LearnSpace: full-stack educational whiteboard platform

Node.js/Express backend + vanilla JS frontend.
Features: real-time collaborative whiteboard (SSE), multi-page support,
LaTeX formulas, shapes/connectors, coordinate systems, number lines,
compass, zoom/pan, Catmull-Rom pencil smoothing, ruler/protractor with
rotation & resize controls, minimap navigation overlay, auto-measurements,
multi-page thumbnails sidebar, PNG export, page templates.
Student/teacher workflows: classes, assignments, library, dashboard.
Mobile responsive. SQLite (better-sqlite3).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/sse.js

@@ -0,0 +1,50 @@
+/* ── SSE registry — shared between controllers ─────────────────────────── */
+const clients = new Map(); // userId -> Set<res>
+const db = require('./db/db');
+
+function addClient(userId, res) {
+  if (!clients.has(userId)) clients.set(userId, new Set());
+  clients.get(userId).add(res);
+}
+
+function removeClient(userId, res) {
+  const set = clients.get(userId);
+  if (!set) return;
+  set.delete(res);
+  if (set.size === 0) clients.delete(userId);
+}
+
+function emit(userId, data) {
+  const conns = clients.get(userId);
+  if (!conns?.size) return;
+  const payload = `data: ${JSON.stringify(data)}\n\n`;
+  for (const res of conns) {
+    try { res.write(payload); } catch {}
+  }
+}
+
+// Heartbeat: detect and remove dead connections every 30s
+setInterval(() => {
+  for (const [userId, conns] of clients) {
+    for (const res of conns) {
+      try {
+        if (res.writableEnded || res.destroyed) { conns.delete(res); continue; }
+        res.write(': heartbeat\n\n');
+      } catch { conns.delete(res); }
+    }
+    if (conns.size === 0) clients.delete(userId);
+  }
+}, 30_000).unref();
+
+/* Broadcast to all members of a class */
+function emitToClass(classId, data) {
+  const members = db.prepare('SELECT user_id FROM class_members WHERE class_id=?').all(classId);
+  for (const { user_id } of members) emit(user_id, data);
+}
+
+/* Returns array of user IDs currently connected via SSE */
+function getOnlineUserIds() {
+  return [...clients.keys()];
+}
+
+module.exports = { addClient, removeClient, emit, emitToClass, getOnlineUserIds };