feat: add deterministic VoronoiCells partition

VoxelEngine.Tests/WorldGen/VoronoiCellsTests.cs

@@ -0,0 +1,36 @@
+using VoxelEngine.World.WorldGen;
+using Xunit;
+
+namespace VoxelEngine.Tests.WorldGen;
+
+public class VoronoiCellsTests
+{
+    [Fact]
+    public void Nearest_IsDeterministic()
+    {
+        var a = new VoronoiCells(42, 96f);
+        var b = new VoronoiCells(42, 96f);
+        Assert.Equal(a.Nearest(123.4f, 56.7f), b.Nearest(123.4f, 56.7f));
+    }
+
+    [Fact]
+    public void Nearest_IsLocallyConstant()
+    {
+        var cells = new VoronoiCells(42, 96f);
+        // Two points a tiny step apart almost always share a cell.
+        var c1 = cells.Nearest(500f, 500f);
+        var c2 = cells.Nearest(500.01f, 500.01f);
+        Assert.Equal(c1.Id, c2.Id);
+    }
+
+    [Fact]
+    public void DifferentSeeds_DifferAtSomePoint()
+    {
+        var a = new VoronoiCells(1, 96f);
+        var b = new VoronoiCells(2, 96f);
+        bool anyDiff = false;
+        for (int i = 0; i < 50 && !anyDiff; i++)
+            anyDiff = a.Nearest(i * 31f, i * 17f).Id != b.Nearest(i * 31f, i * 17f).Id;
+        Assert.True(anyDiff);
+    }
+}

World/WorldGen/VoronoiCells.cs

@@ -0,0 +1,60 @@
+namespace VoxelEngine.World.WorldGen;
+
+// A jittered-grid Voronoi partition. Pure integer-hash based, fully deterministic.
+public sealed class VoronoiCells
+{
+    public readonly record struct Cell(int Id, float CenterX, float CenterZ);
+
+    private readonly int _seed;
+    private readonly float _cellSize;
+
+    public VoronoiCells(int seed, float cellSize)
+    {
+        this._seed = seed;
+        this._cellSize = cellSize;
+    }
+
+    public Cell Nearest(float x, float z)
+    {
+        int gx = (int)System.MathF.Floor(x / this._cellSize);
+        int gz = (int)System.MathF.Floor(z / this._cellSize);
+
+        float bestDist = float.MaxValue;
+        int bestId = 0;
+        float bestCx = 0f, bestCz = 0f;
+
+        for (int oz = -1; oz <= 1; oz++)
+        for (int ox = -1; ox <= 1; ox++)
+        {
+            int cgx = gx + ox;
+            int cgz = gz + oz;
+            float jitterX = VoronoiCells.Jitter(cgx, cgz, this._seed);
+            float jitterZ = VoronoiCells.Jitter(cgx, cgz, this._seed + 9871);
+            float cx = (cgx + jitterX) * this._cellSize;
+            float cz = (cgz + jitterZ) * this._cellSize;
+            float dx = cx - x, dz = cz - z;
+            float dist = dx * dx + dz * dz;
+            if (dist < bestDist)
+            {
+                bestDist = dist;
+                bestId = (int)VoronoiCells.Hash(cgx, cgz, this._seed);
+                bestCx = cx;
+                bestCz = cz;
+            }
+        }
+        return new Cell(bestId, bestCx, bestCz);
+    }
+
+    private static float Jitter(int x, int z, int seed)
+        => (VoronoiCells.Hash(x, z, seed) & 0xFFFFFF) / (float)0x1000000; // [0,1)
+
+    private static uint Hash(int x, int z, int seed)
+    {
+        unchecked
+        {
+            uint h = (uint)(x * 374761393 + z * 668265263 + seed * 362437);
+            h = (h ^ (h >> 13)) * 1274126177u;
+            return h ^ (h >> 16);
+        }
+    }
+}