CultMath: the fixture cases separate each lattice-loop mutant from the shader text

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

packages/cultmath/tests/CultMath.Tests/GlslMirrorTests.Mutants.cs

@@ -0,0 +1,59 @@
+using System.Reflection;
+using System.Text.RegularExpressions;
+using Xunit;
+
+namespace CultMath.Tests;
+
+public sealed partial class GlslMirrorTests
+{
+    private const string Weighted = "cultmath_weighted_lattice_cells(";
+    private const string SmoothStep = "cultmath_lattice_smooth_step(";
+
+    // Edits of the shader text that each break one rule of the lattice loop. M7 (the exit one ring late) is absent: by the
+    // window-independence proof a later exit visits more cells and changes nothing, so no case can kill it.
+    private static readonly (string Name, string Function, string Old, string New, int Kills)[] LatticeMutants =
+    {
+        ("M1 exit one ring early", Weighted, "0.5 * (float)(k - 1) >= max(f2", "0.5 * (float)k >= max(f2", 3),
+        ("M2 exit ignores the smoothing band", Weighted, ">= max(f2, f1 + 2.0 * smoothing))", ">= f2)", 1),
+        ("M3 no pre-normalisation of the normal", Weighted, "normal = stretchNormal / scale;", "normal = stretchNormal;", 1),
+        ("M4 no unit length for the normal", Weighted, "normal = normal / length(normal);", "normal = normal * 1.0;", 1),
+        ("M5 smooth step without its clamp", SmoothStep, "return min(next, f1 + smoothing);", "return next;", 1),
+        ("M6 box pass skips its first row", Weighted, "for (int dz = -reach; dz <= reach; dz++)", "for (int dz = -reach + 1; dz <= reach; dz++)", 1),
+    };
+
+    /// <summary>
+    /// Each rule of the lattice loop is pinned by the committed fixture's own cases: the shader text with one rule broken (a
+    /// mutant mirror, the text a device runs) returns another result than the fixture on at least the stated number of
+    /// cases of the family that exercises it, the exit one ring early on at least three. The device judge holds the same
+    /// cases to their recorded bound, so a mutant these cases separate fails it too.
+    /// </summary>
+    [Fact]
+    public void TheFixtureCasesSeparateEveryLatticeMutantFromTheShader()
+    {
+        var families = FixtureFamilies();
+        foreach (var (name, function, old, replacement, kills) in LatticeMutants)
+        {
+            var (assembly, errors) = HlslSourceCompatibilityTests.CompileShaderMirror(mutate: text =>
+            {
+                Assert.True(text.Split(old).Length == 2, $"{name}: the shader text no longer holds exactly one \"{old}\"");
+                return text.Replace(old, replacement);
+            });
+            Assert.True(assembly is not null, name + ": " + string.Join(Environment.NewLine, errors));
+            var shaderType = assembly!.GetType("CultMathHlsl.HlslShader")!;
+            var shader = Activator.CreateInstance(shaderType);
+            var mirror = shaderType.GetMethod(function[..^1], BindingFlags.Instance | BindingFlags.NonPublic)!;
+
+            var f = Array.FindIndex(Families, family => family.Glsl.Contains(function));
+            var method = Resolve(Families[f].Glsl);
+            var killed = 0;
+            foreach (var entry in families[f]!["cases"]!.AsArray())
+            {
+                var sides = Sides(entry);
+                if (Hex(new[] { mirror.Invoke(shader, Arguments(method, sides[0]))! }) != sides[1])
+                    killed++;
+            }
+
+            Assert.True(killed >= kills, $"{name}: the {Families[f].Glsl} fixture cases separate it on {killed} of {Points}, fewer than {kills}");
+        }
+    }
+}

packages/cultmath/tests/CultMath.Tests/HlslSourceCompatibilityTests.cs

@@ -782,9 +782,11 @@ public sealed class HlslSourceCompatibilityTests
         return "using CultMath;\nusing static CultMath.math;\nnamespace CultMathHlsl;\npublic class HlslShader\n{\n" + members + source + "\n}\n";
     }
 
-    internal static (Assembly? Assembly, IReadOnlyList<string> Errors) CompileShaderMirror(string members = "", IReadOnlyDictionary<string, int>? defines = null)
+    internal static (Assembly? Assembly, IReadOnlyList<string> Errors) CompileShaderMirror(string members = "", IReadOnlyDictionary<string, int>? defines = null, Func<string, string>? mutate = null)
     {
         var hlsl = ReadShaderSource(FindCultMathRoot());
+        if (mutate is not null)
+            hlsl = mutate(hlsl);
         var tree = CSharpSyntaxTree.ParseText(TransformHlslToCSharp(hlsl, members, defines));
         var references = ((string)AppContext.GetData("TRUSTED_PLATFORM_ASSEMBLIES")!)
             .Split(Path.PathSeparator)