Add L4 predictor queue skeleton

src/Nitload.Common/HighPerformance/Memory/L4.PredictorQueue.cs

@@ -1,4 +1,11 @@
-using System;
+// -----------------------------------------------------------------------------
+// file="L4.PredictorQueue"
+// Project: StgSharp
+// Copyright (c) Nitload.
+// SPDX-License-Identifier: MIT
+// -----------------------------------------------------------------------------
+
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
@@ -6,7 +13,97 @@ using System.Threading.Tasks;
 
 namespace Nitload.Common.HighPerformance.Memory
 {
-    public class L4
+    public partial class L4
     {
+
+        private readonly IL4Predictor[] _ring = new IL4Predictor[256];
+        private volatile int _consume;
+        private volatile int _enter;
+        private volatile int _predict;
+
+        private int _queue_sync;
+
+        internal void DropPredictor(
+                      IL4Predictor predictor
+        )
+        {
+            if (predictor is null) {
+                return;
+            }
+            SpinWait sp = new();
+            while (Interlocked.CompareExchange(ref _queue_sync, 1, 0) != 0) {
+                sp.SpinOnce();
+            }
+
+            try
+            {
+                unchecked
+                {
+                    for (int i = _consume; i < _predict; i++)
+                    {
+                        byte idx = (byte)i;
+                        if (ReferenceEquals(predictor, _ring[idx]))
+                        {
+                            int end = _predict;
+                            for (int move = i; move < end - 1; move++)
+                            {
+                                _ring[(byte)move] = _ring[(byte)(move + 1)];
+                            }
+                            _ring[(byte)(end - 1)] = default!;
+                            _predict = end - 1;
+                            return;
+                        }
+                    }
+                }
+            }
+            finally
+            {
+                Interlocked.Exchange(ref _queue_sync, 0);
+            }
+        }
+
+        internal void RegistPredictor(
+                      IL4Predictor predictor
+        )
+        {
+            if (predictor is null) {
+                return;
+            }
+            SpinWait sp = new();
+            while (Interlocked.CompareExchange(ref _queue_sync, 1, 0) != 0) {
+                sp.SpinOnce();
+            }
+
+            WaitForSpare();
+
+            _consume++;
+
+            byte idx = (byte)_consume;
+            _ring[idx] = predictor;
+        }
+
+        private void WaitForSpare()
+        {
+            int spin_count = 0;
+
+            // wait until the queue has space
+            while (_enter - _consume >= 256)
+            {
+                switch (spin_count)
+                {
+                    case < 1024:
+                        spin_count++;
+                        break;
+                    case < 65536:
+                        spin_count++;
+                        Thread.SpinWait(1);
+                        break;
+                    default:
+                        Thread.Sleep(1);
+                        break;
+                }
+            }
+        }
+
     }
 }