lets see . . .

server/src/services/replayBuildService.js

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+const { execFile } = require('child_process');
+const fsp = require('fs/promises');
+const os = require('os');
+const path = require('path');
+const { promisify } = require('util');
+const logger = require('../globals/logger').child('replayBuild');
+const { replaySegmentsDir, segmentSeconds } = require('./replaySegmentManager');
+
+const execFileAsync = promisify(execFile);
+
+const REPLAY_DURATION_MS = 20000;
+const REPLAY_FPS = 15;
+const MAX_REPLAY_WIDTH = 1280;
+const MAX_REPLAY_HEIGHT = 720;
+const REPLAY_MAX_BYTES = Math.floor(9.5 * 1024 * 1024);
+
+function buildGridLayout(count) {
+  const cols = Math.ceil(Math.sqrt(count));
+  const rows = Math.ceil(count / cols);
+  return { cols, rows };
+}
+
+function clampEven(value) {
+  return Math.max(2, Math.floor(value / 2) * 2);
+}
+
+function buildScalePadFilter(tileWidth, tileHeight) {
+  return `scale=${tileWidth}:${tileHeight}:force_original_aspect_ratio=decrease:flags=lanczos,pad=${tileWidth}:${tileHeight}:(ow-iw)/2:(oh-ih)/2:color=black,setsar=1`;
+}
+
+async function probeMaxFrameSize(paths) {
+  let maxWidth = 0;
+  let maxHeight = 0;
+  for (const filePath of paths) {
+    try {
+      const { stdout } = await execFileAsync('ffprobe', [
+        '-v',
+        'error',
+        '-select_streams',
+        'v:0',
+        '-show_entries',
+        'stream=width,height',
+        '-of',
+        'csv=p=0',
+        filePath,
+      ]);
+      const [widthRaw, heightRaw] = stdout.trim().split(',');
+      const width = Number(widthRaw);
+      const height = Number(heightRaw);
+      if (Number.isFinite(width) && Number.isFinite(height)) {
+        maxWidth = Math.max(maxWidth, width);
+        maxHeight = Math.max(maxHeight, height);
+      }
+    } catch (err) {
+      logger.warn('Failed to probe replay clip size', err.message);
+    }
+  }
+  return { maxWidth, maxHeight };
+}
+
+async function listLatestSegments(sourceKey, neededCount) {
+  const dir = path.join(replaySegmentsDir, sourceKey);
+  const entries = await fsp.readdir(dir, { withFileTypes: true });
+  const files = [];
+  for (const entry of entries) {
+    if (!entry.isFile() || !entry.name.endsWith('.mp4')) continue;
+    const filePath = path.join(dir, entry.name);
+    const stat = await fsp.stat(filePath);
+    files.push({ filePath, mtimeMs: stat.mtimeMs });
+  }
+  files.sort((a, b) => a.mtimeMs - b.mtimeMs);
+  return files.slice(-neededCount).map((file) => file.filePath);
+}
+
+async function buildReplayVideo({ sources = [] } = {}) {
+  if (!sources.length) {
+    throw new Error('No replay sources selected');
+  }
+  const segmentCount = Math.max(1, Math.ceil(REPLAY_DURATION_MS / (segmentSeconds * 1000)));
+  const tmpDir = await fsp.mkdtemp(path.join(os.tmpdir(), 'rover-replay-'));
+  try {
+    const clipPaths = [];
+    for (let i = 0; i < sources.length; i += 1) {
+      const source = sources[i];
+      const key = `${source.type}__${source.id}`;
+      let segmentPaths;
+      try {
+        segmentPaths = await listLatestSegments(key, segmentCount);
+      } catch {
+        throw new Error(`Missing replay segments for ${source.type}:${source.id}`);
+      }
+      if (segmentPaths.length < segmentCount) {
+        throw new Error(`Not enough replay segments for ${source.type}:${source.id}`);
+      }
+      const concatPath = path.join(tmpDir, `concat-${i}.txt`);
+      const concatBody = segmentPaths.map((file) => `file '${file}'`).join('\n');
+      await fsp.writeFile(concatPath, concatBody);
+      const clipPath = path.join(tmpDir, `clip-${i}.mp4`);
+      await execFileAsync('ffmpeg', [
+        '-hide_banner',
+        '-loglevel',
+        'error',
+        '-f',
+        'concat',
+        '-safe',
+        '0',
+        '-i',
+        concatPath,
+        '-c',
+        'copy',
+        clipPath,
+      ]);
+      clipPaths.push(clipPath);
+    }
+
+    const { maxWidth, maxHeight } = await probeMaxFrameSize(clipPaths);
+    const layout = buildGridLayout(clipPaths.length);
+    let tileWidth = maxWidth || 640;
+    let tileHeight = maxHeight || 360;
+    let outputWidth = tileWidth * layout.cols;
+    let outputHeight = tileHeight * layout.rows;
+    if (outputWidth > MAX_REPLAY_WIDTH || outputHeight > MAX_REPLAY_HEIGHT) {
+      const scale = Math.min(MAX_REPLAY_WIDTH / outputWidth, MAX_REPLAY_HEIGHT / outputHeight);
+      tileWidth *= scale;
+      tileHeight *= scale;
+      outputWidth = tileWidth * layout.cols;
+      outputHeight = tileHeight * layout.rows;
+    }
+    tileWidth = clampEven(tileWidth);
+    tileHeight = clampEven(tileHeight);
+    outputWidth = clampEven(tileWidth * layout.cols);
+    outputHeight = clampEven(tileHeight * layout.rows);
+
+    const inputArgs = [];
+    const filterParts = [];
+    const layoutParts = [];
+    for (let i = 0; i < clipPaths.length; i += 1) {
+      inputArgs.push('-i', clipPaths[i]);
+      filterParts.push(`[${i}:v]${buildScalePadFilter(tileWidth, tileHeight)}[v${i}]`);
+      const x = (i % layout.cols) * tileWidth;
+      const y = Math.floor(i / layout.cols) * tileHeight;
+      layoutParts.push(`${x}_${y}`);
+    }
+    if (clipPaths.length === 1) {
+      filterParts.push('[v0]null[v]');
+    } else {
+      filterParts.push(
+        `${clipPaths.map((_, i) => `[v${i}]`).join('')}` +
+          `xstack=inputs=${clipPaths.length}:layout=${layoutParts.join('|')}:fill=black[v]`,
+      );
+    }
+
+    const durationSec = Math.max(1, REPLAY_DURATION_MS / 1000);
+    const targetBitrateKbps = Math.max(300, Math.floor((REPLAY_MAX_BYTES * 8) / durationSec / 1000));
+    const maxrateKbps = Math.floor(targetBitrateKbps * 1.1);
+    const bufsizeKbps = Math.floor(targetBitrateKbps * 2);
+
+    const outPath = path.join(tmpDir, 'replay.mp4');
+    await execFileAsync('ffmpeg', [
+      '-y',
+      '-hide_banner',
+      '-loglevel',
+      'error',
+      ...inputArgs,
+      '-filter_complex',
+      filterParts.join(';'),
+      '-map',
+      '[v]',
+      '-r',
+      String(REPLAY_FPS),
+      '-c:v',
+      'libx264',
+      '-b:v',
+      `${targetBitrateKbps}k`,
+      '-maxrate',
+      `${maxrateKbps}k`,
+      '-bufsize',
+      `${bufsizeKbps}k`,
+      '-pix_fmt',
+      'yuv420p',
+      outPath,
+    ]);
+    const buffer = await fsp.readFile(outPath);
+    return buffer;
+  } finally {
+    try {
+      await fsp.rm(tmpDir, { recursive: true, force: true });
+    } catch (err) {
+      logger.warn('Failed to cleanup replay temp dir', err.message);
+    }
+  }
+}
+
+module.exports = {
+  buildReplayVideo,
+};