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test: add antimeridian splitting tests for low npoints values #76

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71 changes: 71 additions & 0 deletions test/antimeridian.test.ts
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Original file line number Diff line number Diff line change
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import { GreatCircle } from '../src';
import type { MultiLineString, LineString } from 'geojson';

// Routes that cross the antimeridian
const PACIFIC_ROUTES = [
{ name: 'Tokyo → LAX', start: { x: 139.7798, y: 35.5494 }, end: { x: -118.4085, y: 33.9416 } },
{ name: 'Auckland → LAX', start: { x: 174.79, y: -36.85 }, end: { x: -118.41, y: 33.94 } },
{ name: 'Shanghai → SFO', start: { x: 121.81, y: 31.14 }, end: { x: -122.38, y: 37.62 } },
];

function assertSplitAtAntimeridian(coords: number[][][]) {
const seg0 = coords[0];
const seg1 = coords[1];

expect(seg0).toBeDefined();
expect(seg1).toBeDefined();

if (!seg0 || !seg1) return; // narrow for TS

const lastOfFirst = seg0[seg0.length - 1];
const firstOfSecond = seg1[0];

expect(lastOfFirst).toBeDefined();
expect(firstOfSecond).toBeDefined();

if (!lastOfFirst || !firstOfSecond) return; // narrow for TS

// Both sides of the split must be at ±180
expect(Math.abs(lastOfFirst[0] ?? NaN)).toBeCloseTo(180, 1);
expect(Math.abs(firstOfSecond[0] ?? NaN)).toBeCloseTo(180, 1);

// Latitudes must match — no gap at the antimeridian
expect(lastOfFirst[1] ?? NaN).toBeCloseTo(firstOfSecond[1] ?? NaN, 3);
}
Comment on lines +11 to +34
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Copilot AI Mar 30, 2026

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assertSplitAtAntimeridian only checks abs(lon) ≈ 180, so it would still pass if both segment endpoints use the same sign (e.g. 180/180) or if the geometry contains >2 segments with later bad splits. To prevent false positives, assert coords.length === 2, both segments are non-empty, and that the split longitudes are exactly 180 and -180 (opposite signs) for the adjacent endpoints (matching the implementation and existing tests in test/great-circle.test.ts).

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describe('antimeridian splitting', () => {
describe('with npoints=100', () => {
for (const { name, start, end } of PACIFIC_ROUTES) {
test(`${name} produces a split MultiLineString`, () => {
const result = new GreatCircle(start, end).Arc(100, { offset: 10 }).json();
expect(result.geometry.type).toBe('MultiLineString');
assertSplitAtAntimeridian((result.geometry as MultiLineString).coordinates);
});
}
});

Comment on lines +37 to +46
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Copilot AI Mar 30, 2026

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This file duplicates the existing Tokyo/Auckland/Shanghai antimeridian split assertions already present in test/great-circle.test.ts for npoints=100. Consider keeping the new low-npoints coverage but avoid repeating the npoints=100 suite (or move these cases into the existing "Dateline crossing" block) to reduce duplicated fixtures and maintenance overhead.

Suggested change
describe('with npoints=100', () => {
for (const { name, start, end } of PACIFIC_ROUTES) {
test(`${name} produces a split MultiLineString`, () => {
const result = new GreatCircle(start, end).Arc(100, { offset: 10 }).json();
expect(result.geometry.type).toBe('MultiLineString');
assertSplitAtAntimeridian((result.geometry as MultiLineString).coordinates);
});
}
});

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describe('with npoints=10', () => {
for (const { name, start, end } of PACIFIC_ROUTES) {
test(`${name} splits correctly`, () => {
const result = new GreatCircle(start, end).Arc(10, { offset: 10 }).json();
expect(result.geometry.type).toBe('MultiLineString');
assertSplitAtAntimeridian((result.geometry as MultiLineString).coordinates);
});
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Copilot AI Mar 30, 2026

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As written, the npoints=10 tests will fail on the current GreatCircle.Arc implementation (it often returns an unsplit LineString for Tokyo→LAX and Shanghai→SFO because the split detector only triggers for longitude diffs > 360 - offset). If this PR is intended to be tests-only, mark the known-bad cases with test.failing/test.todo (or skip with a link to #75) to keep CI green; otherwise include the corresponding implementation fix in this PR.

Suggested change
test(`${name} splits correctly`, () => {
const result = new GreatCircle(start, end).Arc(10, { offset: 10 }).json();
expect(result.geometry.type).toBe('MultiLineString');
assertSplitAtAntimeridian((result.geometry as MultiLineString).coordinates);
});
if (name === 'Tokyo → LAX' || name === 'Shanghai → SFO') {
test.skip(`${name} splits correctly (known issue, see #75)`, () => {
const result = new GreatCircle(start, end).Arc(10, { offset: 10 }).json();
expect(result.geometry.type).toBe('MultiLineString');
assertSplitAtAntimeridian((result.geometry as MultiLineString).coordinates);
});
} else {
test(`${name} splits correctly`, () => {
const result = new GreatCircle(start, end).Arc(10, { offset: 10 }).json();
expect(result.geometry.type).toBe('MultiLineString');
assertSplitAtAntimeridian((result.geometry as MultiLineString).coordinates);
});
}

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}
});

describe('non-crossing routes are unaffected', () => {
test('Seattle → DC returns a LineString with no longitude jumps', () => {
const result = new GreatCircle({ x: -122, y: 48 }, { x: -77, y: 39 }).Arc(100, { offset: 10 }).json();
expect(result.geometry.type).toBe('LineString');

const coords = (result.geometry as LineString).coordinates;
for (let i = 1; i < coords.length; i++) {
const prev = coords[i - 1];
const curr = coords[i];
if (!prev || !curr) continue;
expect(Math.abs((curr[0] ?? 0) - (prev[0] ?? 0))).toBeLessThan(20);
}
});
});
});
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