From 9a81f5f6858d9599bf2b717b68b322682b3ebbe3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Lema=C5=84czyk?= <marek@lemanczyk.eu>
Date: Wed, 8 Oct 2025 22:45:53 +0200
Subject: [PATCH] Enable parallel by-divisor scanning threads

---
 EvenPerfectBitScanner/Program.cs              |  1 -
 .../MersenneNumberDivisorByDivisorTester.cs   | 28 +++++++++++++++++--
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/EvenPerfectBitScanner/Program.cs b/EvenPerfectBitScanner/Program.cs
index 7da6016..229f5dd 100644
--- a/EvenPerfectBitScanner/Program.cs
+++ b/EvenPerfectBitScanner/Program.cs
@@ -458,7 +458,6 @@ internal static class Program
 
         if (useByDivisor)
         {
-            threadCount = 1;
             blockSize = 1;
         }
 
diff --git a/PerfectNumbers.Core/MersenneNumberDivisorByDivisorTester.cs b/PerfectNumbers.Core/MersenneNumberDivisorByDivisorTester.cs
index a332008..a5fbe57 100644
--- a/PerfectNumbers.Core/MersenneNumberDivisorByDivisorTester.cs
+++ b/PerfectNumbers.Core/MersenneNumberDivisorByDivisorTester.cs
@@ -2,6 +2,7 @@ using System;
 using System.Buffers;
 using System.Collections.Generic;
 using System.Globalization;
+using System.Threading.Tasks;
 using Open.Numeric.Primes;
 
 namespace PerfectNumbers.Core;
@@ -158,7 +159,13 @@ public static class MersenneNumberDivisorByDivisorTester
             return;
         }
 
-        foreach (ulong prime in filteredPrimes)
+        int workerCount = threadCount <= 0 ? Environment.ProcessorCount : threadCount;
+        if (workerCount < 1)
+        {
+            workerCount = 1;
+        }
+
+        void ProcessPrime(ulong prime)
         {
             bool isPrime = tester.IsPrime(prime, out bool divisorsExhausted);
 
@@ -166,11 +173,28 @@ public static class MersenneNumberDivisorByDivisorTester
             {
                 markComposite();
                 printResult(prime, true, true, false);
-                continue;
+                return;
             }
 
             clearComposite();
             printResult(prime, true, divisorsExhausted, true);
         }
+
+        if (workerCount == 1)
+        {
+            foreach (ulong prime in filteredPrimes)
+            {
+                ProcessPrime(prime);
+            }
+        }
+        else
+        {
+            ParallelOptions options = new()
+            {
+                MaxDegreeOfParallelism = workerCount
+            };
+
+            Parallel.ForEach(filteredPrimes, options, ProcessPrime);
+        }
     }
 }