Require filtered primes for by-divisor scanning

EvenPerfectBitScanner/Program.cs

@@ -376,6 +376,12 @@ internal static class Program
 			}
                 }
 
+                if (useByDivisor && string.IsNullOrEmpty(filterFile))
+                {
+                        Console.WriteLine("--mersenne=bydivisor requires --filter-p=<path>.");
+                        return;
+                }
+
                 if (showHelp)
                 {
                         PrintHelp();
@@ -601,6 +607,17 @@ internal static class Program
                         // filter.Sort();
                 }
 
+                if (useByDivisor)
+                {
+                        if (maxP <= 1UL)
+                        {
+                                Console.WriteLine("The filter specified by --filter-p must contain at least one prime exponent greater than 1 for --mersenne=bydivisor.");
+                                return;
+                        }
+
+                        _byDivisorTester!.ConfigureFromMaxPrime(maxP);
+                }
+
                 if (_primeCount >= 2)
                 {
                         _primeFoundAfterInit = true;
@@ -785,7 +802,7 @@ internal static class Program
 		Console.WriteLine("  --use-order            test primality via q order");
 		Console.WriteLine("  --workaround-mod       avoid '%' operator on the GPU");
 		// mod-automaton removed
-		Console.WriteLine("  --filter-p=<path>      process only p from previous run results");
+                Console.WriteLine("  --filter-p=<path>      process only p from previous run results (required for --mersenne=bydivisor)");
 		Console.WriteLine("  --write-batch-size=<value> overwrite frequency of disk writes (default 100 lines)");
 		Console.WriteLine("  --gcd-filter           enable early sieve based on GCD");
 		Console.WriteLine("  --help, -help, --?, -?, /?   show this help message");
@@ -1143,7 +1160,7 @@ internal static class Program
                 searchedMersenne = true;
                 if (_useByDivisorMode)
                 {
-                        return _byDivisorTester!.IsPrime(p, divisorCyclesSearchLimit, out detailedCheck);
+                        return _byDivisorTester!.IsPrime(p, out detailedCheck);
                 }
 
                 if (_useDivisor)

PerfectNumbers.Core.Tests/MersenneNumberDivisorGpuTesterTests.cs

@@ -1,5 +1,6 @@
 using FluentAssertions;
 using PerfectNumbers.Core.Gpu;
+using System;
 using System.Numerics;
 using System.Reflection;
 using Xunit;
@@ -66,14 +67,40 @@ public class MersenneNumberDivisorGpuTesterTests
     public void ByDivisor_tester_tracks_divisors_across_primes()
     {
         var tester = new MersenneNumberDivisorByDivisorGpuTester();
-        tester.IsPrime(5UL, 64UL, out bool exhausted).Should().BeTrue();
+        tester.ConfigureFromMaxPrime(11UL);
+
+        tester.IsPrime(5UL, out bool exhausted).Should().BeTrue();
         exhausted.Should().BeTrue();
 
-        tester.IsPrime(7UL, 96UL, out exhausted).Should().BeTrue();
+        tester.IsPrime(7UL, out exhausted).Should().BeTrue();
         exhausted.Should().BeTrue();
 
-        tester.IsPrime(11UL, 128UL, out exhausted).Should().BeFalse();
+        tester.IsPrime(11UL, out exhausted).Should().BeFalse();
         exhausted.Should().BeTrue();
     }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void ByDivisor_tester_requires_configuration()
+    {
+        var tester = new MersenneNumberDivisorByDivisorGpuTester();
+        Action act = () => tester.IsPrime(5UL, out _);
+        act.Should().Throw<InvalidOperationException>();
+    }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void ByDivisor_tester_respects_filter_based_limit()
+    {
+        var tester = new MersenneNumberDivisorByDivisorGpuTester();
+        tester.ConfigureFromMaxPrime(5UL);
+
+        tester.IsPrime(5UL, out _).Should().BeTrue();
+        tester.IsPrime(7UL, out bool exhausted).Should().BeTrue();
+        exhausted.Should().BeTrue();
+
+        tester.IsPrime(11UL, out bool divisorsExhausted).Should().BeTrue();
+        divisorsExhausted.Should().BeTrue();
+    }
 }
 

PerfectNumbers.Core/Gpu/MersenneNumberDivisorByDivisorGpuTester.cs

@@ -16,6 +16,7 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester
         private readonly List<ulong> _lastPrimes = new();
         private readonly object _sync = new();
         private ulong _divisorLimit;
+        private bool _isConfigured;
 
         private readonly ConcurrentDictionary<Accelerator, Action<Index1D, ulong, ArrayView<ulong>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<byte>>> _updateKernelCache = new();
         private readonly ConcurrentDictionary<Accelerator, Action<Index1D, ulong, ArrayView<ulong>, ArrayView<ulong>, ArrayView<byte>>> _initialKernelCache = new();
@@ -26,12 +27,32 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester
         private Action<Index1D, ulong, ArrayView<ulong>, ArrayView<ulong>, ArrayView<byte>> GetInitialKernel(Accelerator accelerator) =>
                         _initialKernelCache.GetOrAdd(accelerator, acc => acc.LoadAutoGroupedStreamKernel<Index1D, ulong, ArrayView<ulong>, ArrayView<ulong>, ArrayView<byte>>(InitialKernel));
 
-        public bool IsPrime(ulong prime, ulong divisorLimit, out bool divisorsExhausted)
+        public void ConfigureFromMaxPrime(ulong maxPrime)
         {
                 lock (_sync)
                 {
-                        _divisorLimit = divisorLimit;
+                        if (_divisors.Count > 0)
-                        ulong allowedMax = ComputeAllowedMaxDivisor(prime, divisorLimit);
+                        {
+                                _divisors.Clear();
+                                _remainders.Clear();
+                                _lastPrimes.Clear();
+                        }
+
+                        _divisorLimit = ComputeDivisorLimitFromMaxPrime(maxPrime);
+                        _isConfigured = true;
+                }
+        }
+
+        public bool IsPrime(ulong prime, out bool divisorsExhausted)
+        {
+                lock (_sync)
+                {
+                        if (!_isConfigured)
+                        {
+                                throw new InvalidOperationException("ConfigureFromMaxPrime must be called before using the tester.");
+                        }
+
+                        ulong allowedMax = ComputeAllowedMaxDivisor(prime);
                         if (allowedMax < 3UL)
                         {
                                 divisorsExhausted = true;
@@ -177,7 +198,22 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester
                 return composite;
         }
 
-        private static ulong ComputeAllowedMaxDivisor(ulong prime, ulong divisorLimit)
+        private static ulong ComputeDivisorLimitFromMaxPrime(ulong maxPrime)
+        {
+                if (maxPrime <= 1UL)
+                {
+                        return 0UL;
+                }
+
+                if (maxPrime - 1UL >= 64UL)
+                {
+                        return ulong.MaxValue;
+                }
+
+                return (1UL << (int)(maxPrime - 1UL)) - 1UL;
+        }
+
+        private ulong ComputeAllowedMaxDivisor(ulong prime)
         {
                 if (prime <= 1UL)
                 {
@@ -186,11 +222,11 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester
 
                 if (prime - 1UL >= 64UL)
                 {
-                        return divisorLimit;
+                        return _divisorLimit;
                 }
 
                 ulong maxByPrime = (1UL << (int)(prime - 1UL)) - 1UL;
-                return Math.Min(maxByPrime, divisorLimit);
+                return Math.Min(maxByPrime, _divisorLimit);
         }
 
         private static void UpdateKernel(Index1D index, ulong prime, ArrayView<ulong> divisors, ArrayView<ulong> remainders, ArrayView<ulong> lastPrimes, ArrayView<byte> hits)