instruments.test.ts

Blame: the commit that last changed each line · file · history

`e5555695` 2026-09-07 Pavel Safronov	1	/**
	2	* The sightline probe reproduces the table in design.md §1.8 — the two-horizons
	3	* result, which is canon and came out of this calculation in the first place.
	4	*/
	5	import { describe, expect, it } from 'vitest'
	6	import { LENGTH_M, RADIUS_M } from '../src/canon/constants.js'
	7	import { sightline, travel } from '../src/world/instruments.js'
	8	import { landStripCentre } from '../src/canon/geometry.js'
	9	import { setSlatCount } from '../src/canon/slats.js'
	10	import { SLAT_COUNT_DEFAULT } from '../src/canon/constants.js'
	11
	12	const along = (m: number) => sightline({ theta: 0, z: 0 }, { theta: 0, z: m })
	13	/** A point `deg` around the cylinder at the same z — a sightline across the interior. */
	14	const across = (deg: number) =>
	15	sightline({ theta: 0, z: 0 }, { theta: (deg * Math.PI) / 180, z: 0 })
	16
	17	describe('§1.8 looking along the axis', () => {
	18	it('is 31% clear at 100 km', () => expect(along(100_000).transmission).toBeCloseTo(0.31, 2))
	19	it('leaves the far cap a 4% ghost from the midpoint', () => {
	20	const s = along(LENGTH_M / 2)
	21	expect(s.transmission).toBeCloseTo(0.044, 3)
	22	expect(s.clarity).toBe('ghost')
	23	})
	24	it('closes the horizon between 100 and 150 km', () => {
	25	expect(along(100_000).clarity).toBe('hazy')
	26	expect(along(150_000).clarity).toBe('ghost')
	27	})
	28	it('never rises off the floor — the axial floor is flat', () =>
	29	expect(along(100_000).apexHeightM).toBeCloseTo(0, 6))
	30	})
	31
	32	describe('§1.8 looking across the interior', () => {
	33	it('is 78% clear to the point directly overhead, 100 km away', () => {
	34	const s = across(180)
	35	expect(s.directDistanceM).toBeCloseTo(2 * RADIUS_M, 6)
	36	expect(s.transmission).toBeCloseTo(0.78, 2)
	37	expect(s.clarity).toBe('crisp')
	38	})
	39	it('stays crisp all the way round', () => {
	40	for (const d of [30, 60, 90, 135, 180]) expect(across(d).clarity).toBe('crisp')
	41	})
	42	it('rises to 14.6 km at 90° — why looking up beats looking along', () =>
	43	expect(across(90).apexHeightM / 1000).toBeCloseTo(14.6, 1))
	44	})
	45
	46	describe('§1.8 the headline: overhead beats ground at the same distance', () => {
	47	it('is more than twice as clear across the interior at 100 km', () => {
	48	const axial = along(100_000)
	49	const overhead = across(180)
	50	expect(overhead.directDistanceM).toBeCloseTo(axial.directDistanceM, 0)
	51	expect(overhead.transmission).toBeGreaterThan(axial.transmission * 2)
	52	})
	53	it('gets clearer with distance across, the opposite of along', () => {
	54	expect(across(180).transmission).toBeGreaterThan(across(60).transmission)
	55	expect(along(150_000).transmission).toBeLessThan(along(50_000).transmission)
	56	})
	57	})
	58
	59	describe('travel', () => {
	60	it('reports no slat crossing with the hull sealed (D-017)', () => {
	61	const t = travel({ theta: 0, z: 0 }, { theta: Math.PI, z: 0 }, 1.4)
	62	expect(t.crossesSlat).toBe(false)
	63	})
	64
	65	it('times a walk and notices slat crossings when slats exist', () => {
	66	setSlatCount(8)
	67	const t = travel(
	68	{ theta: landStripCentre(0), z: 0 },
	69	{ theta: landStripCentre(1), z: 0 },
	70	1.4,
	71	)
	72	expect(t.crossesSlat).toBe(true)
	73	expect(t.durationS).toBeCloseTo(t.distanceM / 1.4, 6)
	74	setSlatCount(SLAT_COUNT_DEFAULT)
	75	})
	76	})