Determinism & rewind

jolt-ts ships Jolt compiled for cross-platform determinism: given the same starting state and inputs, every machine computes the same result, bit for bit. This demo makes that visible — it snapshots the world, runs for a few seconds, restores the snapshot, and repeats. Every replay retraces the exact same path.

Drag to orbit

❚❚ Pause ↺ Reset

Run demo

Loading physics…

The shapes snap back and replay identically each loop — the property rollback netcode is built on.

How it works

const world = await World.create({ deterministic: "cross-platform", gravity: [0, -9.81, 0] });
// …spawn bodies…

// Serialize the whole simulation state to bytes.
const snapshot = world.saveState(); // Uint8Array

function tick(frame) {
  if (frame > 0 && frame % 260 === 0) {
    world.restoreState(snapshot); // jump back to the start
  }
  world.step(1 / 60);
}

saveState() captures positions, velocities, active state, and contacts. restoreState() puts them all back. Because stepping is deterministic, the replay is identical.

From replay to netcode

This is the core of a rollback loop:

• Keep a ring buffer of saveState() bytes, one per frame.
• When a late input arrives for frame N, restoreState() that frame and re-step to the present with the corrected input.
• saveState/restoreState only move simulation state for bodies that already exist — they never add or remove bodies. Replicate each body spawn/despawn as its own event, applied in the same order on every peer so ids stay in lockstep. See the networking guide.

Related

• Determinism & networking guide — snapshots, recorders, and rollback in depth.