 +129,12 @@ public T this[int i]
 
     public void Clear()
     {
-        Array.Clear(elements, 0, Count);
+        for (int i = 0; i < Count; i++) 
+        {
+            elements[i].ListIndex = -1;
+            elements[i] = null!;
+        }
+
         Count = 0;
         Active = 0;
     }
@@ -188,8 +193,15 @@ public void Remove(T element)
         Debug.Assert(element.ListIndex != -1);
 
         MoveToInactive(element);
+
+        int li = element.ListIndex;
+
         Count -= 1;
-        Swap(Count, element.ListIndex);
+
+        elements[li] = elements[Count];
+        elements[li].ListIndex = li;
+        elements[Count] = null!;
+
         element.ListIndex = -1;
     }
 

src/Jitter2/DataStructures/SlimBag.cs

@@ -160,4 +160,13 @@ public void NullOut()
         Array.Clear(array, counter, nullOut - counter);
         nullOut = counter;
     }
+
+    /// <summary>
+    /// Null out a single position in the internal array.
+    /// </summary>
+    public void NullOutOne()
+    {
+        if(nullOut <= counter) return;
+        array[--nullOut] = default!;
+    }
 }

src/JitterTests/MiscTests.cs

@@ -35,6 +35,31 @@ public static void SlimBagTest()
         bag.NullOut();
         Assert.That(bag[0], Is.Not.EqualTo(null));
         Assert.That(bag[1], Is.EqualTo(null));
+
+        bag.Clear();
+        bag.NullOut();
+
+        bag.Add(new object());
+        bag.Add(new object());
+        bag.Add(new object());
+
+        bag.NullOutOne();
+
+        Assert.That(bag[0], Is.Not.EqualTo(null));
+        Assert.That(bag[1], Is.Not.EqualTo(null));
+        Assert.That(bag[2], Is.Not.EqualTo(null));
+
+        bag.NullOutOne();
+        Assert.That(bag[2], Is.Not.EqualTo(null));
+
+        bag.NullOutOne();
+        Assert.That(bag[2], Is.EqualTo(null));
+
+        bag.NullOutOne();
+
+        Assert.That(bag[0], Is.Not.EqualTo(null));
+        Assert.That(bag[1], Is.Not.EqualTo(null));
+        Assert.That(bag[2], Is.EqualTo(null));
     }
 
     [TestCase]