Rented array struct

src/DotRecast.Core/Buffers/RcRentedArray.cs

@@ -1,31 +1,96 @@
 using System;
 using System.Buffers;
+using System.Collections.Generic;
 using System.Runtime.CompilerServices;
+using System.Threading;
 
 namespace DotRecast.Core.Buffers
 {
     public static class RcRentedArray
     {
+        private sealed class RentIdPool
+        {
+            private int[] _generations;
+            private readonly Queue<int> _freeIds;
+            private int _maxId;
+
+            public RentIdPool(int capacity)
+            {
+                _generations = new int[capacity];
+                _freeIds = new Queue<int>(capacity);
+            }
+
+            internal RentIdGen AcquireId()
+            {
+                if (!_freeIds.TryDequeue(out int id))
+                {
+                    id = _maxId++;
+                    if(_generations.Length <= id)
+                        Array.Resize(ref _generations, _generations.Length << 1);    
+                }
+
+                return new RentIdGen(id, _generations[id]);
+            }
+
+            internal void ReturnId(int id)
+            {
+                _generations[id]++;
+                _freeIds.Enqueue(id);
+            }
+
+            internal int GetGeneration(int id)
+            {
+                return _generations.Length <= id ? 0 : _generations[id];
+            }
+        }
+        
+        public const int START_RENT_ID_POOL_CAPACITY = 16;
+        private static readonly ThreadLocal<RentIdPool> _rentPool = new ThreadLocal<RentIdPool>(() => new RentIdPool(START_RENT_ID_POOL_CAPACITY));
+        
         public static RcRentedArray<T> Rent<T>(int minimumLength)
         {
             var array = ArrayPool<T>.Shared.Rent(minimumLength);
-            return new RcRentedArray<T>(ArrayPool<T>.Shared, array, minimumLength);
+            return new RcRentedArray<T>(ArrayPool<T>.Shared, _rentPool.Value.AcquireId(), array, minimumLength);
+        }
+
+        internal static bool IsDisposed(RentIdGen rentIdGen)
+        {
+            return _rentPool.Value.GetGeneration(rentIdGen.Id) != rentIdGen.Gen;
+        }
+
+        internal static void ReturnId(RentIdGen rentIdGen)
+        {
+            _rentPool.Value.ReturnId(rentIdGen.Id);
         }
     }
 
-    public class RcRentedArray<T> : IDisposable
+    public readonly struct RentIdGen
+    {
+        public readonly int Id;
+        public readonly int Gen;
+
+        public RentIdGen(int id, int gen)
+        {
+            Id = id;
+            Gen = gen;
+        }
+    }
+
+    public struct RcRentedArray<T> : IDisposable
     {
         private ArrayPool<T> _owner;
         private T[] _array;
+        private readonly RentIdGen _rentIdGen;
 
         public int Length { get; }
-        public bool IsDisposed => null == _owner || null == _array;
+        public bool IsDisposed => null == _owner || null == _array || RcRentedArray.IsDisposed(_rentIdGen);
 
-        internal RcRentedArray(ArrayPool<T> owner, T[] array, int length)
+        internal RcRentedArray(ArrayPool<T> owner, RentIdGen rentIdGen, T[] array, int length)
         {
             _owner = owner;
             _array = array;
             Length = length;
+            _rentIdGen = rentIdGen;
         }
 
         public ref T this[int index]
@@ -34,6 +99,8 @@ namespace DotRecast.Core.Buffers
             get
             {
                 RcThrowHelper.ThrowExceptionIfIndexOutOfRange(index, Length);
+                if (IsDisposed)
+                    throw new NullReferenceException();
                 return ref _array[index];
             }
         }
@@ -51,12 +118,14 @@ namespace DotRecast.Core.Buffers
 
         public void Dispose()
         {
-            if (null != _owner && null != _array)
+            if (null != _owner && null != _array && !RcRentedArray.IsDisposed(_rentIdGen))
             {
+                RcRentedArray.ReturnId(_rentIdGen);
                 _owner.Return(_array, true);
+            }
+            
             _owner = null;
             _array = null;
         }
     }
-    }
 }

test/DotRecast.Core.Test/RcRentedArrayTest.cs

@@ -103,4 +103,19 @@ public class RcRentedArrayTest
         Assert.That(r1.IsDisposed, Is.EqualTo(true));
         Assert.That(r1.AsArray(), Is.Null);
     }
+
+    [Test]
+    public void TestIdPoolCapacity()
+    {
+        var buffer = new RcRentedArray<int>[RcRentedArray.START_RENT_ID_POOL_CAPACITY + 2];
+        for (int i = 0; i < buffer.Length; i++)
+        {
+            Assert.DoesNotThrow(() => buffer[i] = RcRentedArray.Rent<int>(4));
+        }
+        
+        for (int i = 0; i < buffer.Length; i++)
+        {
+            Assert.DoesNotThrow(() => buffer[i].Dispose());
+        }
+    }
 }