subtlefixer/index.js

Will adjust SRT files with the specified offset

#!/usr/bin/env node
const
    fs = require('fs'),
    argv = require('minimist')(process.argv.slice(3))
const glob = require("glob");

const NEWLINE = '\r\n' // apparently this is the most used one
const SKIP_ME = {}

let ofs = process.argv[2] // the minus with negative numbers drives minimist crazy
ofs = ofs && Number(ofs.replace(',','.'))

if (!ofs) {
    print('subtlefixer <offset-in-seconds> [<files>] [--overwrite]')
    process.exit()
}
const mask = argv._[0] || '*.srt'

print('Working on',mask)
const { overwrite } = argv
print(overwrite ? 'overwrite mode' : `I'll append "-fixed" to destination files`)
let index = 1
const files = glob.sync(mask)
if (!files.length)
    return print('No matching file found')

for (const fn of files) {
    print('file', fn.startsWith(mask) ? fn.substr(mask.length + 1) : fn, '...')
    const content = fs.readFileSync(fn, {encoding:'utf8', flag:'r'})
    const normalized = content.replace(/\r/g,'') // make it easy
    const pieces = normalized.split('\n\n')
    const newPieces = pieces.map(piece=>{
        if (!piece.trim())
            return ''
        const lines = piece.split('\n')
        const times = lines[1].split(/ *-+> */,2)
        const newTimes = times.map(time=> {
            const [hours,minutes,seconds] = time.split(':')
            const totalSeconds = 3600*hours + 60*minutes + 1*seconds.replace(',','.')
            const newSecs = totalSeconds + ofs
            return newSecs >= 0 && secsToSRT(newSecs)
        })
        if (newTimes.some(x=> !x)) // whole piece must be discarded
            return SKIP_ME
        lines[0] = index++
        lines[1] = newTimes.join(' --> ')
        return lines.join(NEWLINE)
    }).filter(x => x !== SKIP_ME)
    const newContent = newPieces.join(NEWLINE+NEWLINE)
    const dest = overwrite ? fn : fn + '-fixed'
    fs.writeFileSync(dest, newContent)
    const discarded = pieces.length - newPieces.length
    print('done', discarded ? ` (${discarded} discarded)` : '')
}

function print(...args) {
    console.log(...args)
}

function secsToSRT(secs) {
    return new Date(secs*1000).toISOString().split('T')[1]
        .replace('.',',') // "this is the way"
        .substr(0,12) // remove final Z
}