slats.ts

1	/**
2	* Slat count, the one canon parameter adjustable at runtime. See design.md §1.4.
3	*
4	* D-002 fixes the total slat width from the land-area target, so the count only
5	* decides how that width is divided up: Tower's land stays exactly Wisconsin's land
6	* area at any count from 1 to 16. At 0 there are no slats at all — the land becomes
7	* the entire inner surface and nobody in Tower can see a star. That is a different
8	* world, and the app says so rather than silently rounding the coincidence away.
9	*/
10	import {
11	CIRCUMFERENCE_M, INNER_AREA_M2, LAND_AREA_M2, RADIUS_M,
12	SLAT_COUNT_DEFAULT, SLAT_COUNT_MAX, SLAT_COUNT_MIN, SLAT_TOTAL_WIDTH_M,
13	} from './constants.js'
14
15	const STORAGE_KEY = 'tower.slatCount'
16	const TAU = 2 * Math.PI
17
18	let count = SLAT_COUNT_DEFAULT
19	const listeners = new Set<() => void>()
20
21	/** Restore the author's chosen count. Safe where localStorage is unavailable. */
22	export function loadSlatCount(): void {
23	try {
24	const raw = localStorage.getItem(STORAGE_KEY)
25	if (raw !== null) count = clamp(Number(raw))
26	} catch {
27	// Private window or blocked site data: keep the canon default.
28	}
29	}
30
31	const clamp = (n: number): number =>
32	Math.min(SLAT_COUNT_MAX, Math.max(SLAT_COUNT_MIN, Math.round(Number.isFinite(n) ? n : SLAT_COUNT_DEFAULT)))
33
34	export const slatCount = (): number => count
35
36	export function setSlatCount(n: number): void {
37	const next = clamp(n)
38	if (next === count) return
39	count = next
40	try {
41	localStorage.setItem(STORAGE_KEY, String(count))
42	} catch {
43	// Not persisting is survivable; the session still works.
44	}
45	for (const fn of listeners) fn()
46	}
47
48	export function onSlatCountChange(fn: () => void): () => void {
49	listeners.add(fn)
50	return () => listeners.delete(fn)
51	}
52
53	/** Width of one slat. Zero when there are none. */
54	export const slatWidth = (): number => (count === 0 ? 0 : SLAT_TOTAL_WIDTH_M / count)
55	export const slatAngularWidth = (): number => slatWidth() / RADIUS_M
56	/** Angle from one slat to the next. Zero when there are none. */
57	export const slatSpacing = (): number => (count === 0 ? 0 : TAU / count)
58
59	/** Width of one land strip. With no slats the strip is the whole circumference. */
60	export const landStripWidth = (): number =>
61	count === 0 ? CIRCUMFERENCE_M : (CIRCUMFERENCE_M - SLAT_TOTAL_WIDTH_M) / count
62
63	/** Total habitable land. Constant at Wisconsin's land area for any count ≥ 1. */
64	export const landArea = (): number => (count === 0 ? INNER_AREA_M2 : LAND_AREA_M2)
65
66	/** How many distinct land strips exist. With no slats, one continuous surface. */
67	export const landStripCount = (): number => (count === 0 ? 1 : count)