feat: add BiomeMap with per-cell climate classification

VoxelEngine.Tests/WorldGen/BiomeMapTests.cs

@@ -0,0 +1,54 @@
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.WorldGen;
+using Xunit;
+
+namespace VoxelEngine.Tests.WorldGen;
+
+public class BiomeMapTests
+{
+    private static (BiomeMap map, BiomeRegistry biomes) Build(int seed = 42)
+    {
+        var blocks = new BlockRegistry();
+        var biomes = new BiomeRegistry(seed, blocks);
+        var noise = new NoiseSource(seed);
+        var cells = new VoronoiCells(seed, 96f);
+        return (new BiomeMap(noise, cells, biomes), biomes);
+    }
+
+    [Fact]
+    public void Classify_HighElevation_IsMountains()
+    {
+        var (_, biomes) = Build();
+        Assert.Same(biomes.Mountains, BiomeMap.Classify(0f, 0.5f, 0f, biomes));
+    }
+
+    [Fact]
+    public void Classify_HotLowElevation_IsDesert()
+    {
+        var (_, biomes) = Build();
+        Assert.Same(biomes.Desert, BiomeMap.Classify(0.5f, 0f, 0f, biomes));
+    }
+
+    [Fact]
+    public void Classify_TemperateWooded_IsForest()
+    {
+        var (_, biomes) = Build();
+        Assert.Same(biomes.Forest, BiomeMap.Classify(0f, 0f, 0.5f, biomes));
+    }
+
+    [Fact]
+    public void Classify_ColdLow_IsPlains()
+    {
+        var (_, biomes) = Build();
+        Assert.Same(biomes.Plains, BiomeMap.Classify(-0.9f, 0f, 0f, biomes));
+    }
+
+    [Fact]
+    public void BiomeAt_IsConstantWithinACell()
+    {
+        var (map, _) = Build();
+        IBiome a = map.BiomeAt(500f, 500f);
+        IBiome b = map.BiomeAt(500.01f, 500.01f); // same cell
+        Assert.Same(a, b);
+    }
+}

World/WorldGen/BiomeMap.cs

@@ -0,0 +1,36 @@
+namespace VoxelEngine.World.WorldGen;
+
+public sealed class BiomeMap
+{
+    private readonly NoiseSource _noise;
+    private readonly VoronoiCells _cells;
+    private readonly BiomeRegistry _biomes;
+
+    public BiomeMap(NoiseSource noise, VoronoiCells cells, BiomeRegistry biomes)
+    {
+        this._noise = noise;
+        this._cells = cells;
+        this._biomes = biomes;
+    }
+
+    public int CellId(float worldX, float worldZ) => this._cells.Nearest(worldX, worldZ).Id;
+
+    public IBiome BiomeAt(float worldX, float worldZ)
+    {
+        VoronoiCells.Cell cell = this._cells.Nearest(worldX, worldZ);
+        // Sample climate at the cell center so the biome is constant across the cell.
+        float temperature = this._noise.Temperature(cell.CenterX, cell.CenterZ);
+        float elevation = this._noise.Continent(cell.CenterX, cell.CenterZ);
+        float forest = this._noise.Forest(cell.CenterX, cell.CenterZ);
+        return BiomeMap.Classify(temperature, elevation, forest, this._biomes);
+    }
+
+    // Pure climate -> biome lookup. Thresholds in noise units (~[-1,1]).
+    public static IBiome Classify(float temperature, float elevation, float forest, BiomeRegistry biomes)
+    {
+        if (elevation > 0.25f) return biomes.Mountains;          // high ground
+        if (temperature > 0.33f) return biomes.Desert;           // hot, low
+        if (temperature > -0.33f && forest > 0f) return biomes.Forest; // temperate, wooded
+        return biomes.Plains;                                    // temperate/cold, low
+    }
+}