RecyclableList.IndexOf performance improvements.

Recyclable.Collections/ItemRange.cs

@@ -5,7 +5,6 @@ namespace Recyclable.Collections
 	public sealed class ItemRange
 	{
 		public int BlockIndex;
-		public int StartingItemIndex;
 		public long ItemsToSearchCount;
 		internal bool _returned;
 
@@ -13,11 +12,10 @@ namespace Recyclable.Collections
 		{
 		}
 
-		public ItemRange(int blockIndex, int startingItemIndex, long itemsToSearchCount)
+		public ItemRange(int blockIndex, long itemsToSearchCount)
 		{
 			BlockIndex = blockIndex;
 			ItemsToSearchCount = itemsToSearchCount;
-			StartingItemIndex = startingItemIndex;
 		}
 
 		public void Dispose()

Recyclable.Collections/ItemRangesIterator.cs

@@ -8,31 +8,22 @@ namespace Recyclable.Collections
 		[MethodImpl(MethodImplOptions.AggressiveOptimization | MethodImplOptions.NoInlining)]
 		public static void Iterate<TList, T>(in ParallelSynchronizationContext context, in SearchConfiguration<TList, T> search, Func<ParallelSynchronizationContext, SearchConfiguration<TList, T>, ItemRange, bool> action)
 		{
-			int itemIndex = 0;
 			ItemRange itemRange;
 			int blockIndex = search.StartingBlockIndex;
 			long itemsCount = search.ItemsCount;
-			while (itemsCount > search.Step)
+			while (itemsCount > search.BlockSize)
 			{
-				itemRange = new(blockIndex, itemIndex, search.Step);
+				itemRange = new(blockIndex, search.BlockSize);
 				if (!action(context, search, itemRange))
 				{
 					break;
 				}
 
-				itemsCount -= search.Step;
+				itemsCount -= search.BlockSize;
-				//blockIndex = (int)Math.DivRem(itemIndex + step, blockSize, out itemIndex);
+				blockIndex++;
-				blockIndex += (int)((itemIndex + search.Step) >> search.BlockSizePow2BitShift);
-				itemIndex = (int)((itemIndex + search.Step) & search.BlockSizeMinus1);
-				//itemIndex += step;
-				//while (itemIndex >= blockSize)
-				//{
-				//	blockIndex++;
-				//	itemIndex -= blockSize;
-				//}
 			}
 
-			itemRange = new(blockIndex, itemIndex, itemsCount);
+			itemRange = new(blockIndex, itemsCount);
 			_ = action(context, search, itemRange);
 		}
 	}

Recyclable.Collections/Parallel/ParallelSynchronizationContext.cs

@@ -2,7 +2,7 @@ namespace Recyclable.Collections.Parallel
 {
 	public sealed class ParallelSynchronizationContext : IDisposable
 	{
-		public readonly ManualResetEventSlimmer ItemFoundSignal;
+		internal bool _isItemFound;
 
 		public readonly Barrier AllDoneSignal;
 		public long FoundItemIndex;
@@ -11,16 +11,21 @@ namespace Recyclable.Collections.Parallel
 
 		public ParallelSynchronizationContext()
 		{
-			ItemFoundSignal = ManualResetEventSlimmerPool.Create(false);
+			AllDoneSignal = new(0);
-            AllDoneSignal = new(0);
 		}
 
 		public ParallelSynchronizationContext(int participantCount)
 		{
-			ItemFoundSignal = ManualResetEventSlimmerPool.Create(false);
 			AllDoneSignal = new(participantCount);
 		}
 
+		public void SetItemFound()
+		{
+			_isItemFound = true;
+		}
+
+		public bool IsItemFound => _isItemFound;
+
 		public void Dispose()
 		{
 			if (!_returned)
@@ -30,7 +35,6 @@ namespace Recyclable.Collections.Parallel
 			}
 			else
 			{
-				ItemFoundSignal.Dispose();
 				AllDoneSignal.Dispose();
 				GC.SuppressFinalize(this);
 			}

Recyclable.Collections/Parallel/ParallelSynchronizationContextPool.cs

@@ -17,9 +17,9 @@ namespace Recyclable.Collections.Parallel
 				_updateLock.Exit(false);
 			}
 
-			if (context.ItemFoundSignal._isSet)
+			if (context._isItemFound)
 			{
-				context.ItemFoundSignal.Reset();
+				context._isItemFound = false;
 			}
 
 			switch (context.AllDoneSignal.ParticipantsRemaining)
@@ -40,7 +40,7 @@ namespace Recyclable.Collections.Parallel
 
 			if (context.FoundItemIndex >= 0)
 			{
-				context.FoundItemIndex = -1L;
+				context.FoundItemIndex = RecyclableDefaults.ItemNotFoundIndexLong;
 			}
 
 			context._returned = false;

Recyclable.Collections/Pools/ItemRangePool.cs

@@ -7,7 +7,7 @@ namespace Recyclable.Collections.Pools
 		private static readonly SpinLock _updateLock = new(false);
 		public static readonly ObjectPool<ItemRange> Shared = new DefaultObjectPool<ItemRange>(new DefaultPooledObjectPolicy<ItemRange>());
 
-		public static ItemRange Create(int blockIndex, int startingItemIndex, long itemsToSearchCount)
+		public static ItemRange Create(int blockIndex, long itemsToSearchCount)
 		{
 			bool lockTaken = false;
 			_updateLock.Enter(ref lockTaken);
@@ -20,7 +20,6 @@ namespace Recyclable.Collections.Pools
 			itemRange._returned = false;
 			itemRange.BlockIndex = blockIndex;
 			itemRange.ItemsToSearchCount = itemsToSearchCount;
-			itemRange.StartingItemIndex = startingItemIndex;
 			return itemRange;
 		}
 	}

Recyclable.Collections/RecyclableDefaults.cs

@@ -4,7 +4,10 @@
 	{
 		public const int BlockSize = 16_384;
 		public const int Capacity = 1;
+		public const long MinItemsCountForParallelization = 786_432;
 		public const int MinPooledArrayLength = 128;
 		public const int MaxPooledBlockSize = 2_147_483_591;
+		public const long ItemNotFoundIndexLong = -1L;
+		public const int ItemNotFoundIndex = -1;
 	}
 }

Recyclable.Collections/RecyclableList.RecyclableListIndexOfHelpers.cs

@@ -0,0 +1,295 @@
+using System.Runtime.CompilerServices;
+using Recyclable.Collections.Parallel;
+
+namespace Recyclable.Collections
+{
+	public partial class RecyclableList<T>
+	{
+		public static class RecyclableListIndexOfHelpers
+		{
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+			// [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			// [MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.NoOptimization)]
+			private static void ScheduleIndexOfTask(ParallelSynchronizationContext context, RecyclableList<T> list, int blockIndex, T itemToFind)
+			{
+				_ = context.AllDoneSignal.AddParticipant();
+				// & WAS SLOWER
+				// _ = ThreadPool.QueueUserWorkItem<object?>((_) =>
+				_ = Task.Factory.StartNew(() =>
+				{
+					try
+					{
+						if (context._isItemFound)
+						{
+							return;
+						}
+
+						long index = Array.IndexOf(list._memoryBlocks[blockIndex], itemToFind, 0, list._blockSize);
+						if (context._isItemFound)
+						{
+							return;
+						}
+						else if (index >= 0)
+						{
+							context._isItemFound = true;
+							_ = Interlocked.Exchange(ref context.FoundItemIndex, (blockIndex << list._blockSizePow2BitShift) + index);
+							return;
+						}
+
+						index = Array.IndexOf(list._memoryBlocks[blockIndex + 1], itemToFind, 0, list._blockSize);
+						if (context._isItemFound)
+						{
+							return;
+						}
+						else if (index >= 0)
+						{
+							context._isItemFound = true;
+							_ = Interlocked.Exchange(ref context.FoundItemIndex, ((blockIndex + 1) << list._blockSizePow2BitShift) + index);
+							return;
+						}
+
+						index = Array.IndexOf(list._memoryBlocks[blockIndex + 2], itemToFind, 0, list._blockSize);
+						if (context._isItemFound)
+						{
+							return;
+						}
+						else if (index >= 0)
+						{
+							context._isItemFound = true;
+							_ = Interlocked.Exchange(ref context.FoundItemIndex, ((blockIndex + 2) << list._blockSizePow2BitShift) + index);
+							return;
+						}
+
+						index = Array.IndexOf(list._memoryBlocks[blockIndex + 3], itemToFind, 0, list._blockSize);
+						if (!context._isItemFound && index >= 0)
+						{
+							context._isItemFound = true;
+							_ = Interlocked.Exchange(ref context.FoundItemIndex, ((blockIndex + 3) << list._blockSizePow2BitShift) + index);
+						}
+
+						//itemsToSearch -= blockSize;
+						//if (itemsToSearch > 0)
+						//{
+						//	if (itemsToSearch < blockSize)
+						//	{
+						//		blockSize = (int)itemsToSearch;
+						//	}
+
+						//	index = Array.IndexOf(list._memoryBlocks[blockIndex + 2], itemToFind, 0, blockSize);
+						//	if (index >= 0)
+						//	{
+						//		if (!context.ItemFoundSignal.IsSet)
+						//		{
+						//			context.ItemFoundSignal.Set();
+						//			_ = Interlocked.Exchange(ref context.FoundItemIndex, ((blockIndex + 2) << list._blockSizePow2BitShift) + index);
+						//		}
+
+						//		return;
+						//	}
+						//}
+
+						//itemsToSearch -= blockSize;
+						//if (itemsToSearch > 0)
+						//{
+						//	if (itemsToSearch < blockSize)
+						//	{
+						//		blockSize = (int)itemsToSearch;
+						//	}
+
+						//	index = Array.IndexOf(list._memoryBlocks[blockIndex + 3], itemToFind, 0, blockSize);
+						//	if (!context.ItemFoundSignal.IsSet && index >= 0)
+						//	{
+						//		context.ItemFoundSignal.Set();
+						//		_ = Interlocked.Exchange(ref context.FoundItemIndex, ((blockIndex + 3) << list._blockSizePow2BitShift) + index);
+
+						//		return;
+						//	}
+						//}
+					}
+					catch (Exception e)
+					{
+						e.Data.Add("list", list);
+						e.Data.Add("itemToFind", itemToFind);
+						e.Data.Add("blockIndex", blockIndex);
+						_ = Interlocked.Exchange(ref context.Exception, e);
+					}
+					finally
+					{
+						context.AllDoneSignal.RemoveParticipant();
+					}
+				});
+			}
+
+			[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+			// [MethodImpl(MethodImplOptions.AggressiveInlining)]
+			// [MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.NoOptimization)]
+			private static void ScheduleIndexOfTaskSingle(ParallelSynchronizationContext context, RecyclableList<T> list, int blockIndex, T itemToFind)
+			{
+				_ = context.AllDoneSignal.AddParticipant();
+				// & WAS SLOWER
+				// _ = ThreadPool.QueueUserWorkItem<object?>((_) =>
+				_ = Task.Factory.StartNew(() =>
+				{
+					try
+					{
+						if (context._isItemFound)
+						{
+							return;
+						}
+
+						long index = Array.IndexOf(list._memoryBlocks[blockIndex], itemToFind, 0, list._blockSize);
+						if (context._isItemFound)
+						{
+							return;
+						}
+						else if (index >= 0)
+						{
+							context._isItemFound = true;
+							_ = Interlocked.Exchange(ref context.FoundItemIndex, (blockIndex << list._blockSizePow2BitShift) + index);
+						}
+					}
+					catch (Exception e)
+					{
+						_ = Interlocked.Exchange(ref context.Exception, e);
+					}
+					finally
+					{
+						context.AllDoneSignal.RemoveParticipant();
+					}
+				});
+			}
+
+			private const int BlocksForEachTask = 4;
+
+			[MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+			public static long IndexOfParallel(RecyclableList<T> list, in T item)
+			{
+				// & WAS SLOWER
+				//long itemsCount = Array.IndexOf(list._memoryBlocks[0], item, 0, (int)Math.Min(list._blockSize, list._longCount));
+				//if (itemsCount >= 0)
+				//{
+				//	return itemsCount;
+				//}
+
+				//using var context = new ParallelSynchronizationContext(1);
+				ParallelSynchronizationContext context = ParallelSynchronizationContextPool.GetWithOneParticipant();
+				// (long)(_longCount * 0.329) => most efficient step, based on benchmarks
+				//Iterate(this, context, item);
+
+				// TODO: Switch to use _blockSize instead of list._longCountIndeexOfStep for simplifications & better performance.
+
+				// & WAS SLOWER without
+				int blockIndex = 2;
+				// & WAS SLOWER
+				//long itemsCount = list._longCount - list._blockSize;
+				// & WAS SLOWER
+				// int blockSize = _blockSize;
+				// & WAS SLOWER
+				// while ((blockIndex << list._blockSizePow2BitShift) + itemIndex > step)
+				//long blockSize = list._blockSize << 1;
+				int lastBlockWithData = list._lastBlockWithData - BlocksForEachTask;
+				while (blockIndex < lastBlockWithData)
+				{
+					// At this point itemIndex is limited to blockSize range - int.
+					ScheduleIndexOfTask(context, list, blockIndex, item);
+					if (context._isItemFound)
+					{
+						break;
+					}
+
+					blockIndex += BlocksForEachTask;
+				}
+
+				if (!context._isItemFound)
+				{
+					while (blockIndex < list._lastBlockWithData)
+					{
+						ScheduleIndexOfTaskSingle(context, list, blockIndex, item);
+						if (context._isItemFound)
+						{
+							break;
+						}
+
+						blockIndex++;
+					}
+				}
+
+				int itemIndex = Array.IndexOf(list._memoryBlocks[list._lastBlockWithData], item, 0, list._nextItemIndex > 0 ? list._nextItemIndex : list._blockSize);
+				if (itemIndex >= 0)
+				{
+					context._isItemFound = true;
+					_ = Interlocked.Exchange(ref context.FoundItemIndex, ((long)blockIndex << list._blockSizePow2BitShift) + itemIndex);
+				}
+
+				context.AllDoneSignal.SignalAndWait();
+				if (context.Exception == null)
+				{
+					return context.FoundItemIndex;
+				}
+
+				context.Exception.CaptureAndRethrow();
+				return RecyclableDefaults.ItemNotFoundIndexLong;
+			}
+
+			[MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+			public static long IndexOfSequential(RecyclableList<T> list, in T item) //, in T[][] memoryBlocks, int lastBlockIndex, int blockSize, int nextItemIndex)
+			{
+				int itemIndex;
+				// & WAS SLOWER
+				//int lastBlockWithData = list._lastBlockWithData;
+				Span<T[]> memoryBlocksSpan = new(list._memoryBlocks, 0, list._lastBlockWithData + 1);
+				int blockIndex;
+				for (blockIndex = 1; blockIndex < list._lastBlockWithData; blockIndex += 4)
+				{
+					itemIndex = Array.IndexOf(memoryBlocksSpan[blockIndex], item, 0, list._blockSize);
+					if (itemIndex >= 0)
+					{
+						return itemIndex + ((long)blockIndex << list._blockSizePow2BitShift);
+					}
+
+					if (blockIndex + 1 <  list._lastBlockWithData)
+					{
+						itemIndex = Array.IndexOf(memoryBlocksSpan[blockIndex + 1], item, 0, list._blockSize);
+						if (itemIndex >= 0)
+						{
+							return itemIndex + ((long)(blockIndex + 1)<< list._blockSizePow2BitShift);
+						}
+
+						if (blockIndex + 2 <  list._lastBlockWithData)
+						{
+							itemIndex = Array.IndexOf(memoryBlocksSpan[blockIndex + 2], item, 0, list._blockSize);
+							if (itemIndex >= 0)
+							{
+								return itemIndex + ((long)(blockIndex + 2)<< list._blockSizePow2BitShift);
+							}
+
+							if (blockIndex + 3 <  list._lastBlockWithData)
+							{
+								itemIndex = Array.IndexOf(memoryBlocksSpan[blockIndex + 3], item, 0, list._blockSize);
+								if (itemIndex >= 0)
+								{
+									return itemIndex + ((long)(blockIndex + 3)<< list._blockSizePow2BitShift);
+								}
+							}
+						}
+					}
+				}
+
+				//if (blockIndex == list._lastBlockWithData)
+				//{
+					itemIndex = list._nextItemIndex != 0
+						? Array.IndexOf(memoryBlocksSpan[list._lastBlockWithData], item, 0, list._nextItemIndex)
+						: Array.IndexOf(memoryBlocksSpan[list._lastBlockWithData], item, 0, list._blockSize);
+				//}
+				//else
+				//{
+				//	return RecyclableDefaults.ItemNotFoundIndexLong;
+				//}
+
+				return itemIndex >= 0 ? itemIndex + ((long)list._lastBlockWithData << list._blockSizePow2BitShift) : RecyclableDefaults.ItemNotFoundIndexLong;
+			}
+
+			public static int Min(int a, int b, int c) => a < b ? a < c ? a : c : b < c ? b : c;
+		}
+	}
+}

Recyclable.Collections/RecyclableList.cs

@@ -2,19 +2,14 @@
 using System.Collections;
 using System.Numerics;
 using System.Runtime.CompilerServices;
-using Recyclable.Collections.Parallel;
 using Recyclable.Collections.Pools;
 
 [assembly: InternalsVisibleTo("Recyclable.Collections.Benchmarks")]
 
 namespace Recyclable.Collections
 {
-	public class RecyclableList<T> : IDisposable, IList<T>
+	public partial class RecyclableList<T> : IDisposable, IList<T>
 	{
-		private const long ItemNotFoundIndexLong = -1L;
-		private const int ItemNotFoundIndex = -1;
-		private const double OptimalParallelSearchStep = 0.1;// 0.329;
-
 		private static readonly ArrayPool<T[]> _defaultMemoryBlocksPool = ArrayPool<T[]>.Create();
 		private static readonly ArrayPool<T> _defaultBlockArrayPool = ArrayPool<T>.Create();
 		private static readonly T[][] _emptyMemoryBlocksArray = new T[0][];
@@ -24,7 +19,7 @@ namespace Recyclable.Collections
 		internal byte _blockSizePow2BitShift;
 		internal int _blockSizeMinus1;
 		private int _nextItemBlockIndex;
-		private int _nextItemIndex;
+		internal int _nextItemIndex;
 		private int _reservedBlockCount;
 
 		protected static readonly bool NeedsClearing = !typeof(T).IsValueType;
@@ -42,19 +37,15 @@ namespace Recyclable.Collections
 
 		public int Count => checked((int)_longCount);
 		public bool IsReadOnly { get; }
-		public int LastBlockWithData => _nextItemBlockIndex - (_nextItemIndex > 0 ? 0 : 1);
+		private int _lastBlockWithData;
+		public int LastBlockWithData => _lastBlockWithData;
 
-		internal long _longCountIndexOfStep;
 		internal long _longCount;
 
 		public long LongCount
 		{
 			get => _longCount;
-			set
+			set => _longCount = value;
-			{
-				_longCount = value;
-				_longCountIndexOfStep = Math.Max((long)(_longCount * OptimalParallelSearchStep), 1L);
-			}
 		}
 
 		public int ReservedBlockCount => _reservedBlockCount;
@@ -75,7 +66,7 @@ namespace Recyclable.Collections
 			int targetBlockIndex = (int)(index >> list._blockSizePow2BitShift);
 			int targetItemIndex = (int)(index & list._blockSizeMinus1);
 
-			int lastTakenBlockIndex = list.LastBlockWithData;
+			int lastTakenBlockIndex = list._lastBlockWithData;
 			while (sourceBlockIndex < lastTakenBlockIndex || (sourceBlockIndex == lastTakenBlockIndex && (sourceItemIndex < nextItemIndex || sourceBlockIndex != list._nextItemBlockIndex)))
 			{
 				int toCopy = sourceBlockIndex < lastTakenBlockIndex || nextItemIndex == 0
@@ -150,6 +141,8 @@ namespace Recyclable.Collections
 
 					if (!enumerator.MoveNext())
 					{
+						_lastBlockWithData = targetBlockIdx;
+						_longCount += copied + i - _longCount;
 						if (targetItemIdx < blockSize)
 						{
 							_nextItemBlockIndex = targetBlockIdx;
@@ -161,8 +154,6 @@ namespace Recyclable.Collections
 							_nextItemIndex = 0;
 						}
 
-						_longCount += copied + i - _longCount;
-						_longCountIndexOfStep = Math.Max((long)(_longCount * OptimalParallelSearchStep), 1L);
 						return;
 					}
 
@@ -220,153 +211,9 @@ namespace Recyclable.Collections
 			}
 
 			_longCount = requiredCapacity;
-			_longCountIndexOfStep = Math.Max((long)(requiredCapacity * OptimalParallelSearchStep), 1L);
 			_nextItemBlockIndex = targetBlockIdx;
 			_nextItemIndex = targetItemIdx;
-		}
+			_lastBlockWithData = targetBlockIdx - (targetItemIdx > 0 ? 0 : 1);
-
-		// & WAS SLOWER
-		// [MethodImpl(MethodImplOptions.AggressiveOptimization | MethodImplOptions.AggressiveInlining)]
-		private static long DoLongIndexOfParallel(RecyclableList<T> list, in ItemRange itemRange, in T itemToFind, in ManualResetEventSlimmer itemFoundSignal)
-		{
-			// & WAS SLOWER without
-			int blockSize = list._blockSize;
-			int blockIndex = itemRange.BlockIndex;
-			long itemsToSearchCount = itemRange.ItemsToSearchCount;
-			int index = Array.IndexOf(list._memoryBlocks[blockIndex], itemToFind, itemRange.StartingItemIndex, (int)Math.Min(blockSize - itemRange.StartingItemIndex, itemsToSearchCount));
-			if (itemFoundSignal.IsSet)
-			{
-				return ItemNotFoundIndexLong;
-			}
-			else if (index >= 0)
-			{
-				return (blockIndex << list._blockSizePow2BitShift) + index;
-			}
-
-			// We don't need Math.Min here, because if itemsToSearchCount < blockSize - itemRange.StartingItemIndex, then it would mean that
-			// there are no more items and we already scanned all. We want to break the loop. Subtracting a higher value from itemsToSearchCount
-			// will always result in a negative result.
-			itemsToSearchCount -= blockSize - itemRange.StartingItemIndex;
-			if (itemsToSearchCount <= 0)
-			{
-				return ItemNotFoundIndexLong;
-			}
-
-			//T itemToFind = itemRange.ItemToFind;
-			//int lastBlockWithData = itemRange.LastBlockWithData;
-			blockIndex++;
-			while (itemsToSearchCount > blockSize)
-			{
-				index = Array.IndexOf(list._memoryBlocks[blockIndex], itemToFind, 0, blockSize);
-				if (itemFoundSignal.IsSet)
-				{
-					return ItemNotFoundIndexLong;
-				}
-				else if (index >= 0)
-				{
-					return ((long)blockIndex << list._blockSizePow2BitShift) + index;
-				}
-
-				itemsToSearchCount -= blockSize;
-				blockIndex++;
-			}
-
-			index = itemsToSearchCount <= 0 //|| blockIndex > list.LastBlockWithData
-				? ItemNotFoundIndex
-				// : blockIndex == list._nextItemBlockIndex
-				// ? Array.IndexOf(list._memoryBlocks[blockIndex], itemToFind, 0, (int)itemsToSearchCount)
-				: Array.IndexOf(list._memoryBlocks[blockIndex], itemToFind, 0, (int)itemsToSearchCount);
-				// ? Array.IndexOf(memoryBlocks[blockIndex], itemToFind, 0, checked((int)Math.Min(itemRange.List._nextItemIndex, itemsToSearchCount)))
-				//: Array.IndexOf(memoryBlocks[blockIndex], itemToFind, 0, checked((int)Math.Min(blockSize, itemsToSearchCount)));
-
-			return !itemFoundSignal.IsSet && index >= 0 ? ((long)blockIndex << list._blockSizePow2BitShift) + index : ItemNotFoundIndexLong;
-		}
-
-		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		private long DoIndexOfParallel(in T item)
-		{
-			if (_longCount == 0)
-			{
-				return ItemNotFoundIndex;
-			}
-
-			using var context = ParallelSynchronizationContextPool.GetWithOneParticipant();
-			// (long)(_longCount * 0.329) => most efficient step, based on benchmarks
-			Iterate(context, item);
-			context.AllDoneSignal.SignalAndWait();
-			if (context.Exception == null)
-			{
-				return context.FoundItemIndex;
-			}
-
-			context.Exception.CaptureAndRethrow();
-			return ItemNotFoundIndexLong;
-		}
-
-		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		// [MethodImpl(MethodImplOptions.AggressiveInlining)]
-		// [MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.NoOptimization)]
-		private static bool ScheduleIndexOfTask(ParallelSynchronizationContext context, RecyclableList<T> list, T itemToFind, ItemRange itemRange)
-		{
-			_ = context.AllDoneSignal.AddParticipant();
-			// & WAS SLOWER
-			// _ = ThreadPool.QueueUserWorkItem<object?>((_) =>
-			_ = Task.Factory.StartNew(() =>
-			{
-				try
-				{
-					if (context.ItemFoundSignal.IsSet)
-					{
-						return;
-					}
-
-					var index = DoLongIndexOfParallel(list, itemRange, itemToFind, context.ItemFoundSignal);
-					if (index >= 0)
-					{
-						context.ItemFoundSignal.Set();
-						_ = Interlocked.Exchange(ref context.FoundItemIndex, index);
-					}
-				}
-				catch (Exception e)
-				{
-					_ = Interlocked.Exchange(ref context.Exception, e);
-				}
-				finally
-				{
-					context.AllDoneSignal.RemoveParticipant();
-					itemRange.Dispose();
-				}
-			});
-
-			return !context.ItemFoundSignal.IsSet;
-		}
-
-		[MethodImpl(MethodImplOptions.AggressiveOptimization)]
-		protected static long DoIndexOf(T item, in T[][] memoryBlocks, int lastBlockIndex, int blockSize, int nextItemIndex)
-		{
-			Span<T[]> memoryBlocksSpan = new(memoryBlocks, 0, lastBlockIndex + 1);
-			int itemIndex, blockIndex;
-			for (blockIndex = 0; blockIndex < lastBlockIndex; blockIndex++)
-			{
-				itemIndex = Array.IndexOf(memoryBlocksSpan[blockIndex], item, 0, blockSize);
-				if (itemIndex >= 0)
-				{
-					return itemIndex + ((long)blockIndex << (31 - BitOperations.LeadingZeroCount((uint)blockSize)));
-				}
-			}
-
-			if (blockIndex == lastBlockIndex)
-			{
-				itemIndex = nextItemIndex != 0
-					? Array.IndexOf(memoryBlocksSpan[lastBlockIndex], item, 0, nextItemIndex)
-					: Array.IndexOf(memoryBlocksSpan[lastBlockIndex], item, 0, blockSize);
-			}
-			else
-			{
-				return ItemNotFoundIndexLong;
-			}
-
-			return itemIndex >= 0 ? itemIndex + ((long)lastBlockIndex << (31 - BitOperations.LeadingZeroCount((uint)blockSize))) : ItemNotFoundIndexLong;
 		}
 
 		/// <summary>
@@ -482,54 +329,32 @@ namespace Recyclable.Collections
 		// [MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.NoOptimization)]
 		// & WAS SLOWER
 		// [MethodImpl(MethodImplOptions.NoInlining)]
-		private void Iterate(in ParallelSynchronizationContext context, in T itemToFind)
+		//private static void Iterate(RecyclableList<T> list, in ParallelSynchronizationContext context, in T itemToFind)
-		{
+		//{
-			// TODO: Switch to use _blockSize instead of list._longCountIndeexOfStep for simplifications & better performance.
+		//	// TODO: Switch to use _blockSize instead of list._longCountIndeexOfStep for simplifications & better performance.
 
-			// & WAS SLOWER without
+		//	// & WAS SLOWER without
-			ItemRange itemRange;
+		//	int	blockIndex = 0;
-			int	blockIndex = 0;
-			// & WAS SLOWER
-			// int blockSize = _blockSize;
-			long itemIndex = 0;
-			long itemsCount = _longCount;
-			// & WAS SLOWER
-			// int blockSize = _blockSize;
-			// long step = list._longCountIndexOfStep;
-			// & WAS SLOWER
-			// while ((blockIndex << list._blockSizePow2BitShift) + itemIndex > step)
-			// & WAS SLOWER
-			// long longCountIndexOfStep = _longCountIndexOfStep;
-			while (itemsCount > _longCountIndexOfStep)
-			{
-				// At this point itemIndex is limited to blockSize range - int.
-				itemRange = ItemRangePool.Create(blockIndex, (int)itemIndex, _longCountIndexOfStep);
-				if (!ScheduleIndexOfTask(context, this, itemToFind, itemRange))
-				{
-					break;
-				}
 
-				itemsCount -= _longCountIndexOfStep;
+		//	long itemsCount = list._longCount;
+		//	// & WAS SLOWER
+		//	// int blockSize = _blockSize;
+		//	// & WAS SLOWER
+		//	// while ((blockIndex << list._blockSizePow2BitShift) + itemIndex > step)
+		//	while (itemsCount > list._blockSize)
+		//	{
+		//		// At this point itemIndex is limited to blockSize range - int.
+		//		if (!ScheduleIndexOfTask(context, list, blockIndex, list._blockSize, itemToFind))
+		//		{
+		//			break;
+		//		}
 
-				// & WAS SLOWER
+		//		itemsCount -= list._blockSize;
-				// blockIndex = (int)Math.DivRem(itemIndex + step, step, out itemIndex);
+		//		blockIndex++;
-				// & WAS SLOWER
+		//	}
-				// blockIndex += (int)((itemIndex + list._longCountIndexOfStep) >> list._blockSizePow2BitShift);
-				// itemIndex = (int)((itemIndex + list._longCountIndexOfStep) & list._blockSizeMinus1);
 
-				itemIndex += _longCountIndexOfStep;
+		//	_ = ScheduleIndexOfTask(context, list, blockIndex, (int)itemsCount, itemToFind);
-				while (itemIndex >= _blockSize)
+		//}
-				{
-					blockIndex++;
-					itemIndex -= _blockSize;
-				}
-			}
-
-			// & WAS SLOWER
-			// itemRange = new(blockIndex, (int)itemIndex, list._longCount - (blockIndex << list._blockSizePow2BitShift) - itemIndex);
-			itemRange = ItemRangePool.Create(blockIndex, (int)itemIndex, itemsCount);
-			_ = ScheduleIndexOfTask(context, this, itemToFind, itemRange);
-		}
 
 		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		protected static void SetupBlockArrayPooling(RecyclableList<T> list, int blockSize, ArrayPool<T>? blockArrayPool = null)
@@ -648,9 +473,12 @@ namespace Recyclable.Collections
 				_nextItemBlockIndex++;
 				_nextItemIndex = 0;
 			}
+			else if (_nextItemIndex == 1)
+			{
+				_lastBlockWithData++;
+			}
 
 			_longCount++;
-			_longCountIndexOfStep = Math.Max((long)(_longCount * OptimalParallelSearchStep), 1L);
 		}
 
 		public void AddRange(in T[] items)
@@ -693,8 +521,8 @@ namespace Recyclable.Collections
 
 			_nextItemIndex = itemsSpan.Length;
 			_longCount = targetCapacity;
-			_longCountIndexOfStep = Math.Max((long)(targetCapacity * OptimalParallelSearchStep), 1L);
 			_nextItemBlockIndex = targetBlockIndex;
+			_lastBlockWithData = targetBlockIndex - (itemsSpan.Length > 0 ? 0 : 1);
 		}
 
 		public void AddRange(RecyclableArrayList<T> items)
@@ -733,8 +561,8 @@ namespace Recyclable.Collections
 			_nextItemIndex = itemsSpan.Length;
 			itemsSpan.CopyTo(targetBlockArraySpan);
 			_longCount = targetCapacity;
-			_longCountIndexOfStep = Math.Max((long)(targetCapacity * OptimalParallelSearchStep), 1L);
 			_nextItemBlockIndex = targetBlockIndex;
+			_lastBlockWithData = targetBlockIndex - (itemsSpan.Length > 0 ? 0 : 1);
 		}
 
 		public void AddRange(RecyclableList<T> items)
@@ -794,6 +622,7 @@ namespace Recyclable.Collections
 				}
 			}
 
+			_lastBlockWithData = targetBlockIndex;
 			if ((targetCapacity & _blockSizeMinus1) == 0)
 			{
 				_nextItemIndex = 0;
@@ -806,7 +635,6 @@ namespace Recyclable.Collections
 			}
 
 			_longCount = targetCapacity;
-			_longCountIndexOfStep = Math.Max((long)(targetCapacity * OptimalParallelSearchStep), 1L);
 		}
 
 		public void AddRange(List<T> items)
@@ -853,7 +681,7 @@ namespace Recyclable.Collections
 
 			_nextItemBlockIndex = targetBlockIndex;
 			_longCount = targetCapacity;
-			_longCountIndexOfStep = Math.Max((long)(targetCapacity * OptimalParallelSearchStep), 1L);
+			_lastBlockWithData = targetBlockIndex - (_nextItemIndex > 0 ? 0 : 1);
 		}
 
 		public void AddRange(IList<T> items)
@@ -898,9 +726,9 @@ namespace Recyclable.Collections
 
 				itemsSpan.CopyTo(targetBlockArraySpan);
 				_longCount = targetCapacity;
-				_longCountIndexOfStep = Math.Max((long)(targetCapacity * OptimalParallelSearchStep), 1L);
 				_nextItemBlockIndex = targetBlockIndex;
 				_nextItemIndex = itemsSpan.Length;
+				_lastBlockWithData = targetBlockIndex - (itemsSpan.Length > 0 ? 0 : 1);
 			}
 			finally
 			{
@@ -976,8 +804,8 @@ namespace Recyclable.Collections
 			_reservedBlockCount = 0;
 			_nextItemBlockIndex = 0;
 			_nextItemIndex = 0;
+			_lastBlockWithData = RecyclableDefaults.ItemNotFoundIndex;
 			_longCount = 0;
-			_longCountIndexOfStep = 1;
 		}
 
 		[MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -1010,17 +838,81 @@ namespace Recyclable.Collections
 
 		public IEnumerator<T> GetEnumerator() => _memoryBlocks.Enumerate(_blockSize, LongCount).GetEnumerator();
 
-		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		public int IndexOf(T item) => checked((int)DoIndexOfParallel(item));
+		public int IndexOf(T item)
+		{
+			if (_lastBlockWithData == 0)
+			{
+				return Array.IndexOf(_memoryBlocks[0], item, 0, _longCount < _blockSize ? _nextItemIndex : _blockSize);
+			}
+			else if (_longCount == 0)
+			{
+				return RecyclableDefaults.ItemNotFoundIndex;
+			}
+
+			int itemIndex = Array.IndexOf(_memoryBlocks[0], item, 0, _longCount < _blockSize ? _nextItemIndex : _blockSize);
+			if (itemIndex >= 0)
+			{
+				return itemIndex;
+			}
+			else if (_longCount < RecyclableDefaults.MinItemsCountForParallelization)
+			{
+				return (int)RecyclableListIndexOfHelpers.IndexOfSequential(this, item);
+			}
+			else
+			{
+				// & WAS SLOWER
+				// long toSearch = _longCount - _blockSize;
+				if (_longCount > _blockSize)
+				{
+					itemIndex = Array.IndexOf(_memoryBlocks[1], item, 0, (int)Math.Min(_blockSize, _longCount - _blockSize));
+					return itemIndex >= 0 ? _blockSize + itemIndex : (int)RecyclableListIndexOfHelpers.IndexOfParallel(this, item);
+				}
+				else
+				{
+					return RecyclableDefaults.ItemNotFoundIndex;
+				}
+			}
+		}
 
 		public void Insert(int index, T item) => throw new NotSupportedException();
 
 		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
-		public long LongIndexOf(T item) => _longCount == 0
+		public long LongIndexOf(T item)
-				? ItemNotFoundIndexLong
+		{
-				: _longCount <= _blockSize
+			if (_lastBlockWithData == 0)
-					? Array.IndexOf(_memoryBlocks[0], item, 0, checked((int)_longCount))
+			{
-					: DoIndexOfParallel(item);
+				return Array.IndexOf(_memoryBlocks[0], item, 0, _longCount < _blockSize ? _nextItemIndex : _blockSize);
+			}
+			else if (_longCount == 0)
+			{
+				return RecyclableDefaults.ItemNotFoundIndex;
+			}
+
+			int itemIndex = Array.IndexOf(_memoryBlocks[0], item, 0, _longCount < _blockSize ? _nextItemIndex : _blockSize);
+			if (itemIndex >= 0)
+			{
+				return itemIndex;
+			}
+			else if (_longCount < RecyclableDefaults.MinItemsCountForParallelization)
+			{
+				return RecyclableListIndexOfHelpers.IndexOfSequential(this, item);
+			}
+			else
+			{
+				// & WAS SLOWER
+				// long toSearch = _longCount - _blockSize;
+				if (_longCount > _blockSize)
+				{
+					itemIndex = Array.IndexOf(_memoryBlocks[1], item, 0, (int)Math.Min(_blockSize, _longCount - _blockSize));
+					return itemIndex >= 0 ? _blockSize + itemIndex : RecyclableListIndexOfHelpers.IndexOfParallel(this, item);
+				}
+				else
+				{
+					return RecyclableDefaults.ItemNotFoundIndex;
+				}
+			}
+		}
 
 		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
 		public bool Remove(T item)
@@ -1029,7 +921,6 @@ namespace Recyclable.Collections
 			if (index >= 0)
 			{
 				_longCount--;
-				_longCountIndexOfStep = Math.Max((long)(_longCount * OptimalParallelSearchStep), 1L);
 
 				if (index < _longCount)
 				{
@@ -1039,6 +930,10 @@ namespace Recyclable.Collections
 				if (_nextItemIndex > 0)
 				{
 					_nextItemIndex--;
+					if (_nextItemIndex == 0)
+					{
+						_lastBlockWithData--;
+					}
 				}
 				else
 				{
@@ -1080,6 +975,8 @@ namespace Recyclable.Collections
 			{
 				_nextItemBlockIndex--;
 			}
+
+			_lastBlockWithData--;
 		}
 
 		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
@@ -1091,7 +988,6 @@ namespace Recyclable.Collections
 			}
 
 			_longCount--;
-			_longCountIndexOfStep = Math.Max((long)(_longCount * OptimalParallelSearchStep), 1L);
 
 			if (index < _longCount)
 			{
@@ -1101,6 +997,10 @@ namespace Recyclable.Collections
 			if (_nextItemIndex > 0)
 			{
 				_nextItemIndex--;
+				if (_nextItemIndex == 0)
+				{
+					_lastBlockWithData--;
+				}
 			}
 			else
 			{
@@ -1125,7 +1025,6 @@ namespace Recyclable.Collections
 			}
 
 			_longCount--;
-			_longCountIndexOfStep = Math.Max((long)Math.Floor(_longCount * OptimalParallelSearchStep), 1L);
 
 			if (index < _longCount)
 			{
@@ -1135,6 +1034,10 @@ namespace Recyclable.Collections
 			if (_nextItemIndex > 0)
 			{
 				_nextItemIndex--;
+				if (_nextItemIndex == 0)
+				{
+					_lastBlockWithData--;
+				}
 			}
 			else
 			{

Recyclable.Collections/SearchConfiguration.cs

@@ -8,7 +8,7 @@ namespace Recyclable.Collections
         public byte BlockSizePow2BitShift { get; init; }
         public long ItemsCount { get; init; }
         public int StartingBlockIndex { get; init; }
-        public long Step { get; init; }
+        public long BlockSize { get; init; }
 
         // public SearchConfiguration(TList list, in TItem itemToFind)
         // {
@@ -23,7 +23,7 @@ namespace Recyclable.Collections
             BlockSizeMinus1 = list._blockSizeMinus1,
             BlockSizePow2BitShift = list._blockSizePow2BitShift,
             ItemsCount = list._longCount,
-            Step = list._longCountIndexOfStep
+            BlockSize = list._blockSize
         };
     }
 }

Recyclable.CollectionsTests/ItemRangesIteratorTests.cs

@@ -9,7 +9,7 @@ namespace Recyclable.CollectionsTests
 	{
 		[Theory]
 		[MemberData(nameof(TestCases))]
-		public void IterateShouldYieldAllItems(int startingBlockIndex, int blockSize, long itemsCount, int step, IEnumerable<(int BlockIndex, int StartingItemIndex, long ItemsCount)> expected)
+		public void IterateShouldYieldAllItems(int startingBlockIndex, int blockSize, long itemsCount, int step, IEnumerable<(int BlockIndex, long ItemsCount)> expected)
 		{
 			// Prepare
 			var mockList = new RecyclableList<long>();
@@ -23,15 +23,15 @@ namespace Recyclable.CollectionsTests
 				BlockSizeMinus1 = blockSize - 1,
 				BlockSizePow2BitShift = blockSizePow2BitShift,
 				ItemsCount = itemsCount,
-				Step = step,
+				BlockSize = step,
 			};
 
-			var actualItemRanges = new List<(int BlockIndex, int StartingItemIndex, long ItemsCount)>();
+			var actualItemRanges = new List<(int BlockIndex, long ItemsCount)>();
 
 			// Act
 			ItemRangesIterator.Iterate(synchronizationContext, search, (context, search, itemRange) =>
 			{
-				actualItemRanges.Add((itemRange.BlockIndex, itemRange.StartingItemIndex, itemRange.ItemsToSearchCount));
+				actualItemRanges.Add((itemRange.BlockIndex, itemRange.ItemsToSearchCount));
 				return true;
 			});
 
@@ -43,102 +43,102 @@ namespace Recyclable.CollectionsTests
 		{
 			new object[]
 			{
-				0, 128, 333, (long)(333 * 0.329), new(int BlockIndex, int StartingItemIndex, long ItemsCount)[]
+				0, 128, 333, (long)(333 * 0.329), new(int BlockIndex, long ItemsCount)[]
 				{
-					(0, 0, 109),
+					(0, 109),
-					(0, 109, 109),
+					(0, 109),
-					(1, 90, 109),
+					(1, 109),
-					(2, 71, 6)
+					(2, 6)
 				}
 			},
 
 			new object[]
 			{
-				0, 2, 10, 2, new (int BlockIndex, int StartingItemIndex, long ItemsCount)[]
+				0, 2, 10, 2, new (int BlockIndex, long ItemsCount)[]
 				{
-					(0, 0, 2),
+					(0, 2),
-					(1, 0, 2),
+					(1, 2),
-					(2, 0, 2),
+					(2, 2),
-					(3, 0, 2),
+					(3, 2),
-					(4, 0, 2),
+					(4, 2),
 				}
 			},
 
 			new object[]
 			{
-				0, 2, 10, 3, new (int BlockIndex, int StartingItemIndex, long ItemsCount)[]
+				0, 2, 10, 3, new (int BlockIndex, long ItemsCount)[]
 				{
-					(0, 0, 3),
+					(0, 3),
-					(1, 1, 3),
+					(1, 3),
-					(3, 0, 3),
+					(3, 3),
-					(4, 1, 1),
+					(4, 1),
 				}
 			},
 
 			new object[]
 			{
-				0, 1, 10, 3, new (int BlockIndex, int StartingItemIndex, long ItemsCount)[]
+				0, 1, 10, 3, new (int BlockIndex, long ItemsCount)[]
 				{
-					(0, 0, 3),
+					(0, 3),
-					(3, 0, 3),
+					(3, 3),
-					(6, 0, 3),
+					(6, 3),
-					(9, 0, 1),
+					(9, 1),
 				}
 			},
 
 			new object[]
 			{
-				0, 2, 10, 4, new (int BlockIndex, int StartingItemIndex, long ItemsCount)[]
+				0, 2, 10, 4, new (int BlockIndex, long ItemsCount)[]
 				{
-					(0, 0, 4),
+					(0, 4),
-					(2, 0, 4),
+					(2, 4),
-					(4, 0, 2),
+					(4, 2),
 				}
 			},
 
 			new object[]
 			{
-				0, 1, 10, 2, new (int BlockIndex, int StartingItemIndex, long ItemsCount)[]
+				0, 1, 10, 2, new (int BlockIndex, long ItemsCount)[]
 				{
-					(0, 0, 2),
+					(0, 2),
-					(2, 0, 2),
+					(2, 2),
-					(4, 0, 2),
+					(4, 2),
-					(6, 0, 2),
+					(6, 2),
-					(8, 0, 2),
+					(8, 2),
 				}
 			},
 
 			new object[]
 			{
-				0, 2, 10, 1, new (int BlockIndex, int StartingItemIndex, long ItemsCount)[]
+				0, 2, 10, 1, new (int BlockIndex, long ItemsCount)[]
 				{
-					(0, 0, 1),
+					(0, 1),
-					(0, 1, 1),
+					(0, 1),
-					(1, 0, 1),
+					(1, 1),
-					(1, 1, 1),
+					(1, 1),
-					(2, 0, 1),
+					(2, 1),
-					(2, 1, 1),
+					(2, 1),
-					(3, 0, 1),
+					(3, 1),
-					(3, 1, 1),
+					(3, 1),
-					(4, 0, 1),
+					(4, 1),
-					(4, 1, 1),
+					(4, 1),
 				}
 			},
 
 			new object[]
 			{
-				0, 1, 10, 1, new (int BlockIndex, int StartingItemIndex, long ItemsCount)[]
+				0, 1, 10, 1, new (int BlockIndex, long ItemsCount)[]
 				{
-					(0, 0, 1),
+					(0, 1),
-					(1, 0, 1),
+					(1, 1),
-					(2, 0, 1),
+					(2, 1),
-					(3, 0, 1),
+					(3, 1),
-					(4, 0, 1),
+					(4, 1),
-					(5, 0, 1),
+					(5, 1),
-					(6, 0, 1),
+					(6, 1),
-					(7, 0, 1),
+					(7, 1),
-					(8, 0, 1),
+					(8, 1),
-					(9, 0, 1),
+					(9, 1),
 				}
 			},
 		};