Add formula engine: data-driven expression evaluator (Core)

Keystone for the gene system. A Formula compiles a string from a def (lexer -> recursive-descent parser -> tree of Func<IFormulaContext,float>) once, then evaluates allocation-free against a variable context. Supports + - * / %, comparisons, && || !, ternary, the constants pi/tau/e, and functions abs sign floor ceil round sqrt exp log sin cos tan min max pow clamp lerp step. Deterministic and side-effect-free so gene-effect formulas stay pure. Covered by FormulaTests (parsing, precedence, functions, logic/ternary, variables, errors). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Split the platform out of Core: new Host library + HeadlessHost
2026-06-12 23:26:06 +03:00 · 2026-06-12 23:04:34 +03:00
40 changed files with 1352 additions and 184 deletions
@@ -27,13 +27,18 @@ is the living showcase — new engine features are demonstrated there.

 Engine libraries (each feature is a namespaced subfolder of its host):

- **`Core`** — game loop, ECS world, scenes, time (`GameClock` with `TimeScale`; `GameSpeed`
-  for discrete pause/1×/3×/6× speed control over the clock, `context.UseGameSpeed(...)`;
-  `Calendar` turning scaled time into in-game days, `context.UseCalendar(secondsPerDay)`;
-  `Climate` — continuous seasonal/daily temperature and season over the calendar,
-  `context.UseClimate(settings)`),
-  plus **Input** (`MrGameEng.Input`: `InputManager`, `ActionMap`, `InputSystem`, in
-  `Core/Input/`). Depends only on MonoGame and Friflo.Engine.ECS.
+- **`Core`** — the platform-free kernel: ECS world, scenes and transition timing, time
+  (`GameClock` with `TimeScale`; `GameSpeed` for discrete pause/1×/3×/6× speed control over
+  the clock, `context.UseGameSpeed(...)`; `Calendar` turning scaled time into in-game days,
+  `context.UseCalendar(secondsPerDay)`; `Climate` — continuous seasonal/daily temperature
+  and season over the calendar, `context.UseClimate(settings)`), services, logging, and
+  **`HeadlessHost`** — a fixed-timestep loop without a window or GPU (dedicated servers,
+  batch simulation, tests). Depends only on Friflo.Engine.ECS — no MonoGame, no platform.
+- **`Host`** — the windowed MonoGame host: `GameHost` (wraps `Game`, owns the window and
+  `GraphicsDeviceManager`, publishes `GraphicsDevice` as a service), visual scene
+  transitions (`OverlayTransition`, `Transitions.Fade/Wipe`, `TransitionRenderer`), plus
+  **Input** (`MrGameEng.Input`: `InputManager`, `ActionMap`, `InputSystem`, in
+  `Host/Input/`). Nothing depends on `Host` except the game itself. → `Core`.
 - **`Graphics`** — custom batched renderer, camera, sprites, plus **Tilemaps**
  (`MrGameEng.Tilemaps`: code-built tile grids rendered through the batcher,
  `scene.UseTilemaps()` after `UseRenderer2D()`, in `Graphics/Tilemaps/`) and **Lighting**
@@ -70,9 +75,13 @@ Engine libraries (each feature is a namespaced subfolder of its host):
  netstandard2.0 analyzer.

 Dependency rule: a library may depend only on `Core` and `Graphics`; `Core` depends only
-on MonoGame and Friflo.Engine.ECS. Features grouped into one library share its package
-set (e.g. `Content` carries both FontStash and StbImage) — keep optional/heavy deps
-(Myra, NVorbis) in their own library so the rest of the engine stays free of them.
+on Friflo.Engine.ECS (no MonoGame — the simulation must run headless). MonoGame is pulled
+in by the platform/graphics libraries (`Host`, `Graphics`, `Audio`). Platform resources
+(e.g. `GraphicsDevice`) are published by hosts as services in `EngineContext.Services`;
+graphics code reaches the device via `context.GetGraphicsDevice()` (extension in
+`Graphics`). Features grouped into one library share its package set (e.g. `Content`
+carries both FontStash and StbImage) — keep optional/heavy deps (Myra, NVorbis) in their
+own library so the rest of the engine stays free of them.

 ## Commands

@@ -1,5 +1,4 @@
 <Project>
-
  <PropertyGroup>
    <LangVersion>latest</LangVersion>
    <Nullable>enable</Nullable>
@@ -13,5 +12,4 @@
  <PropertyGroup Condition="$(MSBuildProjectName.StartsWith('MrGameEng.')) AND !$(MSBuildProjectName.EndsWith('.Tests')) AND !$(MSBuildProjectName.EndsWith('.Generator')) AND !$(MSBuildProjectName.EndsWith('.Sample'))">
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
  </PropertyGroup>
-
 </Project>
@@ -39,6 +39,10 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MrGameEng.UI.Tests", "tests
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MrGameEng.Audio.Tests", "tests\MrGameEng.Audio.Tests\MrGameEng.Audio.Tests.csproj", "{4407F6E6-0B65-41A3-ADFA-B78684A9B918}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MrGameEng.Host", "src\MrGameEng.Host\MrGameEng.Host.csproj", "{59818072-0D2B-4007-A50F-1343FA189EC6}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MrGameEng.Host.Tests", "tests\MrGameEng.Host.Tests\MrGameEng.Host.Tests.csproj", "{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -229,6 +233,30 @@ Global
 		{4407F6E6-0B65-41A3-ADFA-B78684A9B918}.Release|x64.Build.0 = Release|Any CPU
 		{4407F6E6-0B65-41A3-ADFA-B78684A9B918}.Release|x86.ActiveCfg = Release|Any CPU
 		{4407F6E6-0B65-41A3-ADFA-B78684A9B918}.Release|x86.Build.0 = Release|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Debug|x64.Build.0 = Debug|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Debug|x86.Build.0 = Debug|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Release|Any CPU.Build.0 = Release|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Release|x64.ActiveCfg = Release|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Release|x64.Build.0 = Release|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Release|x86.ActiveCfg = Release|Any CPU
+		{59818072-0D2B-4007-A50F-1343FA189EC6}.Release|x86.Build.0 = Release|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Debug|x64.Build.0 = Debug|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Debug|x86.Build.0 = Debug|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Release|Any CPU.Build.0 = Release|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Release|x64.ActiveCfg = Release|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Release|x64.Build.0 = Release|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Release|x86.ActiveCfg = Release|Any CPU
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -249,5 +277,7 @@ Global
 		{C34F2AFF-F2B0-4AC4-A6F9-B114A06FB272} = {0AB3BF05-4346-4AA6-1389-037BE0695223}
 		{7F7D9641-2409-40CB-88A9-56BCE8C90A45} = {0AB3BF05-4346-4AA6-1389-037BE0695223}
 		{4407F6E6-0B65-41A3-ADFA-B78684A9B918} = {0AB3BF05-4346-4AA6-1389-037BE0695223}
+		{59818072-0D2B-4007-A50F-1343FA189EC6} = {827E0CD3-B72D-47B6-A68D-7590B98EB39B}
+		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2} = {0AB3BF05-4346-4AA6-1389-037BE0695223}
 	EndGlobalSection
 EndGlobal
@@ -33,7 +33,8 @@

 | Библиотека (сборка)          | Фичи (неймспейсы) и ответственность                        |
 |------------------------------|------------------------------------------------------------|
-| `MrGameEng.Core`             | Игровой цикл (хост над `Game`), `EntityStore`, `SystemRoot`, сцены, время (`GameClock.TimeScale`; `GameSpeed` — дискретная скорость пауза/1×/3×/6× поверх часов; `Calendar` — игровые дни поверх масштабированного времени, `context.UseCalendar(...)`; `Climate` — непрерывная сезонная/суточная температура и сезон поверх календаря, `context.UseClimate(...)`), жизненный цикл. **Input** (`MrGameEng.Input`, `Core/Input/`): клавиатура, мышь, геймпад, action maps |
+| `MrGameEng.Core`             | Платформо-независимое ядро (зависит только от Friflo): `EngineContext`, `EntityStore`, `SystemRoot`, сцены и переходы (тайминг — `Transition`, `SceneManager`; визуал переходов — в `Host`), время (`GameClock.TimeScale`; `GameSpeed` — дискретная скорость пауза/1×/3×/6× поверх часов; `Calendar` — игровые дни поверх масштабированного времени, `context.UseCalendar(...)`; `Climate` — непрерывная сезонная/суточная температура и сезон поверх календаря, `context.UseClimate(...)`), жизненный цикл, `ServiceRegistry`, `Log`. **`HeadlessHost`** — цикл без окна и GPU на фиксированном тике (`Tick`/`RunTicks`/`Run` с realtime-пейсингом): дедикейтед-серверы, батч-симуляция, тесты |
+| `MrGameEng.Host`             | Оконный MonoGame-хост: `GameHost` (обёртка над `Game` — цикл, окно, `GraphicsDeviceManager`; публикует `GraphicsDevice` сервисом в контексте), визуальные переходы сцен (`OverlayTransition`, фабрики `Transitions.Fade/Wipe`, `TransitionRenderer`). **Input** (`MrGameEng.Input`, `Host/Input/`): клавиатура, мышь, геймпад, action maps |
 | `MrGameEng.Graphics`         | Собственный батчер-рендерер (см. «Рендеринг»), камера, спрайты, анимации, слои. **Tilemaps** (`MrGameEng.Tilemaps`, `Graphics/Tilemaps/`): тайловые карты кодом — `TileGrid` + `TileSet` + компонент `Tilemap`, отрисовка видимых клеток через батчер. **Lighting** (`MrGameEng.Lighting`, `Graphics/Lighting/`): амбиент день/ночь поверх `Calendar` → `Renderer2D.AmbientLight`, `scene.UseDayNight(renderer)`; по-клеточный лайтмап — `LightmapBuilder` (амбиент × окклюзия + точечные `PointLight` с трассировкой теней), накладывается multiply поверх мира через `scene.UseLighting(...)`, сэмплируется `Lighting.SampleAt` |
 | `MrGameEng.Audio`            | Звуковые эффекты и музыка (NVorbis); `AudioManager` с `SoundVolume`/`MasterVolume` (одна ручка на эффекты и музыку) |
 | `MrGameEng.Content`          | Пайплайн контента. **Assets** (`MrGameEng.Assets`): runtime-загрузка без Content Pipeline, кэш, `AssetRef<T>`. **Atlases** (`MrGameEng.Atlases`): текстурные атласы — сборка из дерева картинок (`AtlasBuilder`) и рантайм-загрузка (`TextureAtlas`), CLI `tools/MrGameEng.AtlasTool`. **Mods** (`MrGameEng.Mods`): система модов — порядок загрузки, JSON-дефы, локализация, слияние деревьев контента |
@@ -69,19 +70,27 @@
 ### Правило зависимостей

 ```
-MrGameEng.Audio       ─┐
-MrGameEng.Graphics    ─┼──► MrGameEng.Core ──► MonoGame.Framework.DesktopGL
-                       │                   └──► Friflo.Engine.ECS
+MrGameEng.Host        ─┐
+MrGameEng.Audio       ─┤
+MrGameEng.Graphics    ─┼──► MrGameEng.Core ──► Friflo.Engine.ECS
+                       │
 MrGameEng.Content     ─┤
 MrGameEng.Simulation  ─┤                         (Content — за Texture2DRegion,
 MrGameEng.UI          ─┴──► MrGameEng.Graphics    Simulation/UI — за Transform2D/рендер)
 ```

-Библиотека зависит **только от `Core` и `Graphics`**. `Core` зависит только от MonoGame
-и Friflo. Если двум библиотекам нужен общий тип — он переезжает в `Core` (или, если это
-графический тип, в `Graphics`). `Content` тянет `Graphics` (атласы выдают
-`Texture2DRegion`); `Simulation` тянет `Graphics` (`Collisions` использует `Transform2D`,
-`RectF`); `UI` — только `Core` (Myra рисует своим SpriteBatch).
+Библиотека зависит **только от `Core` и `Graphics`**. `Core` зависит только от Friflo —
+ни MonoGame, ни другой платформы: благодаря этому симуляция запускается и без окна
+(`HeadlessHost`). MonoGame (`MonoGame.Framework.DesktopGL`) тянут платформенные и
+графические библиотеки: `Host`, `Graphics`, `Audio` (и транзитивно их потребители).
+Платформенные ресурсы (например `GraphicsDevice`) хосты публикуют сервисами в
+`EngineContext.Services`; графический код достаёт девайс через
+`context.GetGraphicsDevice()` (расширение в `Graphics`). На `Host` не зависит никто,
+кроме самой игры — это точка входа оконной платформы. Если двум библиотекам нужен общий
+тип — он переезжает в `Core` (или, если это графический тип, в `Graphics`). `Content`
+тянет `Graphics` (атласы выдают `Texture2DRegion`); `Simulation` тянет `Graphics`
+(`Collisions` использует `Transform2D`, `RectF`); `UI` — только `Core` (Myra рисует
+своим SpriteBatch).

 Фичи, собранные в одну библиотеку, делят её набор пакетов (например, `Content` несёт и
 FontStash, и StbImage). Тяжёлые/опциональные зависимости (Myra, NVorbis) держим в
@@ -345,12 +354,15 @@ CLI-обёртка: `dotnet run --project tools/MrGameEng.AtlasTool -- <исто

 - `SceneManager` владеет активной сценой; обычное переключение откладывается до начала
  следующего кадра (сцена никогда не выгружается посреди собственного кадра).
- `Scenes.Switch(scene, Transition.Fade(0.5f))` — переключение с визуальным переходом:
+- `Scenes.Switch(scene, Transitions.Fade(0.5f))` — переключение с визуальным переходом:
  фаза закрытия (старая сцена живёт) → своп при полном покрытии → фаза открытия.
  Тяжёлый `OnLoad` новой сцены скрыт за полностью закрытым экраном.
- Встроенные переходы: `Transition.Fade(duration, color)` и `Transition.Wipe(duration, color)`
-  (шторка). Свои — наследованием от `Transition` (рисование через `TransitionRenderer.Fill`
-  в нормализованных координатах экрана).
+- Тайминг-машина (`Transition` — длительности фаз, покрытие) живёт в `Core` и работает
+  и в headless-контексте; визуал — в `Host`: встроенные `Transitions.Fade(duration, color)`
+  и `Transitions.Wipe(duration, color)` (шторка). Свои — наследованием от
+  `OverlayTransition` (рисование через `TransitionRenderer.Fill` в нормализованных
+  координатах экрана); `GameHost` рисует оверлей поверх сцены, читая
+  `Scenes.ActiveTransition`/`TransitionCoverage`/`TransitionPhase`.
 - Переходы идут по **unscaled**-времени: работают при паузе геймплея (`TimeScale = 0`).
 - Повторный `Switch` во время перехода заменяет целевую сцену, не перезапуская переход.

@@ -4,6 +4,7 @@
  </PropertyGroup>

  <ItemGroup>
+    <PackageReference Include="MonoGame.Framework.DesktopGL" />
    <PackageReference Include="NVorbis" />
  </ItemGroup>

@@ -2,6 +2,7 @@ using FontStashSharp;
 using Microsoft.Xna.Framework.Audio;
 using Microsoft.Xna.Framework.Graphics;
 using MrGameEng.Core;
+using MrGameEng.Graphics;

 namespace MrGameEng.Assets;

@@ -103,7 +104,7 @@ public sealed class AssetManager : IDisposable
    {
        using var stream = File.OpenRead(path);
        return Texture2D.FromStream(
-            context.GraphicsDevice,
+            context.GetGraphicsDevice(),
            stream,
            DefaultColorProcessors.PremultiplyAlpha
        );
@@ -123,7 +124,7 @@ public sealed class AssetManager : IDisposable
    }

    private static Effect LoadEffect(EngineContext context, string path) =>
-        new(context.GraphicsDevice, File.ReadAllBytes(path));
+        new(context.GetGraphicsDevice(), File.ReadAllBytes(path));
 }

 /// <summary>Wires the assets module into the engine.</summary>
@@ -1,5 +1,6 @@
 using MrGameEng.Assets;
 using MrGameEng.Core;
+using MrGameEng.Graphics;

 namespace MrGameEng.Atlases;

@@ -14,6 +15,6 @@ public static class AtlasesEngineExtensions
    public static void UseTextureAtlases(this EngineContext context)
    {
        var assets = context.Services.Get<AssetManager>();
-        assets.RegisterLoader((_, path) => TextureAtlas.Load(context.GraphicsDevice, path));
+        assets.RegisterLoader((_, path) => TextureAtlas.Load(context.GetGraphicsDevice(), path));
    }
 }
@@ -1,10 +1,11 @@
-using Microsoft.Xna.Framework.Graphics;
-
 namespace MrGameEng.Core;

 /// <summary>
-/// Root object handed to scenes and systems: time, scene manager, services and graphics device.
-/// Created by <see cref="GameHost"/>; can also be created standalone for headless tests.
+/// Root object handed to scenes and systems: time, scene manager and services.
+/// Created by a host (the MonoGame game host or <see cref="HeadlessHost"/>); can also be
+/// created standalone for unit tests. Platform resources such as the graphics device are
+/// published through <see cref="Services"/> by hosts that have them — the core itself has
+/// no platform dependencies.
 /// </summary>
 public sealed class EngineContext
 {
@@ -17,38 +18,16 @@ public sealed class EngineContext
    /// <summary>Registry of module services (input, audio, assets, …).</summary>
    public ServiceRegistry Services { get; } = new();

-    /// <summary>
-    /// The graphics device. Available once the host is initialized;
-    /// throws when accessed in a headless context (unit tests).
-    /// </summary>
-    public GraphicsDevice GraphicsDevice =>
-        _graphicsDevice
-        ?? throw new InvalidOperationException(
-            "GraphicsDevice is not available (headless context)."
-        );
-
-    /// <summary>True when a graphics device is attached.</summary>
-    public bool HasGraphicsDevice => _graphicsDevice is not null;
-
-    private GraphicsDevice? _graphicsDevice;
-
-    /// <summary>Creates a context. Games normally never create one themselves — <see cref="GameHost"/> does.</summary>
+    /// <summary>Creates a context. Games normally never create one themselves — the host does.</summary>
    public EngineContext()
    {
        Scenes = new SceneManager(this);
    }

-    internal void AttachGraphicsDevice(GraphicsDevice device) => _graphicsDevice = device;
-
    /// <summary>
-    /// Disposes everything the context owns: registered <see cref="IDisposable"/> services
-    /// and the transition renderer. <paramref name="except"/> (the host itself, also a
-    /// registered service) is skipped — it is being disposed by the caller already.
-    /// Called by <see cref="GameHost.Dispose(bool)"/>.
+    /// Disposes everything the context owns: registered <see cref="IDisposable"/> services.
+    /// Instances in <paramref name="except"/> (the host itself, platform resources the host
+    /// disposes on its own) are skipped. Called by hosts on shutdown.
    /// </summary>
-    internal void DisposeOwnedResources(object except)
-    {
-        Scenes.DisposeRenderer();
-        Services.DisposeServices(except);
-    }
+    internal void DisposeOwnedResources(params object[] except) => Services.DisposeServices(except);
 }
@@ -0,0 +1,60 @@
+namespace MrGameEng.Formulas;
+
+/// <summary>
+/// A compiled arithmetic expression — the keystone of the data-driven gene system. A formula is
+/// parsed once from a string in a def (e.g. a gene effect on a trait) into a delegate tree, then
+/// evaluated many times against an <see cref="IFormulaContext"/> that supplies variable values
+/// (gene values, environment readings, other traits). Evaluation is deterministic and
+/// allocation-free; all the work happens in <see cref="Compile"/>.
+///
+/// <para>Grammar: numbers, variables, the constants <c>pi</c>/<c>tau</c>/<c>e</c>, operators
+/// <c>+ - * / %</c>, comparisons <c>&lt; &gt; &lt;= &gt;= == !=</c>, logical <c>&amp;&amp; || !</c>,
+/// the ternary <c>cond ? a : b</c>, and functions <c>abs sign floor ceil round sqrt exp log sin cos
+/// tan min max pow clamp lerp step</c>. Comparisons and logical operators yield <c>1</c>/<c>0</c>.</para>
+/// </summary>
+public sealed class Formula
+{
+    private static readonly IFormulaContext Empty = new EmptyContext();
+
+    private readonly Func<IFormulaContext, float> _root;
+
+    private Formula(string source, Func<IFormulaContext, float> root)
+    {
+        Source = source;
+        _root = root;
+    }
+
+    /// <summary>The original expression text this formula was compiled from.</summary>
+    public string Source { get; }
+
+    /// <summary>
+    /// Parses and compiles <paramref name="expression"/>. Throws <see cref="FormulaException"/> on any
+    /// lexical or syntactic error, with the offending position.
+    /// </summary>
+    public static Formula Compile(string expression)
+    {
+        ArgumentNullException.ThrowIfNull(expression);
+        var tokens = FormulaLexer.Tokenize(expression);
+        var root = new FormulaParser(tokens).ParseProgram();
+        return new Formula(expression, root);
+    }
+
+    /// <summary>Evaluates the formula, resolving variables through <paramref name="context"/>.</summary>
+    public float Evaluate(IFormulaContext context)
+    {
+        ArgumentNullException.ThrowIfNull(context);
+        return _root(context);
+    }
+
+    /// <summary>
+    /// Evaluates a formula that references no variables. Throws <see cref="FormulaException"/> if it
+    /// turns out to reference one.
+    /// </summary>
+    public float Evaluate() => _root(Empty);
+
+    private sealed class EmptyContext : IFormulaContext
+    {
+        public float Resolve(string name) =>
+            throw new FormulaException($"No context to resolve variable '{name}'.");
+    }
+}
@@ -0,0 +1,28 @@
+namespace MrGameEng.Formulas;
+
+/// <summary>
+/// Supplies variable values to a compiled <see cref="Formula"/>. A context maps a variable name
+/// (a gene value, an environment reading, another trait…) to a number; the formula engine itself
+/// is data-agnostic, so any consumer can back this with whatever lookup it owns.
+/// </summary>
+public interface IFormulaContext
+{
+    /// <summary>
+    /// Returns the value bound to <paramref name="name"/>. Throw (e.g. <see cref="FormulaException"/>)
+    /// if the name is unknown — the engine does not invent a default.
+    /// </summary>
+    float Resolve(string name);
+}
+
+/// <summary>An <see cref="IFormulaContext"/> backed by a lookup delegate — handy for tests and ad-hoc use.</summary>
+public sealed class DelegateFormulaContext : IFormulaContext
+{
+    private readonly Func<string, float> _resolve;
+
+    /// <summary>Wraps <paramref name="resolve"/>; it is called once per variable reference per evaluation.</summary>
+    public DelegateFormulaContext(Func<string, float> resolve) =>
+        _resolve = resolve ?? throw new ArgumentNullException(nameof(resolve));
+
+    /// <inheritdoc />
+    public float Resolve(string name) => _resolve(name);
+}
@@ -0,0 +1,12 @@
+namespace MrGameEng.Formulas;
+
+/// <summary>
+/// Thrown when a <see cref="Formula"/> cannot be lexed or parsed, or when a compiled formula
+/// references a variable the context cannot resolve at evaluation time.
+/// </summary>
+public sealed class FormulaException : Exception
+{
+    /// <summary>Creates the exception with a human-readable <paramref name="message"/>.</summary>
+    public FormulaException(string message)
+        : base(message) { }
+}
@@ -0,0 +1,90 @@
+using Node = System.Func<MrGameEng.Formulas.IFormulaContext, float>;
+
+namespace MrGameEng.Formulas;
+
+/// <summary>
+/// The built-in function set available inside formulas. Each entry validates its argument count at
+/// compile time and returns a <see cref="Node"/> that evaluates its operands then the math. Kept
+/// deterministic and side-effect-free so formulas stay pure.
+/// </summary>
+internal static class FormulaFunctions
+{
+    public static Node Build(string name, List<Node> args)
+    {
+        switch (name)
+        {
+            case "abs":
+                return Unary(name, args, MathF.Abs);
+            case "sign":
+                return Unary(name, args, x => MathF.Sign(x));
+            case "floor":
+                return Unary(name, args, MathF.Floor);
+            case "ceil":
+                return Unary(name, args, MathF.Ceiling);
+            case "round":
+                return Unary(name, args, MathF.Round);
+            case "sqrt":
+                return Unary(name, args, MathF.Sqrt);
+            case "exp":
+                return Unary(name, args, MathF.Exp);
+            case "log":
+                return args.Count == 2
+                    ? Binary(name, args, MathF.Log)
+                    : Unary(name, args, MathF.Log);
+            case "sin":
+                return Unary(name, args, MathF.Sin);
+            case "cos":
+                return Unary(name, args, MathF.Cos);
+            case "tan":
+                return Unary(name, args, MathF.Tan);
+            case "min":
+                return Binary(name, args, MathF.Min);
+            case "max":
+                return Binary(name, args, MathF.Max);
+            case "pow":
+                return Binary(name, args, MathF.Pow);
+            case "clamp":
+                return Ternary(name, args, (x, lo, hi) => Math.Clamp(x, lo, hi));
+            case "lerp":
+                return Ternary(name, args, (a, b, t) => a + (b - a) * t);
+            case "step":
+                return Binary(name, args, (edge, x) => x < edge ? 0f : 1f);
+            default:
+                throw new FormulaException($"Unknown function '{name}'.");
+        }
+    }
+
+    private static Node Unary(string name, List<Node> args, Func<float, float> op)
+    {
+        Require(name, args, 1);
+        var a = args[0];
+        return ctx => op(a(ctx));
+    }
+
+    private static Node Binary(string name, List<Node> args, Func<float, float, float> op)
+    {
+        Require(name, args, 2);
+        var a = args[0];
+        var b = args[1];
+        return ctx => op(a(ctx), b(ctx));
+    }
+
+    private static Node Ternary(string name, List<Node> args, Func<float, float, float, float> op)
+    {
+        Require(name, args, 3);
+        var a = args[0];
+        var b = args[1];
+        var c = args[2];
+        return ctx => op(a(ctx), b(ctx), c(ctx));
+    }
+
+    private static void Require(string name, List<Node> args, int count)
+    {
+        if (args.Count != count)
+        {
+            throw new FormulaException(
+                $"Function '{name}' expects {count} argument(s) but got {args.Count}."
+            );
+        }
+    }
+}
@@ -0,0 +1,217 @@
+using System.Globalization;
+
+namespace MrGameEng.Formulas;
+
+internal enum TokenType
+{
+    Number,
+    Identifier,
+    Plus,
+    Minus,
+    Star,
+    Slash,
+    Percent,
+    LParen,
+    RParen,
+    Comma,
+    Less,
+    Greater,
+    LessEqual,
+    GreaterEqual,
+    EqualEqual,
+    NotEqual,
+    And,
+    Or,
+    Not,
+    Question,
+    Colon,
+    End,
+}
+
+internal readonly struct Token(TokenType type, int position, float number = 0f, string text = "")
+{
+    public TokenType Type { get; } = type;
+    public int Position { get; } = position;
+    public float Number { get; } = number;
+    public string Text { get; } = text;
+}
+
+/// <summary>
+/// Turns a formula string into a flat token list. Pure and allocation-light; recognizes numbers,
+/// identifiers, the arithmetic/comparison/logical operators and the punctuation the parser needs.
+/// </summary>
+internal static class FormulaLexer
+{
+    public static List<Token> Tokenize(string source)
+    {
+        var tokens = new List<Token>();
+        var i = 0;
+        while (i < source.Length)
+        {
+            var c = source[i];
+            if (char.IsWhiteSpace(c))
+            {
+                i++;
+                continue;
+            }
+
+            if (
+                char.IsDigit(c)
+                || (c == '.' && i + 1 < source.Length && char.IsDigit(source[i + 1]))
+            )
+            {
+                var start = i;
+                while (i < source.Length && (char.IsDigit(source[i]) || source[i] == '.'))
+                {
+                    i++;
+                }
+
+                var span = source.AsSpan(start, i - start);
+                if (
+                    !float.TryParse(
+                        span,
+                        NumberStyles.Float,
+                        CultureInfo.InvariantCulture,
+                        out var value
+                    )
+                )
+                {
+                    throw new FormulaException(
+                        $"Invalid number '{span.ToString()}' at position {start}."
+                    );
+                }
+
+                tokens.Add(new Token(TokenType.Number, start, value));
+                continue;
+            }
+
+            if (char.IsLetter(c) || c == '_')
+            {
+                var start = i;
+                while (i < source.Length && (char.IsLetterOrDigit(source[i]) || source[i] == '_'))
+                {
+                    i++;
+                }
+
+                tokens.Add(
+                    new Token(TokenType.Identifier, start, text: source.Substring(start, i - start))
+                );
+                continue;
+            }
+
+            var pos = i;
+            switch (c)
+            {
+                case '+':
+                    tokens.Add(new Token(TokenType.Plus, pos));
+                    i++;
+                    break;
+                case '-':
+                    tokens.Add(new Token(TokenType.Minus, pos));
+                    i++;
+                    break;
+                case '*':
+                    tokens.Add(new Token(TokenType.Star, pos));
+                    i++;
+                    break;
+                case '/':
+                    tokens.Add(new Token(TokenType.Slash, pos));
+                    i++;
+                    break;
+                case '%':
+                    tokens.Add(new Token(TokenType.Percent, pos));
+                    i++;
+                    break;
+                case '(':
+                    tokens.Add(new Token(TokenType.LParen, pos));
+                    i++;
+                    break;
+                case ')':
+                    tokens.Add(new Token(TokenType.RParen, pos));
+                    i++;
+                    break;
+                case ',':
+                    tokens.Add(new Token(TokenType.Comma, pos));
+                    i++;
+                    break;
+                case '?':
+                    tokens.Add(new Token(TokenType.Question, pos));
+                    i++;
+                    break;
+                case ':':
+                    tokens.Add(new Token(TokenType.Colon, pos));
+                    i++;
+                    break;
+                case '<':
+                    i = AddMaybeEqual(tokens, source, i, TokenType.LessEqual, TokenType.Less);
+                    break;
+                case '>':
+                    i = AddMaybeEqual(tokens, source, i, TokenType.GreaterEqual, TokenType.Greater);
+                    break;
+                case '=':
+                    if (Next(source, i) == '=')
+                    {
+                        tokens.Add(new Token(TokenType.EqualEqual, pos));
+                        i += 2;
+                        break;
+                    }
+
+                    throw new FormulaException($"Expected '==' at position {pos}.");
+                case '!':
+                    if (Next(source, i) == '=')
+                    {
+                        tokens.Add(new Token(TokenType.NotEqual, pos));
+                        i += 2;
+                        break;
+                    }
+
+                    tokens.Add(new Token(TokenType.Not, pos));
+                    i++;
+                    break;
+                case '&':
+                    if (Next(source, i) == '&')
+                    {
+                        tokens.Add(new Token(TokenType.And, pos));
+                        i += 2;
+                        break;
+                    }
+
+                    throw new FormulaException($"Expected '&&' at position {pos}.");
+                case '|':
+                    if (Next(source, i) == '|')
+                    {
+                        tokens.Add(new Token(TokenType.Or, pos));
+                        i += 2;
+                        break;
+                    }
+
+                    throw new FormulaException($"Expected '||' at position {pos}.");
+                default:
+                    throw new FormulaException($"Unexpected character '{c}' at position {pos}.");
+            }
+        }
+
+        tokens.Add(new Token(TokenType.End, source.Length));
+        return tokens;
+    }
+
+    private static int AddMaybeEqual(
+        List<Token> tokens,
+        string source,
+        int i,
+        TokenType withEqual,
+        TokenType plain
+    )
+    {
+        if (Next(source, i) == '=')
+        {
+            tokens.Add(new Token(withEqual, i));
+            return i + 2;
+        }
+
+        tokens.Add(new Token(plain, i));
+        return i + 1;
+    }
+
+    private static char Next(string source, int i) => i + 1 < source.Length ? source[i + 1] : '\0';
+}
@@ -0,0 +1,273 @@
+using Node = System.Func<MrGameEng.Formulas.IFormulaContext, float>;
+
+namespace MrGameEng.Formulas;
+
+/// <summary>
+/// Recursive-descent parser that compiles a token list straight into a tree of <see cref="Node"/>
+/// delegates. Parsing (and therefore all closure allocation) happens once; evaluating the returned
+/// node is allocation-free. Precedence, low to high: ternary, <c>||</c>, <c>&amp;&amp;</c>, equality,
+/// comparison, additive, multiplicative, unary, primary.
+/// </summary>
+internal sealed class FormulaParser(List<Token> tokens)
+{
+    private const float True = 1f;
+    private const float False = 0f;
+
+    private int _pos;
+
+    public Node ParseProgram()
+    {
+        var node = ParseTernary();
+        Expect(TokenType.End);
+        return node;
+    }
+
+    private Node ParseTernary()
+    {
+        var condition = ParseOr();
+        if (!Match(TokenType.Question))
+        {
+            return condition;
+        }
+
+        var whenTrue = ParseTernary();
+        Expect(TokenType.Colon);
+        var whenFalse = ParseTernary();
+        return ctx => condition(ctx) != False ? whenTrue(ctx) : whenFalse(ctx);
+    }
+
+    private Node ParseOr()
+    {
+        var left = ParseAnd();
+        while (Match(TokenType.Or))
+        {
+            var right = ParseAnd();
+            var l = left;
+            left = ctx => l(ctx) != False || right(ctx) != False ? True : False;
+        }
+
+        return left;
+    }
+
+    private Node ParseAnd()
+    {
+        var left = ParseEquality();
+        while (Match(TokenType.And))
+        {
+            var right = ParseEquality();
+            var l = left;
+            left = ctx => l(ctx) != False && right(ctx) != False ? True : False;
+        }
+
+        return left;
+    }
+
+    private Node ParseEquality()
+    {
+        var left = ParseComparison();
+        while (true)
+        {
+            if (Match(TokenType.EqualEqual))
+            {
+                var right = ParseComparison();
+                var l = left;
+                left = ctx => l(ctx) == right(ctx) ? True : False;
+            }
+            else if (Match(TokenType.NotEqual))
+            {
+                var right = ParseComparison();
+                var l = left;
+                left = ctx => l(ctx) != right(ctx) ? True : False;
+            }
+            else
+            {
+                return left;
+            }
+        }
+    }
+
+    private Node ParseComparison()
+    {
+        var left = ParseAdditive();
+        while (true)
+        {
+            if (Match(TokenType.Less))
+            {
+                left = Compare(left, ParseAdditive(), (a, b) => a < b);
+            }
+            else if (Match(TokenType.LessEqual))
+            {
+                left = Compare(left, ParseAdditive(), (a, b) => a <= b);
+            }
+            else if (Match(TokenType.Greater))
+            {
+                left = Compare(left, ParseAdditive(), (a, b) => a > b);
+            }
+            else if (Match(TokenType.GreaterEqual))
+            {
+                left = Compare(left, ParseAdditive(), (a, b) => a >= b);
+            }
+            else
+            {
+                return left;
+            }
+        }
+    }
+
+    private Node ParseAdditive()
+    {
+        var left = ParseMultiplicative();
+        while (true)
+        {
+            if (Match(TokenType.Plus))
+            {
+                var right = ParseMultiplicative();
+                var l = left;
+                left = ctx => l(ctx) + right(ctx);
+            }
+            else if (Match(TokenType.Minus))
+            {
+                var right = ParseMultiplicative();
+                var l = left;
+                left = ctx => l(ctx) - right(ctx);
+            }
+            else
+            {
+                return left;
+            }
+        }
+    }
+
+    private Node ParseMultiplicative()
+    {
+        var left = ParseUnary();
+        while (true)
+        {
+            if (Match(TokenType.Star))
+            {
+                var right = ParseUnary();
+                var l = left;
+                left = ctx => l(ctx) * right(ctx);
+            }
+            else if (Match(TokenType.Slash))
+            {
+                var right = ParseUnary();
+                var l = left;
+                left = ctx => l(ctx) / right(ctx);
+            }
+            else if (Match(TokenType.Percent))
+            {
+                var right = ParseUnary();
+                var l = left;
+                left = ctx => l(ctx) % right(ctx);
+            }
+            else
+            {
+                return left;
+            }
+        }
+    }
+
+    private Node ParseUnary()
+    {
+        if (Match(TokenType.Minus))
+        {
+            var operand = ParseUnary();
+            return ctx => -operand(ctx);
+        }
+
+        if (Match(TokenType.Plus))
+        {
+            return ParseUnary();
+        }
+
+        if (Match(TokenType.Not))
+        {
+            var operand = ParseUnary();
+            return ctx => operand(ctx) != False ? False : True;
+        }
+
+        return ParsePrimary();
+    }
+
+    private Node ParsePrimary()
+    {
+        var token = Current;
+        if (Match(TokenType.Number))
+        {
+            var value = token.Number;
+            return _ => value;
+        }
+
+        if (Match(TokenType.LParen))
+        {
+            var inner = ParseTernary();
+            Expect(TokenType.RParen);
+            return inner;
+        }
+
+        if (Match(TokenType.Identifier))
+        {
+            return Peek(TokenType.LParen) ? ParseCall(token.Text) : ParseName(token.Text);
+        }
+
+        throw new FormulaException($"Unexpected token at position {token.Position}.");
+    }
+
+    private Node ParseName(string name)
+    {
+        switch (name)
+        {
+            case "pi":
+                return _ => MathF.PI;
+            case "tau":
+                return _ => MathF.Tau;
+            case "e":
+                return _ => MathF.E;
+            default:
+                return ctx => ctx.Resolve(name);
+        }
+    }
+
+    private Node ParseCall(string name)
+    {
+        Expect(TokenType.LParen);
+        var args = new List<Node>();
+        if (!Peek(TokenType.RParen))
+        {
+            do
+            {
+                args.Add(ParseTernary());
+            } while (Match(TokenType.Comma));
+        }
+
+        Expect(TokenType.RParen);
+        return FormulaFunctions.Build(name, args);
+    }
+
+    private static Node Compare(Node left, Node right, Func<float, float, bool> op) =>
+        ctx => op(left(ctx), right(ctx)) ? True : False;
+
+    private Token Current => tokens[_pos];
+
+    private bool Peek(TokenType type) => Current.Type == type;
+
+    private bool Match(TokenType type)
+    {
+        if (Current.Type != type)
+        {
+            return false;
+        }
+
+        _pos++;
+        return true;
+    }
+
+    private void Expect(TokenType type)
+    {
+        if (!Match(type))
+        {
+            throw new FormulaException($"Expected {type} at position {Current.Position}.");
+        }
+    }
+}
@@ -2,7 +2,7 @@ namespace MrGameEng.Core;

 /// <summary>
 /// Engine time service: per-frame delta, total elapsed time, time scaling and frame counter.
-/// Advanced once per frame by <see cref="GameHost"/>.
+/// Advanced once per frame (or per fixed tick) by the host.
 /// </summary>
 public sealed class GameClock
 {
@@ -0,0 +1,107 @@
+using System.Diagnostics;
+
+namespace MrGameEng.Core;
+
+/// <summary>
+/// A game-loop host without a window, GPU or any platform dependency: drives the active
+/// scene's update phase on a fixed timestep. Suits dedicated servers, batch simulation and
+/// tests. Scenes still own draw systems — they are simply never run, and platform services
+/// (graphics device, window) are absent from <see cref="EngineContext.Services"/>, so only
+/// platform-free modules can be used. The fixed step makes simulation time independent of
+/// wall-clock jitter: N ticks always advance the world by exactly N ×
+/// <see cref="FixedDeltaTime"/> seconds.
+/// </summary>
+public sealed class HeadlessHost : IDisposable
+{
+    /// <summary>Engine context shared with scenes and systems.</summary>
+    public EngineContext Context { get; } = new();
+
+    /// <summary>Fixed simulation step in seconds: 1 / <see cref="HeadlessHostOptions.TicksPerSecond"/>.</summary>
+    public float FixedDeltaTime { get; }
+
+    /// <summary>Ticks completed since the host was created.</summary>
+    public long TickCount => Context.Clock.FrameCount;
+
+    // Отстав сильнее этого, Run ресинкается с настоящим временем вместо лавины тиков.
+    private const double MaxLagSeconds = 1.0;
+
+    private readonly HeadlessHostOptions _options;
+
+    /// <summary>Creates a host that starts with <paramref name="initialScene"/>. The scene
+    /// loads on the first tick, mirroring the windowed host's deferred switch.</summary>
+    public HeadlessHost(HeadlessHostOptions options, Scene initialScene)
+    {
+        if (options.TicksPerSecond <= 0f)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(options),
+                "TicksPerSecond must be positive."
+            );
+        }
+
+        _options = options;
+        FixedDeltaTime = 1f / options.TicksPerSecond;
+        Context.Scenes.Switch(initialScene);
+    }
+
+    /// <summary>Advances the world by exactly one fixed tick.</summary>
+    public void Tick()
+    {
+        Context.Clock.Advance(FixedDeltaTime);
+        Context.Scenes.Update(Context.Clock);
+    }
+
+    /// <summary>Advances the world by <paramref name="count"/> ticks as fast as possible.</summary>
+    public void RunTicks(long count)
+    {
+        for (long i = 0; i < count; i++)
+        {
+            Tick();
+        }
+    }
+
+    /// <summary>
+    /// Runs until <paramref name="cancellationToken"/> is cancelled. With
+    /// <see cref="HeadlessHostOptions.Realtime"/> ticks are paced to the wall clock — the
+    /// loop sleeps when ahead and, having fallen more than a second behind, resyncs instead
+    /// of bursting a catch-up avalanche. Pacing relies on <see cref="Thread.Sleep(TimeSpan)"/>
+    /// and is accurate to a few milliseconds, not exact.
+    /// </summary>
+    public void Run(CancellationToken cancellationToken = default)
+    {
+        if (!_options.Realtime)
+        {
+            while (!cancellationToken.IsCancellationRequested)
+            {
+                Tick();
+            }
+
+            return;
+        }
+
+        var wallClock = Stopwatch.StartNew();
+        var nextTickAt = 0.0;
+        while (!cancellationToken.IsCancellationRequested)
+        {
+            Tick();
+            nextTickAt += FixedDeltaTime;
+            var ahead = nextTickAt - wallClock.Elapsed.TotalSeconds;
+            if (ahead > 0)
+            {
+                Thread.Sleep(TimeSpan.FromSeconds(ahead));
+            }
+            else if (-ahead > MaxLagSeconds)
+            {
+                nextTickAt = wallClock.Elapsed.TotalSeconds;
+            }
+        }
+    }
+
+    /// <summary>Unloads the active scene and disposes context-owned services.</summary>
+    public void Dispose()
+    {
+        Context.Scenes.Switch(null);
+        Context.Scenes.ApplyPending();
+        Context.DisposeOwnedResources();
+    }
+}
@@ -0,0 +1,15 @@
+namespace MrGameEng.Core;
+
+/// <summary>Loop settings for <see cref="HeadlessHost"/>.</summary>
+public sealed class HeadlessHostOptions
+{
+    /// <summary>Fixed simulation rate in ticks per second. Must be positive.</summary>
+    public float TicksPerSecond { get; set; } = 60f;
+
+    /// <summary>
+    /// When true, <see cref="HeadlessHost.Run"/> paces ticks to the wall clock (a dedicated
+    /// server); when false it runs flat out (batch simulation). <see cref="HeadlessHost.RunTicks"/>
+    /// always runs flat out regardless of this setting.
+    /// </summary>
+    public bool Realtime { get; set; } = true;
+}
@@ -4,11 +4,11 @@
  </PropertyGroup>

  <ItemGroup>
-    <PackageReference Include="MonoGame.Framework.DesktopGL" />
    <PackageReference Include="Friflo.Engine.ECS" />
  </ItemGroup>

  <ItemGroup>
    <InternalsVisibleTo Include="MrGameEng.Core.Tests" />
+    <InternalsVisibleTo Include="MrGameEng.Host" />
  </ItemGroup>
 </Project>
@@ -22,13 +22,24 @@ public sealed class SceneManager
    /// <summary>True while a transition is covering or revealing.</summary>
    public bool IsTransitioning => _state != State.Idle;

+    /// <summary>The transition currently covering or revealing, or null while idle.
+    /// Hosts that render overlays read this together with <see cref="TransitionCoverage"/>
+    /// and <see cref="TransitionPhase"/> during their draw phase.</summary>
+    public Transition? ActiveTransition => _state == State.Idle ? null : _transition;
+
+    /// <summary>Coverage of the active transition: 0 = scene fully visible, 1 = fully covered.</summary>
+    public float TransitionCoverage => Math.Clamp(_coverage, 0f, 1f);
+
+    /// <summary>Phase of the active transition. Meaningful only while <see cref="IsTransitioning"/>.</summary>
+    public TransitionPhase TransitionPhase =>
+        _state == State.CoveringOut ? TransitionPhase.Out : TransitionPhase.In;
+
    private readonly EngineContext _context;
    private Scene? _pending;
    private bool _hasPending;
    private Transition? _transition;
    private State _state;
    private float _coverage;
-    private TransitionRenderer? _renderer;

    internal SceneManager(EngineContext context) => _context = context;

@@ -100,27 +111,9 @@ public sealed class SceneManager
        Current?.Update(clock);
    }

-    /// <summary>Draws the active scene and the transition overlay on top. Called by the host.</summary>
-    public void Draw(GameClock clock)
-    {
-        Current?.Draw(clock);
-
-        if (_state == State.Idle || !_context.HasGraphicsDevice)
-        {
-            return;
-        }
-
-        _renderer ??= new TransitionRenderer(_context.GraphicsDevice);
-        var phase = _state == State.CoveringOut ? TransitionPhase.Out : TransitionPhase.In;
-        _transition!.Draw(_renderer, Math.Clamp(_coverage, 0f, 1f), phase);
-    }
-
-    /// <summary>Disposes the lazily created transition renderer. Called on host shutdown.</summary>
-    internal void DisposeRenderer()
-    {
-        _renderer?.Dispose();
-        _renderer = null;
-    }
+    /// <summary>Draws the active scene. Transition overlays are rendered by the host on top,
+    /// from <see cref="ActiveTransition"/> and <see cref="TransitionCoverage"/>.</summary>
+    public void Draw(GameClock clock) => Current?.Draw(clock);

    internal void ApplyPending()
    {
@@ -40,16 +40,17 @@ public sealed class ServiceRegistry

    /// <summary>
    /// Disposes every registered <see cref="IDisposable"/> service (each instance once, even
-    /// when registered under several types) and clears the registry. <paramref name="except"/>
-    /// is skipped. Called on host shutdown.
+    /// when registered under several types) and clears the registry. Instances in
+    /// <paramref name="except"/> are skipped. Called on host shutdown.
    /// </summary>
-    internal void DisposeServices(object? except = null)
+    internal void DisposeServices(params object[] except)
    {
+        var skipped = new HashSet<object>(except, ReferenceEqualityComparer.Instance);
        var disposed = new HashSet<object>(ReferenceEqualityComparer.Instance);
        foreach (var service in _services.Values)
        {
            if (
-                !ReferenceEquals(service, except)
+                !skipped.Contains(service)
                && service is IDisposable disposable
                && disposed.Add(service)
            )
@@ -1,5 +1,3 @@
-using Microsoft.Xna.Framework;
-
 namespace MrGameEng.Core;

 /// <summary>Phase of a scene transition.</summary>
@@ -13,10 +11,12 @@ public enum TransitionPhase
 }

 /// <summary>
-/// Visual transition between scenes. The scene switch itself happens at full coverage,
-/// so a slow <c>OnLoad</c> of the next scene is hidden behind the overlay.
-/// Transitions are stateless and reusable; progress is tracked by <see cref="SceneManager"/>.
-/// Runs on unscaled time, so it works while gameplay is paused.
+/// Timing of a visual transition between scenes. The scene switch itself happens at full
+/// coverage, so a slow <c>OnLoad</c> of the next scene is hidden behind the overlay.
+/// Progress is tracked by <see cref="SceneManager"/> on unscaled time, so transitions work
+/// while gameplay is paused. The core only times the phases — how the overlay looks is
+/// defined by the host (the MonoGame host's <c>OverlayTransition</c> and the
+/// <c>Transitions</c> factories); headless hosts simply let transitions pass invisibly.
 /// </summary>
 public abstract class Transition
 {
@@ -32,49 +32,4 @@ public abstract class Transition
        OutDuration = Math.Max(0f, outDuration);
        InDuration = Math.Max(0f, inDuration);
    }
-
-    /// <summary>
-    /// Draws the overlay. <paramref name="coverage"/> is 0 (scene fully visible) to
-    /// 1 (scene fully covered); <paramref name="phase"/> tells which side of the switch this is.
-    /// </summary>
-    public abstract void Draw(TransitionRenderer renderer, float coverage, TransitionPhase phase);
-
-    /// <summary>Fade through a solid color (black by default). Total duration is split between out and in.</summary>
-    public static Transition Fade(float duration = 0.6f, Color? color = null) =>
-        new FadeTransition(duration / 2f, duration / 2f, color ?? Color.Black);
-
-    /// <summary>A curtain wiping across the screen (black by default). Total duration is split between out and in.</summary>
-    public static Transition Wipe(float duration = 0.6f, Color? color = null) =>
-        new WipeTransition(duration / 2f, duration / 2f, color ?? Color.Black);
-
-    private sealed class FadeTransition(float outDuration, float inDuration, Color color)
-        : Transition(outDuration, inDuration)
-    {
-        public override void Draw(
-            TransitionRenderer renderer,
-            float coverage,
-            TransitionPhase phase
-        ) => renderer.Fill(0f, 0f, 1f, 1f, color, coverage);
-    }
-
-    private sealed class WipeTransition(float outDuration, float inDuration, Color color)
-        : Transition(outDuration, inDuration)
-    {
-        public override void Draw(
-            TransitionRenderer renderer,
-            float coverage,
-            TransitionPhase phase
-        )
-        {
-            // Out: шторка растёт слева направо; In: уезжает дальше вправо.
-            if (phase == TransitionPhase.Out)
-            {
-                renderer.Fill(0f, 0f, coverage, 1f, color);
-            }
-            else
-            {
-                renderer.Fill(1f - coverage, 0f, coverage, 1f, color);
-            }
-        }
-    }
 }
@@ -0,0 +1,19 @@
+using Microsoft.Xna.Framework.Graphics;
+using MrGameEng.Core;
+
+namespace MrGameEng.Graphics;
+
+/// <summary>Graphics-side accessors for the platform-free <see cref="EngineContext"/>.</summary>
+public static class EngineContextGraphicsExtensions
+{
+    /// <summary>
+    /// Returns the <see cref="GraphicsDevice"/> service published by a windowed host.
+    /// Throws in a headless context, where no graphics device exists.
+    /// </summary>
+    public static GraphicsDevice GetGraphicsDevice(this EngineContext context) =>
+        context.Services.GetOrDefault<GraphicsDevice>()
+        ?? throw new InvalidOperationException(
+            "GraphicsDevice is not available: no windowed host has published it "
+                + "(headless context, or graphics are not initialized yet)."
+        );
+}
@@ -42,7 +42,7 @@ public static class SceneLightmapExtensions
        Func<bool[]> occluders
    )
    {
-        var device = scene.Context.GraphicsDevice;
+        var device = scene.Context.GetGraphicsDevice();
        var lightmap = new Lightmap(device, width, height, cellSize, origin);
        var lighting = new Lighting(lightmap);
        scene.Context.Services.Add(lighting);
@@ -3,6 +3,10 @@
    <TargetFramework>net8.0</TargetFramework>
  </PropertyGroup>

+  <ItemGroup>
+    <PackageReference Include="MonoGame.Framework.DesktopGL" />
+  </ItemGroup>
+
  <ItemGroup>
    <InternalsVisibleTo Include="MrGameEng.Graphics.Tests" />
  </ItemGroup>
@@ -22,7 +22,7 @@ public static class SceneGraphicsExtensions
        var renderer = services.GetOrDefault<Renderer2D>();
        if (renderer is null)
        {
-            renderer = new Renderer2D(scene.Context.GraphicsDevice, options);
+            renderer = new Renderer2D(scene.Context.GetGraphicsDevice(), options);
            services.Add(renderer);
        }
        else if (options is not null)
@@ -1,11 +1,16 @@
 using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
+using MrGameEng.Core;

-namespace MrGameEng.Core;
+namespace MrGameEng.Host;

 /// <summary>
-/// The engine's game loop host. Wraps MonoGame's <see cref="Game"/>: owns the
-/// <see cref="EngineContext"/>, advances the <see cref="GameClock"/> and drives the
-/// active scene's update and draw phases.
+/// The engine's windowed game-loop host. Wraps MonoGame's <see cref="Game"/>: owns the
+/// <see cref="EngineContext"/>, advances the <see cref="GameClock"/>, drives the active
+/// scene's update and draw phases and renders scene-transition overlays. The
+/// <see cref="GraphicsDevice"/> is published as a service so graphics modules can reach it
+/// through the context. For a loop without a window or GPU see
+/// <see cref="HeadlessHost"/> in the core.
 /// </summary>
 public class GameHost : Game
 {
@@ -17,6 +22,7 @@ public class GameHost : Game

    private readonly GameHostOptions _options;
    private readonly Scene _initialScene;
+    private TransitionRenderer? _transitionRenderer;

    /// <summary>Creates a host that starts with <paramref name="initialScene"/>.</summary>
    public GameHost(GameHostOptions options, Scene initialScene)
@@ -48,7 +54,7 @@ public class GameHost : Game
    {
        Window.Title = _options.Title;
        Window.AllowUserResizing = _options.AllowResizing;
-        Context.AttachGraphicsDevice(GraphicsDevice);
+        Context.Services.Add(GraphicsDevice);
        Context.Services.Add(Window);
        Context.Services.Add<Game>(this);
        base.Initialize();
@@ -68,9 +74,25 @@ public class GameHost : Game
    {
        GraphicsDevice.Clear(_options.ClearColor);
        Context.Scenes.Draw(Context.Clock);
+        DrawTransitionOverlay();
        base.Draw(gameTime);
    }

+    private void DrawTransitionOverlay()
+    {
+        if (Context.Scenes.ActiveTransition is not OverlayTransition transition)
+        {
+            return;
+        }
+
+        _transitionRenderer ??= new TransitionRenderer(GraphicsDevice);
+        transition.Draw(
+            _transitionRenderer,
+            Context.Scenes.TransitionCoverage,
+            Context.Scenes.TransitionPhase
+        );
+    }
+
    /// <inheritdoc />
    protected override void OnExiting(object sender, ExitingEventArgs args)
    {
@@ -84,7 +106,11 @@ public class GameHost : Game
    {
        if (disposing)
        {
-            Context.DisposeOwnedResources(except: this);
+            _transitionRenderer?.Dispose();
+            _transitionRenderer = null;
+            // GraphicsDevice зарегистрирован как сервис, но им владеет MonoGame:
+            // base.Dispose сам его освобождает, реестру трогать нельзя.
+            Context.DisposeOwnedResources(this, GraphicsDevice);
        }

        base.Dispose(disposing);
@@ -1,6 +1,6 @@
 using Microsoft.Xna.Framework;

-namespace MrGameEng.Core;
+namespace MrGameEng.Host;

 /// <summary>Window and loop settings for <see cref="GameHost"/>.</summary>
 public sealed class GameHostOptions
@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="MonoGame.Framework.DesktopGL" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="MrGameEng.Host.Tests" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\MrGameEng.Core\MrGameEng.Core.csproj" />
+  </ItemGroup>
+</Project>
@@ -1,11 +1,11 @@
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Graphics;

-namespace MrGameEng.Core;
+namespace MrGameEng.Host;

 /// <summary>
-/// Minimal overlay renderer handed to <see cref="Transition.Draw"/>: fills rectangles in
-/// normalized screen coordinates (0..1 on both axes) over the rendered scene.
+/// Minimal overlay renderer handed to <see cref="OverlayTransition.Draw"/>: fills rectangles
+/// in normalized screen coordinates (0..1 on both axes) over the rendered scene.
 /// </summary>
 public sealed class TransitionRenderer : IDisposable
 {
@@ -13,7 +13,7 @@ public sealed class TransitionRenderer : IDisposable
    private readonly BasicEffect _effect;
    private readonly VertexPositionColor[] _vertices = new VertexPositionColor[6];

-    /// <summary>Disposes the GPU effect. Called by <see cref="SceneManager"/> on shutdown.</summary>
+    /// <summary>Disposes the GPU effect. Called by <see cref="GameHost"/> on shutdown.</summary>
    public void Dispose() => _effect.Dispose();

    internal TransitionRenderer(GraphicsDevice device)
@@ -0,0 +1,66 @@
+using Microsoft.Xna.Framework;
+using MrGameEng.Core;
+
+namespace MrGameEng.Host;
+
+/// <summary>
+/// A scene transition that draws a full-screen overlay through a
+/// <see cref="TransitionRenderer"/>. The timing state machine lives in
+/// <see cref="SceneManager"/>; <see cref="GameHost"/> renders the overlay each draw while a
+/// transition is active. Transitions are stateless and reusable.
+/// </summary>
+public abstract class OverlayTransition : Transition
+{
+    /// <summary>Creates a transition with explicit phase durations.</summary>
+    protected OverlayTransition(float outDuration, float inDuration)
+        : base(outDuration, inDuration) { }
+
+    /// <summary>
+    /// Draws the overlay. <paramref name="coverage"/> is 0 (scene fully visible) to
+    /// 1 (scene fully covered); <paramref name="phase"/> tells which side of the switch this is.
+    /// </summary>
+    public abstract void Draw(TransitionRenderer renderer, float coverage, TransitionPhase phase);
+}
+
+/// <summary>Factories for the built-in visual scene transitions.</summary>
+public static class Transitions
+{
+    /// <summary>Fade through a solid color (black by default). Total duration is split between out and in.</summary>
+    public static Transition Fade(float duration = 0.6f, Color? color = null) =>
+        new FadeTransition(duration / 2f, duration / 2f, color ?? Color.Black);
+
+    /// <summary>A curtain wiping across the screen (black by default). Total duration is split between out and in.</summary>
+    public static Transition Wipe(float duration = 0.6f, Color? color = null) =>
+        new WipeTransition(duration / 2f, duration / 2f, color ?? Color.Black);
+
+    private sealed class FadeTransition(float outDuration, float inDuration, Color color)
+        : OverlayTransition(outDuration, inDuration)
+    {
+        public override void Draw(
+            TransitionRenderer renderer,
+            float coverage,
+            TransitionPhase phase
+        ) => renderer.Fill(0f, 0f, 1f, 1f, color, coverage);
+    }
+
+    private sealed class WipeTransition(float outDuration, float inDuration, Color color)
+        : OverlayTransition(outDuration, inDuration)
+    {
+        public override void Draw(
+            TransitionRenderer renderer,
+            float coverage,
+            TransitionPhase phase
+        )
+        {
+            // Out: шторка растёт слева направо; In: уезжает дальше вправо.
+            if (phase == TransitionPhase.Out)
+            {
+                renderer.Fill(0f, 0f, coverage, 1f, color);
+            }
+            else
+            {
+                renderer.Fill(1f - coverage, 0f, coverage, 1f, color);
+            }
+        }
+    }
+}
@@ -0,0 +1,112 @@
+using MrGameEng.Formulas;
+using Xunit;
+
+namespace MrGameEng.Core.Tests;
+
+public class FormulaTests
+{
+    private static IFormulaContext Vars(Dictionary<string, float> values) =>
+        new DelegateFormulaContext(name =>
+            values.TryGetValue(name, out var v)
+                ? v
+                : throw new FormulaException($"unknown '{name}'")
+        );
+
+    [Theory]
+    [InlineData("1 + 2 * 3", 7f)] // precedence
+    [InlineData("(1 + 2) * 3", 9f)] // parentheses
+    [InlineData("10 - 2 - 3", 5f)] // left associativity
+    [InlineData("-2 + 5", 3f)] // unary minus
+    [InlineData("2 * -3", -6f)] // unary minus after operator
+    [InlineData("7 % 3", 1f)] // modulo
+    [InlineData("2.5 * 4", 10f)] // decimals
+    public void Evaluate_Arithmetic_RespectsPrecedence(string expr, float expected)
+    {
+        Assert.Equal(expected, Formula.Compile(expr).Evaluate(), 4);
+    }
+
+    [Theory]
+    [InlineData("min(3, 5)", 3f)]
+    [InlineData("max(3, 5)", 5f)]
+    [InlineData("clamp(12, 0, 10)", 10f)]
+    [InlineData("clamp(-4, 0, 10)", 0f)]
+    [InlineData("lerp(0, 10, 0.25)", 2.5f)]
+    [InlineData("pow(2, 10)", 1024f)]
+    [InlineData("abs(-7)", 7f)]
+    [InlineData("floor(3.9)", 3f)]
+    [InlineData("ceil(3.1)", 4f)]
+    [InlineData("step(5, 7)", 1f)]
+    [InlineData("step(5, 2)", 0f)]
+    public void Evaluate_Functions_ComputeExpected(string expr, float expected)
+    {
+        Assert.Equal(expected, Formula.Compile(expr).Evaluate(), 4);
+    }
+
+    [Fact]
+    public void Evaluate_Constants_AreBuiltIn()
+    {
+        Assert.Equal(MathF.PI, Formula.Compile("pi").Evaluate(), 5);
+        Assert.Equal(MathF.Tau, Formula.Compile("tau").Evaluate(), 5);
+        Assert.Equal(MathF.E, Formula.Compile("e").Evaluate(), 5);
+    }
+
+    [Theory]
+    [InlineData("2 < 3", 1f)]
+    [InlineData("3 <= 3", 1f)]
+    [InlineData("3 > 5", 0f)]
+    [InlineData("4 == 4", 1f)]
+    [InlineData("4 != 4", 0f)]
+    [InlineData("1 && 0", 0f)]
+    [InlineData("0 || 2", 1f)]
+    [InlineData("!0", 1f)]
+    [InlineData("!5", 0f)]
+    public void Evaluate_LogicAndComparison_YieldBooleansAsOneOrZero(string expr, float expected)
+    {
+        Assert.Equal(expected, Formula.Compile(expr).Evaluate(), 4);
+    }
+
+    [Theory]
+    [InlineData("3 > 2 ? 10 : 20", 10f)]
+    [InlineData("3 < 2 ? 10 : 20", 20f)]
+    [InlineData("1 ? 2 ? 3 : 4 : 5", 3f)] // nested ternary
+    public void Evaluate_Ternary_SelectsBranch(string expr, float expected)
+    {
+        Assert.Equal(expected, Formula.Compile(expr).Evaluate(), 4);
+    }
+
+    [Fact]
+    public void Evaluate_Variables_ResolvedThroughContext()
+    {
+        var ctx = Vars(new() { ["light"] = 0.8f, ["optimal"] = 0.5f });
+        var formula = Formula.Compile("clamp(1 - abs(light - optimal), 0, 1)");
+        Assert.Equal(0.7f, formula.Evaluate(ctx), 4);
+    }
+
+    [Fact]
+    public void Evaluate_SameFormulaTwice_IsDeterministic()
+    {
+        var formula = Formula.Compile("sin(t) * 2 + 1");
+        var ctx = Vars(new() { ["t"] = 1.234f });
+        Assert.Equal(formula.Evaluate(ctx), formula.Evaluate(ctx), 6);
+    }
+
+    [Theory]
+    [InlineData("1 +")] // dangling operator
+    [InlineData("(1 + 2")] // unbalanced paren
+    [InlineData("1 2")] // trailing token
+    [InlineData("min(1)")] // wrong arity
+    [InlineData("nope(1)")] // unknown function
+    [InlineData("@")] // bad character
+    [InlineData("1 = 2")] // single equals
+    public void Compile_InvalidExpression_Throws(string expr)
+    {
+        Assert.Throws<FormulaException>(() => Formula.Compile(expr));
+    }
+
+    [Fact]
+    public void Evaluate_UnknownVariableWithoutContext_Throws()
+    {
+        var formula = Formula.Compile("x + 1");
+        Assert.Throws<FormulaException>(() => formula.Evaluate());
+    }
+}
@@ -0,0 +1,117 @@
+using MrGameEng.Core;
+using Xunit;
+
+namespace MrGameEng.Core.Tests;
+
+public class HeadlessHostTests
+{
+    private sealed class CountingScene : Scene
+    {
+        public int LoadCount;
+        public int UnloadCount;
+        public int UpdateCount;
+
+        protected override void OnLoad() => LoadCount++;
+
+        protected override void OnUnload() => UnloadCount++;
+
+        public override void Update(GameClock clock)
+        {
+            UpdateCount++;
+            base.Update(clock);
+        }
+    }
+
+    [Fact]
+    public void RunTicks_AdvancesClock_ByExactFixedStep()
+    {
+        var scene = new CountingScene();
+        using var host = new HeadlessHost(new HeadlessHostOptions { TicksPerSecond = 10f }, scene);
+
+        host.RunTicks(30);
+
+        Assert.Equal(0.1f, host.FixedDeltaTime, 3);
+        Assert.Equal(30, host.TickCount);
+        Assert.Equal(3.0, host.Context.Clock.TotalTime, 3);
+        Assert.Equal(30, scene.UpdateCount);
+    }
+
+    [Fact]
+    public void FirstTick_LoadsTheInitialScene()
+    {
+        var scene = new CountingScene();
+        using var host = new HeadlessHost(new HeadlessHostOptions(), scene);
+
+        Assert.Equal(0, scene.LoadCount);
+
+        host.Tick();
+
+        Assert.Equal(1, scene.LoadCount);
+        Assert.Same(scene, host.Context.Scenes.Current);
+    }
+
+    [Fact]
+    public void Dispose_UnloadsTheActiveScene()
+    {
+        var scene = new CountingScene();
+        var host = new HeadlessHost(new HeadlessHostOptions(), scene);
+        host.Tick();
+
+        host.Dispose();
+
+        Assert.Equal(1, scene.UnloadCount);
+        Assert.Null(host.Context.Scenes.Current);
+    }
+
+    [Fact]
+    public void Run_StopsWhenCancelled()
+    {
+        using var cancellation = new CancellationTokenSource();
+        var scene = new CancellingScene(cancellation, afterTicks: 5);
+        using var host = new HeadlessHost(new HeadlessHostOptions { Realtime = false }, scene);
+
+        host.Run(cancellation.Token);
+
+        Assert.Equal(5, scene.UpdateCount);
+    }
+
+    [Fact]
+    public void TimeScale_StillApplies_OnTopOfFixedStep()
+    {
+        var scene = new CountingScene();
+        using var host = new HeadlessHost(new HeadlessHostOptions { TicksPerSecond = 10f }, scene);
+        host.Context.Clock.TimeScale = 3f;
+
+        host.RunTicks(10);
+
+        Assert.Equal(3.0, host.Context.Clock.TotalTime, 3);
+        Assert.Equal(1.0, host.Context.Clock.UnscaledTotalTime, 3);
+    }
+
+    [Fact]
+    public void NonPositiveTickRate_Throws()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() =>
+            new HeadlessHost(new HeadlessHostOptions { TicksPerSecond = 0f }, new CountingScene())
+        );
+    }
+
+    private sealed class CancellingScene(CancellationTokenSource cancellation, int afterTicks)
+        : Scene
+    {
+        public int UpdateCount;
+
+        protected override void OnLoad() { }
+
+        public override void Update(GameClock clock)
+        {
+            UpdateCount++;
+            if (UpdateCount >= afterTicks)
+            {
+                cancellation.Cancel();
+            }
+
+            base.Update(clock);
+        }
+    }
+}
@@ -15,6 +15,14 @@ public class SceneTransitionTests
        protected override void OnUnload() => UnloadCount++;
    }

+    // Тайминг-машина живёт в ядре и не зависит от визуала перехода —
+    // тестируем на голой заглушке с длительностями, как у Fade.
+    private sealed class TimedTransition(float outDuration, float inDuration)
+        : Transition(outDuration, inDuration);
+
+    private static Transition Fade(float duration) =>
+        new TimedTransition(duration / 2f, duration / 2f);
+
    private static void Tick(EngineContext context, float seconds)
    {
        context.Clock.Advance(seconds);
@@ -31,7 +39,7 @@ public class SceneTransitionTests
        Tick(context, 0.016f);

        // Fade(1.0) → фаза закрытия 0.5 c, фаза открытия 0.5 c.
-        context.Scenes.Switch(second, Transition.Fade(1f));
+        context.Scenes.Switch(second, Fade(1f));
        Tick(context, 0.2f);

        Assert.True(context.Scenes.IsTransitioning);
@@ -61,7 +69,7 @@ public class SceneTransitionTests
        Tick(context, 0.016f);
        context.Clock.TimeScale = 0f; // игра на паузе

-        context.Scenes.Switch(second, Transition.Fade(0.2f));
+        context.Scenes.Switch(second, Fade(0.2f));
        Tick(context, 0.15f);
        Tick(context, 0.15f);

@@ -79,7 +87,7 @@ public class SceneTransitionTests
        context.Scenes.Switch(first);
        Tick(context, 0.016f);

-        context.Scenes.Switch(second, Transition.Fade(1f));
+        context.Scenes.Switch(second, Fade(1f));
        Tick(context, 0.1f);
        context.Scenes.Switch(third); // передумали, пока экран закрывается

@@ -99,12 +107,12 @@ public class SceneTransitionTests
        context.Scenes.Switch(first);
        Tick(context, 0.016f);

-        context.Scenes.Switch(second, Transition.Fade(1f)); // 0.5 c закрытие + 0.5 c открытие
+        context.Scenes.Switch(second, Fade(1f)); // 0.5 c закрытие + 0.5 c открытие
        Tick(context, 0.6f); // закрыто, своп на second, началось открытие
        Assert.Same(second, context.Scenes.Current);

        Tick(context, 0.25f); // открытие наполовину (coverage ~0.5)
-        context.Scenes.Switch(third, Transition.Fade(1f)); // передумали во время открытия
+        context.Scenes.Switch(third, Fade(1f)); // передумали во время открытия

        Tick(context, 0.05f); // экран снова закрывается — свопа ещё нет
        Assert.Same(second, context.Scenes.Current);
@@ -124,7 +132,7 @@ public class SceneTransitionTests
        context.Scenes.Switch(first);
        Tick(context, 0.016f);

-        context.Scenes.Switch(second, Transition.Fade(0f));
+        context.Scenes.Switch(second, Fade(0f));
        Tick(context, 0.016f);
        Tick(context, 0.016f);

@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <OutputType>Exe</OutputType>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
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+    <PackageReference Include="xunit.runner.visualstudio" />
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+  <ItemGroup>
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+  </ItemGroup>
+</Project>