Shift time in .srt subtitles file

shiftsrt.js

#!/bin/node
/*jshint esversion: 9*/

/**
 * Time-shift video subtitles in a .srt file.
 * 
 * Usage:
 *      shiftsrt.js seconds input_file|- [output_file|-]
 * 
 * Examples:
 *      shiftsrt.js 13 movie.srt movie-shifted.srt
 *      shiftsrt.js -13 movie.srt # -> movie_(-13).srt
 *      shiftsrt.js -13 movie.srt - | grep -P '^\d+:\d+:\d+' # output only shifted time intervals
 *      grep -P '^\d+:\d+:\d+' movie.srt | shiftsrt.js 13 - > movie-13.srt # input only time intervals
 * 
 * @gist https://gist.github.com/duzun/bbfc015ecc44618e134b9b0e8f731375
 * @author Dumitru Uzun (https://DUzun.Me)
 * @version 1.0.0
 */

const fs = require('fs');
const path = require('path');

const timeReg = /^([0-9]{2,2})\:([0-9]{2,2})\:([0-9]{2,2}),([0-9]{0,3})/;

const timeShift = parseInt(process.argv[2]);
const fn = process.argv[3];
let destFn = process.argv[4];

if(!destFn) {
    if(fn === '-') {
        destFn = '-';
    }
    else {
        destFn = path.parse(fn);
        destFn = path.join(destFn.dir, `${destFn.name}_(${timeShift})${destFn.ext}`);
    }
}

log(`${fn} -> ${destFn}`);

const _in = fn === '-' ? process.stdin : fs.createReadStream(fn, 'utf8');
const out = destFn === '-' ? process.stdout : fs.createWriteStream(destFn);

let buf = '';
let counter = 0;

_in.on('data', (data) => {
    let ln = (buf + data).split('\n');
    buf = ln.pop();
    out.write(ln.map(mapLine).join('\n') + '\n');
});

_in.on('end', () => {
    if(buf.length); {
        out.write(mapLine(buf));
    }

    if(!out._isStdio) {
        out.close();
    }

    log(`${counter} rows transformed`);
});

function log(...args) {
    if(destFn !== '-') {
        console.log(...args);
    }
}

function mapLine(ln) {
    if(!ln.trim() || !timeReg.test(ln)) return ln;
    ++counter;

    return ln.split(' ').map((str) => {
        let n = readTime(str);
        if(!n) return str;
        return writeTime(n + timeShift);
    }).join(' ');
}

function readTime(str) {
    let m = str.match(timeReg);
    if(!m) return;
    return m[1] * 60 * 60 + m[2] * 60 + +m[3] + (m[4]||0) / 1000;
}

function writeTime(num) {
    let msec = (num * 1000 % 1000) | 0;
    let sec = (num % 60) | 0;
    num = (num / 60) | 0;
    let min = num % 60;
    num = (num / 60) | 0;
    let hours = num;
    return `${pad(hours)}:${pad(min)}:${pad(sec)},${pad(msec,3)}`;
}

function pad(n, len=2) {
    return String(n).padStart(len,'0');
}

/* movie.srt = <<EOF

1
00:01:38,240 --> 00:01:39,850
We're screwed.

2
00:01:39,950 --> 00:01:41,590
No more free Wi-Fi.

3
00:01:43,020 --> 00:01:44,020
Hey, Ki-jung!

EOF
*/