Concatenates multiple srt files together, adjusting the times

cat-srt.js

#!/usr/bin/env node
//
// Concatenates multiple srt files together, adjusting the times
//
// Requires ffprobe and an mp4 file along with each srt file to calculate time offsets  
//
// Usage: cat-srt.js video1.srt video2.srt video3.srt output.srt
//
const fs = require('fs');
const path = require('path');
const execSync = require('child_process').execSync;

const BOM = '\uFEFF';

const infiles = process.argv.slice(2, -1);
const outfile = process.argv[process.argv.length - 1];

const TIME_PATTERN = /^(\d{2}):(\d{2}):(\d{2}),(\d{3})$/;

function getStreamDuration(file) {
  const seconds = execSync(
    `ffprobe -v error -of default=noprint_wrappers=1:nokey=1 -show_entries format=duration ${file}`
  );
  return Math.floor(parseFloat(seconds.toString()) * 1000);
}

function toMillis(timeStr) {
  const groups = TIME_PATTERN.exec(timeStr);
  const hours = parseInt(groups[1]);
  const minutes = parseInt(groups[2]);
  const seconds = parseInt(groups[3]);
  const milliseconds = parseInt(groups[4]);
  return milliseconds + seconds * 1000 + minutes * 60 * 1000 + hours * 60 * 60 * 1000;
}

function zeroPad(n, digits) {
  return ('0'.repeat(digits) + n).slice(-digits);
}

function toTimeStr(millis) {
  const milliseconds = zeroPad(millis % 1000, 3);
  const seconds = zeroPad(Math.floor(millis / 1000) % 60, 2);
  const minutes = zeroPad(Math.floor(millis / (60 * 1000)) % 60, 2)
  const hours = zeroPad(Math.floor(millis / (60 * 60 * 1000)), 2);
  return `${hours}:${minutes}:${seconds},${milliseconds}`;
}

function shiftTimes(line, offset) {
  return line.split(' --> ')
    .map(s => toMillis(s.trim()))
    .map(t => t + offset)
    .map(toTimeStr)
    .join(' --> ');
}

const output = [];

let slotId = 1;
let offset = 0;

for (file of infiles) {
  const duration = getStreamDuration(path.basename(file, '.srt') + '.mp4');
  let input = fs.readFileSync(file, { encoding: 'utf8' });
  if (input.charAt(0) === BOM) {
    input = input.substring(1);
  }
  const lines = input.split('\n');
  for (let i = 0; i < lines.length; i += 4) {
    if (/^\d+$/.test(lines[i])) {
      output.push(slotId++);
      output.push(shiftTimes(lines[i + 1], offset));
      output.push(lines[i + 2] || '…');  // replace empty text to avoid issues
      output.push(lines[i + 3]);
    }
  }
  offset += duration;
}

fs.writeFileSync(outfile, BOM + output.join('\n') + '\n', { encoding: 'utf8' });