Reuse stack buffers when lowering divisor exponents

PerfectNumbers.Core/ExponentRemainderStepper.cs

@@ -27,6 +27,11 @@ internal struct ExponentRemainderStepper
 
     public ulong PreviousExponent => _previousExponent;
 
+    public bool MatchesDivisor(in MontgomeryDivisorData divisor)
+    {
+        return _modulus == divisor.Modulus && _nPrime == divisor.NPrime && _montgomeryOne == divisor.MontgomeryOne;
+    }
+
     public void Reset()
     {
         _previousExponent = 0UL;

PerfectNumbers.Core/MersenneDivisorCycles.cs

@@ -304,7 +304,8 @@ public class MersenneDivisorCycles
         //     return false;
         // }
 
-        Dictionary<ulong, int> factorCounts = ThreadStaticPools.RentFactorCountDictionary();
+        // Each divisor candidate is evaluated once along the scan, so a fresh factor map per call avoids keeping an unused cache.
+        Dictionary<ulong, int> factorCounts = new Dictionary<ulong, int>();
         bool success = false;
 
         if (twoCount > 0)
@@ -322,7 +323,6 @@ public class MersenneDivisorCycles
             primeOrderFailed = false;
         }
 
-        ThreadStaticPools.ReturnFactorCountDictionary(factorCounts);
         return success;
     }
 
@@ -335,12 +335,11 @@ public class MersenneDivisorCycles
             return true;
         }
 
-        Dictionary<ulong, int> scratch = ThreadStaticPools.RentFactorScratchDictionary();
+        // Factorizations are local to a single reduction attempt; there is no reuse across calls.
+        Dictionary<ulong, int> scratch = new Dictionary<ulong, int>();
         bool success = TryFactorIntoCountsInternal(value, scratch);
         if (!success)
         {
-            // The scan recalculates factors every time; the old cache never produced hits.
-            ThreadStaticPools.ReturnFactorScratchDictionary(scratch);
             return false;
         }
 
@@ -350,7 +349,6 @@ public class MersenneDivisorCycles
             AddFactor(counts, pair.Key, pair.Value);
         }
 
-        ThreadStaticPools.ReturnFactorScratchDictionary(scratch);
         return true;
     }
 
@@ -459,9 +457,8 @@ public class MersenneDivisorCycles
         }
 
         int count = factorCounts.Count;
-        ulong[] primes = ArrayPool<ulong>.Shared.Rent(count);
+        ulong[] primes = ThreadStaticPools.UlongPool.Rent(count);
-        try
+
-        {
         int index = 0;
         foreach (KeyValuePair<ulong, int> entry in factorCounts)
         {
@@ -472,35 +469,103 @@ public class MersenneDivisorCycles
         Array.Sort(primes, 0, count);
 
         ulong order = initialOrder;
+
+        int stackMultiplicity = 0;
+        for (int i = 0; i < count; i++)
+        {
+            int multiplicity = factorCounts[primes[i]];
+            if (multiplicity <= 16 && multiplicity > stackMultiplicity)
+            {
+                stackMultiplicity = multiplicity;
+            }
+        }
+
+        Span<ulong> stackCandidates = stackMultiplicity > 0 ? stackalloc ulong[stackMultiplicity] : Span<ulong>.Empty;
+        Span<bool> stackEvaluations = stackMultiplicity > 0 ? stackalloc bool[stackMultiplicity] : Span<bool>.Empty;
+
         for (int i = 0; i < count; i++)
         {
             ulong prime = primes[i];
             int multiplicity = factorCounts[prime];
+            // Multiplicities collected by TryFactorIntoCountsInternal are strictly positive along the scanner path.
+            // Keeping the guard commented documents the assumption so it does not resurface in future reviews.
+            // if (multiplicity <= 0)
+            // {
+            //     continue;
+            // }
 
-                for (int iteration = 0; iteration < multiplicity; iteration++)
+            if (multiplicity <= 16)
             {
-                    if (order % prime != 0UL)
+                ProcessReduceOrderPrime(stackCandidates.Slice(0, multiplicity), stackEvaluations.Slice(0, multiplicity), ref order, prime, multiplicity, divisorData);
+            }
+            else
             {
-                        break;
+                ulong[] rentedCandidates = ThreadStaticPools.UlongPool.Rent(multiplicity);
-                    }
+                bool[] rentedEvaluations = ThreadStaticPools.BoolPool.Rent(multiplicity);
-
+                ProcessReduceOrderPrime(rentedCandidates.AsSpan(0, multiplicity), rentedEvaluations.AsSpan(0, multiplicity), ref order, prime, multiplicity, divisorData);
-                    ulong candidate = order / prime;
+                ThreadStaticPools.UlongPool.Return(rentedCandidates, clearArray: false);
-                    if (candidate.Pow2MontgomeryModWindowedCpu(divisorData, keepMontgomery: false) == 1UL)
+                ThreadStaticPools.BoolPool.Return(rentedEvaluations, clearArray: false);
-                    {
-                        order = candidate;
-                        continue;
-                    }
-
-                    break;
             }
         }
 
+        ThreadStaticPools.UlongPool.Return(primes, clearArray: false);
         return order;
     }
-        finally
+
+    private static void ProcessReduceOrderPrime(Span<ulong> candidateBuffer, Span<bool> evaluationBuffer, ref ulong order, ulong prime, int multiplicity, in MontgomeryDivisorData divisorData)
     {
-            ArrayPool<ulong>.Shared.Return(primes, clearArray: false);
+        ulong working = order;
+        int actual = 0;
+        while (actual < multiplicity)
+        {
+            if (working < prime)
+            {
+                break;
             }
+
+            ulong reduced = working / prime;
+            if (reduced == 0UL)
+            {
+                break;
+            }
+
+            candidateBuffer[actual] = reduced;
+            working = reduced;
+            actual++;
+        }
+
+        if (actual == 0)
+        {
+            return;
+        }
+
+        Span<ulong> candidates = candidateBuffer.Slice(0, actual);
+        Span<bool> evaluations = evaluationBuffer.Slice(0, actual);
+
+        for (int left = 0, right = actual - 1; left < right; left++, right--)
+        {
+            ulong tmp = candidates[left];
+            candidates[left] = candidates[right];
+            candidates[right] = tmp;
+        }
+
+        ExponentRemainderStepper stepper = ThreadStaticPools.RentExponentStepper(divisorData);
+        for (int j = 0; j < actual; j++)
+        {
+            evaluations[j] = stepper.ComputeNextIsUnity(candidates[j]);
+        }
+
+        for (int j = actual - 1; j >= 0; j--)
+        {
+            if (!evaluations[j])
+            {
+                break;
+            }
+
+            order = candidates[j];
+        }
+
+        ThreadStaticPools.ReturnExponentStepper(stepper);
     }
 
     private static ulong PollardRho64(ulong n)

PerfectNumbers.Core/PrimeOrderCalculator.Cpu.cs

@@ -110,27 +110,70 @@ internal static partial class PrimeOrderCalculator
 	private static bool TrySpecialMaxCpu(ulong phi, ulong prime, PartialFactorResult factors, in MontgomeryDivisorData divisorData)
 	{
 		int length = factors.Count;
+		// The phi factorization on the scanner path always yields at least one entry for phi >= 2,
+		// preventing the zero-length case from occurring outside synthetic tests.
+		// if (length == 0)
+		// {
+		//     return true;
+		// }
+
 		ReadOnlySpan<FactorEntry> factorSpan = new(factors.Factors, 0, length);
-		for (int i = 0; i < length; i++)
+
+		if (length <= 32)
 		{
-			ulong factor = factorSpan[i].Value;
+			Span<ulong> stackBuffer = stackalloc ulong[length];
-			// Don't uncomment this. If there is division by zero, that means something wrong.
+			return EvaluateSpecialMaxCandidates(stackBuffer, factorSpan, phi, prime, divisorData);
-			// if (factor == 0UL)
+		}
+
+		ulong[] rented = ThreadStaticPools.UlongPool.Rent(length);
+		bool result = EvaluateSpecialMaxCandidates(rented.AsSpan(0, length), factorSpan, phi, prime, divisorData);
+		ThreadStaticPools.UlongPool.Return(rented, clearArray: false);
+		return result;
+	}
+
+	private static bool EvaluateSpecialMaxCandidates(Span<ulong> buffer, ReadOnlySpan<FactorEntry> factors, ulong phi, ulong prime, in MontgomeryDivisorData divisorData)
+	{
+		int actual = 0;
+		int factorCount = factors.Length;
+		for (int i = 0; i < factorCount; i++)
+		{
+			ulong factor = factors[i].Value;
+			// The partial factor pipeline never feeds zero or oversized factors while scanning candidate orders.
+			// if (factor == 0UL || factor > phi)
 			// {
 			// 	continue;
 			// }
 
-			// TODO: Can we optimize it thanks to knowing the factors, differences between factors, factors and / or phi
-			// being monotonically increasing etc. For example, if phi increases and factor increases, what does it tell us
-			// about the value of reduced?  Or, maybe wee can use residue stepping to calculate the residue instead of calling
-			// Pow2equalsOneCpu? Maybe we don't need to calculate it from scratch?
 			ulong reduced = phi / factor;
-			if (Pow2EqualsOneCpu(reduced, prime, divisorData))
+			if (reduced == 0UL)
 			{
+				continue;
+			}
+
+			buffer[actual] = reduced;
+			actual++;
+		}
+
+		if (actual == 0)
+		{
+			return true;
+		}
+
+		Span<ulong> candidates = buffer.Slice(0, actual);
+		candidates.Sort();
+
+		ExponentRemainderStepper stepper = ThreadStaticPools.RentExponentStepper(divisorData);
+		int candidateCount = candidates.Length;
+		for (int i = 0; i < candidateCount; i++)
+		{
+			if (stepper.ComputeNextIsUnity(candidates[i]))
+			{
+				ThreadStaticPools.ReturnExponentStepper(stepper);
 				return false;
 			}
 		}
 
+		ThreadStaticPools.ReturnExponentStepper(stepper);
 		return true;
 	}
 
@@ -160,8 +203,6 @@ internal static partial class PrimeOrderCalculator
 		Span<FactorEntry> buffer = tempArray;
 		factorSpan.CopyTo(buffer);
 
-		// bool isPrime = PrimeTester.IsPrimeInternal(factors.Cofactor, CancellationToken.None);
-		// bool isPrime = Open.Numeric.Primes.Prime.Numbers.IsPrime(factors.Cofactor);
 		if (!factors.FullyFactored && factors.Cofactor > 1UL && factors.CofactorIsPrime)
 		{
 			buffer[length] = new FactorEntry(factors.Cofactor, 1, true);
@@ -170,32 +211,105 @@ internal static partial class PrimeOrderCalculator
 
 		buffer.Slice(0, length).Sort(static (a, b) => a.Value.CompareTo(b.Value));
 
+		ExponentRemainderStepper stepper = ThreadStaticPools.RentExponentStepper(divisorData);
+
+		int maxStackExponent = 0;
+		for (int i = 0; i < length; i++)
+		{
+			int exponent = buffer[i].Exponent;
+			if (exponent <= 16 && exponent > maxStackExponent)
+			{
+				maxStackExponent = exponent;
+			}
+		}
+
+		Span<ulong> stackCandidates = maxStackExponent > 0 ? stackalloc ulong[maxStackExponent] : Span<ulong>.Empty;
+		Span<bool> stackEvaluations = maxStackExponent > 0 ? stackalloc bool[maxStackExponent] : Span<bool>.Empty;
+
 		for (int i = 0; i < length; i++)
 		{
 			ulong primeFactor = buffer[i].Value;
 			int exponent = buffer[i].Exponent;
-			for (int iteration = 0; iteration < exponent; iteration++)
+			// Factor exponents produced by Pollard-Rho and trial division are always positive in the scanner flow.
-			{
+			// if (exponent <= 0)
-				if ((order % primeFactor) != 0UL)
+			// {
-				{
+			// 	continue;
-					break;
+			// }
-				}
 
-				ulong reduced = order / primeFactor;
+			if (exponent <= 16)
-				if (Pow2EqualsOneCpu(reduced, prime, divisorData))
 			{
-					order = reduced;
+				ProcessExponentLoweringPrime(stackCandidates.Slice(0, exponent), stackEvaluations.Slice(0, exponent), ref order, primeFactor, exponent, ref stepper);
-					continue;
 			}
-
+			else
-				break;
+			{
+				ulong[] rentedCandidates = ThreadStaticPools.UlongPool.Rent(exponent);
+				bool[] rentedEvaluations = ThreadStaticPools.BoolPool.Rent(exponent);
+				ProcessExponentLoweringPrime(rentedCandidates.AsSpan(0, exponent), rentedEvaluations.AsSpan(0, exponent), ref order, primeFactor, exponent, ref stepper);
+				ThreadStaticPools.UlongPool.Return(rentedCandidates, clearArray: false);
+				ThreadStaticPools.BoolPool.Return(rentedEvaluations, clearArray: false);
 			}
 		}
 
+		ThreadStaticPools.ReturnExponentStepper(stepper);
 		pool.Return(tempArray, clearArray: false);
 		return order;
 	}
 
+	private static void ProcessExponentLoweringPrime(Span<ulong> candidateBuffer, Span<bool> evaluationBuffer, ref ulong order, ulong primeFactor, int exponent, ref ExponentRemainderStepper stepper)
+	{
+		ulong working = order;
+		int actual = 0;
+		while (actual < exponent)
+		{
+			if (working < primeFactor)
+			{
+				break;
+			}
+
+			ulong reduced = working / primeFactor;
+			if (reduced == 0UL)
+			{
+				break;
+			}
+
+			candidateBuffer[actual] = reduced;
+			working = reduced;
+			actual++;
+		}
+
+		if (actual == 0)
+		{
+			return;
+		}
+
+		Span<ulong> candidates = candidateBuffer.Slice(0, actual);
+		Span<bool> evaluations = evaluationBuffer.Slice(0, actual);
+
+		for (int left = 0, right = actual - 1; left < right; left++, right--)
+		{
+			ulong tmp = candidates[left];
+			candidates[left] = candidates[right];
+			candidates[right] = tmp;
+		}
+
+		stepper.Reset();
+
+		for (int j = 0; j < actual; j++)
+		{
+			evaluations[j] = stepper.ComputeNextIsUnity(candidates[j]);
+		}
+
+		for (int j = actual - 1; j >= 0; j--)
+		{
+			if (!evaluations[j])
+			{
+				break;
+			}
+
+			order = candidates[j];
+		}
+	}
+
 	private static bool TryConfirmOrderCpu(
 		ulong prime,
 		ulong order,
@@ -253,30 +367,95 @@ internal static partial class PrimeOrderCalculator
 	{
 		ReadOnlySpan<FactorEntry> span = factorization.Factors!;
 		int length = factorization.Count;
+
+		int maxStackExponent = 0;
+		for (int i = 0; i < length; i++)
+		{
+			int exponent = span[i].Exponent;
+			if (exponent <= 32 && exponent > maxStackExponent)
+			{
+				maxStackExponent = exponent;
+			}
+		}
+
+		Span<ulong> stackBuffer = maxStackExponent > 0 ? stackalloc ulong[maxStackExponent] : Span<ulong>.Empty;
+
 		for (int i = 0; i < length; i++)
 		{
 			ulong primeFactor = span[i].Value;
-			ulong reduced = order;
 			int exponent = span[i].Exponent;
-			for (int iteration = 0; iteration < exponent; iteration++)
+			// Factor exponents emitted by TryConfirmCandidateCpu stay positive for production workloads.
-			{
+			// if (exponent <= 0)
-				// TODO: Benchmark and / or implement DivRem like solution or residue stepper?
+			// {
-				if ((reduced % primeFactor) != 0UL)
+			// 	continue;
-				{
+			// }
-					break;
-				}
 
-				reduced /= primeFactor;
+			if (exponent <= 32)
-				if (Pow2EqualsOneCpu(reduced, prime, divisorData))
+			{
+				if (ValidateOrderForFactor(stackBuffer.Slice(0, exponent), primeFactor, exponent, order, divisorData))
 				{
 					return false;
 				}
 			}
+			else
+			{
+				ulong[] rentedCandidates = ThreadStaticPools.UlongPool.Rent(exponent);
+				if (ValidateOrderForFactor(rentedCandidates.AsSpan(0, exponent), primeFactor, exponent, order, divisorData))
+				{
+					ThreadStaticPools.UlongPool.Return(rentedCandidates, clearArray: false);
+					return false;
+				}
+				ThreadStaticPools.UlongPool.Return(rentedCandidates, clearArray: false);
+			}
 		}
 
 		return true;
 	}
 
+	private static bool ValidateOrderForFactor(Span<ulong> buffer, ulong primeFactor, int exponent, ulong order, in MontgomeryDivisorData divisorData)
+	{
+		ulong working = order;
+		int actual = 0;
+		while (actual < exponent)
+		{
+			if (working < primeFactor)
+			{
+				break;
+			}
+
+			ulong reduced = working / primeFactor;
+			if (reduced == 0UL)
+			{
+				break;
+			}
+
+			buffer[actual] = reduced;
+			working = reduced;
+			actual++;
+		}
+
+		if (actual == 0)
+		{
+			return false;
+		}
+
+		Span<ulong> candidates = buffer.Slice(0, actual);
+		candidates.Reverse();
+
+		ExponentRemainderStepper stepper = ThreadStaticPools.RentExponentStepper(divisorData);
+		for (int j = 0; j < actual; j++)
+		{
+			if (stepper.ComputeNextIsUnity(candidates[j]))
+			{
+				ThreadStaticPools.ReturnExponentStepper(stepper);
+				return true;
+			}
+		}
+
+		ThreadStaticPools.ReturnExponentStepper(stepper);
+		return false;
+	}
+
 	private static bool TryHeuristicFinishCpu(
 		ulong prime,
 		ulong order,
@@ -340,6 +519,7 @@ internal static partial class PrimeOrderCalculator
 			int candidateCount = candidates.Count;
 			bool allowGpuBatch = true;
 			Span<ulong> candidateSpan = CollectionsMarshal.AsSpan(candidates);
+			ExponentRemainderStepper powStepper = ThreadStaticPools.RentExponentStepper(divisorData);
 
 			// DebugLog(() => $"Checking candidates ({candidateCount} candidates, {powBudget} pow budget)");
 			int index = 0;
@@ -412,7 +592,7 @@ internal static partial class PrimeOrderCalculator
 					}
 					else
 					{
-						equalsOne = Pow2EqualsOneCpu(candidate, prime, divisorData);
+						equalsOne = powStepper.ComputeNextIsUnity(candidate);
 					}
 					if (!equalsOne)
 					{
@@ -430,6 +610,7 @@ internal static partial class PrimeOrderCalculator
 					}
 
 					ThreadStaticPools.ReturnUlongList(candidates);
+					ThreadStaticPools.ReturnExponentStepper(powStepper);
 					result = candidate;
 					return true;
 				}
@@ -443,6 +624,7 @@ internal static partial class PrimeOrderCalculator
 			}
 
 			ThreadStaticPools.ReturnUlongList(candidates);
+			ThreadStaticPools.ReturnExponentStepper(powStepper);
 			// DebugLog("No candidate confirmed");
 			return false;
 		}
@@ -635,8 +817,6 @@ internal static partial class PrimeOrderCalculator
 					return false;
 				}
 
-				// bool isPrime = Open.Numeric.Primes.Prime.Numbers.IsPrime(factorization.Cofactor);
-				// bool isPrime = PrimeTester.IsPrimeInternal(factorization.Cofactor, CancellationToken.None);
 				if (!factorization.CofactorIsPrime)
 				{
 					return false;
@@ -649,29 +829,45 @@ internal static partial class PrimeOrderCalculator
 
 			ReadOnlySpan<FactorEntry> span = factorization.Factors;
 			int length = factorization.Count;
+
+			int maxStackExponent = 0;
+			for (int i = 0; i < length; i++)
+			{
+				int exponent = span[i].Exponent;
+				if (exponent <= 32 && exponent > maxStackExponent)
+				{
+					maxStackExponent = exponent;
+				}
+			}
+
+			Span<ulong> stackBuffer = maxStackExponent > 0 ? stackalloc ulong[maxStackExponent] : Span<ulong>.Empty;
+
 			for (int i = 0; i < length; i++)
 			{
 				ulong primeFactor = span[i].Value;
-				ulong reduced = candidate;
+				int exponent = span[i].Exponent;
-				for (int iteration = 0; iteration < span[i].Exponent; iteration++)
+				// Factorization entries for candidate confirmation are always positive here.
-				{
+				// if (exponent <= 0)
-					// TODO: Measure and implement DivRem like solution or residue stepper to reuse calculation results from previous iteration
+				// {
-					if ((reduced % primeFactor) != 0UL)
+				// 	continue;
-					{
+				// }
-						break;
-					}
 
-					reduced /= primeFactor;
+				if (exponent <= 32)
-					if (powUsed >= powBudget && powBudget > 0)
+				{
+					if (CheckCandidateViolation(stackBuffer.Slice(0, exponent), primeFactor, exponent, candidate, prime, divisorData, ref powUsed, powBudget))
 					{
 						return false;
 					}
-
+				}
-					powUsed++;
+				else
-					if (Pow2EqualsOneCpu(reduced, prime, divisorData))
 				{
+					ulong[] rentedCandidates = ThreadStaticPools.UlongPool.Rent(exponent);
+					if (CheckCandidateViolation(rentedCandidates.AsSpan(0, exponent), primeFactor, exponent, candidate, prime, divisorData, ref powUsed, powBudget))
+					{
+						ThreadStaticPools.UlongPool.Return(rentedCandidates, clearArray: false);
 						return false;
 					}
+					ThreadStaticPools.UlongPool.Return(rentedCandidates, clearArray: false);
 				}
 			}
 
@@ -683,6 +879,58 @@ internal static partial class PrimeOrderCalculator
 		}
 	}
 
+	private static bool CheckCandidateViolation(Span<ulong> buffer, ulong primeFactor, int exponent, ulong candidate, ulong prime, in MontgomeryDivisorData divisorData, ref int powUsed, int powBudget)
+	{
+		ulong working = candidate;
+		int actual = 0;
+		while (actual < exponent)
+		{
+			if (working < primeFactor)
+			{
+				break;
+			}
+
+			ulong reduced = working / primeFactor;
+			if (reduced == 0UL)
+			{
+				break;
+			}
+
+			buffer[actual] = reduced;
+			working = reduced;
+			actual++;
+		}
+
+		if (actual == 0)
+		{
+			return false;
+		}
+
+		Span<ulong> candidates = buffer.Slice(0, actual);
+		candidates.Reverse();
+
+		ExponentRemainderStepper stepper = ThreadStaticPools.RentExponentStepper(divisorData);
+		for (int j = 0; j < actual; j++)
+		{
+			if (powUsed >= powBudget && powBudget > 0)
+			{
+				ThreadStaticPools.ReturnExponentStepper(stepper);
+				return true;
+			}
+
+			powUsed++;
+			if (stepper.ComputeNextIsUnity(candidates[j]))
+			{
+				ThreadStaticPools.ReturnExponentStepper(stepper);
+				return true;
+			}
+		}
+
+		ThreadStaticPools.ReturnExponentStepper(stepper);
+		return false;
+	}
+
+
 	private static bool Pow2EqualsOneCpu(ulong exponent, ulong prime, in MontgomeryDivisorData divisorData)
 	{
 		if (IsGpuPow2Allowed)

PerfectNumbers.Core/ThreadStaticPools.cs

@@ -1,4 +1,5 @@
 using System.Buffers;
+using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using PerfectNumbers.Core.Cpu;
 
@@ -30,7 +31,6 @@ namespace PerfectNumbers.Core
             }
         }
 
-
         [ThreadStatic]
         private static MersenneCpuDivisorScanSession? _mersenneCpuDivisorSession;
 
@@ -66,6 +66,18 @@ namespace PerfectNumbers.Core
             }
         }
 
+        [ThreadStatic]
+        private static ArrayPool<bool>? _boolPool;
+
+        public static ArrayPool<bool> BoolPool
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get
+            {
+                return _boolPool ??= ArrayPool<bool>.Create();
+            }
+        }
+
         [ThreadStatic]
         private static ArrayPool<byte>? _bytePool;
 
@@ -102,7 +114,6 @@ namespace PerfectNumbers.Core
             }
         }
 
-
         [ThreadStatic]
         private static List<List<ulong>>? _ulongListPool;
 
@@ -110,9 +121,12 @@ namespace PerfectNumbers.Core
         public static List<ulong> RentUlongList(int capacityHint)
         {
             List<List<ulong>>? pool = _ulongListPool;
-            if (pool is not null && pool.Count > 0)
+            if (pool is not null)
             {
-                int lastIndex = pool.Count - 1;
+                int poolCount = pool.Count;
+                if (poolCount > 0)
+                {
+                    int lastIndex = poolCount - 1;
                     List<ulong> list = pool[lastIndex];
                     pool.RemoveAt(lastIndex);
                     if (list.Count > 0)
@@ -127,6 +141,7 @@ namespace PerfectNumbers.Core
 
                     return list;
                 }
+            }
 
             return new List<ulong>(capacityHint);
         }
@@ -151,9 +166,12 @@ namespace PerfectNumbers.Core
         internal static List<PrimeOrderCalculator.PendingEntry> RentPrimeOrderPendingEntryList(int capacityHint)
         {
             List<List<PrimeOrderCalculator.PendingEntry>>? pool = _primeOrderPendingEntryListPool;
-            if (pool is not null && pool.Count > 0)
+            if (pool is not null)
             {
-                int lastIndex = pool.Count - 1;
+                int poolCount = pool.Count;
+                if (poolCount > 0)
+                {
+                    int lastIndex = poolCount - 1;
                     List<PrimeOrderCalculator.PendingEntry> list = pool[lastIndex];
                     pool.RemoveAt(lastIndex);
                     if (list.Count > 0)
@@ -168,6 +186,7 @@ namespace PerfectNumbers.Core
 
                     return list;
                 }
+            }
 
             return new List<PrimeOrderCalculator.PendingEntry>(capacityHint);
         }
@@ -192,23 +211,29 @@ namespace PerfectNumbers.Core
         public static Stack<ulong> RentUlongStack(int capacityHint)
         {
             List<Stack<ulong>>? pool = _ulongStackPool;
-            if (pool is not null && pool.Count > 0)
+            if (pool is not null)
             {
-                int lastIndex = pool.Count - 1;
+                int poolCount = pool.Count;
+                if (poolCount > 0)
+                {
+                    int lastIndex = poolCount - 1;
                     Stack<ulong> stack = pool[lastIndex];
                     pool.RemoveAt(lastIndex);
-                if (stack.Count > 0)
+                    int stackCount = stack.Count;
+                    if (stackCount > 0)
                     {
                         stack.Clear();
+                        stackCount = 0;
                     }
 
-                if (stack.Count < capacityHint)
+                    if (stackCount < capacityHint)
                     {
                         stack.EnsureCapacity(capacityHint);
                     }
 
                     return stack;
                 }
+            }
 
             return new Stack<ulong>(capacityHint);
         }
@@ -226,6 +251,7 @@ namespace PerfectNumbers.Core
             pool.Add(stack);
         }
 
+        // The factor-count scratch caches were removed because divisor factorizations never repeat across scan iterations.
         [ThreadStatic]
         private static List<Dictionary<ulong, int>>? _ulongIntDictionaryPool;
 
@@ -233,9 +259,12 @@ namespace PerfectNumbers.Core
         public static Dictionary<ulong, int> RentUlongIntDictionary(int capacityHint)
         {
             List<Dictionary<ulong, int>>? pool = _ulongIntDictionaryPool;
-            if (pool is not null && pool.Count > 0)
+            if (pool is not null)
             {
-                int lastIndex = pool.Count - 1;
+                int poolCount = pool.Count;
+                if (poolCount > 0)
+                {
+                    int lastIndex = poolCount - 1;
                     Dictionary<ulong, int> dictionary = pool[lastIndex];
                     pool.RemoveAt(lastIndex);
                     if (dictionary.Count > 0)
@@ -243,13 +272,11 @@ namespace PerfectNumbers.Core
                         dictionary.Clear();
                     }
 
-                if (dictionary.Count < capacityHint)
-                {
                     dictionary.EnsureCapacity(capacityHint);
-                }
 
                     return dictionary;
                 }
+            }
 
             return new Dictionary<ulong, int>(capacityHint);
         }
@@ -268,51 +295,31 @@ namespace PerfectNumbers.Core
         }
 
         [ThreadStatic]
-        private static Dictionary<ulong, int>? _factorCountDictionary;
+        private static ExponentRemainderStepper _exponentRemainderStepper;
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static Dictionary<ulong, int> RentFactorCountDictionary()
-        {
-            Dictionary<ulong, int>? dictionary = _factorCountDictionary;
-            if (dictionary is null)
-            {
-                return new Dictionary<ulong, int>(8);
-            }
-
-            _factorCountDictionary = null;
-            dictionary.Clear();
-            return dictionary;
-        }
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static void ReturnFactorCountDictionary(Dictionary<ulong, int> dictionary)
-        {
-            dictionary.Clear();
-            _factorCountDictionary = dictionary;
-        }
 
         [ThreadStatic]
-        private static Dictionary<ulong, int>? _factorScratchDictionary;
+        private static bool _hasExponentRemainderStepper;
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static Dictionary<ulong, int> RentFactorScratchDictionary()
+        internal static ExponentRemainderStepper RentExponentStepper(in MontgomeryDivisorData divisorData)
         {
-            Dictionary<ulong, int>? dictionary = _factorScratchDictionary;
+            if (_hasExponentRemainderStepper && _exponentRemainderStepper.MatchesDivisor(divisorData))
-            if (dictionary is null)
             {
-                return new Dictionary<ulong, int>(8);
+                ExponentRemainderStepper stepper = _exponentRemainderStepper;
+                _hasExponentRemainderStepper = false;
+                stepper.Reset();
+                return stepper;
             }
 
-            _factorScratchDictionary = null;
+            return new ExponentRemainderStepper(divisorData);
-            dictionary.Clear();
-            return dictionary;
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static void ReturnFactorScratchDictionary(Dictionary<ulong, int> dictionary)
+        internal static void ReturnExponentStepper(ExponentRemainderStepper stepper)
         {
-            dictionary.Clear();
+            stepper.Reset();
-            _factorScratchDictionary = dictionary;
+            _exponentRemainderStepper = stepper;
+            _hasExponentRemainderStepper = true;
         }
     }
 }