Add TODOs for divisor metadata caching

PerfectNumbers.Core/Cpu/MersenneNumberLucasLehmerCpuTester.cs

@@ -3,39 +3,43 @@ namespace PerfectNumbers.Core;
 public class MersenneNumberLucasLehmerCpuTester
 {
     public bool IsPrime(ulong exponent)
-	{
+    {
         // Early rejections aligned with incremental/order sieves, but safe for small p:
         // - If 3 | p and p != 3, then 7 | M_p -> composite.
         // - If p ≡ 1 (mod 4) and p shares a factor with (p-1), reject fast.
         // - If p is divisible by divisors specific to numbers ending with 1 or 7, reject fast.
+        // TODO: Replace these `%` checks with Mod3/Mod5/Mod7/Mod11 helpers once Lucas–Lehmer CPU filtering
+        // shares the benchmarked bitmask implementations and avoids slow modulo instructions in the hot path.
         if (
             ((exponent % 3UL) == 0UL && exponent != 3UL) ||
             ((exponent % 5UL) == 0UL && exponent != 5UL) ||
             ((exponent % 7UL) == 0UL && exponent != 7UL) ||
             ((exponent % 11UL) == 0UL && exponent != 11UL)
         )
-		{
-			return false;
-		}
+        {
+            return false;
+        }
 
-		if ((exponent & 3UL) == 1UL && exponent.SharesFactorWithExponentMinusOne())
-		{
-			return false;
-		}
+        if ((exponent & 3UL) == 1UL && exponent.SharesFactorWithExponentMinusOne())
+        {
+            return false;
+        }
 
-		UInt128 s = UInt128Numbers.Four, two = UInt128Numbers.Two,
-			 m = (UInt128.One << (int)exponent) - UInt128.One;
+        UInt128 s = UInt128Numbers.Four, two = UInt128Numbers.Two,
+            m = (UInt128.One << (int)exponent) - UInt128.One;
 
-		UInt128 limit = exponent - UInt128Numbers.Two;
-		for (UInt128 i = UInt128.Zero; i < limit; i++)
-		{
-			s = s.PowModWithCycle(two, MersenneDivisorCycles.GetCycle(m)) - two;
-			if (s < UInt128.Zero)
-			{
-				s += m;
-			}
-		}
+        UInt128 limit = exponent - UInt128Numbers.Two;
+        for (UInt128 i = UInt128.Zero; i < limit; i++)
+        {
+            // TODO: Port this Lucas–Lehmer powmod to the ProcessEightBitWindows helper so the CPU
+            // fallback benefits from the same windowed ladder that halves runtime in the Pow2 benchmarks.
+            s = s.PowModWithCycle(two, MersenneDivisorCycles.GetCycle(m)) - two;
+            if (s < UInt128.Zero)
+            {
+                s += m;
+            }
+        }
 
-		return s == UInt128.Zero;
-	}
+        return s == UInt128.Zero;
+    }
 }