Merge pull request #38 from mlemanczyk/codex/fix-residue-method-iteration-issue

Fix residue iteration with filtered primes

EvenPerfectBitScanner.Tests/ResidueCandidateEnumerationTests.cs

@@ -0,0 +1,77 @@
+using System;
+using System.Threading;
+using EvenPerfectBitScanner;
+using PerfectNumbers.Core;
+using Xunit;
+
+namespace EvenPerfectBitScanner.Tests;
+
+public class ResidueCandidateEnumerationTests
+{
+    [Fact]
+    public void ModResidueTracker_does_not_flag_small_primes_as_composite()
+    {
+        var tracker = new ModResidueTracker(ResidueModel.Identity, initialNumber: 13UL, initialized: true);
+        var primes = PrimesGenerator.SmallPrimes;
+        var primesPow2 = PrimesGenerator.SmallPrimesPow2;
+
+        ulong p = 13UL;
+        ulong remainder = p % 6UL;
+
+        for (int i = 0; i < 2000; i++)
+        {
+            tracker.BeginMerge(p);
+            bool composite = false;
+            for (int j = 0; j < primes.Length; j++)
+            {
+                if (primesPow2[j] > p)
+                {
+                    break;
+                }
+
+                tracker.MergeOrAppend(p, primes[j], out bool divisible);
+                if (divisible)
+                {
+                    composite = true;
+                    break;
+                }
+            }
+
+            bool isPrime = PrimeTester.IsPrimeInternal(p, CancellationToken.None);
+            if (isPrime && composite)
+            {
+                throw new InvalidOperationException($"Prime {p} marked as composite.");
+            }
+
+            p = Program.TransformPAdd(p, ref remainder);
+        }
+    }
+
+    [Fact]
+    public void TransformPAdd_visits_both_prime_residue_classes()
+    {
+        ulong p = 13UL;
+        ulong remainder = p % 6UL;
+        bool sawOneModSix = false;
+        bool sawFiveModSix = false;
+
+        for (int i = 0; i < 256; i++)
+        {
+            ulong residue = p % 6UL;
+            if (residue == 1UL)
+            {
+                sawOneModSix = true;
+            }
+            else if (residue == 5UL)
+            {
+                sawFiveModSix = true;
+            }
+
+            p = Program.TransformPAdd(p, ref remainder);
+        }
+
+        Assert.True(sawOneModSix, "Sequence never produced numbers congruent to 1 mod 6.");
+        Assert.True(sawFiveModSix, "Sequence never produced numbers congruent to 5 mod 6.");
+    }
+
+}

EvenPerfectBitScanner/Program.cs

@@ -554,6 +554,12 @@ internal static class Program
 		}
 
 		bool useFilter = !string.IsNullOrEmpty(filterFile);
+		if (useFilter && !useBitTransform)
+		{
+			// When replaying a filtered list of p values, iterate strictly along the prime sequence.
+			// This keeps candidate generation aligned with the recorded primes regardless of residue stepping.
+			_transformP = &TransformPAddPrimes;
+		}
 		HashSet<ulong> filter = [];
 		ulong maxP = 0UL;
 		if (useFilter)
@@ -950,9 +956,11 @@ internal static class Program
 		}
 
 		ulong next = (value << 1) | 1UL;
-		remainder = (remainder << 1) + 1UL;
-		remainder -= (remainder >= 6UL) ? 6UL : 0UL;
-		remainder -= (remainder >= 6UL) ? 6UL : 0UL;
+                remainder = (remainder << 1) + 1UL;
+                while (remainder >= 6UL)
+                {
+                        remainder -= 6UL;
+                }
 
 		value = remainder switch
 		{
@@ -970,8 +978,11 @@ internal static class Program
 			return original;
 		}
 
-		remainder += value;
-		remainder -= (remainder >= 6UL) ? 6UL : 0UL;
+                remainder += value;
+                while (remainder >= 6UL)
+                {
+                        remainder -= 6UL;
+                }
 
 		return next + value;
 	}
@@ -988,8 +999,11 @@ internal static class Program
 			return next;
 		}
 
-		remainder += diff;
-		remainder -= (remainder >= 6UL) ? 6UL : 0UL;
+                remainder += diff;
+                while (remainder >= 6UL)
+                {
+                        remainder -= 6UL;
+                }
 
 		return next + diff;
 	}
@@ -1017,8 +1031,11 @@ internal static class Program
 				}
 
 				candidate += diff;
-				addRemainder += diff;
-				addRemainder -= (addRemainder >= 6UL) ? 6UL : 0UL;
+                                addRemainder += diff;
+                                while (addRemainder >= 6UL)
+                                {
+                                        addRemainder -= 6UL;
+                                }
 			}
 
 			if (advancePrime)