fix(geo): wrap longitude differences across the antimeridian in point distances (#1304)

Wrap longitude differences in annotation proximity checks and nearby-place distances so points across the antimeridian remain close.

Original contribution by quanticsoul4772.

Co-authored-by: quanticsoul4772 <[email protected]>

src/sources/featureGeometry.js

@@ -250,10 +250,13 @@ export function approximateAreaM2(coords) {
   return Math.abs(area) / 2;
 }
 
+/** Local equirectangular distance in metres, the short way round 180°. */
 export function approximateDistanceM(latA, lonA, latB, lonB) {
   const latScale = 111_320;
   const lonScale = latScale * Math.cos(((latA + latB) / 2) * (Math.PI / 180));
-  return Math.hypot((latB - latA) * latScale, (lonB - lonA) * lonScale);
+  let dLon = lonB - lonA;
+  dLon -= 360 * Math.round(dLon / 360);
+  return Math.hypot((latB - latA) * latScale, dLon * lonScale);
 }
 
 export function pointInPolygon(lon, lat, coords) {

src/sources/featureGeometry.test.mjs

@@ -0,0 +1,15 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { approximateDistanceM } from './featureGeometry.js';
+
+test('approximate distance takes the short way round 180°', () => {
+  // Lau Islands (-179.0) to Savusavu (179.33): about 178 km apart.
+  const across = approximateDistanceM(-17.5, -179.0, -16.78, 179.33);
+  assert.ok(across > 150_000 && across < 200_000, String(across));
+  assert.equal(
+    Math.round(approximateDistanceM(0, 179.9, 0, -179.9)),
+    Math.round(approximateDistanceM(0, -0.1, 0, 0.1)),
+  );
+  // Away from 180° nothing changes.
+  assert.equal(Math.round(approximateDistanceM(0, 0, 0, 1)), 111_320);
+});