Sync Subtitles Node.js

const fs = require('fs')
const path_in = './<original>.srt'
const path_out = './<output>.srt'
const input_encoding = 'latin1'
const output_encoding = 'latin1'

const src = fs.readFileSync(path_in).toString(input_encoding)

const parseTime = (time) => {
    const [h, m, s] = time
        .replace(',', '.')
        .split(':')
        .map(Number)
    return (h*60 + m)*60 + s
}

const toTime = (parsed) => {
    const s = parsed % 60
    parsed = Math.round((parsed - s)/60)
    const m = parsed % 60
    const h = Math.round((parsed - m)/60)
    return `${
        h.toString().padStart(2,0)
    }:${
        m.toString().padStart(2,0)
    }:${
        s.toFixed(3).padStart(6,0).replace('.', ',')
    }`
}

// Calculate speed change and displacement
// const cur_t0 = parseTime('00:02:03,924')
// const cur_t1 = parseTime('00:44:42,015')
// const fix_t0 = parseTime('00:02:03,924')
// const fix_t1 = parseTime('00:44:42,015')
// const mul = (fix_t1 - fix_t0)/(cur_t1 - cur_t0)
// const sum = fix_t0 - cur_t0*mul

// Define displacement with no speed change
const mul = 1
const sum = 6.5

const res = src
    .replace(/\r/g, '')
    .trim()
    .split(/\n\n/)
    .map(sub => {
        let [, n, a, b, text ] = sub
            .trim()
            .match(/^(\d+)\s+([^\s]+) --> ([^\s]+)\s+((.|\s)*)$/)
        a = toTime(parseTime(a)*mul + sum)
        b = toTime(parseTime(b)*mul + sum)
        return `${n}\n${a} --> ${b}\n${text}\n\n`
    })
    .join('')
    .replace(/\n/g, '\r\n')

fs.writeFileSync(path_out, Buffer.from(res, output_encoding))