Initial commit.

PerfectNumbers.Core.Tests/MersenneResidueStepperTests.cs

@@ -0,0 +1,37 @@
+using FluentAssertions;
+using Xunit;
+
+namespace PerfectNumbers.Core.Tests;
+
+public class MersenneResidueStepperTests
+{
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void Step_advances_residue()
+    {
+        var stepper = new MersenneResidueStepper(13, 4, 2);
+
+        stepper.State().Should().Be((4UL, 2UL));
+
+        stepper.Step(3).Should().Be((7UL, 10UL));
+    }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void To_moves_directly_to_target_exponent()
+    {
+        var stepper = new MersenneResidueStepper(13, 4, 2);
+
+        stepper.To(10).Should().Be((10UL, 9UL));
+    }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void Constructor_rejects_zero_modulus()
+    {
+        var act = () => new MersenneResidueStepper(0, 0, 0);
+
+        act.Should().Throw<ArgumentException>();
+    }
+}
+