refactor(geo): define shared distance contract

src/geoDistance.test.mjs

@@ -0,0 +1,78 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  EARTH_RADIUS_KM,
+  greatCircleKm,
+  greatCircleMeters,
+  isGeoPoint,
+} from './geoDistance.js';
+
+const AUSTIN = Object.freeze({ lat: 30.2672, lon: -97.7431 });
+const HOUSTON = Object.freeze({ lat: 29.7604, lon: -95.3698 });
+
+test('great-circle identity and symmetry', () => {
+  assert.equal(greatCircleKm(AUSTIN, AUSTIN), 0);
+  assert.equal(greatCircleKm(AUSTIN, HOUSTON), greatCircleKm(HOUSTON, AUSTIN));
+});
+
+test('great-circle one-degree calibration and canonical units', () => {
+  const kilometres = greatCircleKm({ lat: 0, lon: 0 }, { lat: 1, lon: 0 });
+  assert.ok(Math.abs(kilometres - 111.195) < 0.001);
+  assert.equal(
+    greatCircleMeters({ lat: 0, lon: 0 }, { lat: 1, lon: 0 }),
+    kilometres * 1000,
+  );
+});
+
+test('great-circle handles dateline crossing and antipodal points', () => {
+  const acrossDateline = greatCircleKm(
+    { lat: 0, lon: 179 },
+    { lat: 0, lon: -179 },
+  );
+  assert.ok(Math.abs(acrossDateline - 222.39) < 0.01);
+  assert.ok(
+    Math.abs(
+      greatCircleKm({ lat: 0, lon: 0 }, { lat: 0, lon: 180 }) -
+        Math.PI * EARTH_RADIUS_KM,
+    ) < 1e-9,
+  );
+});
+
+test('canonical points require explicit latitude and longitude keys', () => {
+  assert.equal(isGeoPoint({ lat: 10, lon: 80 }), true);
+  assert.equal(isGeoPoint({ latitude: 10, longitude: 80 }), false);
+  assert.equal(isGeoPoint([10, 80]), false);
+  assert.equal(
+    greatCircleKm(
+      { latitude: 10, longitude: 80 },
+      { latitude: 20, longitude: 70 },
+    ),
+    Infinity,
+  );
+});
+
+test('latitude-before-longitude regression rejects a swapped Austin point', () => {
+  assert.ok(Number.isFinite(greatCircleKm(AUSTIN, HOUSTON)));
+  assert.equal(
+    greatCircleKm(
+      { lat: AUSTIN.lon, lon: AUSTIN.lat },
+      { lat: HOUSTON.lon, lon: HOUSTON.lat },
+    ),
+    Infinity,
+  );
+});
+
+test('invalid, missing, non-finite, and out-of-range points return Infinity', () => {
+  for (const point of [
+    null,
+    {},
+    { lat: 0 },
+    { lat: NaN, lon: 0 },
+    { lat: 0, lon: Infinity },
+    { lat: 90.0001, lon: 0 },
+    { lat: 0, lon: -180.0001 },
+  ]) {
+    assert.equal(greatCircleKm(point, AUSTIN), Infinity);
+    assert.equal(greatCircleMeters(AUSTIN, point), Infinity);
+  }
+});