feat(world-gen): add WormHash deterministic mixing function

VoxelEngine.Tests/WorldGen/WormHashTests.cs

@@ -0,0 +1,40 @@
+using VoxelEngine.World.WorldGen;
+using Xunit;
+
+namespace VoxelEngine.Tests.WorldGen;
+
+public class WormHashTests
+{
+    [Fact]
+    public void Hash_IsDeterministic_ForSameInputs()
+    {
+        uint a = WormHash.Hash(3, -2, 7, seed: 1337);
+        uint b = WormHash.Hash(3, -2, 7, seed: 1337);
+        Assert.Equal(a, b);
+    }
+
+    [Fact]
+    public void Hash_DiffersForDifferentCells()
+    {
+        uint a = WormHash.Hash(3, -2, 7, seed: 1337);
+        uint b = WormHash.Hash(3, -2, 8, seed: 1337);
+        Assert.NotEqual(a, b);
+    }
+
+    [Fact]
+    public void Hash_DiffersForDifferentSeeds()
+    {
+        uint a = WormHash.Hash(3, -2, 7, seed: 1337);
+        uint b = WormHash.Hash(3, -2, 7, seed: 9999);
+        Assert.NotEqual(a, b);
+    }
+
+    [Fact]
+    public void Hash_HandlesNegativeCoords()
+    {
+        // Just shouldn't throw and shouldn't collide trivially with the positive case.
+        uint a = WormHash.Hash(-1, -1, -1, seed: 1);
+        uint b = WormHash.Hash(1, 1, 1, seed: 1);
+        Assert.NotEqual(a, b);
+    }
+}

VoxelEngine.csproj

@@ -22,4 +22,8 @@
         </None>
     </ItemGroup>
 
+    <ItemGroup>
+        <InternalsVisibleTo Include="VoxelEngine.Tests" />
+    </ItemGroup>
+
 </Project>

World/WorldGen/WormHash.cs

@@ -0,0 +1,21 @@
+namespace VoxelEngine.World.WorldGen;
+
+// Deterministic 32-bit mixing hash for worm-cell coordinates.
+// Cell coords can be negative; cast through unchecked to preserve bit pattern.
+internal static class WormHash
+{
+    public static uint Hash(int cellX, int cellY, int cellZ, int seed)
+    {
+        unchecked
+        {
+            uint h = (uint)seed;
+            h ^= (uint)cellX * 73856093u;
+            h ^= (uint)cellY * 19349663u;
+            h ^= (uint)cellZ * 83492791u;
+            h ^= h >> 13;
+            h *= 0x5bd1e995u;
+            h ^= h >> 15;
+            return h;
+        }
+    }
+}