Allow exhaustive divisor scans with 128-bit limits

PerfectNumbers.Core.Tests/MersenneNumberDivisorGpuTesterTests.cs

@@ -1,5 +1,7 @@
 using FluentAssertions;
 using PerfectNumbers.Core.Gpu;
+using System.Numerics;
+using System.Reflection;
 using Xunit;
 
 namespace PerfectNumbers.Core.Tests;
@@ -17,4 +19,46 @@ public class MersenneNumberDivisorGpuTesterTests
         var tester = new MersenneNumberDivisorGpuTester();
         tester.IsDivisible(exponent, divisor).Should().Be(expected);
     }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void IsDivisible_handles_large_divisors()
+    {
+        var tester = new MersenneNumberDivisorGpuTester();
+        UInt128 divisor = (UInt128.One << 65) - UInt128.One;
+        tester.IsDivisible(65UL, divisor).Should().BeTrue();
+        tester.IsDivisible(65UL, divisor + 2).Should().BeFalse();
+    }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void IsPrime_sets_divisorsExhausted_false_when_search_range_not_exhausted()
+    {
+        var tester = new MersenneNumberDivisorGpuTester();
+        typeof(MersenneNumberDivisorGpuTester)
+            .GetField("_divisorCandidates", BindingFlags.NonPublic | BindingFlags.Static)!
+            .SetValue(null, Array.Empty<(ulong, uint)>());
+
+        tester.IsPrime(11UL, UInt128.Zero, 0UL, out bool divisorsExhausted).Should().BeTrue();
+        divisorsExhausted.Should().BeFalse();
+    }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void IsPrime_sets_divisorsExhausted_true_when_divisible_by_specified_divisor()
+    {
+        var tester = new MersenneNumberDivisorGpuTester();
+        tester.IsPrime(3UL, 7UL, 0UL, out bool divisorsExhausted).Should().BeFalse();
+        divisorsExhausted.Should().BeTrue();
+    }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void IsPrime_accepts_large_search_limits()
+    {
+        var tester = new MersenneNumberDivisorGpuTester();
+        tester.IsPrime(3UL, 7UL, ulong.MaxValue, out bool exhausted).Should().BeFalse();
+        exhausted.Should().BeTrue();
+    }
 }
+