Merge pull request #135 from mlemanczyk/codex/optimize-prime-filtering-method

Optimize filtering strategy for previous Mersenne results

PerfectNumbers.Core/MersenneNumberDivisorByDivisorTester.cs

@@ -38,6 +38,9 @@ public static class MersenneNumberDivisorByDivisorTester
         if (previousResults is not null && previousResults.Count > 0)
         {
             int recordedCount = previousResults.Count;
+            bool preferSortedMerge = recordedCount <= 4096 || (recordedCount * 4L) <= candidateCount;
+            if (preferSortedMerge)
+            {
                 ulong[] recordedCandidates = ArrayPool<ulong>.Shared.Rent(recordedCount);
                 try
                 {
@@ -75,7 +78,7 @@ public static class MersenneNumberDivisorByDivisorTester
                     if (writeIndex < candidateCount)
                     {
                         candidates.RemoveRange(writeIndex, candidateCount - writeIndex);
-                    candidateSpan = CollectionsMarshal.AsSpan(candidates);
+                        candidateSpan = CollectionsMarshal.AsSpan(candidates); // candidateSpan now tracks the reduced list.
                         candidateCount = candidateSpan.Length;
                     }
                 }
@@ -84,6 +87,30 @@ public static class MersenneNumberDivisorByDivisorTester
                     ArrayPool<ulong>.Shared.Return(recordedCandidates, clearArray: true);
                 }
             }
+            else
+            {
+                int writeIndex = 0;
+                for (int readIndex = 0; readIndex < candidateCount; readIndex++)
+                {
+                    ulong candidate = candidateSpan[readIndex];
+
+                    if (previousResults.ContainsKey(candidate))
+                    {
+                        skippedByPreviousResults++;
+                        continue;
+                    }
+
+                    candidateSpan[writeIndex++] = candidate;
+                }
+
+                if (writeIndex < candidateCount)
+                {
+                    candidates.RemoveRange(writeIndex, candidateCount - writeIndex);
+                    candidateSpan = CollectionsMarshal.AsSpan(candidates); // candidateSpan now tracks the reduced list.
+                    candidateCount = candidateSpan.Length;
+                }
+            }
+        }
 
         if (skippedByPreviousResults > 0)
         {