refactor(geo): define shared distance contract

src/geoDistance.js

@@ -0,0 +1,70 @@
+/**
+ * Canonical geographic point used by shared spatial utilities.
+ *
+ * @typedef {{lat: number, lon: number}} GeoPoint
+ */
+
+/** IUGG mean spherical Earth radius, in kilometres. */
+export const EARTH_RADIUS_KM = 6371.0088;
+
+const DEGREES_TO_RADIANS = Math.PI / 180;
+
+/**
+ * Whether a value is a canonical, finite WGS84-style point in degrees.
+ * Longitudes are not implicitly wrapped because an out-of-range value usually
+ * indicates a broken coordinate contract at the caller.
+ *
+ * @param {*} point
+ * @returns {point is GeoPoint}
+ */
+export function isGeoPoint(point) {
+  return (
+    Number.isFinite(point?.lat) &&
+    point.lat >= -90 &&
+    point.lat <= 90 &&
+    Number.isFinite(point?.lon) &&
+    point.lon >= -180 &&
+    point.lon <= 180
+  );
+}
+
+/**
+ * Great-circle distance in kilometres between two `{ lat, lon }` points.
+ *
+ * `{ lat, lon }` is the canonical shared coordinate representation. Invalid,
+ * missing, non-finite, or out-of-range coordinates return `Infinity`, which
+ * keeps invalid points out of threshold and nearest-neighbour selections.
+ *
+ * @param {GeoPoint} from
+ * @param {GeoPoint} to
+ * @returns {number}
+ */
+export function greatCircleKm(from, to) {
+  if (!isGeoPoint(from) || !isGeoPoint(to)) return Number.POSITIVE_INFINITY;
+
+  const latitude1 = from.lat * DEGREES_TO_RADIANS;
+  const latitude2 = to.lat * DEGREES_TO_RADIANS;
+  const latitudeDelta = (to.lat - from.lat) * DEGREES_TO_RADIANS;
+  const longitudeDelta = (to.lon - from.lon) * DEGREES_TO_RADIANS;
+  const haversine =
+    Math.sin(latitudeDelta / 2) ** 2 +
+    Math.cos(latitude1) *
+      Math.cos(latitude2) *
+      Math.sin(longitudeDelta / 2) ** 2;
+  const bounded = Math.min(1, Math.max(0, haversine));
+  return (
+    2 * EARTH_RADIUS_KM * Math.atan2(Math.sqrt(bounded), Math.sqrt(1 - bounded))
+  );
+}
+
+/**
+ * Great-circle distance in metres, with the same point and invalid-input
+ * contract as {@link greatCircleKm}.
+ *
+ * @param {GeoPoint} from
+ * @param {GeoPoint} to
+ * @returns {number}
+ */
+export function greatCircleMeters(from, to) {
+  return greatCircleKm(from, to) * 1000;
+}