xpTURN.Klotho

Deterministic Multiplayer Simulation Framework for Unity and Godot

⚠️ Experimental Release This project is under active development and has not yet reached a stable stage. Public APIs, serialization formats, and network protocols may change without notice. Production use is not recommended.

A deterministic-simulation framework supporting Client-Side Prediction (CSP), Rollback, Frame Synchronization, Server-Driven mode, and Replay. The simulation core is engine-agnostic pure C# with Unity and Godot (.NET) adapters on top. By excluding floating-point and building the simulation solely on 32.32 fixed-point (FP64) and a deterministic RNG (Xorshift128+), it guarantees full reproducibility across platforms and compilers.

Klotho weaves the simulation, one frame at a time.

How synchronization works, in one paragraph: determinism is the keystone — given the same ordered inputs, every peer computes byte-identical state, so the network carries inputs only and verification reduces to comparing a hash. On that foundation each peer keeps two timelines over the same tick axis: a Verified chain (ticks where every player's real input is known — immutable) and a Predicted chain (ticks run ahead using guessed remote input — provisional). Like CPU branch prediction, the simulation advances immediately on predicted input instead of waiting; when a real input contradicts a guess, the engine restores a snapshot and re-simulates (rollback), which is cheap precisely because state is a pure function of inputs. Input delay and adaptive timing buffers minimize how often that happens, and when determinism genuinely breaks, a graded recovery ladder (hash check → rollback → full-state resync → corrective reset) restores agreement. The same machinery serves both P2P lockstep (peers hold equal authority) and Server-Driven (the server owns the verified chain). Full rationale: Docs/SynchronizationDesign.md.

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Key Features

Area	Contents
Network Models	P2P Lockstep · Rollback + Client Prediction · Server-Driven (Authoritative) · Dedicated Server · Spectator · Late Join · Reconnect
Deterministic Math	FP64 (32.32 fixed-point) · FPVector2/3/4 · FPQuaternion · FPMatrix · LUT/CORDIC-based trigonometry · DeterministicRandom (Xorshift128+)
Physics	FPPhysicsWorld · Broadphase (SpatialGrid) · Narrowphase · CCD (Sweep) · Constraint Solver · Joints · Triggers · Static BVH
Navigation	FPNavMesh · A* (triangle graph) · Funnel (SSFA) · ORCA avoidance · ECS-integrated NavAgentComponent
ECS	Sparse-set ComponentStorage<T> · Frame (single byte[] heap) · FilterWithout/Filter<T1..T5> · FrameRingBuffer · SystemRunner
Serialization / Source Generator	SpanWriter/Reader (ref struct, GC-free) · automatic code generation via [KlothoComponent] / [KlothoSerializable] / [KlothoDataAsset]
Data Assets	IDataAsset · DataAssetRegistry · DataAssetRef · JSON serialization (xpTURN.Klotho.DataAsset.Json)
Replay	Record / playback / seek / variable speed · LZ4 compression (K4os.Compression.LZ4)
Verification Tools	SyncTestRunner (GGPO-style determinism verification) · DeterminismVerificationRunner · benchmark suite
Unity Integration	USimulationConfig · USessionConfig · View layer (EntityViewFactory / EntityViewUpdater / EntityView, BindBehaviour / ViewFlags, VerifiedFrameInterpolator) · KlothoSessionDriver (MonoBehaviour) · KlothoConnectionAsync (UniTask)
Godot Integration	GodotSimulationConfig · GodotSessionConfig (Resource) · View layer (EntityViewFactory / EntityViewUpdaterNode / EntityViewNode, VerifiedFrameInterpolator) · GodotSessionDriver (Node) · GodotConnectionAsync (Task) · GodotDebugSink / GodotLogSink

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Suitable Genres

Genres that benefit most from Klotho's deterministic simulation features.

Best Fit (core targets)

• Fighting games — frame-perfect inputs + rollback netcode
• Platform fighters / arena brawlers
• 2–4 player PvP action — optimal for small-roster P2P rollback
• Tactics / turn-based SRPG — determinism + replay + sync verification
• Real-time strategy (RTS) — lockstep frame sync
• MOBA / top-down arena — ECS, physics, and navigation all included
• Twin-stick shooters / co-op shooters — deterministic physics + ORCA avoidance
• Auto-battlers — deterministic simulation + shareable replays

Good Fit (structurally well-suited)

• Roguelike / roguelite (PvE co-op) — deterministic RNG (Xorshift128+) for shared seeds and replays
• Card / board / deckbuilder PvP — low input volume, strong verification and replay
• Puzzle PvP / falling-block versus
• Racing (small scale) — fixed-point physics guarantees determinism for small grids
• Top-down survival action

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Architecture Overview

┌─────────────────────────────────────────────────────────────┐
│                    Game Application                         │
│   (ISimulation impl: EcsSimulation or custom Simulation)    │
└───────────────────────────┬─────────────────────────────────┘
                            │
         ┌──────────────────┼──────────────────┐
         ▼                  ▼                  ▼
  ┌───────────────┐   ┌──────────────┐   ┌────────────────────┐
  │ KlothoEngine  │   │ ReplaySystem │   │KlothoNetworkService│
  │ (orchestrator)│◄  │ (record/play)│   │                    │
  └──────┬────────┘   └──────────────┘   └─────────┬──────────┘
         │                                         │
    ┌────┴────┐                              ┌─────┴──────┐
    ▼         ▼                              ▼            ▼
 ISimulation  InputBuffer               INetworkTransport  NetworkMessages
    │
    ├── (ECS) Frame — EntityManager + ComponentStorage[]
    ├── SystemRunner — ISystem[]
    ├── FrameRingBuffer (snapshots / rollback)
    └── DataAssetRegistry

Three-layer separation:

• Klotho engine layer — Pure C#, engine-independent (no UnityEngine / Godot references). The same binary runs under the Unity and Godot (.NET) adapters and on the server side (.NET console / ASP.NET).
• Simulation transport layer — UDP transport over the INetworkTransport abstraction (Input / InputAck / SyncCheck / Handshake). LiteNetLib is provided as the default reference implementation; replaceable with any other library.
• Game service layer — Lobby / matchmaking / authentication (external gRPC, etc.; integrated outside this project).

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Tech Stack

• Unity 2022.3+ — scripting backends: Mono / IL2CPP (AOT-safe; serialization via source generation, no runtime codegen/reflection)
• Godot 4.4+ (mono / .NET, net8.0) — the Godot adapter ships as addons/klotho/ (prebuilt core DLL + adapter source). See Installation — Godot.
• C# language level — Unity targets C# 8.0 with the xpTURN.Polyfill package supplying the runtime-attribute shims required by C# 11; the Godot adapter is net8.0 / LangVersion latest and needs no polyfill (native init / required).
• UniTask — async on Unity only (Cysharp). The Godot adapter uses the standard library Task — no UniTask dependency.
• xpTURN.Klotho.Logging (IKLogger) — in-house structured logging (no external logging dependency). Optional MEL interop via the Plugins~/Logging.Mel sample adapter (Microsoft.Extensions.Logging.Abstractions DLL is consumer-provided).
• LiteNetLib — default reference implementation for UDP transport (MIT, pure C#). On Unity it is vendored as source under Runtime/ThirdParty/LiteNetLib.v2.1.4; on Godot it arrives as the NuGet package LiteNetLib 2.1.4. The transport layer is abstracted via INetworkTransport and is replaceable.
• Newtonsoft.Json — DataAsset JSON serialization (Unity: com.unity.nuget.newtonsoft-json · Godot: NuGet Newtonsoft.Json 13.0.3)
• K4os.Compression.LZ4 — replay compression (Unity: vendored under Runtime/ThirdParty/ · Godot: NuGet K4os.Compression.LZ4 1.3.8)

Details: Docs/BaseLibraries.md

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Installation

Klotho installs differently per engine — follow the matching guide. Each covers the client first, then the optional dedicated server (a plain .NET host on the shared engine-agnostic core):

• Installation — Unity — UPM git URLs + Polyfill activation, then Server~ project references for a dedicated server.
• Installation — Godot (.NET) — the addons/klotho/ folder + a one-line Klotho.props import, then the (engine-agnostic) dedicated server.

The in-repo projects under Samples/ double as install references — see the Documentation Map for per-sample walkthroughs.

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Repository Layout

Klotho lives at the repository top level under com.xpturn.klotho/. Unity consumers install the package via UPM; Godot consumers use the Godot~/ adapter packaged as an addons/klotho/ folder (see Installation). The in-repo Unity samples consume the package through a file: manifest reference to the same top-level package.

<repo root>/
├── README.md  ·  CHANGELOG.md  ·  LICENSE
├── com.xpturn.klotho/             ← ★ framework package
│   ├── package.json
│   ├── Runtime/                   engine-agnostic core + Unity adapter
│   │   ├── Core/                  engine · session · network
│   │   ├── Logging/               IKLogger (in-house)
│   │   ├── Gameplay/              built-in components / systems
│   │   ├── Diagnostics/           fault injection · metrics
│   │   ├── Input/                 input buffer / predictor
│   │   ├── Network/               transport · server · spectator
│   │   ├── State/                 snapshot manager
│   │   ├── Serialization/         SpanWriter/Reader
│   │   ├── Replay/                record / playback (LZ4)
│   │   ├── ECS/                   Frame · components · systems
│   │   ├── Deterministic/         FP64 · physics · navigation
│   │   ├── Unity/                 Unity adapter
│   │   ├── LiteNetLib/            UDP transport
│   │   └── ThirdParty/            vendored deps
│   ├── Godot~/                    Godot (.NET) adapter
│   ├── Editor/                    Unity-only editor tools
│   ├── Plugins/Analyzers/         source generator
│   ├── Prefabs/                   debug prefabs (Unity)
│   ├── Plugins~/Logging.Mel/      MEL interop sample
│   └── Server~/                   dedicated-server projects
│
├── Samples/                       ← standalone samples
│   ├── Brawler/                   4-player fighter (Unity)
│   ├── P2pSample/                 minimal P2P (Unity)
│   ├── SdSample/                  minimal Server-Driven (Unity)
│   ├── GodotP2pSample/            minimal P2P (Godot)
│   ├── GodotSdSample/             minimal Server-Driven (Godot)
│   └── LoggingMelConsole/         .NET logging sample
│
├── Docs/                          ← documentation
└── Tools/                         ← .NET tooling (internal)
    ├── KlothoGenerator/           source generator
    ├── KlothoGenerator.Tests/     generator tests
    ├── DeterminismVerification/   determinism verifier
    ├── PhysicsDeterminismProbe/   FP determinism probe
    └── gen.sh                     generator build script

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Quick Start

The four-step path — define a component → implement a system → wire callbacks → create & drive a session — is the same shape on both engines, but the session-driving and view layers are engine-specific. Pick the matching walkthrough (each is self-contained, with full code):

• Quick Start — Unity — MonoBehaviour controller, KlothoSessionDriver, ScriptableObject configs, UniTask joins, EntityViewFactory / EntityViewUpdater / EntityView.
• Quick Start — Godot (.NET) — Node controller, GodotSessionDriver (_Process), Resource configs, standard Task joins, EntityViewNode / EntityViewUpdaterNode (.tscn).

Steps 1–3 (component / system / callbacks) are engine-agnostic core; KlothoSessionFlow exposes the same entry points (StartHostAndListen / JoinP2PAsync / JoinServerDrivenAsync / ReconnectAsync / SpectateAsync / StartReplayFromFile) on both — Unity wraps the async ones in UniTask, Godot in standard Task. Session creation is observed through the single IKlothoSessionObserver.OnSessionCreated(session, SessionEntryKind kind) — branch on kind, not simCfg.Mode.

Detailed guides: Docs/GameDevWorkflow.md, Docs/GameDevAPI.md

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Sample

Samples/Brawler — a 4-player fighting-game sample

• ECS-based combat / movement / skills / cooldowns / knockback / items / traps
• HFSM-based bot AI (BotHFSMRoot / BotActions / BotDecisions)
• Camera integration with Unity Cinemachine
• Supports both P2P and ServerDriven modes
• LZ4-compressed replay record / playback

Docs: Docs/Samples/Brawler.md

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Documentation Map

Document	Contents
Docs/Installation.Unity.md · Docs/Installation.Godot.md	Engine-specific install (client + dedicated server)
Docs/QuickStart.Unity.md · Docs/QuickStart.Godot.md	Engine-specific 5-step quick starts (component → system → callbacks → session → view)
Docs/FEATURES.md	Full feature list
Docs/Specification.md	Engine specification (state machines · configuration · events · message protocol · formats)
Docs/SynchronizationDesign.md	Synchronization design direction (determinism · two-chain model · prediction/rollback · timing · authority models · recovery ladder)
Docs/GameDevWorkflow.md	Game-developer workflow (step-by-step)
Docs/GameDevAPI.md	Game-developer API status
Docs/SimulationConfigGuide.md	SimulationConfig recommended-value guide (per genre / platform)
Docs/BaseLibraries.md	List of base libraries used
Docs/Navigation.md	Deterministic navigation (FPNavMesh · A* · Funnel · ORCA)
Docs/Samples/	Sample walkthroughs — Brawler, P2pSample, SdSample
Docs/Samples/GodotSdSample.md · Docs/Samples/GodotP2pSample.md	Godot (.NET) sample walkthroughs (architecture + Godot-specific gotchas)

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Design Principles

• Determinism first — no float; all simulation state is FP64 / integer / bool
• Zero-GC oriented — ref struct, object pools, cached fields, no LINQ
• Engine independence — the core is pure C# (no UnityEngine / Godot references); engine integration (Unity / Godot) lives in adapter layers on top of the shared core
• Minimal bandwidth — only inputs (commands) are sent; no state synchronization (only hash verification)
• Layer separation — strict separation between simulation callbacks (deterministic) and view callbacks (non-deterministic)