rng.ts

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	}