Use field-based exponent tracking for steppers

PerfectNumbers.Core/ExponentRemainderStepperCpu.cs

@@ -10,7 +10,7 @@ internal struct ExponentRemainderStepperCpu
     private readonly ulong _modulus;
     private readonly ulong _nPrime;
     private readonly ulong _montgomeryOne;
-    private ulong _previousExponent;
+    public ulong PreviousExponent;
     private ulong _currentMontgomery;
 
     public ExponentRemainderStepperCpu(in MontgomeryDivisorData divisor)
@@ -19,12 +19,10 @@ internal struct ExponentRemainderStepperCpu
         _modulus = divisor.Modulus;
         _nPrime = divisor.NPrime;
         _montgomeryOne = divisor.MontgomeryOne;
-        _previousExponent = 0UL;
+        PreviousExponent = 0UL;
         _currentMontgomery = divisor.MontgomeryOne;
     }
 
-    public ulong PreviousExponent => _previousExponent;
-
     public bool MatchesDivisor(in MontgomeryDivisorData divisor)
     {
         return _modulus == divisor.Modulus && _nPrime == divisor.NPrime && _montgomeryOne == divisor.MontgomeryOne;
@@ -36,7 +34,7 @@ internal struct ExponentRemainderStepperCpu
     /// </summary>
     public void Reset()
     {
-        _previousExponent = 0UL;
+        PreviousExponent = 0UL;
         _currentMontgomery = _montgomeryOne;
     }
 
@@ -71,13 +69,13 @@ internal struct ExponentRemainderStepperCpu
     /// <returns>The canonical residue produced by <c>2^exponent mod divisor</c>.</returns>
     public ulong ComputeNextCpu(ulong exponent)
     {
-        ulong delta = exponent - _previousExponent;
+        ulong delta = exponent - PreviousExponent;
         // TODO: Once divisor cycle lengths are mandatory, pull the delta multiplier from the
         // single-block divisor-cycle snapshot so we can skip the powmod entirely and reuse the
         // cached Montgomery residue ladder highlighted in MersenneDivisorCycleLengthGpuBenchmarks.
         ulong multiplier = delta.Pow2MontgomeryModWindowedCpu(_divisor, keepMontgomery: true);
         _currentMontgomery = _currentMontgomery.MontgomeryMultiply(multiplier, _modulus, _nPrime);
-        _previousExponent = exponent;
+        PreviousExponent = exponent;
         return ReduceCurrent();
     }
 
@@ -90,12 +88,12 @@ internal struct ExponentRemainderStepperCpu
     /// <returns><see langword="true"/> when the canonical residue equals one.</returns>
     public bool ComputeNextIsUnity(ulong exponent)
     {
-        ulong delta = exponent - _previousExponent;
+        ulong delta = exponent - PreviousExponent;
         // TODO: Reuse the divisor-cycle derived Montgomery delta once the cache exposes single-cycle
         // lookups so this branch also avoids recomputing powmods when the snapshot lacks the divisor.
         ulong multiplier = delta.Pow2MontgomeryModWindowedCpu(_divisor, keepMontgomery: true);
         _currentMontgomery = _currentMontgomery.MontgomeryMultiply(multiplier, _modulus, _nPrime);
-        _previousExponent = exponent;
+        PreviousExponent = exponent;
         return _currentMontgomery == _montgomeryOne;
     }
 
@@ -110,7 +108,7 @@ internal struct ExponentRemainderStepperCpu
     public bool TryInitializeFromMontgomeryResult(ulong exponent, ulong montgomeryResult, out bool isUnity)
     {
         _currentMontgomery = montgomeryResult;
-        _previousExponent = exponent;
+        PreviousExponent = exponent;
         isUnity = _currentMontgomery == _montgomeryOne;
         return true;
     }
@@ -129,7 +127,7 @@ internal struct ExponentRemainderStepperCpu
         // of relying on the caller-provided delta so incremental scans remain in sync with the snapshot
         // without computing additional cycles or mutating cache state.
         _currentMontgomery = _currentMontgomery.MontgomeryMultiply(montgomeryDelta, _modulus, _nPrime);
-        _previousExponent = exponent;
+        PreviousExponent = exponent;
         isUnity = _currentMontgomery == _montgomeryOne;
         return true;
     }
@@ -137,7 +135,7 @@ internal struct ExponentRemainderStepperCpu
     private void InitializeCpuState(ulong exponent)
     {
         _currentMontgomery = exponent.Pow2MontgomeryModWindowedCpu(_divisor, keepMontgomery: true);
-        _previousExponent = exponent;
+        PreviousExponent = exponent;
     }
 
     private ulong ReduceCurrent() => _currentMontgomery.MontgomeryMultiply(1UL, _modulus, _nPrime);