geometry.test.ts

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`ec7e78a8` 2026-09-07 Pavel Safronov	1	import { afterEach, beforeEach, describe, expect, it } from 'vitest'
	2	import { CIRCUMFERENCE_M, LENGTH_M, RADIUS_M } from '../src/canon/constants.js'
	3	import { setSlatCount, slatCount } from '../src/canon/slats.js'
	4	import { SLAT_COUNT_DEFAULT } from '../src/canon/constants.js'
	5	import * as G from '../src/canon/geometry.js'
	6
	7	describe('theta wrapping', () => {
	8	it('normalizes into [0, 2π)', () => {
	9	expect(G.normalizeTheta(-0.5)).toBeCloseTo(2 * Math.PI - 0.5, 10)
	10	expect(G.normalizeTheta(7 * Math.PI)).toBeCloseTo(Math.PI, 10)
	11	})
	12	it('takes the short way across the seam', () => {
	13	expect(G.deltaTheta(0.1, 2 * Math.PI - 0.1)).toBeCloseTo(-0.2, 10)
	14	expect(G.deltaTheta(2 * Math.PI - 0.1, 0.1)).toBeCloseTo(0.2, 10)
	15	})
	16	})
	17
	18	describe('distances', () => {
	19	it('measures surface distance across the seam, not the long way round', () => {
	20	const a = { theta: 0.05, z: 0 }
	21	const b = { theta: 2 * Math.PI - 0.05, z: 0 }
	22	expect(G.surfaceDistance(a, b)).toBeCloseTo(0.1 * RADIUS_M, 6)
	23	})
	24	it('halfway round is half the circumference on the surface', () => {
	25	const d = G.surfaceDistance({ theta: 0, z: 0 }, { theta: Math.PI, z: 0 })
	26	expect(d).toBeCloseTo(CIRCUMFERENCE_M / 2, 6)
	27	})
	28	it('the chord straight overhead is the diameter, and shorter than walking', () => {
	29	const a = { theta: 0, z: 0 }
	30	const b = { theta: Math.PI, z: 0 }
	31	expect(G.directDistance(a, b)).toBeCloseTo(2 * RADIUS_M, 6)
	32	expect(G.directDistance(a, b)).toBeLessThan(G.surfaceDistance(a, b))
	33	})
	34	it('rises the full radius to reach the point directly overhead', () => {
	35	expect(G.chordApexHeight({ theta: 0, z: 0 }, { theta: Math.PI, z: 0 })).toBeCloseTo(RADIUS_M, 6)
	36	expect(G.chordApexHeight({ theta: 0, z: 0 }, { theta: 0, z: 1000 })).toBeCloseTo(0, 6)
	37	})
	38	})
	39
	40	describe('slats and land strips', () => {
	41	// Canon is a sealed hull (D-017); these exercise the exploration mode.
	42	beforeEach(() => setSlatCount(8))
	43	afterEach(() => setSlatCount(SLAT_COUNT_DEFAULT))
	44
	45	it('puts land strip centres on land and slat centres on slats', () => {
	46	for (let i = 0; i < slatCount(); i++) {
	47	expect(G.isSlat(G.landStripCentre(i))).toBe(false)
	48	expect(G.isSlat(G.slatCentre(i))).toBe(true)
	49	expect(G.landStripIndex(G.landStripCentre(i))).toBe(i)
	50	expect(G.landStripIndex(G.slatCentre(i))).toBeNull()
	51	}
	52	})
	53	it('reports a slat crossing when moving between adjacent strips', () => {
	54	expect(G.routeCrossesSlat(
	55	{ theta: G.landStripCentre(0), z: 0 },
	56	{ theta: G.landStripCentre(1), z: 0 },
	57	)).toBe(true)
	58	})
	59	it('reports no crossing when moving along one strip', () => {
	60	expect(G.routeCrossesSlat(
	61	{ theta: G.landStripCentre(0), z: 0 },
	62	{ theta: G.landStripCentre(0), z: 200_000 },
	63	)).toBe(false)
	64	})
	65	})
	66
	67	describe('map projection', () => {
	68	it('round-trips surface points exactly — a cylinder unrolls without distortion', () => {
	69	for (const p of [
	70	{ theta: 0, z: 0 },
	71	{ theta: 1.234, z: 271_000 },
	72	{ theta: 2 * Math.PI - 1e-9, z: LENGTH_M },
	73	]) {
	74	const back = G.fromMap(G.toMap(p))
	75	expect(back.theta).toBeCloseTo(G.normalizeTheta(p.theta), 9)
	76	expect(back.z).toBeCloseTo(p.z, 6)
	77	}
	78	})
	79	it('spans the world', () => {
	80	expect(G.MAP_WIDTH_M).toBe(LENGTH_M)
	81	expect(G.MAP_HEIGHT_M).toBeCloseTo(CIRCUMFERENCE_M, 9)
	82	})
	83	})