rng.ts

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`5551eb37` 2026-10-20 Pavel Safronov	1	/**
	2	* Seeded randomness. Every random choice in the game comes from a stream derived from the run
	3	* seed by purpose and level (PLAN.md §12), so an extra bomb in one place never changes what
	4	* happens somewhere else, and the same seed always gives the same levels.
	5	*/
	6
	7	/** FNV-1a over the parts joined by `:`. A 32-bit unsigned integer. */
	8	export function hashSeed(...parts: (string \| number)[]): number {
	9	let h = 0x811c9dc5
	10	for (const ch of parts.join(':')) {
	11	h ^= ch.codePointAt(0)!
	12	h = Math.imul(h, 0x01000193)
	13	}
	14	return h >>> 0
	15	}
	16
	17	/**
	18	* mulberry32. Its whole state is one 32-bit number, kept in `state`, so a stream can be stored
	19	* in the game state and resumed exactly.
	20	*/
	21	export class Rng {
	22	constructor(public state: number) {}
	23
	24	/** A float in [0, 1). */
	25	next(): number {
	26	this.state = (this.state + 0x6d2b79f5) >>> 0
	27	let t = this.state
	28	t = Math.imul(t ^ (t >>> 15), t \| 1)
	29	t ^= t + Math.imul(t ^ (t >>> 7), t \| 61)
	30	return ((t ^ (t >>> 14)) >>> 0) / 4294967296
	31	}
	32
	33	/** An integer in [0, n). */
	34	int(n: number): number {
	35	return Math.floor(this.next() * n)
	36	}
	37	}
	38
	39	export function stream(seed: number, purpose: string, level: number): Rng {
	40	return new Rng(hashSeed(seed, purpose, level))
	41	}