Add diagnostics for removed factorization cache

PerfectNumbers.Core/Cpu/MersenneNumberDivisorByDivisorCpuTester.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using PerfectNumbers.Core.Gpu;
 
@@ -11,6 +12,12 @@ public sealed class MersenneNumberDivisorByDivisorCpuTester : IMersenneNumberDiv
     private ulong _lastStatusDivisor;
     private int _batchSize = 1_024;
 
+    private const int FactorCachePrimeTrackingLimit = 500_000;
+    private static readonly HashSet<ulong> FactorCacheTrackedPrimes = new();
+    private static readonly object FactorCachePrimeLock = new();
+    private static ulong FactorCachePrimeHitCount;
+    private static bool FactorCachePrimeTrackingDisabled;
+
     [ThreadStatic]
     private static GpuUInt128WorkSet _divisorScanGpuWorkSet;
 
@@ -115,6 +122,8 @@ public sealed class MersenneNumberDivisorByDivisorCpuTester : IMersenneNumberDiv
         processedCount = 0UL;
         processedAll = false;
 
+        ObserveFactorCachePrime(prime);
+
         // The EvenPerfectBitScanner feeds primes >= 138,000,000 here, so allowedMax >= 3 in production runs.
         // Keeping the guard commented out documents the reasoning for benchmarks and tests.
         // if (allowedMax < 3UL)
@@ -254,6 +263,43 @@ public sealed class MersenneNumberDivisorByDivisorCpuTester : IMersenneNumberDiv
         return false;
     }
 
+    private static void ObserveFactorCachePrime(ulong prime)
+    {
+        bool logHit = false;
+        ulong hitNumber = 0UL;
+        bool trackingDisabledNow = false;
+
+        lock (FactorCachePrimeLock)
+        {
+            if (FactorCachePrimeTrackingDisabled)
+            {
+                return;
+            }
+
+            if (!FactorCacheTrackedPrimes.Add(prime))
+            {
+                FactorCachePrimeHitCount++;
+                hitNumber = FactorCachePrimeHitCount;
+                logHit = true;
+            }
+            else if (FactorCacheTrackedPrimes.Count >= FactorCachePrimeTrackingLimit)
+            {
+                FactorCachePrimeTrackingDisabled = true;
+                FactorCacheTrackedPrimes.Clear();
+                trackingDisabledNow = true;
+            }
+        }
+
+        if (logHit)
+        {
+            Console.WriteLine($"Factor cache hit for exponent {prime} ({hitNumber})");
+        }
+        else if (trackingDisabledNow)
+        {
+            Console.WriteLine("Factor cache hit tracking disabled after exceeding the tracking limit.");
+        }
+    }
+
     private static byte CheckDivisor(ulong prime, ulong divisorCycle, in MontgomeryDivisorData divisorData)
     {
         ulong residue = prime.Pow2MontgomeryModWithCycleCpu(divisorCycle, divisorData);