const { v4: uuidv4 } = require('uuid');
const crypto = require('crypto');
const path = require('path');
const fs = require('fs');
const { db, pruneTelemetry, pruneScreenshots } = require('../db/database');
const config = require('../config');
const heartbeat = require('../services/heartbeat');
const { getUserPlan, getUserDeviceCount } = require('../middleware/subscription');

// In-memory store for latest screenshot per device (avoids disk writes during streaming)
let lastScreenshots = {};

// Generate a random device token
function generateDeviceToken() {
  return crypto.randomBytes(32).toString('hex');
}

// Validate device_id + device_token pair. Returns true if valid.
function validateDeviceToken(deviceId, token) {
  if (!deviceId || !token) return false;
  const row = db.prepare('SELECT device_token FROM devices WHERE id = ?').get(deviceId);
  if (!row || !row.device_token) return false;
  // Constant-time comparison to prevent timing attacks
  try {
    return crypto.timingSafeEqual(Buffer.from(row.device_token), Buffer.from(token));
  } catch {
    return false;
  }
}

function getClientIp(socket) {
  const forwarded = socket.handshake.headers['x-forwarded-for'];
  if (forwarded) return forwarded.split(',')[0].trim();
  return socket.handshake.address;
}

function logDeviceStatus(deviceId, status) {
  try {
    db.prepare('INSERT INTO device_status_log (device_id, status) VALUES (?, ?)').run(deviceId, status);
    // Prune entries older than 7 days
    db.prepare("DELETE FROM device_status_log WHERE device_id = ? AND timestamp < strftime('%s','now') - 604800").run(deviceId);
  } catch (e) { /* table might not exist yet */ }
}


// Build playlist payload with layout and zones
// Reads from published_snapshot (Phase 3) so draft edits don't affect live devices
function buildPlaylistPayload(deviceId) {
  const device = db.prepare('SELECT playlist_id, layout_id, orientation, wall_id FROM devices WHERE id = ?').get(deviceId);

  let assignments = [];
  if (device?.playlist_id) {
    const playlist = db.prepare('SELECT published_snapshot FROM playlists WHERE id = ?').get(device.playlist_id);
    if (playlist?.published_snapshot) {
      try { assignments = JSON.parse(playlist.published_snapshot); } catch (e) { assignments = []; }
    }
  }

  let layout = null;
  if (device?.layout_id) {
    layout = db.prepare('SELECT * FROM layouts WHERE id = ?').get(device.layout_id);
    if (layout) {
      layout.zones = db.prepare('SELECT * FROM layout_zones WHERE layout_id = ? ORDER BY sort_order').all(layout.id);
    }
  }

  // Wall membership flips the player into wall mode. The renderer needs two
  // rectangles in canvas-space: this device's screen rect, and the wall's
  // player rect. The intersection is what this screen displays. The leader
  // drives playback; followers track via wall:sync.
  let wall_config = null;
  if (device?.wall_id) {
    const wall = db.prepare('SELECT * FROM video_walls WHERE id = ?').get(device.wall_id);
    const pos = db.prepare('SELECT * FROM video_wall_devices WHERE wall_id = ? AND device_id = ?').get(device.wall_id, deviceId);
    if (wall && pos) {
      const baseW = 320, baseH = 180;
      const bezelH = wall.bezel_h_mm || 0;
      const bezelV = wall.bezel_v_mm || 0;

      // Backfill canvas rect from grid math when canvas_* is unset (legacy
      // walls that haven't been touched by the new editor yet). Coords are
      // rounded to integers so sub-pixel drift can't cause two visually
      // identical rects to compute different stage offsets.
      const screenRect = {
        x: Math.round(pos.canvas_x ?? (pos.grid_col * (baseW + bezelH))),
        y: Math.round(pos.canvas_y ?? (pos.grid_row * (baseH + bezelV))),
        w: Math.round(pos.canvas_width ?? baseW),
        h: Math.round(pos.canvas_height ?? baseH),
      };

      // Player rect defaults to the bounding box of all screens on the wall.
      let playerRect;
      if (wall.player_x !== null && wall.player_x !== undefined) {
        playerRect = { x: wall.player_x, y: wall.player_y, w: wall.player_width, h: wall.player_height };
      } else {
        const all = db.prepare('SELECT * FROM video_wall_devices WHERE wall_id = ?').all(wall.id);
        let x = Infinity, y = Infinity, x2 = -Infinity, y2 = -Infinity;
        for (const p of all) {
          const px = p.canvas_x ?? (p.grid_col * (baseW + bezelH));
          const py = p.canvas_y ?? (p.grid_row * (baseH + bezelV));
          const pw = p.canvas_width ?? baseW;
          const ph = p.canvas_height ?? baseH;
          if (px < x) x = px;
          if (py < y) y = py;
          if (px + pw > x2) x2 = px + pw;
          if (py + ph > y2) y2 = py + ph;
        }
        playerRect = isFinite(x)
          ? { x, y, w: x2 - x, h: y2 - y }
          : { x: 0, y: 0, w: baseW, h: baseH };
      }
      // Round the player rect too — same rationale.
      playerRect = {
        x: Math.round(playerRect.x), y: Math.round(playerRect.y),
        w: Math.round(playerRect.w), h: Math.round(playerRect.h),
      };

      wall_config = {
        wall_id: wall.id,
        screen_rect: screenRect,
        player_rect: playerRect,
        is_leader: wall.leader_device_id === deviceId,
        rotation: pos.rotation || 0,
      };
    }
  }

  return { assignments, layout, orientation: device?.orientation || 'landscape', wall_config };
}

// Check if a device should show trial expired screen
function checkDeviceAccess(deviceId) {
  const device = db.prepare('SELECT * FROM devices WHERE id = ?').get(deviceId);
  if (!device || !device.user_id) return { allowed: true };

  const plan = getUserPlan(device.user_id);
  if (!plan) return { allowed: true };

  // Check if trial expired and over free limit
  if (plan.trial_started && !plan.trial_active && plan.plan_name === 'free') {
    const deviceCount = getUserDeviceCount(device.user_id);
    // Get this device's position (ordered by created_at)
    const userDevices = db.prepare('SELECT id FROM devices WHERE user_id = ? ORDER BY created_at ASC').all(device.user_id);
    const deviceIndex = userDevices.findIndex(d => d.id === deviceId);

    // Only the first device (within free limit) is allowed
    if (deviceIndex >= plan.max_devices) {
      return {
        allowed: false,
        reason: 'trial_expired',
        message: 'Trial Expired',
        detail: 'Upgrade your plan to continue using this display.',
      };
    }
  }

  // Check if over plan device limit (non-trial)
  if (!plan.trial_started && plan.max_devices > 0) {
    const userDevices = db.prepare('SELECT id FROM devices WHERE user_id = ? ORDER BY created_at ASC').all(device.user_id);
    const deviceIndex = userDevices.findIndex(d => d.id === deviceId);
    if (deviceIndex >= plan.max_devices) {
      return {
        allowed: false,
        reason: 'device_limit',
        message: 'Device Limit Reached',
        detail: 'Upgrade your plan to activate this display.',
      };
    }
  }

  return { allowed: true };
}

module.exports = function setupDeviceSocket(io) {
  // Expose helpers for use by route handlers
  module.exports.lastScreenshots = lastScreenshots;
  module.exports.buildPlaylistPayload = buildPlaylistPayload;
  module.exports.generateDeviceToken = generateDeviceToken;
  const deviceNs = io.of('/device');
  const dashboardNs = io.of('/dashboard');

  // Disconnect any existing socket that is currently registered for this device_id.
  // Called when a fresh registration comes in for the same device so the old (likely
  // half-dead) socket can't fire its disconnect handler and clobber the new entry.
  function evictPriorSocket(deviceId, exceptSocketId) {
    const prior = heartbeat.getConnection(deviceId);
    if (!prior || prior.socketId === exceptSocketId) return;
    const oldSocket = deviceNs.sockets.get(prior.socketId);
    if (oldSocket) {
      console.log(`Evicting prior socket ${prior.socketId} for device ${deviceId}`);
      try { oldSocket.disconnect(true); } catch (_) {}
    }
  }

  deviceNs.on('connection', (socket) => {
    console.log(`Device socket connected: ${socket.id}`);
    let currentDeviceId = null;
    let authenticated = false; // Track whether this socket has been authenticated

    // Device registers with a pairing code (first time) or device_id + device_token (reconnect)
    socket.on('device:register', (data) => {
      const { pairing_code, device_id, device_token, device_info, fingerprint } = data;

      // Track device fingerprint to prevent reinstall abuse
      if (fingerprint) {
        try {
          const existing = db.prepare('SELECT * FROM device_fingerprints WHERE fingerprint = ?').get(fingerprint);
          if (existing) {
            db.prepare("UPDATE device_fingerprints SET last_seen = strftime('%s','now'), device_id = ? WHERE fingerprint = ?")
              .run(device_id || existing.device_id, fingerprint);
            // If this fingerprint was previously registered to a different device, block the new registration
            if (!device_id && existing.device_id && pairing_code) {
              // Someone reinstalled - link them back to existing device
              const oldDevice = db.prepare('SELECT * FROM devices WHERE id = ?').get(existing.device_id);
              if (oldDevice) {
                // Fingerprint reclaim guard: a leaked/duplicated fingerprint shouldn't be enough
                // to take over a live device. Reject the reclaim if the device is currently
                // online OR has been online within the last 24h — by then a real reinstall has
                // had plenty of time to come back, but a credential thief is more likely caught.
                const liveConn = heartbeat.getConnection(existing.device_id);
                const RECLAIM_GRACE_SECONDS = 24 * 60 * 60;
                const lastBeat = oldDevice.last_heartbeat || 0;
                const secondsSince = Math.floor(Date.now() / 1000) - lastBeat;
                if (liveConn || (oldDevice.status === 'online') || secondsSince < RECLAIM_GRACE_SECONDS) {
                  console.warn(`Fingerprint reclaim rejected for ${existing.device_id}: device active (status=${oldDevice.status}, ${secondsSince}s since last heartbeat, liveConn=${!!liveConn})`);
                  socket.emit('device:auth-error', {
                    error: 'This display is currently active. If you reinstalled the app, the original device must be offline for 24 hours before its slot can be reclaimed.'
                  });
                  return;
                }

                // Fingerprint matched — this is a reinstalled app reconnecting to its old device.
                // Issue a fresh token so the app can authenticate going forward.
                const newToken = generateDeviceToken();
                db.prepare('UPDATE devices SET device_token = ? WHERE id = ?').run(newToken, existing.device_id);
                console.log(`Fingerprint match: linking reinstalled app to existing device ${existing.device_id} (new token issued)`);
                authenticated = true;
                evictPriorSocket(existing.device_id, socket.id);
                db.prepare("UPDATE devices SET status = 'online', last_heartbeat = strftime('%s','now'), ip_address = ?, updated_at = strftime('%s','now') WHERE id = ?")
                  .run(getClientIp(socket), existing.device_id);
                socket.emit('device:registered', { device_id: existing.device_id, device_token: newToken, status: 'online' });
                // If device was already claimed by a user, tell the player it's paired
                if (oldDevice.user_id) {
                  socket.emit('device:paired', { name: oldDevice.name || 'Display' });
                }
                currentDeviceId = existing.device_id;
                heartbeat.registerConnection(existing.device_id, socket.id);
                socket.join(existing.device_id);
                logDeviceStatus(existing.device_id, 'online');
                dashboardNs.emit('dashboard:device-status', { device_id: existing.device_id, status: 'online' });
                // Send playlist
                const access = checkDeviceAccess(existing.device_id);
                if (!access.allowed) {
                  socket.emit('device:playlist-update', { assignments: [], suspended: true, message: access.message, detail: access.detail });
                } else {
                  socket.emit('device:playlist-update', buildPlaylistPayload(existing.device_id));
                }
                return;
              }
            }
          } else if (device_id || pairing_code) {
            db.prepare("INSERT OR IGNORE INTO device_fingerprints (fingerprint, device_id) VALUES (?, ?)")
              .run(fingerprint, device_id || null);
          }
        } catch (e) {
          console.error('Fingerprint tracking error:', e.message);
        }
      }

      if (device_id) {
        // Reconnecting known device — require valid token
        const device = db.prepare('SELECT * FROM devices WHERE id = ?').get(device_id);
        if (device) {
          // Validate device token (skip for legacy devices that don't have a token yet)
          if (device.device_token && !validateDeviceToken(device_id, device_token)) {
            console.warn(`Invalid device token for ${device_id} from ${getClientIp(socket)} — received_len=${(device_token || '').length}, stored_len=${device.device_token.length}, received_prefix=${(device_token || '').substring(0, 8)}, stored_prefix=${device.device_token.substring(0, 8)}`);
            socket.emit('device:auth-error', { error: 'Invalid device token' });
            return;
          }

          currentDeviceId = device_id;
          authenticated = true;
          evictPriorSocket(device_id, socket.id);
          db.prepare("UPDATE devices SET status = 'online', last_heartbeat = strftime('%s','now'), ip_address = ?, updated_at = strftime('%s','now') WHERE id = ?")
            .run(getClientIp(socket), device_id);

          // Generate token for legacy devices that don't have one yet
          let tokenToSend = device.device_token;
          if (!tokenToSend) {
            tokenToSend = generateDeviceToken();
            db.prepare('UPDATE devices SET device_token = ? WHERE id = ?').run(tokenToSend, device_id);
          }

          if (device_info) {
            db.prepare('UPDATE devices SET android_version = ?, app_version = ?, screen_width = ?, screen_height = ? WHERE id = ?')
              .run(device_info.android_version, device_info.app_version, device_info.screen_width, device_info.screen_height, device_id);
          }

          heartbeat.registerConnection(device_id, socket.id);
          socket.join(device_id);
          socket.emit('device:registered', { device_id, device_token: tokenToSend, status: 'online' });
          logDeviceStatus(device_id, 'online');

          // If this device is part of a wall, re-evaluate leadership.
          // Preferred leader = online member with smallest (canvas_x +
          // canvas_y), falling back to grid 0,0. If the original leader
          // (top-left tile) is back, they reclaim the role and peers re-sync.
          if (device.wall_id) {
            try {
              const wall = db.prepare('SELECT * FROM video_walls WHERE id = ?').get(device.wall_id);
              if (wall) {
                const candidates = db.prepare(`
                  SELECT vwd.device_id, vwd.canvas_x, vwd.canvas_y, vwd.grid_col, vwd.grid_row
                  FROM video_wall_devices vwd
                  JOIN devices d ON d.id = vwd.device_id
                  WHERE vwd.wall_id = ? AND d.status = 'online'
                `).all(wall.id);
                if (candidates.length > 0) {
                  const score = (c) => (c.canvas_x ?? c.grid_col * 320) + (c.canvas_y ?? c.grid_row * 180);
                  candidates.sort((a, b) => score(a) - score(b));
                  const preferredLeader = candidates[0].device_id;
                  if (wall.leader_device_id !== preferredLeader) {
                    db.prepare('UPDATE video_walls SET leader_device_id = ? WHERE id = ?').run(preferredLeader, wall.id);
                    console.log(`Wall ${wall.id} leader reassigned to ${preferredLeader} on reconnect`);
                    // Re-push payload to every member so role flags refresh.
                    const members = db.prepare('SELECT device_id FROM video_wall_devices WHERE wall_id = ?').all(wall.id);
                    for (const m of members) {
                      if (m.device_id !== device_id) {
                        deviceNs.to(m.device_id).emit('device:playlist-update', buildPlaylistPayload(m.device_id));
                      }
                    }
                  }
                }
              }
            } catch (e) { console.error('Wall leader reclaim failed:', e.message); }
          }

          // Check subscription/trial status before sending playlist
          const access = checkDeviceAccess(device_id);
          if (!access.allowed) {
            socket.emit('device:playlist-update', { assignments: [], suspended: true, message: access.message, detail: access.detail });
          } else {
            socket.emit('device:playlist-update', buildPlaylistPayload(device_id));
          }

          dashboardNs.emit('dashboard:device-status', { device_id, status: 'online' });
          console.log(`Device reconnected: ${device_id}`);
          return;
        }

        // Device ID not found in database - tell device to re-provision
        console.log(`Device ${device_id} not found in database, sending unpaired`);
        socket.emit('device:unpaired', { reason: 'not_found' });
        return;
      }

      if (pairing_code) {
        // New device registering with pairing code — generate a device_token
        const id = uuidv4();
        const newToken = generateDeviceToken();
        currentDeviceId = id;
        authenticated = true;

        db.prepare(`
          INSERT INTO devices (id, pairing_code, device_token, status, ip_address, android_version, app_version, screen_width, screen_height, last_heartbeat)
          VALUES (?, ?, ?, 'provisioning', ?, ?, ?, ?, ?, strftime('%s','now'))
        `).run(
          id, pairing_code, newToken, getClientIp(socket),
          device_info?.android_version || null,
          device_info?.app_version || null,
          device_info?.screen_width || null,
          device_info?.screen_height || null
        );

        heartbeat.registerConnection(id, socket.id);
        socket.join(id);
        socket.emit('device:registered', { device_id: id, device_token: newToken, status: 'provisioning' });

        dashboardNs.emit('dashboard:device-added', db.prepare('SELECT * FROM devices WHERE id = ?').get(id));
        console.log(`New device registered: ${id} with pairing code: ${pairing_code}`);
      }
    });

    // Require authentication for all events after register
    function requireDeviceAuth() {
      if (!authenticated || !currentDeviceId) {
        socket.emit('device:auth-error', { error: 'Not authenticated. Send device:register first.' });
        return false;
      }
      return true;
    }

    // Heartbeat with telemetry
    socket.on('device:heartbeat', (data) => {
      if (!requireDeviceAuth()) return;
      const { device_id, telemetry } = data;
      if (!device_id || device_id !== currentDeviceId) return;

      currentDeviceId = device_id;
      heartbeat.updateHeartbeat(device_id);

      db.prepare("UPDATE devices SET status = 'online', last_heartbeat = strftime('%s','now'), updated_at = strftime('%s','now') WHERE id = ?")
        .run(device_id);

      if (telemetry) {
        db.prepare(`
          INSERT INTO device_telemetry (device_id, battery_level, battery_charging, storage_free_mb, storage_total_mb,
            ram_free_mb, ram_total_mb, cpu_usage, wifi_ssid, wifi_rssi, uptime_seconds)
          VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
        `).run(
          device_id,
          telemetry.battery_level ?? null,
          telemetry.battery_charging ? 1 : 0,
          telemetry.storage_free_mb ?? null,
          telemetry.storage_total_mb ?? null,
          telemetry.ram_free_mb ?? null,
          telemetry.ram_total_mb ?? null,
          telemetry.cpu_usage ?? null,
          telemetry.wifi_ssid ?? null,
          telemetry.wifi_rssi ?? null,
          telemetry.uptime_seconds ?? null
        );
        pruneTelemetry(device_id);

        dashboardNs.emit('dashboard:device-status', {
          device_id,
          status: 'online',
          telemetry
        });
      }
    });

    // Screenshot received from device - relay via WebSocket, keep latest in memory
    socket.on('device:screenshot', (data) => {
      if (!requireDeviceAuth()) return;
      const { device_id, image_b64 } = data;
      if (!device_id || device_id !== currentDeviceId || !image_b64) return;
      // Validate screenshot size (max 2MB base64 ≈ 1.5MB image)
      if (image_b64.length > 2 * 1024 * 1024) return;

      // Store latest screenshot in memory (for Now Playing preview and offline snapshot)
      if (!lastScreenshots) lastScreenshots = {};
      lastScreenshots[device_id] = image_b64;

      // Relay directly to dashboard - no disk write
      try {
        dashboardNs.emit('dashboard:screenshot-ready', {
          device_id,
          image_data: `data:image/jpeg;base64,${image_b64}`,
          timestamp: Date.now()
        });
      } catch (err) {
        console.error('Screenshot save error:', err);
      }
    });

    // Content download acknowledgement
    socket.on('device:content-ack', (data) => {
      if (!requireDeviceAuth()) return;
      const { device_id, content_id, status } = data;
      if (device_id !== currentDeviceId) return;
      console.log(`Device ${device_id} content ${content_id}: ${status}`);
      dashboardNs.emit('dashboard:content-ack', { device_id, content_id, status });
    });

    // Playback state update
    socket.on('device:playback-state', (data) => {
      if (!requireDeviceAuth()) return;
      dashboardNs.emit('dashboard:playback-state', data);
    });

    // Play event logging (proof-of-play)
    socket.on('device:play-event', (data) => {
      if (!requireDeviceAuth()) return;
      const { device_id, event, content_id, content_name, zone_id, completed, duration_sec } = data;
      if (device_id !== currentDeviceId) return;
      try {
        if (event === 'play_start') {
          db.prepare(`
            INSERT INTO play_logs (device_id, content_id, zone_id, content_name, started_at, trigger_type)
            VALUES (?, ?, ?, ?, strftime('%s','now'), 'playlist')
          `).run(device_id, content_id || null, zone_id || null, content_name || 'Unknown');
          // Forward to dashboard so it can render a per-device progress bar.
          // Server-side timestamp avoids clock-skew between player and dashboard.
          dashboardNs.emit('dashboard:playback-progress', {
            device_id,
            content_id: content_id || null,
            content_name: content_name || null,
            duration_sec: typeof duration_sec === 'number' && duration_sec > 0 ? duration_sec : null,
            started_at: Date.now(),
          });
        } else if (event === 'play_end') {
          db.prepare(`
            UPDATE play_logs SET ended_at = strftime('%s','now'),
              duration_sec = strftime('%s','now') - started_at,
              completed = ?
            WHERE id = (
              SELECT id FROM play_logs WHERE device_id = ? AND content_id = ? AND ended_at IS NULL
              ORDER BY started_at DESC LIMIT 1
            )
          `).run(completed ? 1 : 0, device_id, content_id);
        }
      } catch (err) {
        console.error('Play log error:', err.message);
      }
    });

    // Video wall sync relay. Sender must be a member of the wall it claims —
    // otherwise an authenticated device could inject sync packets into a wall
    // it doesn't belong to (jitter/DoS that wall's playback). Exclusion uses
    // currentDeviceId, never the client-supplied data.device_id.
    socket.on('wall:sync', (data) => {
      if (!requireDeviceAuth()) return;
      if (!data?.wall_id) return;
      const isMember = db.prepare(
        'SELECT 1 FROM video_wall_devices WHERE wall_id = ? AND device_id = ?'
      ).get(data.wall_id, currentDeviceId);
      if (!isMember) return;
      const wallDevices = db.prepare(
        'SELECT device_id FROM video_wall_devices WHERE wall_id = ? AND device_id != ?'
      ).all(data.wall_id, currentDeviceId);
      // Stamp device_id with the authenticated id so followers can trust it.
      const payload = { ...data, device_id: currentDeviceId };
      for (const wd of wallDevices) {
        deviceNs.to(wd.device_id).emit('wall:sync', payload);
      }
    });

    // A follower asks for an immediate position update from the leader.
    // Used on (re)connect so the follower doesn't drift for ~1s waiting on
    // the next periodic wall:sync tick. Server forwards only to the leader,
    // and only when the requester is actually a member of the named wall.
    socket.on('wall:sync-request', (data) => {
      if (!requireDeviceAuth()) return;
      if (!data?.wall_id) return;
      const isMember = db.prepare(
        'SELECT 1 FROM video_wall_devices WHERE wall_id = ? AND device_id = ?'
      ).get(data.wall_id, currentDeviceId);
      if (!isMember) return;
      const wall = db.prepare('SELECT leader_device_id FROM video_walls WHERE id = ?').get(data.wall_id);
      if (!wall?.leader_device_id || wall.leader_device_id === currentDeviceId) return;
      deviceNs.to(wall.leader_device_id).emit('wall:sync-request', {
        wall_id: data.wall_id,
        requested_by: currentDeviceId,
      });
    });

    socket.on('disconnect', () => {
      if (currentDeviceId) {
        // If a newer socket has already taken over this device_id, this is a stale
        // disconnect from a replaced socket — skip the offline transition so we don't
        // flip an actively-connected device offline or clobber the new heartbeat entry.
        const activeConn = heartbeat.getConnection(currentDeviceId);
        if (activeConn && activeConn.socketId !== socket.id) {
          console.log(`Stale disconnect for ${currentDeviceId} (socket ${socket.id}); active is ${activeConn.socketId}, skipping offline`);
          return;
        }

        console.log(`Device disconnected: ${currentDeviceId}`);
        db.prepare("UPDATE devices SET status = 'offline', updated_at = strftime('%s','now') WHERE id = ?")
          .run(currentDeviceId);
        heartbeat.removeConnection(currentDeviceId);
        logDeviceStatus(currentDeviceId, 'offline');
        dashboardNs.emit('dashboard:device-status', { device_id: currentDeviceId, status: 'offline' });

        // If this device was leading a wall, reassign leadership to the next
        // online member so playback stays driven. Without this the wall freezes
        // when the leader drops.
        try {
          const wall = db.prepare('SELECT id FROM video_walls WHERE leader_device_id = ?').get(currentDeviceId);
          if (wall) {
            const candidates = db.prepare(`
              SELECT vwd.device_id FROM video_wall_devices vwd
              JOIN devices d ON d.id = vwd.device_id
              WHERE vwd.wall_id = ? AND d.status = 'online' AND vwd.device_id != ?
              ORDER BY vwd.grid_row, vwd.grid_col LIMIT 1
            `).all(wall.id, currentDeviceId);
            const newLeader = candidates[0]?.device_id || null;
            db.prepare('UPDATE video_walls SET leader_device_id = ? WHERE id = ?').run(newLeader, wall.id);
            // Notify the new leader (and refresh peers' is_leader flags).
            const members = db.prepare('SELECT device_id FROM video_wall_devices WHERE wall_id = ?').all(wall.id);
            for (const m of members) {
              if (m.device_id !== currentDeviceId) {
                deviceNs.to(m.device_id).emit('device:playlist-update', buildPlaylistPayload(m.device_id));
              }
            }
          }
        } catch (e) { console.error('Wall leader reassign failed:', e.message); }

        // Save last screenshot to disk as offline snapshot
        const lastB64 = lastScreenshots[currentDeviceId];
        if (lastB64) {
          try {
            const filename = `${currentDeviceId}_latest.jpg`;
            const buffer = Buffer.from(lastB64, 'base64');
            fs.writeFileSync(path.join(config.screenshotsDir, filename), buffer);
            // Upsert screenshot record
            const existing = db.prepare('SELECT id FROM screenshots WHERE device_id = ?').get(currentDeviceId);
            if (existing) {
              db.prepare('UPDATE screenshots SET filepath = ?, captured_at = strftime(\'%s\',\'now\') WHERE device_id = ?')
                .run(filename, currentDeviceId);
            } else {
              db.prepare('INSERT INTO screenshots (device_id, filepath) VALUES (?, ?)').run(currentDeviceId, filename);
            }
          } catch (e) {
            console.error('Failed to save offline screenshot:', e.message);
          }
          delete lastScreenshots[currentDeviceId];
        }
      }
    });
  });

  return deviceNs;
};