Consolidate multiply helpers and guard GPU fallbacks

PerfectNumbers.Core.Tests/ULongExtensionsMultiplyShiftTests.cs

@@ -0,0 +1,67 @@
+using System.Numerics;
+using FluentAssertions;
+using PerfectNumbers.Core;
+using Xunit;
+
+namespace PerfectNumbers.Core.Tests;
+
+[Trait("Category", "Fast")]
+public class ULongExtensionsMultiplyShiftTests
+{
+    [Theory]
+    [InlineData(0UL, 0UL, 1)]
+    [InlineData(1UL, 3UL, 1)]
+    [InlineData(ulong.MaxValue, 3UL, 3)]
+    [InlineData(0xFFFF_FFFF_FFFF_FFFFUL, 5UL, 4)]
+    [InlineData(0x8000_0000_0000_0000UL, 7UL, 5)]
+    public void MultiplyShiftRight_matches_big_integer(ulong value, ulong multiplier, int shift)
+    {
+        BigInteger expected = ((BigInteger)value * multiplier) >> shift;
+
+        ulong result = ULongExtensions.MultiplyShiftRight(value, multiplier, shift);
+
+        result.Should().Be((ulong)expected);
+    }
+
+    [Theory]
+    [InlineData(0UL, 0UL, 1)]
+    [InlineData(1UL, 3UL, 1)]
+    [InlineData(ulong.MaxValue, 3UL, 3)]
+    [InlineData(0xFFFF_FFFF_FFFF_FFFFUL, 5UL, 4)]
+    [InlineData(0x8000_0000_0000_0000UL, 7UL, 5)]
+    [InlineData(0xDEAD_BEEF_DEAD_BEEFUL, 11UL, 63)]
+    public void MultiplyShiftRightShiftFirst_matches_big_integer(ulong value, ulong multiplier, int shift)
+    {
+        BigInteger expected = ((BigInteger)value * multiplier) >> shift;
+
+        ulong result = ULongExtensions.MultiplyShiftRightShiftFirst(value, multiplier, shift);
+
+        result.Should().Be((ulong)expected);
+    }
+
+    [Fact]
+    public void MultiplyShiftRightShiftFirst_returns_zero_when_shift_is_large()
+    {
+        ULongExtensions.MultiplyShiftRightShiftFirst(ulong.MaxValue, 123UL, 64).Should().Be(0UL);
+    }
+
+    [Theory]
+    [InlineData(0UL, ulong.MaxValue, -1)]
+    [InlineData(1UL, 1UL, -3)]
+    public void MultiplyShiftRight_throws_when_shift_is_negative(ulong value, ulong multiplier, int shift)
+    {
+        Action act = () => ULongExtensions.MultiplyShiftRight(value, multiplier, shift);
+
+        act.Should().Throw<ArgumentOutOfRangeException>();
+    }
+
+    [Theory]
+    [InlineData(0UL, ulong.MaxValue, -1)]
+    [InlineData(1UL, 1UL, -3)]
+    public void MultiplyShiftRightShiftFirst_throws_when_shift_is_negative(ulong value, ulong multiplier, int shift)
+    {
+        Action act = () => ULongExtensions.MultiplyShiftRightShiftFirst(value, multiplier, shift);
+
+        act.Should().Throw<ArgumentOutOfRangeException>();
+    }
+}