Turn order and lockstep

TurnOrderState is a neutral sequential-seat cursor. It tracks seat order, direction, one-based turn and round counters, queued skips, and FIFO extra turns without owning phases, actions, reactions, or elimination rules.

let order = createTurnOrder(['north', 'south']);
order = queueSkip(order, 'south');
order = advanceTurn(order); // north remains active; south's skip is consumed
order = queueExtraTurn(order, 'north');

The helpers are immutable:

• advanceTurn consumes a queued extra turn before rotating normally and consumes queued skips while searching for an eligible seat;
• reverseTurnOrder changes direction without moving the current seat;
• eliminateSeat removes a seat and advances when it was active;
• reorderSeats preserves the active seat while accepting a permutation of the current seats; and
• activeSeat returns the current seat or throws when none remain.

Products own the exact moment an action, response window, or phase advances the cursor. Store the returned order in authoritative state so replay observes the same sequence.

Participation and outcomes

TurnView.participation explicitly distinguishes:

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

The older activeSeat field remains sequential compatibility sugar. 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. Required solo status remains available for old consumers.

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),
});

When ticks have gaps, applyEmptyTick is called for each missing tick. Without that callback, empty ticks intentionally have no effect. A supplied action may omit seat and receive its group seat; a conflicting seat is rejected. When the reducer implements applyIntents, every canonical action at one tick is passed as a single atomic batch. Legacy reducers without that method receive the same actions serially in canonical order.

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.