Merge pull request #88 from mlemanczyk/codex/fix-divisor-cycle-handling-in-gpu-mode

Filter disallowed divisors when scanning by divisor

EvenPerfectBitScanner/Program.cs

@@ -1019,13 +1019,10 @@ internal static class Program
                 ref long nextDivisorBits = ref Unsafe.As<ulong, long>(ref nextDivisor);
                 ulong blockStride = unchecked(DivisorAllocationBlockSize * 2);
 
-		if (localDivisorsRemaining > 0)
+                if (TryAcquireLocalDivisor(ref localDivisorCursor, ref localDivisorsRemaining, out ulong localDivisor))
                 {
-			ulong currentValue = localDivisorCursor;
-			localDivisorCursor += 2UL;
-			localDivisorsRemaining--;
                         exhausted = false;
-			return currentValue;
+                        return localDivisor;
                 }
 
                 while (true)
@@ -1074,11 +1071,55 @@ internal static class Program
                                 continue;
                         }
 
-			localDivisorCursor = currentValue + 2UL;
-			localDivisorsRemaining = count - 1;
+                        localDivisorCursor = currentValue;
+                        localDivisorsRemaining = count;
+
+                        if (TryAcquireLocalDivisor(ref localDivisorCursor, ref localDivisorsRemaining, out ulong candidate))
+                        {
                                 exhausted = false;
-			return currentValue;
+                                return candidate;
                         }
+
+                        // No valid divisor was found in this range; attempt to acquire the next block.
+                        localDivisorCursor = 0UL;
+                        localDivisorsRemaining = 0;
+                }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static bool TryAcquireLocalDivisor(ref ulong cursor, ref int remaining, out ulong divisor)
+        {
+                while (remaining > 0)
+                {
+                        ulong candidate = cursor;
+                        cursor += 2UL;
+                        remaining--;
+
+                        if (IsAllowedDivisorCandidate(candidate))
+                        {
+                                divisor = candidate;
+                                return true;
+                        }
+                }
+
+                divisor = 0UL;
+                return false;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static bool IsAllowedDivisorCandidate(ulong divisor)
+        {
+                if ((divisor & 1UL) == 0UL)
+                {
+                        return false;
+                }
+
+                if ((divisor % 3UL) == 0UL || (divisor % 5UL) == 0UL || (divisor % 7UL) == 0UL || (divisor % 11UL) == 0UL)
+                {
+                        return false;
+                }
+
+                return true;
         }
 
 	private static int BuildPrimeBuffer(ulong divisor, in ulong[] primeValues, in ulong[] allowedMaxValues, int[] stateFlags, ulong[] primeBuffer, int[] indexBuffer, PendingResult[] completionsBuffer, ref int completionsCount, ref int remainingStates, long[] activeStateMask, ref int activeStartIndex)