BitContradiction improvements.

PerfectNumbers.Core.Tests/BitContradictionSolverTests.cs

@@ -1,3 +1,4 @@
+using System.Numerics;
 using FluentAssertions;
 using PerfectNumbers.Core.ByDivisor;
 using Xunit;
@@ -22,7 +23,7 @@ public class BitContradictionSolverTests
     [Fact]
     public void TryPropagateCarry_FailsWhenParityUnreachable()
     {
-		CarryRange carry = CarryRange.Single(0);
+        CarryRange carry = CarryRange.Single(0);
         bool success = TryPropagateCarry(
             currentCarry: ref carry,
             forcedOnes: 0,
@@ -34,10 +35,34 @@ public class BitContradictionSolverTests
         carry.Max.Should().Be(0);
     }
 
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void ShouldPruneWithFinalPow2Shift_ReturnsTrue_WhenShiftedResidueStaysBelowModulusAndResidueIsNotOne()
+    {
+        bool shouldPrune = ShouldPruneWithFinalPow2Shift(
+            prefixResidue: new BigInteger(3),
+            modulus: new BigInteger(1000),
+            tailShift: 8);
+
+        shouldPrune.Should().BeTrue();
+    }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void ShouldPruneWithFinalPow2Shift_ReturnsFalse_WhenResidueIsOne()
+    {
+        bool shouldPrune = ShouldPruneWithFinalPow2Shift(
+            prefixResidue: BigInteger.One,
+            modulus: new BigInteger(1000),
+            tailShift: 8);
+
+        shouldPrune.Should().BeFalse();
+    }
+
     [Fact]
     public void TryPropagateCarry_ComputesNextCarryRange()
     {
-		CarryRange carry = new(0, 1);
+        CarryRange carry = new(0, 1);
         bool success = TryPropagateCarry(
             currentCarry: ref carry,
             forcedOnes: 1,