srt_fill_gaps.js

/******************************************************************************/
'use strict';
/******************************************************************************/
const fs = require('fs');
const path = require('path');

const outCoding = 'utf8';

process.argv.slice(2).forEach(processSRT);
/******************************************************************************/
function processSRT(fpath) {
  const inCoding = guessEncoding(fpath);
  if (!inCoding) return;
  
  const fext = path.extname(fpath);
  if (fext != '.srt') {
    console.error(`Wrong file: ${fpath}`);
    return;
  }
  const fname  = path.basename(fpath, fext);
  
  let ftext = fs.readFileSync(fpath, inCoding)
              .replace(/^\uFEFF/, '').replace(/(\r?\n)+$/, '\r\n');

  const subts = ftext.split(/(?:\r?\n){2,}/);
  
  subts.forEach((s, i) => {
    subts[i] = subts[i].split(/\r?\n/);
    subts[i][1] = subts[i][1].split(' --> ');
    subts[i][1][0] = timecode2ms(subts[i][1][0]);
    subts[i][1][1] = timecode2ms(subts[i][1][1]);
  });
  
  subts[0][1][0] = ms2timecode(subts[0][1][0]);
  subts.forEach((s, i) => {
    if (i < subts.length-1) {
      let   curEnd = subts[i][1][1];
      let   nextStart = subts[i+1][1][0];
      const halfGap = Math.floor((nextStart - curEnd) / 2);
      if (halfGap > 0) {
        curEnd += (halfGap - 1);
        nextStart -= halfGap;
      }
      subts[i][1][1] = ms2timecode(curEnd);
      subts[i+1][1][0] = ms2timecode(nextStart);
    }
  });
  subts[subts.length-1][1][1] = ms2timecode(subts[subts.length-1][1][1]);
  
  subts.forEach((s, i) => {
    subts[i][1] = subts[i][1].join(' --> ');
    subts[i] = subts[i].join('\r\n');
  });

  fs.writeFileSync(path.join(path.dirname(fpath), `${fname}.long${fext}`),
    `\uFEFF${ subts.join('\r\n\r\n').replace(/(\r\n)+$/, '\r\n\r\n') }`, outCoding
  );
  //fs.renameSync(fpath, path.join(path.dirname(fpath), `${fname}.bak${fext}`));
}
/******************************************************************************/
function timecode2ms(timecode) {
  const timeParts = timecode.split(/:|,/);
  return (
    Number(timeParts[3])         +
    Number(timeParts[2]) * 1000  +
    Number(timeParts[1]) * 60000 +
    Number(timeParts[0]) * 3600000);
}
/******************************************************************************/
function ms2timecode(ms) {
  return (
    Math.floor( ms / 3600000)          + ':' +
    Math.floor((ms % 3600000) / 60000) + ':' +
    Math.floor((ms % 60000)   / 1000)  + ',' +
    Math.floor( ms % 1000)
  ).replace(/\b(\d)\b/g, '0$1').replace(/,(\d\d)\b/, ',0$1');
}
/******************************************************************************/
function guessEncoding(fpath) {
  try  {
    const fd = fs.openSync(fpath, 'r');
    const bf =  new Buffer(2).fill(0);
    fs.readSync(fd, bf, 0, 2, 0);
    fs.closeSync(fd);
    return bf[0] === 0xFF && bf[1] === 0xFE ? 'utf16le' : 'utf8';
  } catch(e) {
    console.error(`Error: ${e.message}.`);
    return null;
  }
}
/******************************************************************************/