Merge pull request #39 from mlemanczyk/codex/fix-evenperfectbitscanner-prime-detection

Fix GPU residue false positives for Mersenne primes

EvenPerfectBitScanner.Tests/ProgramTests.cs

@@ -351,6 +351,46 @@ public class ProgramTests
         }
         finally
         {
+            useDivisorField.SetValue(null, false);
+            mersenneField.SetValue(null, null);
+            primeField.SetValue(null, null);
+            residueField.SetValue(null, null);
+            forceCpuProp!.SetValue(null, false);
+        }
+    }
+
+    [Fact]
+    public void Residue_mode_with_default_limits_accepts_large_known_primes()
+    {
+        var useDivisorField = typeof(Program).GetField("_useDivisor", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var mersenneField = typeof(Program).GetField("MersenneTesters", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var primeField = typeof(Program).GetField("PrimeTesters", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var residueField = typeof(Program).GetField("PResidue", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var forceCpuProp = typeof(GpuContextPool).GetProperty("ForceCpu");
+
+        useDivisorField.SetValue(null, false);
+        mersenneField.SetValue(null, new ThreadLocal<MersenneNumberTester>(() => new MersenneNumberTester(
+            useIncremental: true,
+            useResidue: true,
+            maxK: 5_000_000UL,
+            residueDivisorSets: PerfectNumberConstants.ExtraDivisorCycleSearchLimit), trackAllValues: true));
+        primeField.SetValue(null, new ThreadLocal<PrimeTester>(() => new PrimeTester(), trackAllValues: true));
+        residueField.SetValue(null, new ThreadLocal<ModResidueTracker>(() => new ModResidueTracker(ResidueModel.Identity, 2UL, true), trackAllValues: true));
+        forceCpuProp!.SetValue(null, false);
+
+        try
+        {
+            Program.IsEvenPerfectCandidate(107UL, 0UL, out bool searched, out bool detailed).Should().BeTrue();
+            searched.Should().BeTrue();
+            detailed.Should().BeTrue();
+
+            Program.IsEvenPerfectCandidate(127UL, 0UL, out searched, out detailed).Should().BeTrue();
+            searched.Should().BeTrue();
+            detailed.Should().BeTrue();
+        }
+        finally
+        {
+            useDivisorField.SetValue(null, false);
             mersenneField.SetValue(null, null);
             primeField.SetValue(null, null);
             residueField.SetValue(null, null);

PerfectNumbers.Core.Tests/MersenneNumberResidueGpuTesterTests.cs

@@ -21,9 +21,57 @@ public class MersenneNumberResidueGpuTesterTests
         RunCase(tester, 29UL, 37UL, expectedPrime: false);
         RunCase(tester, 31UL, 1_000UL, expectedPrime: true);
         RunCase(tester, 89UL, 1_001UL, expectedPrime: true);
+        RunCase(tester, 107UL, 1_001UL, expectedPrime: true);
         RunCase(tester, 127UL, 1_001UL, expectedPrime: true);
     }
 
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    [Trait("Category", "Fast")]
+    public void Scan_handles_multiple_sets_without_false_positives(bool useGpuOrder)
+    {
+        var tester = new MersenneNumberResidueGpuTester(useGpuOrder);
+
+        bool isPrime = true;
+        bool exhausted = false;
+        UInt128 perSetLimit = 1_666_667UL;
+        UInt128 setCount = 3UL;
+        UInt128 overallLimit = 5_000_000UL;
+
+        tester.Scan(
+            127UL,
+            (UInt128)127UL << 1,
+            LastDigitIsSeven(127UL),
+            perSetLimit,
+            setCount,
+            overallLimit,
+            ref isPrime,
+            ref exhausted);
+
+        isPrime.Should().BeTrue();
+        exhausted.Should().BeTrue();
+
+        isPrime = true;
+        exhausted = false;
+        perSetLimit = 20_000_000UL;
+        setCount = 5UL;
+        overallLimit = 100_000_000UL;
+
+        tester.Scan(
+            107UL,
+            (UInt128)107UL << 1,
+            LastDigitIsSeven(107UL),
+            perSetLimit,
+            setCount,
+            overallLimit,
+            ref isPrime,
+            ref exhausted);
+
+        isPrime.Should().BeTrue();
+        exhausted.Should().BeTrue();
+    }
+
     private static void RunCase(MersenneNumberResidueGpuTester tester, ulong exponent, ulong maxK, bool expectedPrime)
     {
         bool isPrime = true;

PerfectNumbers.Core/Gpu/GpuKernelPool.cs

@@ -195,12 +195,12 @@ public ref struct GpuKernelLease(IDisposable limiter, GpuContextLease gpu, Kerne
         GpuUInt128 q = twoP.Mul64(k) + GpuUInt128.One;
         
         // Small-cycles in-kernel early rejection from device table
-        if (q.High == 0UL && q.Low <= (ulong)(smallCycles.Length - 1))
+        if (q.High == 0UL && q.Low < (ulong)smallCycles.Length)
         {
             uint cyc = smallCycles[(int)q.Low];
-            if (cyc != 0U && (exponent % cyc) == 0UL)
+            if (cyc != 0U && (exponent % cyc) != 0UL)
             {
-                orders[index] = exponent;
+                orders[index] = 0UL;
                 return;
             }
         }
@@ -340,12 +340,12 @@ public ref struct GpuKernelLease(IDisposable limiter, GpuContextLease gpu, Kerne
 
         GpuUInt128 k = kStart + (GpuUInt128)idx;
         GpuUInt128 q = twoP.Mul64(k) + GpuUInt128.One;
-        if (q.High == 0UL && q.Low <= (ulong)(smallCycles.Length - 1))
+        if (q.High == 0UL && q.Low < (ulong)smallCycles.Length)
         {
             uint cyc = smallCycles[(int)q.Low];
-            if (cyc != 0U && (exponent % cyc) == 0UL)
+            if (cyc != 0U && (exponent % cyc) != 0UL)
             {
-                orders[index] = exponent;
+                orders[index] = 0UL;
                 return;
             }
         }
@@ -468,10 +468,10 @@ public ref struct GpuKernelLease(IDisposable limiter, GpuContextLease gpu, Kerne
         GpuUInt128 q = twoP.Mul64(k) + GpuUInt128.One;
 
         // Small-cycles in-kernel early rejection from device table
-        if (q.High == 0UL && q.Low <= (ulong)(smallCycles.Length - 1))
+        if (q.High == 0UL && q.Low < (ulong)smallCycles.Length)
         {
             uint cyc = smallCycles[(int)q.Low];
-            if (cyc != 0U && (exponent % (ulong)cyc) == 0UL)
+            if (cyc != 0U && (exponent % (ulong)cyc) != 0UL)
             {
                 return;
             }
@@ -547,10 +547,10 @@ public ref struct GpuKernelLease(IDisposable limiter, GpuContextLease gpu, Kerne
         GpuUInt128 k = kStart + (GpuUInt128)idx;
         GpuUInt128 q = twoP.Mul64(k) + GpuUInt128.One;
         // Small-cycles in-kernel early rejection from device table
-        if (q.High == 0UL && q.Low <= (ulong)(smallCycles.Length - 1))
+        if (q.High == 0UL && q.Low < (ulong)smallCycles.Length)
         {
             uint cyc = smallCycles[(int)q.Low];
-            if (cyc != 0U && (exponent % (ulong)cyc) == 0UL)
+            if (cyc != 0U && (exponent % (ulong)cyc) != 0UL)
             {
                 return;
             }