Alternative to SemaphoreSlim added

PerfectNumbers.Core/PollingSemaphore.cs

@@ -0,0 +1,103 @@
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core;
+
+/// <summary>
+/// Lightweight semaphore-like guard that uses spinning and optional sleeps instead of locks.
+/// It allows a small transient oversubscription while contending threads back off between checks.
+/// </summary>
+public sealed class PollingSemaphore
+{
+    private readonly int _maximumConcurrency;
+    private readonly TimeSpan _pauseBetweenChecks;
+    private readonly bool _hasPause;
+    private int _currentHolders;
+
+    public PollingSemaphore(int maximumConcurrency, TimeSpan pauseBetweenChecks)
+    {
+        if (maximumConcurrency <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(maximumConcurrency));
+        }
+
+        if (pauseBetweenChecks < TimeSpan.Zero)
+        {
+            throw new ArgumentOutOfRangeException(nameof(pauseBetweenChecks));
+        }
+
+        _maximumConcurrency = maximumConcurrency;
+        _pauseBetweenChecks = pauseBetweenChecks;
+        _hasPause = pauseBetweenChecks > TimeSpan.Zero;
+    }
+
+    public bool TryEnter()
+    {
+        int newValue = Interlocked.Increment(ref _currentHolders);
+
+        if (newValue <= _maximumConcurrency)
+        {
+            return true;
+        }
+
+        Interlocked.Decrement(ref _currentHolders);
+        return false;
+    }
+
+    public bool Wait(TimeSpan timeout, CancellationToken cancellationToken = default)
+    {
+		ArgumentOutOfRangeException.ThrowIfLessThan(timeout, Timeout.InfiniteTimeSpan);
+
+		Stopwatch? stopwatch = timeout == Timeout.InfiniteTimeSpan ? null : Stopwatch.StartNew();
+        SpinWait spinner = new();
+
+        while (true)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            if (TryEnter())
+            {
+                return true;
+            }
+
+            if (stopwatch is not null && stopwatch.Elapsed >= timeout)
+            {
+                return false;
+            }
+
+            if (spinner.Count < 10)
+            {
+                spinner.SpinOnce();
+                continue;
+            }
+
+            if (_hasPause)
+            {
+                Thread.Sleep(_pauseBetweenChecks);
+                spinner.Reset();
+            }
+            else
+            {
+                Thread.Yield();
+            }
+        }
+    }
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	public bool Wait(CancellationToken cancellationToken) => Wait(Timeout.InfiniteTimeSpan, cancellationToken);
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	public bool Wait() => Wait(Timeout.InfiniteTimeSpan, CancellationToken.None);
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	public void Release()
+    {
+        int newValue = Interlocked.Decrement(ref _currentHolders);
+
+        if (newValue < 0)
+        {
+            Interlocked.Increment(ref _currentHolders);
+            throw new SynchronizationLockException("Release called more times than Wait.");
+        }
+    }
+}