Extract divisor validator and rename legacy CPU prime helper

PerfectNumbers.Core/DivisorGenerator.cs

@@ -0,0 +1,38 @@
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core;
+
+internal static class DivisorGenerator
+{
+    private const ushort DecimalMaskWhenLastIsSeven = (1 << 3) | (1 << 7) | (1 << 9);
+    private const ushort DecimalMaskOtherwise = (1 << 1) | (1 << 3) | (1 << 9);
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static bool IsValidDivisor(
+        byte remainder10,
+        byte remainder8,
+        byte remainder3,
+        byte remainder5,
+        byte remainder7,
+        byte remainder11,
+        LastDigit lastDigit)
+    {
+        ushort decimalMask = lastDigit == LastDigit.Seven ? DecimalMaskWhenLastIsSeven : DecimalMaskOtherwise;
+        if (((decimalMask >> remainder10) & 1) == 0)
+        {
+            return false;
+        }
+
+        if (remainder8 != 1 && remainder8 != 7)
+        {
+            return false;
+        }
+
+        if (remainder3 == 0 || remainder5 == 0 || remainder7 == 0 || remainder11 == 0)
+        {
+            return false;
+        }
+
+        return true;
+    }
+}