DivisorSet.BitContradiction mode added to --mersenne=bydivisor.

PerfectNumbers.Core.Tests/BitContradictionSolverTests.cs

@@ -0,0 +1,59 @@
+using FluentAssertions;
+using PerfectNumbers.Core.ByDivisor;
+using Xunit;
+using static PerfectNumbers.Core.ByDivisor.BitContradictionSolver;
+
+namespace PerfectNumbers.Core.Tests;
+
+public class BitContradictionSolverTests
+{
+    [Fact]
+    public void ComputeColumnBounds_TracksForcedAndPossibleOnes()
+    {
+        BitState[] a = [BitState.One, BitState.Unknown];
+        BitState[] q = [BitState.One, BitState.One];
+
+        ColumnBounds bounds = ComputeColumnBounds(a, q, 1);
+
+        bounds.ForcedOnes.Should().Be(1);
+        bounds.PossibleOnes.Should().Be(2);
+    }
+
+    [Fact]
+    public void TryPropagateCarry_FailsWhenParityUnreachable()
+    {
+        CarryRange carry = CarryRange.Single(0);
+
+        bool success = TryPropagateCarry(
+            carry,
+            forcedOnes: 0,
+            possibleOnes: 0,
+            requiredResultBit: 1,
+            out CarryRange next,
+            out ContradictionReason reason);
+
+        success.Should().BeFalse();
+        reason.Should().Be(ContradictionReason.ParityUnreachable);
+        next.Min.Should().Be(0);
+        next.Max.Should().Be(0);
+    }
+
+    [Fact]
+    public void TryPropagateCarry_ComputesNextCarryRange()
+    {
+        CarryRange carry = new(0, 1);
+
+        bool success = TryPropagateCarry(
+            carry,
+            forcedOnes: 1,
+            possibleOnes: 3,
+            requiredResultBit: 1,
+            out CarryRange next,
+            out ContradictionReason reason);
+
+        success.Should().BeTrue();
+        reason.Should().Be(ContradictionReason.None);
+        next.Min.Should().Be(0);
+        next.Max.Should().Be(1);
+    }
+}