The demo bot draws from the seed, the round and the seat

template/src/lib/game/index.ts

@@ -33,12 +33,11 @@ export interface State {
 
 export type Input = { pick: number };
 
-/** Mulberry32: a small seeded generator, so a game replays to the same bot picks. */
-function next(state: State): number {
-	let t = (state.rng += 0x6d2b79f5) >>> 0;
+/** Mulberry32 on a seed: a small generator, so a game replays to the same bot picks. */
+function random(seed: number): number {
+	let t = (seed + 0x6d2b79f5) >>> 0;
 	t = Math.imul(t ^ (t >>> 15), t | 1);
 	t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
-	state.rng = state.rng >>> 0;
 	return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
 }
 
@@ -86,7 +85,8 @@ export const game: GameSpec<State, Input> = {
 	turn: (state) => state.rounds.length + 1,
 	// Every finished round is public; nothing is hidden, so the gallery sees what a seat sees.
 	view: (state, viewer) => (viewer === SPECTATOR ? structuredClone(state) : structuredClone(state)),
-	botInput: (state) => ({ pick: 1 + Math.floor(next(structuredClone(state)) * HIGHEST) }),
+	// A function of the state alone: the seed, the round and the seat, so a replay draws the same pick.
+	botInput: (state, seat) => ({ pick: 1 + Math.floor(random(state.rng + state.rounds.length * 7919 + state.seats.indexOf(seat)) * HIGHEST) }),
 	cleanInput: (raw) => {
 		const pick = Number((raw as { pick?: unknown })?.pick);
 		return { pick: Number.isInteger(pick) && pick >= 1 && pick <= HIGHEST ? pick : 1 };