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 || '/var/lib/rover-snapshots';
const POLL_INTERVAL_MS = 300;

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,
};