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server/src/services/roverSnapshotService.js

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+const EventEmitter = require('events');
+const fs = require('fs/promises');
+const path = require('path');
+const logger = require('../globals/logger').child('roverSnapshot');
+const roverManager = require('./roverManager');
+
+const SNAPSHOT_DIR = process.env.ROVER_SNAPSHOT_DIR || '/run/rover-snapshots';
+const POLL_INTERVAL_MS = 500;
+
+const roverState = new Map(); // id -> { frame, ts, error, failures, fetching, mtimeMs }
+const events = new EventEmitter(); // frame, status
+let pollTimer = null;
+
+function markState(id, updates = {}) {
+  const prev = roverState.get(id) || {};
+  const next = { ...prev, ...updates };
+  roverState.set(id, next);
+  return next;
+}
+
+function getSnapshotPath(id) {
+  return path.join(SNAPSHOT_DIR, `${id}.jpg`);
+}
+
+async function fetchSnapshot(id) {
+  const state = roverState.get(id);
+  if (state?.fetching) return;
+  markState(id, { fetching: true });
+  try {
+    const filePath = getSnapshotPath(id);
+    const stats = await fs.stat(filePath);
+    if (state?.mtimeMs && stats.mtimeMs <= state.mtimeMs) {
+      return;
+    }
+    const buffer = await fs.readFile(filePath);
+    const ts = stats.mtimeMs || Date.now();
+    markState(id, { frame: buffer, ts, error: null, failures: 0, mtimeMs: stats.mtimeMs });
+    events.emit('frame', { id, buffer, ts });
+  } catch (err) {
+    const failures = (state?.failures || 0) + 1;
+    const message = err.code === 'ENOENT' ? 'Snapshot missing' : err.message;
+    markState(id, { error: message, failures });
+    events.emit('status', { id, error: message });
+    if (failures % 20 === 1) {
+      logger.warn('Snapshot read failed', { id, err: message });
+    }
+  } finally {
+    markState(id, { fetching: false });
+  }
+}
+
+function cleanupInactive(activeIds) {
+  roverState.forEach((_, id) => {
+    if (!activeIds.has(id)) {
+      roverState.delete(id);
+    }
+  });
+}
+
+function stopAll() {
+  if (pollTimer) {
+    clearInterval(pollTimer);
+    pollTimer = null;
+  }
+  roverState.clear();
+}
+
+function startAll() {
+  stopAll();
+  pollTimer = setInterval(() => {
+    const roster = roverManager.getRoster();
+    const activeIds = new Set(roster.map((entry) => String(entry.id)));
+    cleanupInactive(activeIds);
+    roster.forEach((entry) => fetchSnapshot(String(entry.id)));
+  }, POLL_INTERVAL_MS);
+  logger.info('Started rover snapshot polling');
+}
+
+function getState(id) {
+  const state = roverState.get(id);
+  if (!state) return null;
+  return {
+    frame: state.frame || null,
+    ts: state.ts || null,
+    error: state.error || null,
+  };
+}
+
+startAll();
+
+module.exports = {
+  roverSnapshotEvents: events,
+  getRoverSnapshotState: getState,
+};