Ticks and lockstep

A tick is the SDK's deterministic simulation boundary. Products own scheduling and higher-level concepts such as player turns, phases, and rounds.

Participation and outcomes

TickView.participation explicitly distinguishes:

{ mode: 'sequential', activeSeat: 'north' }
{ mode: 'simultaneous', seats: ['north', 'south'] }

Multi-seat games may report outcome: { kind: 'ongoing' } or a decided ranking. Rankings use ascending rank and may include a per-seat score; tied seats share a rank.

Deterministic lockstep

LockstepInput groups authored actions by integer tick and seat. canonicalizeLockstepInputs orders groups by tick and then by lexical seat id, while preserving the action order inside each group. resimulate starts from a supplied rollback snapshot and applies that canonical stream:

const state = resimulate(reducer, rollbackSnapshot, inputs, {
  applyEmptyTick: (state, tick) => advanceScheduledSystems(state, tick),
});

Canonical TickReducer implementations receive an empty input batch for each missing tick. The applyEmptyTick option supports older action-at-a-time reducers. A supplied action may omit seat and receive its group seat; a conflicting seat is rejected. Every canonical input at one tick is committed as one batch.

stateDigest(state) provides a compact deterministic FNV-1a digest over JSON by default. For authoritative networking, provide a canonical serializer that sorts unordered maps and excludes presentation-only data. Compare digests only between compatible reducer, content, serializer, and numeric-runtime versions.

Time is input: wall clocks, frame duration, and scheduler timing must not alter reducer state unless converted to an explicit tick action or deterministic state value. Mint ids from deterministic counters or seeded streams, not process randomness. Avoid engine-dependent transcendental floating-point math in cross-runtime lockstep, or quantize its result before it reaches state.