Fundies

2026

A 2D fighting game about spacing, hit confirms and whiff punishes, with rollback netplay, an engine-free deterministic simulation, and a Capcom-style motion-input engine you can try below.

C#
Godot
.NET
Cloudflare Workers

The repository is private, and the game ships as desktop and mobile builds rather than for the web. The input engine below is its real code, ported.

The motion-input engine

Ported from the game

Click the panel, then use the arrows or WASD and press J. Or play a scripted input.

Loading the input engine…

Fundies is a Footsies-like 2D fighter: spacing, hit confirms, whiff punishes and committed special moves, with fireballs and jumping added. Each fighter has 100 HP, there is no guard break, and a fireball parry restores a little health.

The architecture follows from one fact: rollback netcode means the simulation gets re-run. So `Simulation.Step` is a pure function over a flat `GameState` struct with no references or collections — no wall-clock time, no floats, no engine calls, no unseeded randomness. Snapshotting a frame is a plain assignment, which is what makes saving state every single frame affordable. Godot only renders and polls input; it does not simulate.

The same discipline reaches the art. A pose is a pure function of the fighter's state and state frame, so rewinding the frame rewinds the drawing for free. Poses are chunky and held so a two-to-five frame correction usually lands inside one, and cosmetic effects are fire-and-forget events that are suppressed during resimulation.

The motion engine follows a published SF6 input reference: eleven-frame quarter-circle segments, a lenient seven-frame DP, exhaustive matching through intervening inputs, and a five-frame action buffer. Online play is peer to peer, with room codes and IP discovery handled by a small Cloudflare Worker.

What it does

Pure `Simulation.Step` over a flat struct: snapshotting a frame is an assignment
356 tests: determinism, gameplay, rollback and two-peer desync
Capcom-style motion input, checked against its own test vectors
Poses derived from state, so rollback corrects the drawing for free
Room-code matchmaking on a Cloudflare Worker for peer-to-peer netplay

Problems worth writing down

Godot's skeleton, AnimationPlayer, physics, timers and tweens all advance on the engine's own clock and cannot be rewound, so none of them can be allowed to affect a rollback game.

The engine does not simulate. The fight is plain C# over fixed-point numbers, and every pose is computed from the snapshot each frame — the same boundary Footsies itself uses, with Unity in its case rather than Godot.

A strict motion reader rejects inputs players consider clean, and a loose one fires specials by accident — especially a DP out of a walking fireball.

Direction history is stored as runs and matched exhaustively within per-segment windows, so intervening inputs are tolerated and an older valid diagonal is still found. Returning through back and down to forward is read as an intentional fireball, so a walking fireball does not come out as a DP.