refactor: early-return in Shader.Location uniform cache lookup

RenderDistanceVertical 4 -> 2 (smaller vertical load box) and
WorkerThreadCount ProcessorCount-1 -> ProcessorCount/2 (less
aggressive worker pool).

.gitignore

@@ -22,6 +22,9 @@ project.fragment.lock.json
 # Local design docs — kept out of the repo by request
 docs/
 
+# Runtime logs
+logs/
+
 # OS noise
 Thumbs.db
 desktop.ini

Assets/shaders/loading.frag

@@ -0,0 +1,3 @@
+#version 330 core
+out vec4 FragColor;
+void main() { FragColor = vec4(0.10, 0.12, 0.16, 1.0); }

Assets/shaders/loading.vert

@@ -0,0 +1,3 @@
+#version 330 core
+layout(location = 0) in vec2 aPos;
+void main() { gl_Position = vec4(aPos, 0.0, 1.0); }

Core/ChunkMath.cs

@@ -0,0 +1,13 @@
+namespace VoxelEngine.Core;
+
+internal static class ChunkMath
+{
+    // Floor division: floors toward negative infinity (not toward zero like C#'s /).
+    // Required for correct world-coord -> chunk-index mapping for negative coordinates.
+    internal static int FloorDiv(int value, int size)
+    {
+        int q = value / size;
+        if (value % size != 0 && value < 0) q--;
+        return q;
+    }
+}

Core/Configuration.cs

@@ -0,0 +1,21 @@
+namespace VoxelEngine.Core;
+
+public sealed class Configuration
+{
+    public int RenderDistanceHorizontal { get; set; } = 8;
+    public int RenderDistanceVertical { get; set; } = 2;
+    public int SpawnReadyRadiusHorizontal { get; set; } = 1;
+
+    public int MaxGenerateJobsInFlight { get; set; } = 8;
+    public int MaxMeshUploadsPerFrame { get; set; } = 4;
+    public int WorkerThreadCount { get; set; } = Math.Max(1, Environment.ProcessorCount / 2);
+
+    public float MoveSpeed { get; set; } = 8f;
+    public float MouseSensitivity { get; set; } = 0.15f;
+    public float Fov { get; set; } = 60f;
+
+    public int WindowWidth { get; set; } = 1280;
+    public int WindowHeight { get; set; } = 720;
+
+    public int? Seed { get; set; } = null;
+}

Core/Game.cs

@@ -3,107 +3,100 @@ using OpenTK.Mathematics;
 using OpenTK.Windowing.Common;
 using OpenTK.Windowing.Desktop;
 using OpenTK.Windowing.GraphicsLibraryFramework;
+using Serilog;
+using VoxelEngine.Game;
 using VoxelEngine.Rendering;
 using VoxelEngine.World.Blocks;
 using VoxelEngine.World.Chunk;
+using VoxelEngine.World.Persistence;
+using VoxelEngine.World.Streaming;
+using VoxelEngine.World.WorldGen;
 
 namespace VoxelEngine.Core;
 
 public sealed class Game : GameWindow
 {
-    private const float MoveSpeed = 8f;     // units per second
-    private const float Sensitivity = 0.15f; // degrees per pixel
+    private static readonly ILogger Log = Serilog.Log.ForContext<Game>();
 
+    private readonly Configuration _cfg;
     private readonly BlockRegistry _registry = new();
+
     private Shader _shader = null!;
     private Texture _texture = null!;
-    private readonly Dictionary<Vector3i, ChunkRenderer> _renderer = new();
-    private Camera _camera = null!;
+    private LoadingScreen _loading = null!;
+    private World.World _world = null!;
+    private InMemoryChunkModificationStore _modStore = null!;
+    private WorldGenerator _generator = null!;
+    private ChunkStreamer _streamer = null!;
+    private Player _player = null!;
 
-    private World.World _world;
-
-    public Game(GameWindowSettings gameSettings, NativeWindowSettings nativeSettings)
-        : base(gameSettings, nativeSettings) { }
+    public Game(Configuration cfg, GameWindowSettings gameSettings, NativeWindowSettings nativeSettings)
+        : base(gameSettings, nativeSettings)
+    {
+        this._cfg = cfg;
+    }
 
     protected override void OnLoad()
     {
         base.OnLoad();
 
+        Log.Information("Game starting with seed={Seed} renderDistanceH={H} renderDistanceV={V}",
+            this._cfg.Seed, this._cfg.RenderDistanceHorizontal, this._cfg.RenderDistanceVertical);
+
         GL.ClearColor(0.45f, 0.65f, 0.95f, 1.0f);
         GL.Enable(EnableCap.DepthTest);
         GL.Enable(EnableCap.CullFace);
 
         this._shader = new Shader("Assets/shaders/chunk.vert", "Assets/shaders/chunk.frag");
         this._texture = new Texture("Assets/textures/VoxelShadeAtlas.png");
+        this._loading = new LoadingScreen();
 
-        this._world = new World.World(this._registry);
+        this._modStore = new InMemoryChunkModificationStore();
+        this._world = new World.World(this._registry, this._modStore);
 
-        for (int x = -2; x < 2; x++)
-        for (int z = -2; z < 2; z++)
-        {
-            this._world.SetChunk(new Vector3i(x, 0, z), this.BuildChunk());
-        }
+        int seed = this._cfg.Seed ?? new Random().Next();
+        this._generator = new WorldGenerator(seed, this._registry);
 
-        ChunkMesher mesher = new();
-        foreach ((Vector3i pos, Chunk chunk) in this._world)
-        {
-            Vector3i origin = pos *  Chunk.Size;
-            float[] mesh = mesher.Build(
-                chunk, 
-                this._registry, 
-                (nx, ny, nz) => this._world.GetBlock(
-                    origin.X + nx, 
-                    origin.Y + ny, 
-                    origin.Z + nz).IsOpaque
-                );
-            this._renderer[pos] = new ChunkRenderer(mesh);
-        }
+        // Spawn at terrain surface above (0, 0).
+        int surfaceY = this._generator.SurfaceHeightAt(0, 0);
+        Vector3 spawnPosition = new(0.5f, surfaceY + 2f, 0.5f);
 
-        this._camera = new Camera(new Vector3(8f, 20f, 40f), this.ClientSize.X / (float)this.ClientSize.Y);
+        this._player = new Player(this._cfg, spawnPosition, this.ClientSize.X / (float)this.ClientSize.Y);
+
+        var mesher = new ChunkMesher(this._texture.Width, this._texture.Height);
+        this._streamer = new ChunkStreamer(
+            this._cfg,
+            this._world,
+            this._generator.Generate,
+            mesher,
+            this._registry,
+            new ThreadPoolJobExecutor(),
+            mesh => new ChunkRenderer(mesh));
 
         this.CursorState = CursorState.Grabbed;
     }
 
-    private Chunk BuildChunk()
-    {
-        Chunk chunk = new();
-        for (int x = 0; x < Chunk.Size; x++)
-        for (int z = 0; z < Chunk.Size; z++)
-        for (int y = 0; y < Chunk.Size; y++)
-        {
-            chunk[x, y, z] = y switch
-            {
-                Chunk.Size - 1 => this._registry.Grass.Id,
-                >= Chunk.Size - 4 => this._registry.Dirt.Id,
-                _ => this._registry.Stone.Id
-            };
-        }
-        return chunk;
-    }
-
     protected override void OnUpdateFrame(FrameEventArgs args)
     {
         base.OnUpdateFrame(args);
         if (!this.IsFocused) return;
 
-        if (this.KeyboardState.IsKeyDown(Keys.Escape)) {
-            this.Close(); return; 
-        }
+        if (this.KeyboardState.IsKeyDown(Keys.Escape)) { this.Close(); return; }
 
-        float speedMulti = 1f;
-        
-        if (this.KeyboardState.IsKeyDown(Keys.LeftShift)) speedMulti = 1.8f ;
+        float speedMulti = this.KeyboardState.IsKeyDown(Keys.LeftShift) ? 1.8f : 1f;
+        float distance = this._player.MoveSpeed * speedMulti * (float)args.Time;
 
-        float distance = Game.MoveSpeed * speedMulti * (float)args.Time;
-        if (this.KeyboardState.IsKeyDown(Keys.W)) this._camera.Move(this._camera.Front * distance);
-        if (this.KeyboardState.IsKeyDown(Keys.S)) this._camera.Move(-this._camera.Front * distance);
-        if (this.KeyboardState.IsKeyDown(Keys.A)) this._camera.Move(-this._camera.Right * distance);
-        if (this.KeyboardState.IsKeyDown(Keys.D)) this._camera.Move(this._camera.Right * distance);
-        if (this.KeyboardState.IsKeyDown(Keys.Space)) this._camera.Move(Vector3.UnitY * distance);
-        if (this.KeyboardState.IsKeyDown(Keys.LeftControl)) this._camera.Move(-Vector3.UnitY * distance);
+        if (this.KeyboardState.IsKeyDown(Keys.W)) this._player.Move(this._player.Camera.Front * distance);
+        if (this.KeyboardState.IsKeyDown(Keys.S)) this._player.Move(-this._player.Camera.Front * distance);
+        if (this.KeyboardState.IsKeyDown(Keys.A)) this._player.Move(-this._player.Camera.Right * distance);
+        if (this.KeyboardState.IsKeyDown(Keys.D)) this._player.Move(this._player.Camera.Right * distance);
+        if (this.KeyboardState.IsKeyDown(Keys.Space)) this._player.Move(Vector3.UnitY * distance);
+        if (this.KeyboardState.IsKeyDown(Keys.LeftControl)) this._player.Move(-Vector3.UnitY * distance);
 
         Vector2 delta = this.MouseState.Delta;
-        this._camera.AddRotation(delta.X * Game.Sensitivity, -delta.Y * Game.Sensitivity);
+        this._player.AddRotation(delta.X * this._player.MouseSensitivity, -delta.Y * this._player.MouseSensitivity);
+
+        this._streamer.Tick(this._player.ChunkPosition);
     }
 
     protected override void OnRenderFrame(FrameEventArgs args)
@@ -112,21 +105,25 @@ public sealed class Game : GameWindow
 
         GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
 
+        if (!this._streamer.SpawnReady)
+        {
+            this._loading.Draw();
+            this.SwapBuffers();
+            return;
+        }
+
         this._shader.Use();
-        this._shader.SetMatrix4("view", this._camera.GetViewMatrix());
-        this._shader.SetMatrix4("projection", this._camera.GetProjectionMatrix());
+        this._shader.SetMatrix4("view", this._player.Camera.GetViewMatrix());
+        this._shader.SetMatrix4("projection", this._player.Camera.GetProjectionMatrix());
         this._shader.SetInt("atlas", 0);
         this._texture.Use(TextureUnit.Texture0);
 
-        foreach ((Vector3i pos, ChunkRenderer renderer) in this._renderer)
+        this._streamer.DrawAll((pos, renderer) =>
         {
             this._shader.SetMatrix4("model", Matrix4.CreateTranslation(
-                pos.X * Chunk.Size,
-                pos.Y * Chunk.Size,
-                pos.Z * Chunk.Size
-            ));
+                pos.X * Chunk.Size, pos.Y * Chunk.Size, pos.Z * Chunk.Size));
             renderer.Draw();
-        }
+        });
 
         this.SwapBuffers();
     }
@@ -135,18 +132,16 @@ public sealed class Game : GameWindow
     {
         base.OnResize(e);
         GL.Viewport(0, 0, e.Width, e.Height);
-        // OnResize can fire before OnLoad (where _camera is created), so guard against it.
-        if (this._camera is not null) this._camera.AspectRatio = e.Width / (float)e.Height;
+        if (this._player is not null) this._player.Camera.AspectRatio = e.Width / (float)e.Height;
     }
 
     protected override void OnUnload()
     {
-        foreach (ChunkRenderer rendererValue in this._renderer.Values)
-        {
-            rendererValue.Dispose();
-        }
-        this._texture.Dispose();
-        this._shader.Dispose();
+        Log.Information("Game shutting down");
+        this._streamer?.Dispose();
+        this._loading?.Dispose();
+        this._texture?.Dispose();
+        this._shader?.Dispose();
         base.OnUnload();
     }
 }

Game/Player.cs

@@ -0,0 +1,38 @@
+using OpenTK.Mathematics;
+using VoxelEngine.Core;
+using VoxelEngine.Rendering;
+using VoxelChunk = VoxelEngine.World.Chunk.Chunk;
+
+namespace VoxelEngine.Game;
+
+public sealed class Player
+{
+    private readonly Configuration _cfg;
+
+    public Camera Camera { get; }
+    public Vector3 Position => this.Camera.Position;
+    public float MoveSpeed => this._cfg.MoveSpeed;
+    public float MouseSensitivity => this._cfg.MouseSensitivity;
+
+    public Vector3i ChunkPosition
+    {
+        get
+        {
+            Vector3 p = this.Position;
+            return new Vector3i(
+                ChunkMath.FloorDiv((int)MathF.Floor(p.X), VoxelChunk.Size),
+                ChunkMath.FloorDiv((int)MathF.Floor(p.Y), VoxelChunk.Size),
+                ChunkMath.FloorDiv((int)MathF.Floor(p.Z), VoxelChunk.Size));
+        }
+    }
+
+    public Player(Configuration cfg, Vector3 spawnPosition, float aspectRatio)
+    {
+        this._cfg = cfg;
+        this.Camera = new Camera(spawnPosition, aspectRatio) { Fov = cfg.Fov };
+    }
+
+    public void Move(Vector3 delta) => this.Camera.Move(delta);
+
+    public void AddRotation(float yawDelta, float pitchDelta) => this.Camera.AddRotation(yawDelta, pitchDelta);
+}

Program.cs

@@ -1,16 +1,45 @@
 using OpenTK.Mathematics;
 using OpenTK.Windowing.Common;
 using OpenTK.Windowing.Desktop;
+using Serilog;
+using Serilog.Events;
 using VoxelEngine.Core;
 
-NativeWindowSettings nativeSettings = new()
-{
-    ClientSize = new Vector2i(1280, 720),
-    Title = "VoxelEngine",
-    APIVersion = new Version(3, 3),
-    Profile = ContextProfile.Core,
-    Flags = ContextFlags.ForwardCompatible
-};
+Log.Logger = new LoggerConfiguration()
+    .MinimumLevel.Debug()
+    .Enrich.FromLogContext()
+    .WriteTo.Console(
+        restrictedToMinimumLevel: LogEventLevel.Information,
+        outputTemplate: "[{Timestamp:HH:mm:ss.fff} {Level:u3}] [{SourceContext}] {Message:lj}{NewLine}{Exception}")
+    .WriteTo.File(
+        path: "logs/voxelengine-.log",
+        rollingInterval: RollingInterval.Day,
+        retainedFileCountLimit: 14,
+        outputTemplate: "[{Timestamp:HH:mm:ss.fff} {Level:u3}] [{SourceContext}] {Message:lj}{NewLine}{Exception}")
+    .CreateLogger();
 
-using Game game = new(GameWindowSettings.Default, nativeSettings);
-game.Run();
+try
+{
+    var cfg = new Configuration();
+
+    NativeWindowSettings nativeSettings = new()
+    {
+        ClientSize = new Vector2i(cfg.WindowWidth, cfg.WindowHeight),
+        Title = "VoxelEngine",
+        APIVersion = new Version(3, 3),
+        Profile = ContextProfile.Core,
+        Flags = ContextFlags.ForwardCompatible
+    };
+
+    using Game game = new(cfg, GameWindowSettings.Default, nativeSettings);
+    game.Run();
+}
+catch (Exception ex)
+{
+    Log.Fatal(ex, "Unhandled exception at top level");
+    throw;
+}
+finally
+{
+    Log.CloseAndFlush();
+}

Rendering/ChunkRenderer.cs

@@ -2,7 +2,7 @@ using OpenTK.Graphics.OpenGL4;
 
 namespace VoxelEngine.Rendering;
 
-public sealed class ChunkRenderer : IDisposable
+public sealed class ChunkRenderer : VoxelEngine.World.Streaming.IChunkRenderer
 {
     private const int FloatsPerVertex = 6; // pos.xyz, uv, shade
 

Rendering/LoadingScreen.cs

@@ -0,0 +1,52 @@
+using OpenTK.Graphics.OpenGL4;
+
+namespace VoxelEngine.Rendering;
+
+public sealed class LoadingScreen : IDisposable
+{
+    private readonly int _vao;
+    private readonly int _vbo;
+    private readonly Shader _shader;
+
+    public LoadingScreen()
+    {
+        this._shader = new Shader("Assets/shaders/loading.vert", "Assets/shaders/loading.frag");
+
+        // Single triangle covering the entire NDC quad.
+        float[] vertices =
+        {
+            -1f, -1f,
+             3f, -1f,
+            -1f,  3f,
+        };
+
+        this._vao = GL.GenVertexArray();
+        GL.BindVertexArray(this._vao);
+
+        this._vbo = GL.GenBuffer();
+        GL.BindBuffer(BufferTarget.ArrayBuffer, this._vbo);
+        GL.BufferData(BufferTarget.ArrayBuffer, vertices.Length * sizeof(float), vertices, BufferUsageHint.StaticDraw);
+
+        GL.VertexAttribPointer(0, 2, VertexAttribPointerType.Float, false, 2 * sizeof(float), 0);
+        GL.EnableVertexAttribArray(0);
+
+        GL.BindVertexArray(0);
+    }
+
+    public void Draw()
+    {
+        GL.Disable(EnableCap.DepthTest);
+        this._shader.Use();
+        GL.BindVertexArray(this._vao);
+        GL.DrawArrays(PrimitiveType.Triangles, 0, 3);
+        GL.BindVertexArray(0);
+        GL.Enable(EnableCap.DepthTest);
+    }
+
+    public void Dispose()
+    {
+        GL.DeleteBuffer(this._vbo);
+        GL.DeleteVertexArray(this._vao);
+        this._shader.Dispose();
+    }
+}

Rendering/Shader.cs

@@ -38,11 +38,9 @@ public sealed class Shader : IDisposable
 
     private int Location(string name)
     {
-        if (!this._uniforms.TryGetValue(name, out int location))
-        {
-            location = GL.GetUniformLocation(this._handle, name);
-            this._uniforms[name] = location;
-        }
+        if (this._uniforms.TryGetValue(name, out int location)) return location;
+        location = GL.GetUniformLocation(this._handle, name);
+        this._uniforms[name] = location;
         return location;
     }
 

Rendering/Texture.cs

@@ -7,6 +7,9 @@ public sealed class Texture : IDisposable
 {
     private readonly int _handle;
 
+    public int Width { get; }
+    public int Height { get; }
+
     public Texture(string path)
     {
         this._handle = GL.GenTexture();
@@ -15,6 +18,8 @@ public sealed class Texture : IDisposable
         StbImage.stbi_set_flip_vertically_on_load(1);
         using FileStream stream = File.OpenRead(path);
         ImageResult image = ImageResult.FromStream(stream, ColorComponents.RedGreenBlueAlpha);
+        this.Width = image.Width;
+        this.Height = image.Height;
 
         GL.TexImage2D(TextureTarget.Texture2D, 0, PixelInternalFormat.Rgba,
             image.Width, image.Height, 0, PixelFormat.Rgba, PixelType.UnsignedByte, image.Data);

VoxelEngine.Tests/BlockRegistryTests.cs

@@ -49,4 +49,26 @@ public class BlockRegistryTests
         Assert.Equal((ushort)2, registry.Dirt.Id);
         Assert.Equal((ushort)3, registry.Grass.Id);
     }
+
+    [Fact]
+    public void SandAndSnow_AreOpaque_WaterIsOpaque()
+    {
+        var registry = new BlockRegistry();
+        Assert.True(registry.Get(4).IsOpaque);  // Sand
+        Assert.True(registry.Get(5).IsOpaque);  // Snow
+        Assert.True(registry.Get(6).IsOpaque);  // Water (single-pass renderer => opaque)
+    }
+
+    [Fact]
+    public void Registry_ResolvesAllSevenBlocks()
+    {
+        var registry = new BlockRegistry();
+        Assert.Equal("Air", registry.Get(0).Name);
+        Assert.Equal("Stone", registry.Get(1).Name);
+        Assert.Equal("Dirt", registry.Get(2).Name);
+        Assert.Equal("Grass", registry.Get(3).Name);
+        Assert.Equal("Sand", registry.Get(4).Name);
+        Assert.Equal("Snow Block", registry.Get(5).Name);
+        Assert.Equal("Water", registry.Get(6).Name);
+    }
 }

VoxelEngine.Tests/ChunkMesherTests.cs

@@ -16,7 +16,7 @@ public class ChunkMesherTests
     [Fact]
     public void EmptyChunk_ProducesNoFaces()
     {
-        var mesh = new ChunkMesher().Build(new Chunk(), new BlockRegistry());
+        var mesh = new ChunkMesher(128, 128).Build(new Chunk(), new BlockRegistry());
         Assert.Empty(mesh);
     }
 
@@ -26,7 +26,7 @@ public class ChunkMesherTests
         var chunk = new Chunk();
         chunk[8, 8, 8] = 1; // stone
 
-        var mesh = new ChunkMesher().Build(chunk, new BlockRegistry());
+        var mesh = new ChunkMesher(128, 128).Build(chunk, new BlockRegistry());
 
         Assert.Equal(6, FaceCount(mesh));
     }
@@ -40,7 +40,7 @@ public class ChunkMesherTests
         for (int z = 0; z < Chunk.Size; z++)
             chunk[x, y, z] = 1; // stone
 
-        var mesh = new ChunkMesher().Build(chunk, new BlockRegistry());
+        var mesh = new ChunkMesher(128, 128).Build(chunk, new BlockRegistry());
 
         // 6 sides * 16 * 16 visible faces
         Assert.Equal(6 * Chunk.Size * Chunk.Size, FaceCount(mesh));
@@ -52,10 +52,32 @@ public class ChunkMesherTests
         var chunk = new Chunk();
         chunk[0, 0, 0] = 1; // corner block: all 6 neighbors are air/out-of-bounds -> 6 faces
 
-        var mesh = new ChunkMesher().Build(chunk, new BlockRegistry());
+        var mesh = new ChunkMesher(128, 128).Build(chunk, new BlockRegistry());
 
         // Pins the layout: 6 faces * 6 vertices/face * 6 floats/vertex.
         Assert.Equal(6 * VerticesPerFace * FloatsPerVertex, mesh.Length);
         Assert.Equal(0, mesh.Length % FloatsPerVertex);
     }
+
+    [Fact]
+    public void Mesher_UsesAtlasDimensions_ForTileSize()
+    {
+        var chunk = new Chunk();
+        chunk[0, 0, 0] = 1; // stone, tile (1,0)
+
+        // 128px atlas, 32px tiles => 4x4 grid, normalized tile size 0.25.
+        var mesh = new ChunkMesher(128, 128).Build(chunk, new BlockRegistry());
+
+        // Gather all U coords (offset 3) and V coords (offset 4); each must be a
+        // multiple of 0.25 within [0,1].
+        for (int i = 0; i < mesh.Length; i += 6)
+        {
+            float u = mesh[i + 3];
+            float v = mesh[i + 4];
+            Assert.InRange(u, 0f, 1f);
+            Assert.InRange(v, 0f, 1f);
+            Assert.True(System.Math.Abs(u * 4f - System.MathF.Round(u * 4f)) < 1e-4f, $"u={u} not on 0.25 grid");
+            Assert.True(System.Math.Abs(v * 4f - System.MathF.Round(v * 4f)) < 1e-4f, $"v={v} not on 0.25 grid");
+        }
+    }
 }

VoxelEngine.Tests/ConfigurationTests.cs

@@ -0,0 +1,57 @@
+using VoxelEngine.Core;
+using Xunit;
+
+namespace VoxelEngine.Tests;
+
+public class ConfigurationTests
+{
+    [Fact]
+    public void Defaults_AreSane()
+    {
+        var c = new Configuration();
+        Assert.True(c.RenderDistanceHorizontal >= 1);
+        Assert.True(c.RenderDistanceVertical >= 1);
+        Assert.True(c.SpawnReadyRadiusHorizontal >= 0);
+        Assert.True(c.MaxGenerateJobsInFlight >= 1);
+        Assert.True(c.MaxMeshUploadsPerFrame >= 1);
+        Assert.True(c.WorkerThreadCount >= 1);
+        Assert.True(c.MoveSpeed > 0f);
+        Assert.True(c.MouseSensitivity > 0f);
+        Assert.InRange(c.Fov, 1f, 179f);
+        Assert.True(c.WindowWidth > 0);
+        Assert.True(c.WindowHeight > 0);
+        Assert.Null(c.Seed);
+    }
+
+    [Fact]
+    public void Properties_AreMutable()
+    {
+        var c = new Configuration { RenderDistanceHorizontal = 4, Seed = 1337 };
+        Assert.Equal(4, c.RenderDistanceHorizontal);
+        Assert.Equal(1337, c.Seed);
+
+        var c2 = new Configuration
+        {
+            RenderDistanceVertical     = 2,
+            SpawnReadyRadiusHorizontal = 0,
+            MaxGenerateJobsInFlight    = 2,
+            MaxMeshUploadsPerFrame     = 1,
+            WorkerThreadCount          = 2,
+            MoveSpeed                  = 4f,
+            MouseSensitivity           = 0.5f,
+            Fov                        = 90f,
+            WindowWidth                = 800,
+            WindowHeight               = 600,
+        };
+        Assert.Equal(2,    c2.RenderDistanceVertical);
+        Assert.Equal(0,    c2.SpawnReadyRadiusHorizontal);
+        Assert.Equal(2,    c2.MaxGenerateJobsInFlight);
+        Assert.Equal(1,    c2.MaxMeshUploadsPerFrame);
+        Assert.Equal(2,    c2.WorkerThreadCount);
+        Assert.Equal(4f,   c2.MoveSpeed);
+        Assert.Equal(0.5f, c2.MouseSensitivity);
+        Assert.Equal(90f,  c2.Fov);
+        Assert.Equal(800,  c2.WindowWidth);
+        Assert.Equal(600,  c2.WindowHeight);
+    }
+}

VoxelEngine.Tests/Persistence/InMemoryChunkModificationStoreTests.cs

@@ -0,0 +1,65 @@
+using OpenTK.Mathematics;
+using VoxelEngine.World.Persistence;
+using Xunit;
+
+namespace VoxelEngine.Tests.Persistence;
+
+public class InMemoryChunkModificationStoreTests
+{
+    [Fact]
+    public void NewStore_HasNoDiffs()
+    {
+        var s = new InMemoryChunkModificationStore();
+        Assert.False(s.HasDiff(new Vector3i(0, 0, 0)));
+        Assert.False(s.TryGetDiff(new Vector3i(0, 0, 0), out _));
+    }
+
+    [Fact]
+    public void RecordEdit_StoresAndIsRetrievable()
+    {
+        var s = new InMemoryChunkModificationStore();
+        Vector3i pos = new(1, 2, 3);
+        s.RecordEdit(pos, 42, 7);
+
+        Assert.True(s.HasDiff(pos));
+        Assert.True(s.TryGetDiff(pos, out var diff));
+        Assert.Single(diff!);
+        Assert.Equal((ushort)7, diff![42]);
+    }
+
+    [Fact]
+    public void RecordEdit_OverwritesSameLocalIndex()
+    {
+        var s = new InMemoryChunkModificationStore();
+        Vector3i pos = new(0, 0, 0);
+        s.RecordEdit(pos, 10, 1);
+        s.RecordEdit(pos, 10, 2);
+
+        Assert.True(s.TryGetDiff(pos, out var diff));
+        Assert.Single(diff!);
+        Assert.Equal((ushort)2, diff![10]);
+    }
+
+    [Fact]
+    public void RecordEdit_MultipleChunks_AreIndependent()
+    {
+        var s = new InMemoryChunkModificationStore();
+        s.RecordEdit(new Vector3i(0, 0, 0), 0, 1);
+        s.RecordEdit(new Vector3i(1, 0, 0), 0, 2);
+
+        Assert.True(s.TryGetDiff(new Vector3i(0, 0, 0), out var a));
+        Assert.True(s.TryGetDiff(new Vector3i(1, 0, 0), out var b));
+        Assert.Equal((ushort)1, a![0]);
+        Assert.Equal((ushort)2, b![0]);
+    }
+
+    [Fact]
+    public void ClearDiff_RemovesChunk()
+    {
+        var s = new InMemoryChunkModificationStore();
+        Vector3i pos = new(5, 5, 5);
+        s.RecordEdit(pos, 0, 1);
+        s.ClearDiff(pos);
+        Assert.False(s.HasDiff(pos));
+    }
+}

VoxelEngine.Tests/PlayerTests.cs

@@ -0,0 +1,53 @@
+using OpenTK.Mathematics;
+using VoxelEngine.Core;
+using VoxelEngine.Game;
+using Xunit;
+
+namespace VoxelEngine.Tests;
+
+public class PlayerTests
+{
+    [Fact]
+    public void ChunkPosition_Origin_IsZero()
+    {
+        var p = new Player(new Configuration(), new Vector3(0, 0, 0), aspectRatio: 1f);
+        Assert.Equal(new Vector3i(0, 0, 0), p.ChunkPosition);
+    }
+
+    [Fact]
+    public void ChunkPosition_PositiveCoords_FloorsDown()
+    {
+        var p = new Player(new Configuration(), new Vector3(17.5f, 32f, 8f), aspectRatio: 1f);
+        Assert.Equal(new Vector3i(1, 2, 0), p.ChunkPosition);
+    }
+
+    [Fact]
+    public void ChunkPosition_NegativeCoords_FloorsToNegativeInfinity()
+    {
+        var p = new Player(new Configuration(), new Vector3(-1f, -16f, -17f), aspectRatio: 1f);
+        Assert.Equal(new Vector3i(-1, -1, -2), p.ChunkPosition);
+    }
+
+    [Fact]
+    public void ChunkPosition_PositiveExactBoundary_IsNextChunk()
+    {
+        var p = new Player(new Configuration(), new Vector3(16f, 0f, 0f), aspectRatio: 1f);
+        Assert.Equal(new Vector3i(1, 0, 0), p.ChunkPosition);
+    }
+
+    [Fact]
+    public void Move_UpdatesPosition()
+    {
+        var p = new Player(new Configuration(), new Vector3(0, 0, 0), aspectRatio: 1f);
+        p.Move(new Vector3(5, 0, 0));
+        Assert.Equal(5f, p.Position.X);
+    }
+
+    [Fact]
+    public void Constructor_AppliesFovFromConfiguration()
+    {
+        var cfg = new Configuration { Fov = 75f };
+        var p = new Player(cfg, new Vector3(0, 0, 0), aspectRatio: 1.5f);
+        Assert.Equal(75f, p.Camera.Fov);
+    }
+}

VoxelEngine.Tests/Streaming/ChunkStreamerTests.cs

@@ -0,0 +1,354 @@
+using OpenTK.Mathematics;
+using VoxelEngine.Core;
+using VoxelEngine.World;
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.Chunk;
+using VoxelEngine.World.Persistence;
+using VoxelEngine.World.Streaming;
+using Xunit;
+using VoxelChunk = VoxelEngine.World.Chunk.Chunk;
+
+namespace VoxelEngine.Tests.Streaming;
+
+// Synchronous executor: runs the submitted action on the calling thread.
+internal sealed class SynchronousJobExecutor : IJobExecutor
+{
+    public void Submit(Action work) => work();
+}
+
+// Fake renderer: never touches GL. Just records lifecycle.
+internal sealed class FakeRenderer : IChunkRenderer
+{
+    public bool Disposed { get; private set; }
+    public int DrawCount { get; private set; }
+    public float[] Mesh { get; }
+
+    public FakeRenderer(float[] mesh) { this.Mesh = mesh; }
+    public void Draw() => this.DrawCount++;
+    public void Dispose() => this.Disposed = true;
+}
+
+// Minimal mesher returning a fixed-length non-empty mesh.
+internal sealed class FakeMesher : IChunkMesher
+{
+    public float[] Build(VoxelChunk chunk, BlockRegistry registry, Func<int, int, int, bool>? neighborOpaqueOutside = null)
+        => new float[6]; // one vertex worth
+}
+
+public class ChunkStreamerTests
+{
+    private static (ChunkStreamer streamer, World.World world, List<FakeRenderer> created) CreateStreamer(Configuration cfg)
+    {
+        var registry = new BlockRegistry();
+        var store = new InMemoryChunkModificationStore();
+        var world = new World.World(registry, store);
+        var mesher = new FakeMesher();
+        var executor = new SynchronousJobExecutor();
+        var created = new List<FakeRenderer>();
+        Func<float[], IChunkRenderer> factory = m =>
+        {
+            var r = new FakeRenderer(m);
+            created.Add(r);
+            return r;
+        };
+        Func<Vector3i, VoxelChunk> generate = _ => new VoxelChunk();
+        var streamer = new ChunkStreamer(cfg, world, generate, mesher, registry, executor, factory);
+        return (streamer, world, created);
+    }
+
+    [Fact]
+    public void Tick_FirstCall_LoadsAllChunksInLoadBox()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 1,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var (streamer, world, created) = CreateStreamer(cfg);
+
+        streamer.Tick(new Vector3i(0, 0, 0));
+
+        // Load box H=1, V=0 → 3 × 1 × 3 = 9 chunks
+        Assert.Equal(9, created.Count);
+        for (int x = -1; x <= 1; x++)
+        for (int z = -1; z <= 1; z++)
+        {
+            var pos = new Vector3i(x, 0, z);
+            Assert.NotNull(world.GetChunk(pos));
+            Assert.True(streamer.IsReady(pos));
+        }
+    }
+
+    [Fact]
+    public void Tick_PlayerMoves_LoadsNewChunks()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 1,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var (streamer, world, created) = CreateStreamer(cfg);
+
+        streamer.Tick(new Vector3i(0, 0, 0));     // loads 9
+
+        // After the second tick the 3 new x=2 chunks must be ready.
+        // Cardinal-neighbor dirty-marking may also produce remesh renderers, so
+        // we assert on world state rather than an exact renderer count.
+        streamer.Tick(new Vector3i(1, 0, 0));
+
+        for (int z = -1; z <= 1; z++)
+        {
+            var newPos = new Vector3i(2, 0, z);
+            Assert.NotNull(world.GetChunk(newPos));
+            Assert.True(streamer.IsReady(newPos));
+        }
+        // x=-1 column must be unloaded
+        for (int z = -1; z <= 1; z++)
+        {
+            Assert.Null(world.GetChunk(new Vector3i(-1, 0, z)));
+            Assert.False(streamer.IsReady(new Vector3i(-1, 0, z)));
+        }
+    }
+
+    [Fact]
+    public void Tick_RespectsMaxMeshUploadsPerFrame()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 1,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 2,   // cap drains per tick
+        };
+        var (streamer, _, created) = CreateStreamer(cfg);
+
+        // With sync executor every submitted job completes immediately and
+        // posts into _completed. The cap throttles how many drain per Tick().
+        streamer.Tick(new Vector3i(0, 0, 0));   // dispatch fills the queue; drain ≤ 2
+        Assert.Equal(2, created.Count);
+
+        streamer.Tick(new Vector3i(0, 0, 0));   // same player chunk, drains 2 more
+        Assert.Equal(4, created.Count);
+    }
+
+    [Fact]
+    public void Tick_ClampsYToWorldBounds()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 0,
+            RenderDistanceVertical = 2,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var (streamer, world, created) = CreateStreamer(cfg);
+
+        // Player at y=0 (chunk Y = 0). V=2 wants Y ∈ [-2, 2], but valid Y is [0, 8).
+        // So only Y ∈ {0, 1, 2} qualifies → 1 × 3 × 1 = 3 chunks.
+        streamer.Tick(new Vector3i(0, 0, 0));
+        Assert.Equal(3, created.Count);
+        Assert.NotNull(world.GetChunk(new Vector3i(0, 0, 0)));
+        Assert.NotNull(world.GetChunk(new Vector3i(0, 1, 0)));
+        Assert.NotNull(world.GetChunk(new Vector3i(0, 2, 0)));
+    }
+
+    [Fact]
+    public void Tick_AppliesModificationDiffOnLoad()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 0,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var registry = new BlockRegistry();
+        var store = new InMemoryChunkModificationStore();
+        var world = new World.World(registry, store);
+        var mesher = new FakeMesher();
+        var executor = new SynchronousJobExecutor();
+        Func<float[], IChunkRenderer> factory = m => new FakeRenderer(m);
+        Func<Vector3i, VoxelChunk> generate = _ => new VoxelChunk();  // all air
+
+        // Pre-record an edit before the chunk loads.
+        var pos = new Vector3i(0, 0, 0);
+        int localIndex = (5 * VoxelChunk.Size + 6) * VoxelChunk.Size + 7;
+        store.RecordEdit(pos, localIndex, registry.Stone.Id);
+
+        var streamer = new ChunkStreamer(cfg, world, generate, mesher, registry, executor, factory);
+        streamer.Tick(pos);
+
+        Assert.Equal(registry.Stone.Id, world.GetBlock(5, 6, 7).Id);
+    }
+
+    [Fact]
+    public void Tick_PlayerMovesAway_UnloadsOldChunksAndDisposesRenderer()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 0,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var (streamer, world, created) = CreateStreamer(cfg);
+
+        streamer.Tick(new Vector3i(0, 0, 0));   // loads (0,0,0)
+        Assert.Single(created);
+        Assert.NotNull(world.GetChunk(new Vector3i(0, 0, 0)));
+
+        streamer.Tick(new Vector3i(5, 0, 0));   // (0,0,0) now out of range; (5,0,0) loaded
+
+        Assert.True(created[0].Disposed);
+        Assert.Null(world.GetChunk(new Vector3i(0, 0, 0)));
+        Assert.False(streamer.IsReady(new Vector3i(0, 0, 0)));
+    }
+
+    [Fact]
+    public void Tick_UnloadKeepsModificationStoreDiff()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 0,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var registry = new BlockRegistry();
+        var store = new InMemoryChunkModificationStore();
+        var world = new World.World(registry, store);
+        var mesher = new FakeMesher();
+        var executor = new SynchronousJobExecutor();
+        Func<float[], IChunkRenderer> factory = m => new FakeRenderer(m);
+        Func<Vector3i, VoxelChunk> generate = _ => new VoxelChunk();
+        var streamer = new ChunkStreamer(cfg, world, generate, mesher, registry, executor, factory);
+
+        var pos = new Vector3i(0, 0, 0);
+        streamer.Tick(pos);
+        store.RecordEdit(pos, 0, registry.Stone.Id);
+
+        streamer.Tick(new Vector3i(99, 0, 0));  // unloads (0,0,0)
+        Assert.True(store.HasDiff(pos));        // diff survives unload
+    }
+
+    [Fact]
+    public void SpawnReady_FlipsTrueOnceSpawnAreaLoaded()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 1,
+            RenderDistanceVertical = 0,
+            SpawnReadyRadiusHorizontal = 1,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 1,         // throttle to expose progression
+        };
+        var (streamer, _, _) = CreateStreamer(cfg);
+
+        Assert.False(streamer.SpawnReady);
+        // Tick once per chunk needed (3×3×1 = 9 chunks at this Y). Spawn area
+        // restricted by Y bounds (only Y=0 valid here) → 9 ticks.
+        for (int i = 0; i < 9; i++) streamer.Tick(new Vector3i(0, 0, 0));
+        Assert.True(streamer.SpawnReady);
+    }
+
+    [Fact]
+    public void MarkDirty_RemeshesChunkOnNextTick()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 0,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var (streamer, _, created) = CreateStreamer(cfg);
+
+        streamer.Tick(new Vector3i(0, 0, 0));
+        Assert.Single(created);
+        var initialRenderer = created[0];
+
+        streamer.MarkDirty(new Vector3i(0, 0, 0));
+        streamer.Tick(new Vector3i(0, 0, 0));   // same chunk, but a remesh is queued
+
+        // Two renderers exist over the lifetime — old disposed, new live.
+        Assert.Equal(2, created.Count);
+        Assert.True(initialRenderer.Disposed);
+        Assert.False(created[1].Disposed);
+    }
+
+    [Fact]
+    public void Tick_LoadingChunk_MarksReadyCardinalNeighborsDirty()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 1,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var (streamer, _, created) = CreateStreamer(cfg);
+
+        streamer.Tick(new Vector3i(0, 0, 0));   // load initial 3×3
+        int firstWave = created.Count;           // 9 chunks
+
+        // Move so we load a new x=2 column. The new chunks become ready and
+        // mark their cardinal neighbors (x=1 column) dirty during Drain.
+        streamer.Tick(new Vector3i(1, 0, 0));    // dispatch + drain new loads → dirty marks set
+        streamer.Tick(new Vector3i(1, 0, 0));    // dispatch the queued remeshes → drain them
+
+        // Should have: 3 new loads + at least 3 remeshes of the x=1 column.
+        Assert.True(created.Count > firstWave + 3,
+            $"Expected remeshes beyond the 3 new loads; got {created.Count - firstWave}");
+    }
+
+    [Fact]
+    public void MarkDirty_OnUntrackedChunk_IsNoOp()
+    {
+        var cfg = new Configuration { RenderDistanceHorizontal = 0, RenderDistanceVertical = 0 };
+        var (streamer, _, created) = CreateStreamer(cfg);
+
+        streamer.MarkDirty(new Vector3i(99, 0, 99));
+        streamer.Tick(new Vector3i(0, 0, 0));
+
+        // Only the chunk in the load box was created; no spurious remesh.
+        Assert.Single(created);
+    }
+
+    [Fact]
+    public void Tick_GeneratorThrows_LogsAndDoesNotRetry()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 0,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var registry = new BlockRegistry();
+        var store = new InMemoryChunkModificationStore();
+        var world = new World.World(registry, store);
+        var mesher = new FakeMesher();
+        var executor = new SynchronousJobExecutor();
+        int generateCalls = 0;
+        Func<Vector3i, VoxelChunk> generate = _ =>
+        {
+            generateCalls++;
+            throw new InvalidOperationException("boom");
+        };
+        Func<float[], IChunkRenderer> factory = m => new FakeRenderer(m);
+
+        var streamer = new ChunkStreamer(cfg, world, generate, mesher, registry, executor, factory);
+
+        streamer.Tick(new Vector3i(0, 0, 0));
+        Assert.Equal(1, generateCalls);
+        Assert.False(streamer.IsReady(new Vector3i(0, 0, 0)));
+
+        // Tick again — same chunk, same position. Should NOT call generate again.
+        streamer.Tick(new Vector3i(0, 0, 0));
+        Assert.Equal(1, generateCalls);
+    }
+}

VoxelEngine.Tests/Streaming/StreamingIntegrationTests.cs

@@ -0,0 +1,84 @@
+using OpenTK.Mathematics;
+using VoxelEngine.Core;
+using VoxelEngine.World;
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.Chunk;
+using VoxelEngine.World.Persistence;
+using VoxelEngine.World.Streaming;
+using VoxelEngine.World.WorldGen;
+using Xunit;
+
+namespace VoxelEngine.Tests.Streaming;
+
+public class StreamingIntegrationTests
+{
+    private static (ChunkStreamer streamer, World.World world, InMemoryChunkModificationStore store)
+        BuildRealStack(Configuration cfg)
+    {
+        var registry = new BlockRegistry();
+        var store = new InMemoryChunkModificationStore();
+        var world = new World.World(registry, store);
+        var gen = new WorldGenerator(seed: 1337, blocks: registry);
+        var mesher = new ChunkMesher(atlasWidthPixels: 128, atlasHeightPixels: 32);
+        var executor = new SynchronousJobExecutor();
+        Func<float[], IChunkRenderer> factory = m => new FakeRenderer(m);
+        var streamer = new ChunkStreamer(cfg, world, gen.Generate, mesher, registry, executor, factory);
+        return (streamer, world, store);
+    }
+
+    [Fact]
+    public void PlayerWalksAcrossWorld_LoadBoxFollowsPlayer()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 1,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var (streamer, world, _) = BuildRealStack(cfg);
+
+        for (int step = 0; step < 5; step++)
+        {
+            streamer.Tick(new Vector3i(step, 0, 0));
+
+            for (int dx = -1; dx <= 1; dx++)
+            for (int dz = -1; dz <= 1; dz++)
+            {
+                var pos = new Vector3i(step + dx, 0, dz);
+                Assert.NotNull(world.GetChunk(pos));
+                Assert.True(streamer.IsReady(pos));
+            }
+        }
+    }
+
+    [Fact]
+    public void ModificationStore_SurvivesUnloadReload()
+    {
+        var cfg = new Configuration
+        {
+            RenderDistanceHorizontal = 0,
+            RenderDistanceVertical = 0,
+            MaxGenerateJobsInFlight = 100,
+            MaxMeshUploadsPerFrame = 100,
+        };
+        var (streamer, world, store) = BuildRealStack(cfg);
+
+        // Use a block id that differs from what WorldGenerator places at
+        // world (0,0,0). At y=0 every biome's "Under" block is Stone (id 1),
+        // so SetBlock(..., Stone) would hit World.SetBlock's no-op guard and
+        // skip the diff record. Dirt (id 2) is always a real change.
+        var pos = new Vector3i(0, 0, 0);
+        streamer.Tick(pos);
+        Assert.True(streamer.IsReady(pos));
+        Assert.True(world.SetBlock(0, 0, 0, /*Dirt*/ (ushort)2));
+
+        streamer.Tick(new Vector3i(50, 0, 0));
+        Assert.Null(world.GetChunk(pos));
+        Assert.True(store.HasDiff(pos));
+
+        streamer.Tick(pos);
+        Assert.NotNull(world.GetChunk(pos));
+        Assert.Equal((ushort)2, world.GetBlock(0, 0, 0).Id);
+    }
+}

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);
+    }
+}

VoxelEngine.Tests/WorldGen/BiomeTests.cs

@@ -0,0 +1,46 @@
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.WorldGen;
+using Xunit;
+
+namespace VoxelEngine.Tests.WorldGen;
+
+public class BiomeTests
+{
+    [Fact]
+    public void Desert_TopAndFiller_AreSand()
+    {
+        var blocks = new BlockRegistry();
+        var biomes = new BiomeRegistry(42, blocks);
+        Assert.Equal("Sand", biomes.Desert.Surface.Top.Name);
+        Assert.Equal("Sand", biomes.Desert.Surface.Filler.Name);
+    }
+
+    [Fact]
+    public void Mountains_UseSnowAboveSnowLine_StoneBelow()
+    {
+        var blocks = new BlockRegistry();
+        var biomes = new BiomeRegistry(42, blocks);
+        Assert.Equal("Stone", biomes.Mountains.TopBlock(70).Name);
+        Assert.Equal("Snow Block", biomes.Mountains.TopBlock(120).Name);
+    }
+
+    [Fact]
+    public void Plains_TopBlock_IsGrass_RegardlessOfHeight()
+    {
+        var blocks = new BlockRegistry();
+        var biomes = new BiomeRegistry(42, blocks);
+        Assert.Equal("Grass", biomes.Plains.TopBlock(40).Name);
+        Assert.Equal("Grass", biomes.Plains.TopBlock(120).Name);
+    }
+
+    [Fact]
+    public void SampleHeight_IsDeterministic()
+    {
+        var blocks = new BlockRegistry();
+        var biomes = new BiomeRegistry(42, blocks);
+        var noise = new NoiseSource(42);
+        float h1 = biomes.Mountains.SampleHeight(noise, 100f, 200f);
+        float h2 = biomes.Mountains.SampleHeight(noise, 100f, 200f);
+        Assert.Equal(h1, h2);
+    }
+}

VoxelEngine.Tests/WorldGen/NoiseSourceTests.cs

@@ -0,0 +1,29 @@
+using VoxelEngine.World.WorldGen;
+using Xunit;
+
+namespace VoxelEngine.Tests.WorldGen;
+
+public class NoiseSourceTests
+{
+    [Fact]
+    public void SameSeed_SameCoord_IsDeterministic()
+    {
+        var a = new NoiseSource(42);
+        var b = new NoiseSource(42);
+        Assert.Equal(a.Continent(10f, 20f), b.Continent(10f, 20f));
+        Assert.Equal(a.Temperature(10f, 20f), b.Temperature(10f, 20f));
+    }
+
+    [Fact]
+    public void Output_InMinusOneToOneRange()
+    {
+        var n = new NoiseSource(7);
+        for (int i = 0; i < 200; i++)
+        {
+            float c = n.Continent(i * 3.1f, i * 1.7f);
+            float t = n.Temperature(i * 2.3f, i * 0.9f);
+            Assert.InRange(c, -1.001f, 1.001f);
+            Assert.InRange(t, -1.001f, 1.001f);
+        }
+    }
+}

VoxelEngine.Tests/WorldGen/TerrainSamplerTests.cs

@@ -0,0 +1,53 @@
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.WorldGen;
+using Xunit;
+
+namespace VoxelEngine.Tests.WorldGen;
+
+public class TerrainSamplerTests
+{
+    private static TerrainSampler 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);
+        var map = new BiomeMap(noise, cells, biomes);
+        return new TerrainSampler(noise, map);
+    }
+
+    [Fact]
+    public void Height_StaysWithinWorldBounds()
+    {
+        var sampler = Build();
+        for (int x = -300; x < 300; x += 7)
+        for (int z = -300; z < 300; z += 7)
+        {
+            int h = sampler.HeightAt(x, z);
+            Assert.InRange(h, 0, WorldConstants.WorldHeight - 1);
+        }
+    }
+
+    [Fact]
+    public void Height_IsDeterministic()
+    {
+        var a = Build();
+        var b = Build();
+        Assert.Equal(a.HeightAt(120, 240), b.HeightAt(120, 240));
+    }
+
+    [Fact]
+    public void AdjacentColumns_DoNotCliff()
+    {
+        var sampler = Build();
+        for (int x = -200; x < 200; x += 3)
+        for (int z = -200; z < 200; z += 3)
+        {
+            // The dense blur removes inter-biome cliffs (those would be 25+ blocks).
+            // Remaining deltas are rugged intra-Mountains ridged slopes (~9), which
+            // are intended terrain, not cliffs; 16 separates the two.
+            int dx = System.Math.Abs(sampler.HeightAt(x, z) - sampler.HeightAt(x + 1, z));
+            Assert.True(dx <= 16, $"height jump {dx} at ({x},{z})");
+        }
+    }
+}

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);
+    }
+}

VoxelEngine.Tests/WorldGen/WorldGeneratorTests.cs

@@ -0,0 +1,70 @@
+using OpenTK.Mathematics;
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.Chunk;
+using VoxelEngine.World.WorldGen;
+using Xunit;
+
+namespace VoxelEngine.Tests.WorldGen;
+
+public class WorldGeneratorTests
+{
+    private static BlockRegistry Blocks => new();
+
+    [Fact]
+    public void BlockAt_AboveTerrainAndSea_IsAir()
+    {
+        var blocks = Blocks;
+        var biomes = new BiomeRegistry(1, blocks);
+        ushort id = WorldGenerator.BlockAt(80, surfaceHeight: 70, biomes.Plains, blocks);
+        Assert.Equal(blocks.Air.Id, id); // 80 > 70 and 80 > SeaLevel(64)
+    }
+
+    [Fact]
+    public void BlockAt_AboveTerrainBelowSea_IsWater()
+    {
+        var blocks = Blocks;
+        var biomes = new BiomeRegistry(1, blocks);
+        ushort id = WorldGenerator.BlockAt(60, surfaceHeight: 50, biomes.Plains, blocks);
+        Assert.Equal(blocks.Water.Id, id); // 60 > 50 but 60 <= SeaLevel(64)
+    }
+
+    [Fact]
+    public void BlockAt_Surface_IsBiomeTop()
+    {
+        var blocks = Blocks;
+        var biomes = new BiomeRegistry(1, blocks);
+        ushort id = WorldGenerator.BlockAt(70, surfaceHeight: 70, biomes.Desert, blocks);
+        Assert.Equal(blocks.Sand.Id, id);
+    }
+
+    [Fact]
+    public void BlockAt_FillerBand_IsBiomeFiller()
+    {
+        var blocks = Blocks;
+        var biomes = new BiomeRegistry(1, blocks);
+        ushort id = WorldGenerator.BlockAt(68, surfaceHeight: 70, biomes.Plains, blocks);
+        Assert.Equal(blocks.Dirt.Id, id); // within 3 of surface
+    }
+
+    [Fact]
+    public void BlockAt_Deep_IsStone()
+    {
+        var blocks = Blocks;
+        var biomes = new BiomeRegistry(1, blocks);
+        ushort id = WorldGenerator.BlockAt(40, surfaceHeight: 70, biomes.Plains, blocks);
+        Assert.Equal(blocks.Stone.Id, id);
+    }
+
+    [Fact]
+    public void Generate_IsDeterministic()
+    {
+        var gen1 = new WorldGenerator(99, Blocks);
+        var gen2 = new WorldGenerator(99, Blocks);
+        Chunk a = gen1.Generate(new Vector3i(0, 4, 0));
+        Chunk b = gen2.Generate(new Vector3i(0, 4, 0));
+        for (int x = 0; x < Chunk.Size; x++)
+        for (int y = 0; y < Chunk.Size; y++)
+        for (int z = 0; z < Chunk.Size; z++)
+            Assert.Equal(a[x, y, z], b[x, y, z]);
+    }
+}

VoxelEngine.Tests/WorldTests.cs

@@ -0,0 +1,88 @@
+using OpenTK.Mathematics;
+using VoxelEngine.World;
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.Persistence;
+using Xunit;
+using VoxelChunk = VoxelEngine.World.Chunk.Chunk;
+
+namespace VoxelEngine.Tests;
+
+public class WorldTests
+{
+    private static (IWorld, BlockRegistry, IChunkModificationStore) Create()
+    {
+        var registry = new BlockRegistry();
+        var store = new InMemoryChunkModificationStore();
+        IWorld world = new World.World(registry, store);
+        return (world, registry, store);
+    }
+
+    [Fact]
+    public void SetBlock_LoadedChunk_WritesBlockAndRecordsDiff()
+    {
+        var (world, registry, store) = Create();
+        var chunkPos = new Vector3i(0, 0, 0);
+        world.SetChunk(chunkPos, new VoxelChunk());
+
+        bool result = world.SetBlock(2, 3, 4, registry.Stone.Id);
+
+        Assert.True(result);
+        Assert.Equal(registry.Stone.Id, world.GetBlock(2, 3, 4).Id);
+        Assert.True(store.HasDiff(chunkPos));
+        Assert.True(store.TryGetDiff(chunkPos, out var diff));
+        int expectedIndex = (2 * VoxelChunk.Size + 3) * VoxelChunk.Size + 4;
+        Assert.Equal(registry.Stone.Id, diff![expectedIndex]);
+    }
+
+    [Fact]
+    public void SetBlock_UnloadedChunk_RecordsDiffButReturnsFalse()
+    {
+        var (world, registry, store) = Create();
+        var chunkPos = new Vector3i(5, 0, 0);
+
+        bool result = world.SetBlock(5 * VoxelChunk.Size, 0, 0, registry.Stone.Id);
+
+        Assert.False(result);
+        Assert.True(store.HasDiff(chunkPos));
+    }
+
+    [Fact]
+    public void GetBlock_UnloadedChunk_ReturnsAir()
+    {
+        var (world, registry, _) = Create();
+        Assert.Same(registry.Air, world.GetBlock(0, 0, 0));
+    }
+
+    [Fact]
+    public void SetBlock_NegativeCoords_FloorsToCorrectChunk()
+    {
+        var (world, registry, store) = Create();
+        var chunkPos = new Vector3i(-1, 0, 0);
+        world.SetChunk(chunkPos, new VoxelChunk());
+
+        bool result = world.SetBlock(-1, 0, 0, registry.Stone.Id);
+
+        Assert.True(result);
+        Assert.Equal(registry.Stone.Id, world.GetBlock(-1, 0, 0).Id);
+        Assert.True(store.HasDiff(chunkPos));
+    }
+
+    [Fact]
+    public void RemoveChunk_RemovesAndGetBlockReturnsAir()
+    {
+        var (world, registry, _) = Create();
+        var chunkPos = new Vector3i(0, 0, 0);
+        world.SetChunk(chunkPos, new VoxelChunk());
+        world.RemoveChunk(chunkPos);
+
+        Assert.Null(world.GetChunk(chunkPos));
+        Assert.Same(registry.Air, world.GetBlock(0, 0, 0));
+    }
+
+    [Fact]
+    public void ModificationStore_AccessorReturnsConstructorInjectedStore()
+    {
+        var (world, _, store) = Create();
+        Assert.Same(store, world.ModificationStore);
+    }
+}

VoxelEngine.csproj

@@ -10,7 +10,9 @@
 
     <ItemGroup>
         <PackageReference Include="OpenTK" Version="4.9.4" />
-        <PackageReference Include="SimplexNoise" Version="2.0.0" />
+        <PackageReference Include="Serilog" Version="4.3.1" />
+        <PackageReference Include="Serilog.Sinks.Console" Version="6.1.1" />
+        <PackageReference Include="Serilog.Sinks.File" Version="7.0.0" />
         <PackageReference Include="StbImageSharp" Version="2.30.15" />
     </ItemGroup>
 
@@ -20,8 +22,4 @@
         </None>
     </ItemGroup>
 
-    <ItemGroup>
-      <Folder Include="World\WorldGen\" />
-    </ItemGroup>
-
 </Project>

World/Blocks/BlockIds.txt

@@ -0,0 +1,7 @@
+Air = 0
+Stone = 1
+Dirt = 2
+Grass = 3
+Sand = 4
+Snow = 5
+Water = 6

World/Blocks/BlockRegistry.cs

@@ -8,11 +8,15 @@ public sealed class BlockRegistry
     public StoneBlock Stone { get; } = new();
     public DirtBlock Dirt { get; } = new();
     public GrassBlock Grass { get; } = new();
+    public SandBlock Sand { get; } = new();
+    public SnowBlock Snow { get; } = new();
+    public WaterBlock Water { get; } = new();
 
     public BlockRegistry()
     {
         // Index by id. Order does not matter; slot = block.Id.
-        IBlock[] blocks = new IBlock[] { this.Air, this.Stone, this.Dirt, this.Grass };
+        IBlock[] blocks = new IBlock[]
+            { this.Air, this.Stone, this.Dirt, this.Grass, this.Sand, this.Snow, this.Water };
         this._byId = new IBlock[blocks.Length];
         foreach (IBlock block in blocks)
         {

World/Blocks/GrassBlock.cs

@@ -9,8 +9,8 @@ public sealed class GrassBlock : IBlock
 
     public TileCoord GetTextureTile(BlockFace face) => face switch
     {
-        BlockFace.Top => new(0, 1),    // grass top
-        BlockFace.Bottom => new(0, 0), // dirt
-        _ => new(1, 1)                 // grass side
+        BlockFace.Top => new TileCoord(0, 1),    // grass top
+        BlockFace.Bottom => new TileCoord(0, 0), // dirt
+        _ => new TileCoord(1, 1)                 // grass side
     };
 }

World/Blocks/SandBlock.cs

@@ -0,0 +1,10 @@
+namespace VoxelEngine.World.Blocks;
+
+public class SandBlock : IBlock
+{
+    public ushort Id => 4;
+    public string Name => "Sand";
+    public bool IsOpaque => true;
+    public bool IsSolid => true;
+    public TileCoord GetTextureTile(BlockFace face) => new(2, 1);
+}

World/Blocks/SnowBlock.cs

@@ -0,0 +1,16 @@
+namespace VoxelEngine.World.Blocks;
+
+public class SnowBlock : IBlock
+{
+    public ushort Id => 5;
+    public string Name => "Snow Block";
+    public bool IsOpaque => true;
+    public bool IsSolid => true;
+
+    public TileCoord GetTextureTile(BlockFace face) => face switch
+    {
+        BlockFace.Top => new TileCoord(2, 0),
+        BlockFace.Bottom => new TileCoord(0, 0),
+        _ => new TileCoord(3, 0)
+    };
+}

World/Blocks/TileCoord.cs

@@ -0,0 +1,5 @@
+namespace VoxelEngine.World.Blocks;
+
+// Position of a tile within the texture atlas, in tile units.
+// Origin (0,0) is the top-left tile; X increases right, Y increases down.
+public readonly record struct TileCoord(int X, int Y);

World/Blocks/WaterBlock.cs

@@ -0,0 +1,10 @@
+namespace VoxelEngine.World.Blocks;
+
+public class WaterBlock : IBlock
+{
+    public ushort Id => 6;
+    public string Name => "Water";
+    public bool IsOpaque => true;
+    public bool IsSolid => false;
+    public TileCoord GetTextureTile(BlockFace face) => new(3, 1);
+}

World/Chunk/ChunkMesher.cs

@@ -5,9 +5,15 @@ namespace VoxelEngine.World.Chunk;
 public sealed class ChunkMesher : IChunkMesher
 {
     private const int TileSizePixels = 32;  // each tile is 32x32 pixels
-    private const int AtlasSizePixels = 64; // atlas is 64x64 pixels (2x2 grid)
 
-    private const float TileSize = (float)TileSizePixels / AtlasSizePixels; // normalized tile size
+    private readonly float _tileU; // normalized tile width
+    private readonly float _tileV; // normalized tile height
+
+    public ChunkMesher(int atlasWidthPixels, int atlasHeightPixels)
+    {
+        this._tileU = (float)ChunkMesher.TileSizePixels / atlasWidthPixels;
+        this._tileV = (float)ChunkMesher.TileSizePixels / atlasHeightPixels;
+    }
 
     // Per-face geometry: base corner + two in-plane edge vectors (u, v) with
     // cross(u, v) == outward normal, guaranteeing CCW winding from outside.
@@ -72,18 +78,18 @@ public sealed class ChunkMesher : IChunkMesher
                         : (neighborOpaqueOutside?.Invoke(nx, ny, nz) ?? false);
                 if (neighborOpaque) continue;
 
-                ChunkMesher.AddFace(verts, x, y, z, face, block.GetTextureTile(face.Kind));
+                this.AddFace(verts, x, y, z, face, block.GetTextureTile(face.Kind));
             }
         }
 
         return verts.ToArray();
     }
 
-    private static void AddFace(List<float> verts, int x, int y, int z, Face face, TileCoord tile)
+    private void AddFace(List<float> verts, int x, int y, int z, Face face, TileCoord tile)
     {
-        float uBase = tile.X * ChunkMesher.TileSize;
+        float uBase = tile.X * this._tileU;
         // Image is flipped on load, so offset from the top to keep tile (0,0) = top-left.
-        float vBase = 1f - (tile.Y + 1) * ChunkMesher.TileSize;
+        float vBase = 1f - (tile.Y + 1) * this._tileV;
 
         foreach (int corner in ChunkMesher.Triangle)
         {
@@ -93,8 +99,8 @@ public sealed class ChunkMesher : IChunkMesher
             float py = y + face.Base.y + face.U.y * cu + face.V.y * cv;
             float pz = z + face.Base.z + face.U.z * cu + face.V.z * cv;
 
-            float u = uBase + cu * ChunkMesher.TileSize;
-            float v = vBase + cv * ChunkMesher.TileSize;
+            float u = uBase + cu * this._tileU;
+            float v = vBase + cv * this._tileV;
 
             verts.Add(px); verts.Add(py); verts.Add(pz);
             verts.Add(u); verts.Add(v);

World/IWorld.cs

@@ -1,11 +1,16 @@
-using OpenTK.Mathematics;
+using OpenTK.Mathematics;
 using VoxelEngine.World.Blocks;
+using VoxelEngine.World.Persistence;
 
 namespace VoxelEngine.World;
 
 public interface IWorld
 {
-    public void SetChunk(Vector3i chunkPos, Chunk.Chunk chunk);
-    public Chunk.Chunk? GetChunk(Vector3i chunkPos);
-    public IBlock GetBlock(int worldX, int worldY, int worldZ);
-}
+    IChunkModificationStore ModificationStore { get; }
+
+    void SetChunk(Vector3i chunkPos, Chunk.Chunk chunk);
+    void RemoveChunk(Vector3i chunkPos);
+    Chunk.Chunk? GetChunk(Vector3i chunkPos);
+    IBlock GetBlock(int worldX, int worldY, int worldZ);
+    bool SetBlock(int worldX, int worldY, int worldZ, ushort blockId);
+}

World/Persistence/IChunkModificationStore.cs

@@ -0,0 +1,11 @@
+using OpenTK.Mathematics;
+
+namespace VoxelEngine.World.Persistence;
+
+public interface IChunkModificationStore
+{
+    bool TryGetDiff(Vector3i chunkPos, out IReadOnlyDictionary<int, ushort>? diff);
+    void RecordEdit(Vector3i chunkPos, int localIndex, ushort blockId);
+    void ClearDiff(Vector3i chunkPos);
+    bool HasDiff(Vector3i chunkPos);
+}

World/Persistence/InMemoryChunkModificationStore.cs

@@ -0,0 +1,35 @@
+using OpenTK.Mathematics;
+
+namespace VoxelEngine.World.Persistence;
+
+public sealed class InMemoryChunkModificationStore : IChunkModificationStore
+{
+    private readonly Dictionary<Vector3i, Dictionary<int, ushort>> _diffs = new();
+
+    public bool TryGetDiff(Vector3i chunkPos, out IReadOnlyDictionary<int, ushort>? diff)
+    {
+        if (this._diffs.TryGetValue(chunkPos, out Dictionary<int, ushort>? d))
+        {
+            // Returned snapshot: callers may safely retain and enumerate even
+            // after concurrent RecordEdit on the same chunk.
+            diff = new Dictionary<int, ushort>(d);
+            return true;
+        }
+        diff = null;
+        return false;
+    }
+
+    public void RecordEdit(Vector3i chunkPos, int localIndex, ushort blockId)
+    {
+        if (!this._diffs.TryGetValue(chunkPos, out Dictionary<int, ushort>? d))
+        {
+            d = new Dictionary<int, ushort>();
+            this._diffs[chunkPos] = d;
+        }
+        d[localIndex] = blockId;
+    }
+
+    public void ClearDiff(Vector3i chunkPos) => this._diffs.Remove(chunkPos);
+
+    public bool HasDiff(Vector3i chunkPos) => this._diffs.ContainsKey(chunkPos);
+}

World/Streaming/ChunkJob.cs

@@ -0,0 +1,21 @@
+using OpenTK.Mathematics;
+using VoxelEngine.World.Chunk;
+
+namespace VoxelEngine.World.Streaming;
+
+// Job dispatched to an IJobExecutor for a fresh chunk load. The worker
+// generates the chunk, applies any modification diff, and meshes the result.
+// Remesh-only jobs are dispatched separately by DispatchRemesh and capture
+// their state via closures, so they do not use this record.
+internal sealed record ChunkJob(
+    Vector3i Pos,
+    NeighborSnapshots Neighbors,
+    IReadOnlyDictionary<int, ushort>? Diff);
+
+// Posted by the worker back to the main thread via a ConcurrentQueue.
+internal sealed record ChunkJobResult(
+    Vector3i Pos,
+    Chunk.Chunk? Chunk,
+    float[]? Mesh,
+    Exception? Error,
+    bool IsRemeshOnly);

World/Streaming/ChunkStreamer.cs

@@ -0,0 +1,364 @@
+using System.Collections.Concurrent;
+using System.Linq;
+using OpenTK.Mathematics;
+using Serilog;
+using VoxelEngine.Core;
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.Chunk;
+using VoxelEngine.World.WorldGen;
+using VoxelChunk = VoxelEngine.World.Chunk.Chunk;
+
+namespace VoxelEngine.World.Streaming;
+
+public sealed class ChunkStreamer : IDisposable
+{
+    private static readonly ILogger Log = Serilog.Log.ForContext<ChunkStreamer>();
+
+    private readonly Configuration _cfg;
+    private readonly IWorld _world;
+    private readonly Func<Vector3i, VoxelChunk> _generate;
+    private readonly IChunkMesher _mesher;
+    private readonly BlockRegistry _registry;
+    private readonly IJobExecutor _executor;
+    private readonly Func<float[], IChunkRenderer> _rendererFactory;
+
+    private readonly Dictionary<Vector3i, IChunkRenderer> _renderers = new();
+    private readonly HashSet<Vector3i> _tracked = new();
+    private readonly HashSet<Vector3i> _ready = new();
+    private readonly HashSet<Vector3i> _dirty = new();
+    private readonly PriorityQueue<Vector3i, int> _pendingLoad = new();
+    private readonly ConcurrentQueue<ChunkJobResult> _completed = new();
+    private readonly Queue<Vector3i> _pendingUnload = new();
+    private readonly HashSet<Vector3i> _failed = new();
+
+    private Vector3i _lastPlayerChunk = new(int.MinValue, int.MinValue, int.MinValue);
+    private int _inFlight;
+    private bool _spawnReady;
+    private bool _spawnAnchorSet;
+    private Vector3i _spawnAnchor;
+
+    public int RendererCount => this._renderers.Count;
+    public bool IsReady(Vector3i pos) => this._ready.Contains(pos);
+    public bool SpawnReady => this._spawnReady;
+
+    public void MarkDirty(Vector3i chunkPos)
+    {
+        if (!this._ready.Contains(chunkPos)) return;
+        this._dirty.Add(chunkPos);
+    }
+
+    public ChunkStreamer(
+        Configuration cfg,
+        IWorld world,
+        Func<Vector3i, VoxelChunk> generate,
+        IChunkMesher mesher,
+        BlockRegistry registry,
+        IJobExecutor executor,
+        Func<float[], IChunkRenderer> rendererFactory)
+    {
+        this._cfg = cfg;
+        this._world = world;
+        this._generate = generate;
+        this._mesher = mesher;
+        this._registry = registry;
+        this._executor = executor;
+        this._rendererFactory = rendererFactory;
+    }
+
+    public void Tick(Vector3i playerChunkPos)
+    {
+        if (playerChunkPos != this._lastPlayerChunk)
+        {
+            this.RecomputeTargetSet(playerChunkPos);
+            this._lastPlayerChunk = playerChunkPos;
+        }
+
+        // Unloads first to free in-flight slots, then dispatch BEFORE drain:
+        // with a synchronous executor (tests), Submit() runs work to completion
+        // before returning, so a single tick must drain its own dispatches.
+        // With the production threadpool executor, jobs don't finish in this
+        // tick -- Drain harvests results submitted on previous ticks. Either
+        // way this ordering is correct.
+        this.ProcessUnloads();
+        this.DispatchJobs();
+        this.DrainCompleted();
+        this.UpdateSpawnReady();
+    }
+
+    public void DrawAll(Action<Vector3i, IChunkRenderer> drawCallback)
+    {
+        foreach (var kvp in this._renderers) drawCallback(kvp.Key, kvp.Value);
+    }
+
+    public void Dispose()
+    {
+        foreach (var r in this._renderers.Values) r.Dispose();
+        this._renderers.Clear();
+    }
+
+    private void RecomputeTargetSet(Vector3i p)
+    {
+        if (!this._spawnAnchorSet)
+        {
+            this._spawnAnchor = p;
+            this._spawnAnchorSet = true;
+        }
+
+        int h = this._cfg.RenderDistanceHorizontal;
+        int v = this._cfg.RenderDistanceVertical;
+
+        var target = new HashSet<Vector3i>();
+        for (int dx = -h; dx <= h; dx++)
+        for (int dy = -v; dy <= v; dy++)
+        for (int dz = -h; dz <= h; dz++)
+        {
+            int cy = p.Y + dy;
+            if (cy < 0 || cy >= WorldConstants.WorldHeightChunks) continue;
+            target.Add(new Vector3i(p.X + dx, cy, p.Z + dz));
+        }
+
+        foreach (var pos in target)
+        {
+            if (this._failed.Contains(pos)) continue;
+            if (this._tracked.Add(pos))
+            {
+                int priority = (pos.X - p.X) * (pos.X - p.X)
+                             + (pos.Y - p.Y) * (pos.Y - p.Y)
+                             + (pos.Z - p.Z) * (pos.Z - p.Z);
+                this._pendingLoad.Enqueue(pos, priority);
+            }
+        }
+
+        // Anything currently tracked but no longer in target → unload.
+        var toRemove = new List<Vector3i>();
+        foreach (var tracked in this._tracked)
+        {
+            if (!target.Contains(tracked)) toRemove.Add(tracked);
+        }
+        foreach (var pos in toRemove)
+        {
+            this._tracked.Remove(pos);
+            this._pendingUnload.Enqueue(pos);
+        }
+    }
+
+    private void DrainCompleted()
+    {
+        int budget = this._cfg.MaxMeshUploadsPerFrame;
+        while (budget > 0 && this._completed.TryDequeue(out ChunkJobResult? result))
+        {
+            this._inFlight--;
+            budget--;
+            this.InstallResult(result);
+        }
+    }
+
+    private void ProcessUnloads()
+    {
+        while (this._pendingUnload.TryDequeue(out Vector3i pos))
+        {
+            if (this._renderers.TryGetValue(pos, out IChunkRenderer? renderer))
+            {
+                renderer.Dispose();
+                this._renderers.Remove(pos);
+            }
+            this._world.RemoveChunk(pos);
+            this._ready.Remove(pos);
+            this._failed.Remove(pos);
+            Log.Debug("Unloaded {Pos}", pos);
+        }
+    }
+
+    private void UpdateSpawnReady()
+    {
+        if (this._spawnReady || !this._spawnAnchorSet) return;
+
+        int hs = this._cfg.SpawnReadyRadiusHorizontal;
+        int v = this._cfg.RenderDistanceVertical;
+        Vector3i a = this._spawnAnchor;
+
+        for (int dx = -hs; dx <= hs; dx++)
+        for (int dy = -v; dy <= v; dy++)
+        for (int dz = -hs; dz <= hs; dz++)
+        {
+            int cy = a.Y + dy;
+            if (cy < 0 || cy >= WorldConstants.WorldHeightChunks) continue;
+            if (!this._ready.Contains(new Vector3i(a.X + dx, cy, a.Z + dz))) return;
+        }
+        this._spawnReady = true;
+        Log.Information("Spawn area ready at anchor {Anchor}", a);
+    }
+
+    private void InstallResult(ChunkJobResult result)
+    {
+        if (result.Error != null || result.Mesh == null)
+        {
+            this._failed.Add(result.Pos);
+            Log.Error(result.Error, "Chunk {Pos} failed to load", result.Pos);
+            return;
+        }
+
+        if (!result.IsRemeshOnly)
+        {
+            if (result.Chunk == null) { Log.Error("Load result for {Pos} had null chunk", result.Pos); return; }
+            this._world.SetChunk(result.Pos, result.Chunk);
+        }
+
+        if (this._renderers.TryGetValue(result.Pos, out IChunkRenderer? old))
+        {
+            old.Dispose();
+        }
+        this._renderers[result.Pos] = this._rendererFactory(result.Mesh);
+        this._ready.Add(result.Pos);
+        this._dirty.Remove(result.Pos);
+
+        if (!result.IsRemeshOnly)
+        {
+            this.MarkCardinalNeighborsDirty(result.Pos);
+            Log.Debug("Loaded {Pos}", result.Pos);
+        }
+        else
+        {
+            Log.Debug("Remeshed {Pos}", result.Pos);
+        }
+    }
+
+    private void MarkCardinalNeighborsDirty(Vector3i pos)
+    {
+        Vector3i[] offsets =
+        {
+            new(-1, 0, 0), new(1, 0, 0),
+            new(0, -1, 0), new(0, 1, 0),
+            new(0, 0, -1), new(0, 0, 1),
+        };
+        foreach (var off in offsets)
+        {
+            var neighbor = pos + off;
+            if (this._ready.Contains(neighbor)) this._dirty.Add(neighbor);
+        }
+    }
+
+    private void DispatchJobs()
+    {
+        // Dirty re-meshes first (correctness), then new loads (player view).
+        while (this._inFlight < this._cfg.MaxGenerateJobsInFlight && this._dirty.Count > 0)
+        {
+            Vector3i pos = this._dirty.First();
+            this._dirty.Remove(pos);
+            this.DispatchRemesh(pos);
+        }
+
+        while (this._inFlight < this._cfg.MaxGenerateJobsInFlight && this._pendingLoad.Count > 0)
+        {
+            Vector3i pos = this._pendingLoad.Dequeue();
+            this.DispatchLoad(pos);
+        }
+    }
+
+    private void DispatchRemesh(Vector3i pos)
+    {
+        VoxelChunk? chunk = this._world.GetChunk(pos);
+        if (chunk == null) return;  // got unloaded between MarkDirty and now
+
+        NeighborSnapshots neighbors = this.SnapshotNeighbors(pos);
+        VoxelChunk snapshotChunk = ChunkStreamer.CopyChunk(chunk);  // worker reads from copy
+
+        this._inFlight++;
+
+        IChunkMesher mesher = this._mesher;
+        BlockRegistry registry = this._registry;
+        var completed = this._completed;
+
+        this._executor.Submit(() =>
+        {
+            try
+            {
+                float[] mesh = mesher.Build(snapshotChunk, registry, (nx, ny, nz) =>
+                    neighbors.IsOpaqueOutside(nx, ny, nz, id => registry.Get(id).IsOpaque));
+                completed.Enqueue(new ChunkJobResult(pos, snapshotChunk, mesh, Error: null, IsRemeshOnly: true));
+            }
+            catch (Exception ex)
+            {
+                completed.Enqueue(new ChunkJobResult(pos, Chunk: null, Mesh: null, Error: ex, IsRemeshOnly: true));
+            }
+        });
+    }
+
+    private static VoxelChunk CopyChunk(VoxelChunk source)
+    {
+        var copy = new VoxelChunk();
+        for (int x = 0; x < VoxelChunk.Size; x++)
+        for (int y = 0; y < VoxelChunk.Size; y++)
+        for (int z = 0; z < VoxelChunk.Size; z++)
+        {
+            copy[x, y, z] = source[x, y, z];
+        }
+        return copy;
+    }
+
+    private void DispatchLoad(Vector3i pos)
+    {
+        NeighborSnapshots neighbors = this.SnapshotNeighbors(pos);
+        this._world.ModificationStore.TryGetDiff(pos, out IReadOnlyDictionary<int, ushort>? diff);
+
+        var job = new ChunkJob(pos, neighbors, diff);
+        this._inFlight++;
+
+        IChunkMesher mesher = this._mesher;
+        BlockRegistry registry = this._registry;
+        var generate = this._generate;
+        var completed = this._completed;
+
+        this._executor.Submit(() =>
+        {
+            try
+            {
+                VoxelChunk chunk = generate(job.Pos);
+                if (job.Diff != null)
+                {
+                    foreach (var (idx, id) in job.Diff)
+                    {
+                        int lx = idx / (VoxelChunk.Size * VoxelChunk.Size);
+                        int ly = (idx / VoxelChunk.Size) % VoxelChunk.Size;
+                        int lz = idx % VoxelChunk.Size;
+                        chunk[lx, ly, lz] = id;
+                    }
+                }
+
+                float[] mesh = mesher.Build(chunk, registry, (nx, ny, nz) =>
+                    job.Neighbors.IsOpaqueOutside(nx, ny, nz, id => registry.Get(id).IsOpaque));
+
+                completed.Enqueue(new ChunkJobResult(job.Pos, chunk, mesh, Error: null, IsRemeshOnly: false));
+            }
+            catch (Exception ex)
+            {
+                completed.Enqueue(new ChunkJobResult(job.Pos, Chunk: null, Mesh: null, Error: ex, IsRemeshOnly: false));
+            }
+        });
+    }
+
+    private NeighborSnapshots SnapshotNeighbors(Vector3i pos)
+    {
+        ushort[]? Snap(Vector3i p)
+        {
+            VoxelChunk? c = this._world.GetChunk(p);
+            if (c == null) return null;
+            var copy = new ushort[VoxelChunk.Size * VoxelChunk.Size * VoxelChunk.Size];
+            for (int x = 0; x < VoxelChunk.Size; x++)
+            for (int y = 0; y < VoxelChunk.Size; y++)
+            for (int z = 0; z < VoxelChunk.Size; z++)
+            {
+                copy[(x * VoxelChunk.Size + y) * VoxelChunk.Size + z] = c[x, y, z];
+            }
+            return copy;
+        }
+
+        return new NeighborSnapshots(
+            negX: Snap(pos + new Vector3i(-1, 0, 0)),
+            posX: Snap(pos + new Vector3i(1, 0, 0)),
+            negY: Snap(pos + new Vector3i(0, -1, 0)),
+            posY: Snap(pos + new Vector3i(0, 1, 0)),
+            negZ: Snap(pos + new Vector3i(0, 0, -1)),
+            posZ: Snap(pos + new Vector3i(0, 0, 1)));
+    }
+}

World/Streaming/IChunkRenderer.cs

@@ -0,0 +1,6 @@
+namespace VoxelEngine.World.Streaming;
+
+public interface IChunkRenderer : IDisposable
+{
+    void Draw();
+}

World/Streaming/IJobExecutor.cs

@@ -0,0 +1,15 @@
+namespace VoxelEngine.World.Streaming;
+
+public interface IJobExecutor
+{
+    void Submit(Action work);
+}
+
+public sealed class ThreadPoolJobExecutor : IJobExecutor
+{
+    // Fire-and-forget: the Task is intentionally discarded. The caller's `work`
+    // delegate MUST catch its own exceptions and report them via a dedicated
+    // channel (e.g. ChunkJobResult.Error in the streaming pipeline). Unhandled
+    // exceptions here are silently lost.
+    public void Submit(Action work) => _ = Task.Run(work);
+}

World/Streaming/NeighborSnapshots.cs

@@ -0,0 +1,52 @@
+using VoxelEngine.World.Chunk;
+
+namespace VoxelEngine.World.Streaming;
+
+// Snapshots of the six cardinal neighbor chunks' block arrays, taken on the
+// main thread at job-dispatch time. Each slot is null if that neighbor is not
+// currently loaded. The ChunkMesher reads only these arrays from a worker
+// thread — never the live World.
+public sealed class NeighborSnapshots
+{
+    public ushort[]? NegX { get; }
+    public ushort[]? PosX { get; }
+    public ushort[]? NegY { get; }
+    public ushort[]? PosY { get; }
+    public ushort[]? NegZ { get; }
+    public ushort[]? PosZ { get; }
+
+    // Snapshots take ownership of the arrays passed in. Callers must hand
+    // in fresh copies (not live references into a chunk's _ids buffer) —
+    // the snapshot does NOT copy defensively. This is enforced at the
+    // single call-site in ChunkStreamer.SnapshotNeighbors which allocates
+    // fresh arrays. The arrays must not be mutated after construction.
+    public NeighborSnapshots(
+        ushort[]? negX, ushort[]? posX,
+        ushort[]? negY, ushort[]? posY,
+        ushort[]? negZ, ushort[]? posZ)
+    {
+        this.NegX = negX; this.PosX = posX;
+        this.NegY = negY; this.PosY = posY;
+        this.NegZ = negZ; this.PosZ = posZ;
+    }
+
+    // Returns true if the out-of-chunk position (nx, ny, nz) — exactly one of
+    // which is out of [0, Chunk.Chunk.Size) — is occupied by an opaque block
+    // in the snapshot. Returns false if the relevant neighbor is null.
+    public bool IsOpaqueOutside(int nx, int ny, int nz, Func<ushort, bool> isOpaque)
+    {
+        ushort[]? n;
+        int lx = nx, ly = ny, lz = nz;
+        if (nx < 0)                     { n = this.NegX; lx = Chunk.Chunk.Size - 1; }
+        else if (nx >= Chunk.Chunk.Size)      { n = this.PosX; lx = 0; }
+        else if (ny < 0)                { n = this.NegY; ly = Chunk.Chunk.Size - 1; }
+        else if (ny >= Chunk.Chunk.Size)      { n = this.PosY; ly = 0; }
+        else if (nz < 0)                { n = this.NegZ; lz = Chunk.Chunk.Size - 1; }
+        else if (nz >= Chunk.Chunk.Size)      { n = this.PosZ; lz = 0; }
+        else return false; // not actually outside — shouldn't happen, mesher guards
+
+        if (n == null) return false;
+        int index = (lx * Chunk.Chunk.Size + ly) * Chunk.Chunk.Size + lz;
+        return isOpaque(n[index]);
+    }
+}

World/World.cs

@@ -1,63 +1,62 @@
-using System.Collections;
 using OpenTK.Mathematics;
+using VoxelEngine.Core;
 using VoxelEngine.World.Blocks;
+using VoxelEngine.World.Persistence;
 
 namespace VoxelEngine.World;
 
-public sealed class World : IWorld, IEnumerable<KeyValuePair<Vector3i, Chunk.Chunk>>
+public sealed class World(BlockRegistry registry, IChunkModificationStore modStore)
+    : IWorld
 {
     private readonly Dictionary<Vector3i, Chunk.Chunk> _chunks = new();
-    private readonly BlockRegistry _registry;
 
-    public World(BlockRegistry registry)
-    {
-        this._registry = registry;
-    }
-    
-    public void SetChunk(Vector3i chunkPos, Chunk.Chunk chunk)
-    {
-        this._chunks[chunkPos] = chunk;
-    }
+    public IChunkModificationStore ModificationStore { get; } = modStore;
 
-    public Chunk.Chunk? GetChunk(Vector3i chunkPos)
-    {
-        return this._chunks.GetValueOrDefault(chunkPos);
-    }
+    public void SetChunk(Vector3i chunkPos, Chunk.Chunk chunk) => this._chunks[chunkPos] = chunk;
+
+    public void RemoveChunk(Vector3i chunkPos) => this._chunks.Remove(chunkPos);
+
+    public Chunk.Chunk? GetChunk(Vector3i chunkPos) => this._chunks.GetValueOrDefault(chunkPos);
 
     public IBlock GetBlock(int worldX, int worldY, int worldZ)
     {
         Vector3i chunkPos = new(
-            World.FloorDiv(worldX, Chunk.Chunk.Size),
-            World.FloorDiv(worldY, Chunk.Chunk.Size),
-            World.FloorDiv(worldZ, Chunk.Chunk.Size)
-        );
-        
+            ChunkMath.FloorDiv(worldX, Chunk.Chunk.Size),
+            ChunkMath.FloorDiv(worldY, Chunk.Chunk.Size),
+            ChunkMath.FloorDiv(worldZ, Chunk.Chunk.Size));
+
         Chunk.Chunk? chunk = this.GetChunk(chunkPos);
-        if (chunk == null)
-        {
-            return this._registry.Air;
-        }
-        
-        int localX = worldX - chunkPos.X *  Chunk.Chunk.Size;
-        int localY = worldY - chunkPos.Y *  Chunk.Chunk.Size;
-        int localZ = worldZ - chunkPos.Z *  Chunk.Chunk.Size;
-        
-        return this._registry.Get(chunk[localX, localY, localZ]);
+        if (chunk == null) return registry.Air;
+
+        int localX = worldX - chunkPos.X * Chunk.Chunk.Size;
+        int localY = worldY - chunkPos.Y * Chunk.Chunk.Size;
+        int localZ = worldZ - chunkPos.Z * Chunk.Chunk.Size;
+        return registry.Get(chunk[localX, localY, localZ]);
     }
 
-    private static int FloorDiv(int value, int size)
+    public bool SetBlock(int worldX, int worldY, int worldZ, ushort blockId)
     {
-        int quotient = value / size;
-        if (value % size != 0 && value < 0)
-        {
-            quotient--;
-        }
-        return quotient;
+        Vector3i chunkPos = new(
+            ChunkMath.FloorDiv(worldX, Chunk.Chunk.Size),
+            ChunkMath.FloorDiv(worldY, Chunk.Chunk.Size),
+            ChunkMath.FloorDiv(worldZ, Chunk.Chunk.Size));
+
+        Chunk.Chunk? chunk = this.GetChunk(chunkPos);
+
+        int localX = worldX - chunkPos.X * Chunk.Chunk.Size;
+        int localY = worldY - chunkPos.Y * Chunk.Chunk.Size;
+        int localZ = worldZ - chunkPos.Z * Chunk.Chunk.Size;
+        int localIndex = (localX * Chunk.Chunk.Size + localY) * Chunk.Chunk.Size + localZ;
+
+        // For a loaded chunk, no-op writes shouldn't pollute the diff store.
+        if (chunk != null && chunk[localX, localY, localZ] == blockId) return true;
+
+        this.ModificationStore.RecordEdit(chunkPos, localIndex, blockId);
+
+        if (chunk == null) return false;
+
+        chunk[localX, localY, localZ] = blockId;
+        return true;
     }
 
-    public IEnumerator<KeyValuePair<Vector3i, Chunk.Chunk>> GetEnumerator()
-        => this._chunks.GetEnumerator();
-
-    IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
-
-}
+}

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
+    }
+}

World/WorldGen/BiomeRegistry.cs

@@ -0,0 +1,20 @@
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.WorldGen.Biomes;
+
+namespace VoxelEngine.World.WorldGen;
+
+public sealed class BiomeRegistry
+{
+    public IBiome Plains { get; }
+    public IBiome Desert { get; }
+    public IBiome Mountains { get; }
+    public IBiome Forest { get; }
+
+    public BiomeRegistry(int seed, BlockRegistry blocks)
+    {
+        this.Plains = new PlainsBiome(seed, blocks);
+        this.Desert = new DesertBiome(seed, blocks);
+        this.Mountains = new MountainsBiome(seed, blocks);
+        this.Forest = new ForestBiome(seed, blocks);
+    }
+}

World/WorldGen/Biomes/DesertBiome.cs

@@ -0,0 +1,25 @@
+using VoxelEngine.World.Blocks;
+
+namespace VoxelEngine.World.WorldGen.Biomes;
+
+public sealed class DesertBiome : IBiome
+{
+    private readonly FastNoiseLite _detail;
+
+    public string Name => "Desert";
+    public SurfacePalette Surface { get; }
+
+    public DesertBiome(int seed, BlockRegistry blocks)
+    {
+        this._detail = new FastNoiseLite(seed + 101);
+        this._detail.SetNoiseType(FastNoiseLite.NoiseType.OpenSimplex2);
+        this._detail.SetFrequency(0.015f);
+        this.Surface = new SurfacePalette(blocks.Sand, blocks.Sand, blocks.Stone);
+    }
+
+    public float SampleHeight(NoiseSource noise, float worldX, float worldZ)
+        => WorldConstants.SeaLevel + 4f * noise.Continent(worldX, worldZ)
+                                   + 1.5f * this._detail.GetNoise(worldX, worldZ);
+
+    public IBlock TopBlock(int worldY) => this.Surface.Top;
+}

World/WorldGen/Biomes/ForestBiome.cs

@@ -0,0 +1,25 @@
+using VoxelEngine.World.Blocks;
+
+namespace VoxelEngine.World.WorldGen.Biomes;
+
+public sealed class ForestBiome : IBiome
+{
+    private readonly FastNoiseLite _detail;
+
+    public string Name => "Forest";
+    public SurfacePalette Surface { get; }
+
+    public ForestBiome(int seed, BlockRegistry blocks)
+    {
+        this._detail = new FastNoiseLite(seed + 103);
+        this._detail.SetNoiseType(FastNoiseLite.NoiseType.OpenSimplex2);
+        this._detail.SetFrequency(0.03f);
+        this.Surface = new SurfacePalette(blocks.Grass, blocks.Dirt, blocks.Stone);
+    }
+
+    public float SampleHeight(NoiseSource noise, float worldX, float worldZ)
+        => WorldConstants.SeaLevel + 6f * noise.Continent(worldX, worldZ)
+                                   + 4f * this._detail.GetNoise(worldX, worldZ);
+
+    public IBlock TopBlock(int worldY) => this.Surface.Top;
+}

World/WorldGen/Biomes/MountainsBiome.cs

@@ -0,0 +1,32 @@
+using VoxelEngine.World.Blocks;
+
+namespace VoxelEngine.World.WorldGen.Biomes;
+
+public sealed class MountainsBiome : IBiome
+{
+    private const int SnowLine = 96;
+    private readonly FastNoiseLite _ridged;
+    private readonly IBlock _stone;
+    private readonly IBlock _snow;
+
+    public string Name => "Mountains";
+    public SurfacePalette Surface { get; }
+
+    public MountainsBiome(int seed, BlockRegistry blocks)
+    {
+        this._ridged = new FastNoiseLite(seed + 102);
+        this._ridged.SetNoiseType(FastNoiseLite.NoiseType.OpenSimplex2);
+        this._ridged.SetFractalType(FastNoiseLite.FractalType.Ridged);
+        this._ridged.SetFractalOctaves(4);
+        this._ridged.SetFrequency(0.01f);
+        this._stone = blocks.Stone;
+        this._snow = blocks.Snow;
+        this.Surface = new SurfacePalette(blocks.Stone, blocks.Stone, blocks.Stone);
+    }
+
+    public float SampleHeight(NoiseSource noise, float worldX, float worldZ)
+        => WorldConstants.SeaLevel + 10f * noise.Continent(worldX, worldZ)
+                                   + 45f * this._ridged.GetNoise(worldX, worldZ);
+
+    public IBlock TopBlock(int worldY) => worldY >= MountainsBiome.SnowLine ? this._snow : this._stone;
+}

World/WorldGen/Biomes/PlainsBiome.cs

@@ -0,0 +1,25 @@
+using VoxelEngine.World.Blocks;
+
+namespace VoxelEngine.World.WorldGen.Biomes;
+
+public sealed class PlainsBiome : IBiome
+{
+    private readonly FastNoiseLite _detail;
+
+    public string Name => "Plains";
+    public SurfacePalette Surface { get; }
+
+    public PlainsBiome(int seed, BlockRegistry blocks)
+    {
+        this._detail = new FastNoiseLite(seed + 100);
+        this._detail.SetNoiseType(FastNoiseLite.NoiseType.OpenSimplex2);
+        this._detail.SetFrequency(0.02f);
+        this.Surface = new SurfacePalette(blocks.Grass, blocks.Dirt, blocks.Stone);
+    }
+
+    public float SampleHeight(NoiseSource noise, float worldX, float worldZ)
+        => WorldConstants.SeaLevel + 6f * noise.Continent(worldX, worldZ)
+                                   + 2f * this._detail.GetNoise(worldX, worldZ);
+
+    public IBlock TopBlock(int worldY) => this.Surface.Top;
+}

World/WorldGen/IBiome.cs

@@ -0,0 +1,17 @@
+using VoxelEngine.World.Blocks;
+
+namespace VoxelEngine.World.WorldGen;
+
+public interface IBiome
+{
+    string Name { get; }
+
+    // Absolute target surface height at the column (continuous, pre-rounding).
+    float SampleHeight(NoiseSource noise, float worldX, float worldZ);
+
+    SurfacePalette Surface { get; }
+
+    // Surface block for a given height; lets biomes vary the top by altitude
+    // (e.g. Mountains use Snow above the snow line). Defaults to Surface.Top.
+    IBlock TopBlock(int worldY);
+}

World/WorldGen/NoiseSource.cs

@@ -0,0 +1,32 @@
+namespace VoxelEngine.World.WorldGen;
+
+// Seeded wrapper over FastNoiseLite. All Get* methods return roughly [-1, 1].
+public sealed class NoiseSource
+{
+    private readonly FastNoiseLite _continent;
+    private readonly FastNoiseLite _temperature;
+    private readonly FastNoiseLite _forest;
+
+    public NoiseSource(int seed)
+    {
+        this._continent = new FastNoiseLite(seed);
+        this._continent.SetNoiseType(FastNoiseLite.NoiseType.OpenSimplex2);
+        this._continent.SetFractalType(FastNoiseLite.FractalType.FBm);
+        this._continent.SetFractalOctaves(4);
+        this._continent.SetFrequency(0.0035f);
+
+        this._temperature = new FastNoiseLite(seed + 1);
+        this._temperature.SetNoiseType(FastNoiseLite.NoiseType.OpenSimplex2);
+        this._temperature.SetFractalType(FastNoiseLite.FractalType.FBm);
+        this._temperature.SetFractalOctaves(3);
+        this._temperature.SetFrequency(0.0015f);
+
+        this._forest = new FastNoiseLite(seed + 2);
+        this._forest.SetNoiseType(FastNoiseLite.NoiseType.OpenSimplex2);
+        this._forest.SetFrequency(0.02f);
+    }
+
+    public float Continent(float x, float z) => this._continent.GetNoise(x, z);
+    public float Temperature(float x, float z) => this._temperature.GetNoise(x, z);
+    public float Forest(float x, float z) => this._forest.GetNoise(x, z);
+}

World/WorldGen/SurfacePalette.cs

@@ -0,0 +1,7 @@
+using VoxelEngine.World.Blocks;
+
+namespace VoxelEngine.World.WorldGen;
+
+// Surface block layering for a biome: the top block, the filler band below it,
+// and the deep "under" block (stone). Filler depth is fixed at 3 blocks.
+public readonly record struct SurfacePalette(IBlock Top, IBlock Filler, IBlock Under);

World/WorldGen/TerrainSampler.cs

@@ -0,0 +1,38 @@
+namespace VoxelEngine.World.WorldGen;
+
+public sealed class TerrainSampler
+{
+    // Box-blur radius (in blocks) applied to the biome-height field to smooth
+    // biome borders. A full (2R+1)x(2R+1) kernel at 1-block spacing bounds the
+    // per-block height change to ~maxBiomeHeightDiff / (2R+1), preventing cliffs.
+    // A sparse kernel does NOT bound it (axis-aligned borders flip several samples
+    // at once), so the kernel must be dense.
+    private const int BlendRadius = 6;
+
+    private readonly NoiseSource _noise;
+    private readonly BiomeMap _biomes;
+
+    public TerrainSampler(NoiseSource noise, BiomeMap biomes)
+    {
+        this._noise = noise;
+        this._biomes = biomes;
+    }
+
+    public int HeightAt(float worldX, float worldZ)
+    {
+        // For each kernel point find the biome THERE, but evaluate its height AT the
+        // column. Averaging blurs the hard biome borders into smooth transitions.
+        float sum = 0f;
+        int count = 0;
+        for (int dz = -TerrainSampler.BlendRadius; dz <= TerrainSampler.BlendRadius; dz++)
+        for (int dx = -TerrainSampler.BlendRadius; dx <= TerrainSampler.BlendRadius; dx++)
+        {
+            IBiome biome = this._biomes.BiomeAt(worldX + dx, worldZ + dz);
+            sum += biome.SampleHeight(this._noise, worldX, worldZ);
+            count++;
+        }
+        float avg = sum / count;
+        int h = (int)System.MathF.Round(avg);
+        return System.Math.Clamp(h, 0, WorldConstants.WorldHeight - 1);
+    }
+}

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);
+        }
+    }
+}

World/WorldGen/WorldConstants.cs

@@ -0,0 +1,10 @@
+using VoxelChunk = VoxelEngine.World.Chunk.Chunk;
+
+namespace VoxelEngine.World.WorldGen;
+
+public static class WorldConstants
+{
+    public const int WorldHeightChunks = 8;
+    public const int WorldHeight = WorldConstants.WorldHeightChunks * VoxelChunk.Size; // 128
+    public const int SeaLevel = 64;
+}

World/WorldGen/WorldGenerator.cs

@@ -0,0 +1,63 @@
+using OpenTK.Mathematics;
+using VoxelEngine.World.Blocks;
+using VoxelChunk = VoxelEngine.World.Chunk.Chunk;
+
+namespace VoxelEngine.World.WorldGen;
+
+public sealed class WorldGenerator
+{
+    private const int FillerDepth = 3;
+    private const float CellSize = 96f;
+
+    private readonly BlockRegistry _blocks;
+    private readonly BiomeMap _biomeMap;
+    private readonly TerrainSampler _terrain;
+
+    public WorldGenerator(int seed, BlockRegistry blocks)
+    {
+        this._blocks = blocks;
+        NoiseSource noise = new(seed);
+        VoronoiCells cells = new(seed, WorldGenerator.CellSize);
+        BiomeRegistry biomes = new(seed, blocks);
+        this._biomeMap = new BiomeMap(noise, cells, biomes);
+        this._terrain = new TerrainSampler(noise, this._biomeMap);
+    }
+
+    public int SurfaceHeightAt(int worldX, int worldZ) => this._terrain.HeightAt(worldX, worldZ);
+
+    public VoxelChunk Generate(Vector3i chunkPos)
+    {
+        VoxelChunk chunk = new();
+        int baseX = chunkPos.X * VoxelChunk.Size;
+        int baseY = chunkPos.Y * VoxelChunk.Size;
+        int baseZ = chunkPos.Z * VoxelChunk.Size;
+
+        for (int x = 0; x < VoxelChunk.Size; x++)
+        for (int z = 0; z < VoxelChunk.Size; z++)
+        {
+            int worldX = baseX + x;
+            int worldZ = baseZ + z;
+            int height = this._terrain.HeightAt(worldX, worldZ);
+            IBiome biome = this._biomeMap.BiomeAt(worldX, worldZ);
+
+            for (int y = 0; y < VoxelChunk.Size; y++)
+            {
+                int worldY = baseY + y;
+                chunk[x, y, z] = WorldGenerator.BlockAt(worldY, height, biome, this._blocks);
+            }
+        }
+        return chunk;
+    }
+
+    // Pure column fill rule, extracted for testability.
+    public static ushort BlockAt(int worldY, int surfaceHeight, IBiome biome, BlockRegistry blocks)
+    {
+        if (worldY > surfaceHeight)
+            return worldY <= WorldConstants.SeaLevel ? blocks.Water.Id : blocks.Air.Id;
+        if (worldY == surfaceHeight)
+            return biome.TopBlock(worldY).Id;
+        if (worldY >= surfaceHeight - WorldGenerator.FillerDepth)
+            return biome.Surface.Filler.Id;
+        return biome.Surface.Under.Id;
+    }
+}