fix(map): group collocated user markers

apps/web/src/lib/user-location-map.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { parseUserLocation, projectWorldPoint } from "./user-location-map";
+import { groupMapLocations, parseUserLocation, projectWorldPoint } from "./user-location-map";
 
 describe("user location map", () => {
   it("extracts a valid approximate location from cached IP intelligence", () => {
@@ -24,6 +24,28 @@ describe("user location map", () => {
     expect(parseUserLocation({ location: { city: "Unknown" } })).toBeNull();
   });
 
+  it("groups users sharing approximate coordinates without hiding their identities", () => {
+    const groups = groupMapLocations([
+      { userId: "one", nickname: "Dani (Steve)", latitude: 37.4056, longitude: -122.0775 },
+      { userId: "two", nickname: "Alex (AlexMC)", latitude: 37.4057, longitude: -122.0774 },
+      { userId: "three", nickname: "Sam (Notch)", latitude: 51.5, longitude: -0.12 },
+    ]);
+
+    expect(groups).toHaveLength(2);
+    expect(groups[0]).toMatchObject({ count: 2, nicknames: ["Alex (AlexMC)", "Dani (Steve)"] });
+    expect(groups[0]?.locations.map((location) => location.userId)).toEqual(["one", "two"]);
+    expect(groups[1]).toMatchObject({ count: 1, nicknames: ["Sam (Notch)"] });
+  });
+
+  it("normalizes signed zero and the antimeridian before grouping", () => {
+    const groups = groupMapLocations([
+      { nickname: "West", latitude: -0.004, longitude: 180 },
+      { nickname: "East", latitude: 0.004, longitude: -180 },
+    ]);
+    expect(groups).toHaveLength(1);
+    expect(groups[0]).toMatchObject({ count: 2, key: "0:-180", latitude: 0, longitude: -180 });
+  });
+
   it("projects longitude and latitude into an equirectangular SVG", () => {
     expect(projectWorldPoint(0, 0, 800, 400)).toEqual({ x: 400, y: 200 });
     expect(projectWorldPoint(90, 180, 800, 400)).toEqual({ x: 800, y: 0 });

apps/web/src/lib/user-location-map.ts

@@ -33,6 +33,32 @@ export function parseUserLocation(value: unknown): ParsedUserLocation | null {
   return { latitude, longitude, label: label || "Approximate location unavailable" };
 }
 
+export function groupMapLocations<T extends {
+  latitude: number;
+  longitude: number;
+  nickname: string;
+}>(locations: T[]) {
+  const grouped = new Map<string, { latitude: number; longitude: number; locations: T[] }>();
+  for (const location of locations) {
+    const roundedLatitude = Number(location.latitude.toFixed(2));
+    const latitude = roundedLatitude === 0 ? 0 : roundedLatitude;
+    const roundedLongitude = Number(location.longitude.toFixed(2));
+    const longitude = Math.abs(roundedLongitude) === 180 ? -180 : roundedLongitude;
+    const key = `${latitude}:${longitude}`;
+    const group = grouped.get(key);
+    if (group) group.locations.push(location);
+    else grouped.set(key, { latitude, longitude, locations: [location] });
+  }
+  return [...grouped.entries()].map(([key, group]) => ({
+    key,
+    latitude: group.latitude,
+    longitude: group.longitude,
+    count: group.locations.length,
+    nicknames: [...group.locations.map((location) => location.nickname)].sort((left, right) => left.localeCompare(right)),
+    locations: group.locations,
+  }));
+}
+
 export function projectWorldPoint(latitude: number, longitude: number, width: number, height: number) {
   return {
     x: ((longitude + 180) / 360) * width,