Restore GPU-backed UInt128 pow2 Montgomery helper

PerfectNumbers.Core.Tests/Gpu/GpuRationalTests.cs

@@ -0,0 +1,82 @@
+using FluentAssertions;
+using PerfectNumbers.Core.Gpu;
+using Xunit;
+
+namespace PerfectNumbers.Core.Tests.Gpu;
+
+public class GpuRationalTests
+{
+    [Fact, Trait("Category", "Fast")]
+    public void Constructor_ReducesFraction()
+    {
+        GpuRational value = new(6UL, 8UL);
+        value.Numerator.Should().Be(new GpuUInt128(3UL));
+        value.Denominator.Should().Be(new GpuUInt128(4UL));
+    }
+
+    [Fact, Trait("Category", "Fast")]
+    public void Addition_ReturnsExpectedValue()
+    {
+        GpuRational left = new(1UL, 3UL);
+        GpuRational right = new(1UL, 6UL);
+
+        GpuRational sum = left + right;
+
+        sum.Numerator.Should().Be(new GpuUInt128(1UL));
+        sum.Denominator.Should().Be(new GpuUInt128(2UL));
+    }
+
+    [Fact, Trait("Category", "Fast")]
+    public void Subtraction_ReturnsExpectedValue()
+    {
+        GpuRational left = new(5UL, 6UL);
+        GpuRational right = new(1UL, 6UL);
+
+        GpuRational difference = left - right;
+
+        difference.Numerator.Should().Be(new GpuUInt128(2UL));
+        difference.Denominator.Should().Be(new GpuUInt128(3UL));
+    }
+
+    [Fact, Trait("Category", "Fast")]
+    public void Multiplication_ReturnsExpectedValue()
+    {
+        GpuRational left = new(3UL, 4UL);
+        GpuRational right = new(2UL, 3UL);
+
+        GpuRational product = left * right;
+
+        product.Numerator.Should().Be(new GpuUInt128(1UL));
+        product.Denominator.Should().Be(new GpuUInt128(2UL));
+    }
+
+    [Fact, Trait("Category", "Fast")]
+    public void Division_ReturnsExpectedValue()
+    {
+        GpuRational left = new(3UL, 5UL);
+        GpuRational right = new(9UL, 10UL);
+
+        GpuRational quotient = left / right;
+
+        quotient.Numerator.Should().Be(new GpuUInt128(2UL));
+        quotient.Denominator.Should().Be(new GpuUInt128(3UL));
+    }
+
+    [Fact, Trait("Category", "Fast")]
+    public void CompareTo_DetectsOrdering()
+    {
+        GpuRational smaller = new(1UL, 4UL);
+        GpuRational larger = new(2UL, 3UL);
+
+        smaller.CompareTo(larger).Should().BeLessThan(0);
+        larger.CompareTo(smaller).Should().BeGreaterThan(0);
+        smaller.CompareTo(new GpuRational(1UL, 4UL)).Should().Be(0);
+    }
+
+    [Fact, Trait("Category", "Fast")]
+    public void ToDouble_ReturnsExpectedResult()
+    {
+        GpuRational value = new(1UL, 5UL);
+        value.ToDouble().Should().BeApproximately(0.2, 1e-12);
+    }
+}