BigInteger-friendly steppers added. MersenneResidueStepper converted to struct. Manual residue tracking replaced with steppers. Modulo operations replaced with steppers.

PerfectNumbers.Core/BigIntegerExponentStepper.cs

@@ -0,0 +1,60 @@
+using System.Numerics;
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core;
+
+/// <summary>
+/// Tracks successive residues of <c>baseValue^exponent mod modulus</c> for ascending exponents using <see cref="BigInteger"/> inputs.
+/// This avoids recomputing full modular exponentiation when stepping forward by a known delta.
+/// </summary>
+public struct BigIntegerExponentStepper(BigInteger modulus, BigInteger baseValue)
+{
+    private readonly BigInteger _modulus = modulus;
+    private readonly BigInteger _baseValue = baseValue % modulus;
+    private BigInteger PreviousExponent = BigInteger.Zero;
+    private BigInteger CurrentResidue = BigInteger.One % modulus;
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public BigInteger Initialize(BigInteger exponent)
+    {
+        CurrentResidue = BigInteger.ModPow(_baseValue, exponent, _modulus);
+        PreviousExponent = exponent;
+        return CurrentResidue;
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public bool InitializeIsUnity(BigInteger exponent)
+    {
+        Initialize(exponent);
+        return CurrentResidue.IsOne;
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public BigInteger ComputeNext(BigInteger exponent)
+    {
+        if (exponent == PreviousExponent)
+        {
+            return CurrentResidue;
+        }
+
+        BigInteger delta = exponent - PreviousExponent;
+        BigInteger deltaResidue = BigInteger.ModPow(_baseValue, delta, _modulus);
+        CurrentResidue = (CurrentResidue * deltaResidue) % _modulus;
+        PreviousExponent = exponent;
+        return CurrentResidue;
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public bool ComputeNextIsUnity(BigInteger exponent)
+    {
+        ComputeNext(exponent);
+        return CurrentResidue.IsOne;
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public void Reset()
+    {
+        PreviousExponent = BigInteger.Zero;
+        CurrentResidue = BigInteger.One % _modulus;
+    }
+}