// ==UserScript==
// @name           Youtube 双语字幕下载 v2 (中文+任选的一门双语,比如英语) 
// @include        https://*youtube.com/*
// @author         Cheng Zheng
// @require        http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js
// @version        2
// @grant GM_xmlhttpRequest
// @description   字幕格式是 "中文 \n 英语"（\n 是换行符的意思）
// @namespace https://greasyfork.org/users/5711
// @downloadURL none
// ==/UserScript==

/*
  作者联系方式:
    QQ 1003211008
    邮件 guokrfans@gmail.com
    Github@1c7

  使用场景:
    此文件仅针对于 Tampermonkey (Chrome 上的一款插件)
    需要安装在 Tampermonkey 里

  解决什么问题：
    下载中外双语的字幕，格式是 中文 \n 外语, \n 是换行符的意思
  
  术语说明：
    auto 自动字幕
    closed 完整字幕 (或者叫人工字幕也可以)

  时间: 
    初次写于 2020-10-7，大量代码来自同一作者之前写的另一个脚本
*/

// text for display
var NO_SUBTITLE = '没有字幕';
var HAVE_SUBTITLE = '下载双语字幕 (中文 + 外语)';
const NEW_LINE = '\n'

// initialize
var first_load = true; // indicate if first load this webpage or not
var youtube_playerResponse_1c7 = null; // for auto subtitle
unsafeWindow.caption_array = []; // store all subtitle

// trigger when first load
$(document).ready(function () {
  start();
});

// Explain this function: we repeatly try if certain HTML element exist, 
// if it does, we call init()
// if it doesn't, stop trying after certain time
function start() {
  var retry_count = 0;
  var RETRY_LIMIT = 20;
  // use "setInterval" is because "$(document).ready()" still not enough, still too early
  // 330 work for me.
  if (new_material_design_version()) {
    var material_checkExist = setInterval(function () {
      if (document.querySelectorAll('.title.style-scope.ytd-video-primary-info-renderer').length) {
        init();
        clearInterval(material_checkExist);
      }
      retry_count = retry_count + 1;
      if (retry_count > RETRY_LIMIT) {
        clearInterval(material_checkExist);
      }
    }, 330);
  } else {
    var checkExist = setInterval(function () {
      if ($('#watch7-headline').length) {
        init();
        clearInterval(checkExist);
      }
      retry_count = retry_count + 1;
      if (retry_count > RETRY_LIMIT) {
        clearInterval(checkExist);
      }
    }, 330);
  }
}

// trigger when loading new page 
// (actually this would also trigger when first loading, that's not what we want, that's why we need to use firsr_load === false)
// (new Material design version would trigger this "yt-navigate-finish" event. old version would not.)
var body = document.getElementsByTagName("body")[0];
body.addEventListener("yt-navigate-finish", function (event) {
  if (current_page_is_video_page() === false) {
    return;
  }
  youtube_playerResponse_1c7 = event.detail.response.playerResponse; // for auto subtitle
  unsafeWindow.caption_array = []; // clean up (important, otherwise would have more and more item and cause error)

  // if use click to another page, init again to get correct subtitle
  if (first_load === false) {
    remove_subtitle_download_button();
    init();
  }
});

// trigger when loading new page
// (old version would trigger "spfdone" event. new Material design version not sure yet.)
window.addEventListener("spfdone", function (e) {
  if (current_page_is_video_page()) {
    remove_subtitle_download_button();
    var checkExist = setInterval(function () {
      if ($('#watch7-headline').length) {
        init();
        clearInterval(checkExist);
      }
    }, 330);
  }
});

// return true / false
// Detect [new version UI(material design)] OR [old version UI]
// I tested this, accurated.
function new_material_design_version() {
  var old_title_element = document.getElementById('watch7-headline');
  if (old_title_element) {
    return false;
  } else {
    return true;
  }
}

// return true / false
function current_page_is_video_page() {
  return get_video_id() !== null;
}

// return string like "RW1ChiWyiZQ",  from "https://www.youtube.com/watch?v=RW1ChiWyiZQ"
// or null
function get_video_id() {
  return getURLParameter('v');
}

//https://stackoverflow.com/questions/11582512/how-to-get-url-parameters-with-javascript/11582513#11582513
function getURLParameter(name) {
  return decodeURIComponent((new RegExp('[?|&]' + name + '=' + '([^&;]+?)(&|#|;|$)').exec(location.search) || [null, ''])[1].replace(/\+/g, '%20')) || null;
}

function remove_subtitle_download_button() {
  $('#youtube-subtitle-downloader-by-1c7').remove();
}

function init() {
  inject_our_script();
  first_load = false;
}

function inject_our_script() {
  var div = document.createElement('div'),
    select = document.createElement('select'),
    option = document.createElement('option'),
    controls = document.getElementById('watch7-headline'); // Youtube video title DIV

  div.setAttribute('style', `display: table; 
margin-top:4px;
border: 1px solid rgb(0, 183, 90); 
cursor: pointer; color: rgb(255, 255, 255); 
border-top-left-radius: 3px; 
border-top-right-radius: 3px; 
border-bottom-right-radius: 3px; 
border-bottom-left-radius: 3px; 
background-color: #00B75A;
`);

  div.id = 'youtube-subtitle-downloader-by-1c7';
  div.title = 'Youtube Subtitle Download v16'; // display when cursor hover

  select.id = 'captions_selector';
  select.disabled = true;
  select.setAttribute('style', `display:block; 
border: 1px solid rgb(0, 183, 90); 
cursor: pointer; 
color: rgb(255, 255, 255); 
background-color: #00B75A;
padding: 4px;
`);

  option.textContent = 'Loading...';
  option.selected = true;
  select.appendChild(option);

  // 下拉菜单里，选择一项后触发下载
  select.addEventListener('change', function () {
    download_subtitle(this);
  }, false);

  div.appendChild(select); // put <select> into <div>

  // put the div into page: new material design
  var title_element = document.querySelectorAll('.title.style-scope.ytd-video-primary-info-renderer');
  if (title_element) {
    $(title_element[0]).after(div);
  }
  // put the div into page: old version
  if (controls) {
    controls.appendChild(div);
  }

  load_language_list(select);

  // <a> element is for download
  var a = document.createElement('a');
  a.style.cssText = 'display:none;';
  a.setAttribute("id", "ForSubtitleDownload");
  var body = document.getElementsByTagName('body')[0];
  body.appendChild(a);
}


// 输入: url
// 输出: SRT Array
async function auto_sub_in_chinese_fmt_json3_to_srt(url) {
  var json = await get(url);
  var srt_array = []
  // 先处理中文，转成 SRT 中间格式, 一个句子一个句子的形式，开始时间，结束时间，文字，都是对的
  var events = json.events;
  for (let index = 0; index < events.length; index++) {
    const event = events[index];
    // console.log(event);
    var tStartMs = event.tStartMs
    var dDurationMs = event.dDurationMs
    var segs = event.segs
    // console.log(tStartMs)
    // console.log(dDurationMs)
    // console.log(segs)
    var text = segs[0].utf8;
    // console.log(text)

    var item = {
      startTime: ms_to_srt(tStartMs),
      endTime: ms_to_srt(tStartMs + dDurationMs),
      text: text,

      tStartMs: tStartMs,
      dDurationMs: dDurationMs,
    }
    srt_array.push(item);
  }
  return srt_array
}

// 下载自动字幕的中英双语
async function download_auto_subtitle(file_name) {
  // 1. English Auto Sub in json3 format
  var auto_sub_url = get_auto_subtitle_xml_url();
  var format_json3_url = auto_sub_url + '&fmt=json3'
  var en_auto_sub = await get(format_json3_url); // 格式参考 Youtube-Subtitle-Downloader/fmt=json3/en.json

  // 2. 自动字幕的翻译中文
  var cn_url = format_json3_url + '&tlang=zh-Hans'
  var cn_srt = await auto_sub_in_chinese_fmt_json3_to_srt(cn_url) // 格式参考 Youtube-Subtitle-Downloader/fmt=json3/zh-Hans.json

  // 3. 处理英文，插入到句子里, 也就是插入到 cn_srt 的每个元素里（新加一个属性叫 words)
  var events = en_auto_sub.events;
  for (let i = 0; i < events.length; i++) { // loop events
    let event = events[i];
    if (event.aAppend == 1) { // 这样的元素内部只有一个换行符，对我们没有意义，跳过
      continue
    }
    var segs = event.segs
    if (segs == undefined) { // 这样的元素也没意义，跳过
      continue
    }
    var tStartMs = event.tStartMs
    var dDurationMs = event.dDurationMs

    for (let j = 0; j < segs.length; j++) { // loop segs
      const seg = segs[j];
      var word = seg.utf8 // 词的内容

      var word_offset = seg.tOffsetMs === undefined ? 0 : seg.tOffsetMs;
      var word_start_time = tStartMs + word_offset // 词的开始时间

      for (let z = 0; z < cn_srt.length; z++) { // loop each word and put into cn_srt
        const srt_line = cn_srt[z];
        var line_start_time_ms = srt_line.tStartMs
        var line_end_time_ms = srt_line.tStartMs + dDurationMs

        // 如果词的开始时间，刚好处于这个句子的 [开始时间, 结束时间] 区间之内
        if (word_start_time >= line_start_time_ms && word_start_time <= line_end_time_ms) {
          // push 到 words 数组里
          if (cn_srt[z].words === undefined) {
            cn_srt[z].words = [word]
          } else {
            var final_word = ` ${word.trim()}` // 去掉单词本身的空格（不管有没有）然后给单词前面加一个空格
            cn_srt[z].words.push(final_word)
          }
        }
      }
    }
  }

  // console.log(cn_srt)
  // 到了这一步，cn_srt 的每一个 item 应该是：
  // var item_example = {
  //   "startTime": "00:00:06,640",
  //   "endTime": "00:00:09,760",
  //   "text": "在与朋友的长时间交谈中以及与陌生人的简短交谈中",
  //   "tStartMs": 6640,
  //   "dDurationMs": 3120,
  //   "words": ["in", " a", " long", " conversation", " with", " a", " friend", " and", "a", " short", " chat", " with", " a", " stranger", "the", " endless", " streams"]
  // }

  // 最后保存下来
  var srt_string = auto_sub_dual_language_to_srt(cn_srt) // 结合中文和英文
  downloadString(srt_string, "text/plain", file_name);
}

function auto_sub_dual_language_to_srt(srt_array) {
  // var srt_array_item_example = {
  //   "startTime": "00:00:06,640",
  //   "endTime": "00:00:09,760",
  //   "text": "在与朋友的长时间交谈中以及与陌生人的简短交谈中",
  //   "tStartMs": 6640,
  //   "dDurationMs": 3120,
  //   "words": ["in", " a", " long", " conversation", " with", " a", " friend", " and", "a", " short", " chat", " with", " a", " stranger", "the", " endless", " streams"]
  // }

  var result_array = []
  for (let i = 0; i < srt_array.length; i++) {
    const line = srt_array[i];
    var text = line.text + NEW_LINE + line.words.join(''); // 中文 \n 英文
    var item = {
      startTime: line.startTime,
      endTime: line.endTime,
      text: text
    }
    result_array.push(item)
  }

  var srt_string = object_array_to_SRT_string(result_array)
  return srt_string
}

// 把毫秒转成 srt 时间
// 代码来源网络
function ms_to_srt($milliseconds) {
  var $seconds = Math.floor($milliseconds / 1000);
  var $minutes = Math.floor($seconds / 60);
  var $hours = Math.floor($minutes / 60);
  var $milliseconds = $milliseconds % 1000;
  var $seconds = $seconds % 60;
  var $minutes = $minutes % 60;
  return ($hours < 10 ? '0' : '') + $hours + ':' +
    ($minutes < 10 ? '0' : '') + $minutes + ':' +
    ($seconds < 10 ? '0' : '') + $seconds + ',' +
    ($milliseconds < 100 ? '0' : '') + ($milliseconds < 10 ? '0' : '') + $milliseconds;
}

// Trigger when user select <option>
async function download_subtitle(selector) {
  // if user select first <option>
  // we just return, do nothing.
  if (selector.selectedIndex == 0) {
    return;
  }

  // 核心概念
  // 对于完整字幕而言，英文和中文的时间轴是一致的，只需要一行行的 match 即可

  // 但是对于自动字幕就不是这样了，"自动字幕的英文"只能拿到一个个单词的开始时间和结束时间
  // "自动字幕的中文"只能拿到一个个句子
  // 现在的做法是，先拿到中文，处理成 SRT 格式，
  // 然后去拿英文，然后把英文的每个词，拿去和中文的每个句子的开始时间和结束时间进行对比
  // 如果"英文单词的开始时间"在"中文句子的开始-结束时间"区间内，那么认为这个英文单词属于这一句中文

  var caption = caption_array[selector.selectedIndex - 1]; // because first <option> is for display, so index-1 
  var lang_code = caption.lang_code;
  var lang_name = caption.lang_name;

  // if user choose auto subtitle // 如果用户选的是自动字幕
  if (caption.lang_code == 'AUTO') {
    var file_name = get_file_name(lang_name);
    download_auto_subtitle(file_name);
    selector.options[0].selected = true; // after download, select first <option>
    return
  }

  // 如果用户选的是完整字幕
  // 原文
  // sub mean "subtitle"
  var sub_original_url = await get_closed_subtitle_url(lang_code)
  var sub_original_xml = await get(sub_original_url);

  // 中文
  var sub_translated_url = sub_original_url + "&tlang=" + "zh-Hans"
  var sub_translated_xml = await get(sub_translated_url);

  // 根据时间轴融合这俩
  var sub_original_srt = parse_youtube_XML_to_object_list(sub_original_xml)
  var sub_translated_srt = parse_youtube_XML_to_object_list(sub_translated_xml)
  // 'sub_original_srt' and 'sub_translated_srt' have the same length

  var dual_language_srt = []
  for (let index = 0; index < sub_original_srt.length; index++) {
    const original = sub_original_srt[index];
    const translated = sub_translated_srt[index];
    var text = translated.text + NEW_LINE + original.text; // 中文 \n 英文
    var item = {
      startTime: original.startTime,
      endTime: original.endTime,
      text: text
    }
    dual_language_srt.push(item)
  }

  var srt_string = object_array_to_SRT_string(dual_language_srt)
  var title = get_file_name(lang_name);
  downloadString(srt_string, "text/plain", title);

  // after download, select first <option>
  selector.options[0].selected = true;
}


// Return something like: "(English)How Did Python Become A Data Science Powerhouse?.srt"
function get_file_name(x) {
  return '(' + x + ')' + document.title + '.srt';
}

// detect if "auto subtitle" and "closed subtitle" exist
// and add <option> into <select>
function load_language_list(select) {
  // auto
  var auto_subtitle_exist = false;

  // closed
  var closed_subtitle_exist = false;
  var captions = null;

  // get auto subtitle
  var auto_subtitle_url = get_auto_subtitle_xml_url();
  if (auto_subtitle_url != false) {
    auto_subtitle_exist = true;
  }

  // get closed subtitle
  var list_url = 'https://video.google.com/timedtext?v=' + get_video_id() + '&type=list&hl=zh-CN';
  // https://video.google.com/timedtext?v=if36bqHypqk&type=list&hl=en // 英文
  // https://video.google.com/timedtext?v=n1zpnN-6pZQ&type=list&hl=zh-CN // 中文

  GM_xmlhttpRequest({
    method: 'GET',
    url: list_url,
    onload: function (xhr) {
      captions = new DOMParser().parseFromString(xhr.responseText, "text/xml").getElementsByTagName('track');
      if (captions.length != 0) {
        closed_subtitle_exist = true;
      }

      // if no subtitle at all, just say no and stop
      if (auto_subtitle_exist == false && closed_subtitle_exist == false) {
        select.options[0].textContent = NO_SUBTITLE;
        disable_download_button();
        return false;
      }

      // if at least one type of subtitle exist
      select.options[0].textContent = HAVE_SUBTITLE;
      select.disabled = false;

      var caption = null; // for inside loop
      var option = null; // for <option>
      var caption_info = null; // for our custom object

      // 自动字幕
      if (auto_subtitle_exist) {
        var auto_sub_name = get_auto_subtitle_name()
        // var lang_name = auto_sub_name + " (自动字幕暂不支持双语) "
        var lang_name = `中文 + ${auto_sub_name}`
        caption_info = {
          lang_code: 'AUTO', // later we use this to know if it's auto subtitle
          lang_name: lang_name // for display only
        };
        caption_array.push(caption_info);

        option = document.createElement('option');
        option.textContent = caption_info.lang_name;
        select.appendChild(option);
      }

      // if closed_subtitle_exist
      if (closed_subtitle_exist) {
        for (var i = 0, il = captions.length; i < il; i++) {
          caption = captions[i];
          // console.log(caption); // <track id="0" name="" lang_code="en" lang_original="English" lang_translated="English" lang_default="true"/>
          var lang_code = caption.getAttribute('lang_code')
          var lang_translated = caption.getAttribute('lang_translated')
          var lang_name = `中文 + ${lang_translated}`
          caption_info = {
            lang_code: lang_code, // for AJAX request
            lang_name: lang_name, // display to user
          };
          caption_array.push(caption_info);
          // 注意这里是加到 caption_array, 一个全局变量, 待会要靠它来下载
          option = document.createElement('option');
          option.textContent = caption_info.lang_name;
          select.appendChild(option);
        }
      }
    }
  });
}

function disable_download_button() {
  $('#youtube-subtitle-downloader-by-1c7')
    .css('border', '#95a5a6')
    .css('cursor', 'not-allowed')
    .css('background-color', '#95a5a6');
  $('#captions_selector')
    .css('border', '#95a5a6')
    .css('cursor', 'not-allowed')
    .css('background-color', '#95a5a6');

  if (new_material_design_version()) {
    $('#youtube-subtitle-downloader-by-1c7').css('padding', '6px');
  } else {
    $('#youtube-subtitle-downloader-by-1c7').css('padding', '5px');
  }
}

// 处理时间. 比如 start="671.33"  start="37.64"  start="12" start="23.029"
// 处理成 srt 时间, 比如 00:00:00,090    00:00:08,460    00:10:29,350
function process_time(s) {
  s = s.toFixed(3);
  // 超棒的函数, 不论是整数还是小数都给弄成3位小数形式
  // 举个柚子:
  // 671.33 -> 671.330
  // 671 -> 671.000
  // 注意函数会四舍五入. 具体读文档

  var array = s.split('.');
  // 把开始时间根据句号分割
  // 671.330 会分割成数组: [671, 330]

  var Hour = 0;
  var Minute = 0;
  var Second = array[0]; // 671
  var MilliSecond = array[1]; // 330
  // 先声明下变量, 待会把这几个拼好就行了

  // 我们来处理秒数.  把"分钟"和"小时"除出来
  if (Second >= 60) {
    Minute = Math.floor(Second / 60);
    Second = Second - Minute * 60;
    // 把 秒 拆成 分钟和秒, 比如121秒, 拆成2分钟1秒

    Hour = Math.floor(Minute / 60);
    Minute = Minute - Hour * 60;
    // 把 分钟 拆成 小时和分钟, 比如700分钟, 拆成11小时40分钟
  }
  // 分钟，如果位数不够两位就变成两位，下面两个if语句的作用也是一样。
  if (Minute < 10) {
    Minute = '0' + Minute;
  }
  // 小时
  if (Hour < 10) {
    Hour = '0' + Hour;
  }
  // 秒
  if (Second < 10) {
    Second = '0' + Second;
  }
  return Hour + ':' + Minute + ':' + Second + ',' + MilliSecond;
}

// Copy from: https://gist.github.com/danallison/3ec9d5314788b337b682
// Thanks! https://github.com/danallison
// Work in Chrome 66
// Test passed: 2018-5-19
function downloadString(text, fileType, fileName) {
  var blob = new Blob([text], {
    type: fileType
  });
  var a = document.createElement('a');
  a.download = fileName;
  a.href = URL.createObjectURL(blob);
  a.dataset.downloadurl = [fileType, a.download, a.href].join(':');
  a.style.display = "none";
  document.body.appendChild(a);
  a.click();
  document.body.removeChild(a);
  setTimeout(function () {
    URL.revokeObjectURL(a.href);
  }, 1500);
}

// https://css-tricks.com/snippets/javascript/unescape-html-in-js/
// turn HTML entity back to text, example: &quot; should be "
function htmlDecode(input) {
  var e = document.createElement('div');
  e.class = 'dummy-element-for-tampermonkey-Youtube-Subtitle-Downloader-script-to-decode-html-entity';
  e.innerHTML = input;
  return e.childNodes.length === 0 ? "" : e.childNodes[0].nodeValue;
}

// return URL or null;
// later we can send a AJAX and get XML subtitle
function get_auto_subtitle_xml_url() {
  try {
    // get JSON
    var json = '';
    if (typeof youtube_playerResponse_1c7 !== "undefined" && youtube_playerResponse_1c7 !== null && youtube_playerResponse_1c7 !== '') {
      json = youtube_playerResponse_1c7;
    } else {
      var raw_string = ytplayer.config.args.player_response;
      json = JSON.parse(raw_string);
    }

    // get data from JSON
    var captionTracks = json.captions.playerCaptionsTracklistRenderer.captionTracks;
    for (var index in captionTracks) {
      var caption = captionTracks[index];
      if (typeof caption.kind === 'string' && caption.kind == 'asr') {
        return captionTracks[index].baseUrl;
      }
      // ASR – A caption track generated using automatic speech recognition.
      // https://developers.google.com/youtube/v3/docs/captions
    }
  } catch (error) {
    return false;
  }
}

function get_auto_subtitle(callback) {
  var url = get_auto_subtitle_xml_url();
  get_from_url(url, callback);
}

async function get_closed_subtitle_url(lang_code) {
  try {
    var json = '';
    if (typeof youtube_playerResponse_1c7 !== "undefined" && youtube_playerResponse_1c7 !== null && youtube_playerResponse_1c7 !== '') {
      json = youtube_playerResponse_1c7;
    } else {
      var raw_string = ytplayer.config.args.player_response;
      json = JSON.parse(raw_string);
    }

    // get data from JSON
    var captionTracks = json.captions.playerCaptionsTracklistRenderer.captionTracks;
    for (var index in captionTracks) {
      var caption = captionTracks[index];
      if (caption.languageCode === lang_code && caption.kind != 'asr') {
        var url = captionTracks[index].baseUrl;
        return url
      }
    }
  } catch (error) {
    console.log(error);
    return false;
  }
}

// Usage: var result = await get(url)
function get(url) {
  return $.ajax({
    url: url,
    type: 'get',
    success: function (r) {
      return r
    },
    fail: function (error) {
      return error
    }
  });
}

// GET request a URL
function get_from_url(url, callback) {
  $.ajax({
    url: url,
    type: 'get',
    success: function (r) {
      callback(r); // pass result to callback
    },
    fail: function (error) {
      callback(false); // or pass false to callback
    }
  });
}

// input: XML
// output: Array of object
// each object look like: 
/*
  {
    startTime: "",
    endTime: "",
    text: ""
  }
*/
// it's intermediate representation for SRT
function parse_youtube_XML_to_object_list(youtube_xml_string) {
  if (youtube_xml_string === '' || youtube_xml_string === undefined || youtube_xml_string === null) {
    return false;
  }
  var result_array = []
  var text_nodes = youtube_xml_string.getElementsByTagName('text');
  var len = text_nodes.length;
  for (var i = 0; i < len; i++) {
    var text = text_nodes[i].textContent.toString();
    text = text.replace(/(<([^>]+)>)/ig, ""); // remove all html tag.
    text = htmlDecode(text);

    var start = text_nodes[i].getAttribute('start');
    var end = '';

    if (i + 1 >= len) {
      end = parseFloat(text_nodes[i].getAttribute('start')) + parseFloat(text_nodes[i].getAttribute('dur'));
    } else {
      end = text_nodes[i + 1].getAttribute('start');
    }

    var start_time = process_time(parseFloat(start));
    var end_time = process_time(parseFloat(end));

    var item = {
      startTime: start_time,
      endTime: end_time,
      text: text
    }
    result_array.push(item)
  }

  return result_array
}


function object_array_to_SRT_string(object_array) {
  var result = '';
  var BOM = '\uFEFF';
  result = BOM + result; // store final SRT result

  for (var i = 0; i < object_array.length; i++) {
    var item = object_array[i]
    var index = i + 1;
    var start_time = item.startTime
    var end_time = item.endTime
    var text = item.text

    var new_line = "\n";
    result = result + index + new_line;

    result = result + start_time;
    result = result + ' --> ';
    result = result + end_time + new_line;

    result = result + text + new_line + new_line;
  }

  return result;
}

// Youtube return XML. we want SRT  
// input: Youtube XML format
// output: SRT format
function parse_youtube_XML_to_SRT(youtube_xml_string) {
  if (youtube_xml_string === '' || youtube_xml_string === undefined || youtube_xml_string === null) {
    return false;
  }

  var result = '';
  var BOM = '\uFEFF';
  result = BOM + result; // store final SRT result

  var text = youtube_xml_string.getElementsByTagName('text');
  var len = text.length;
  for (var i = 0; i < len; i++) {
    var index = i + 1;
    var content = text[i].textContent.toString();
    content = content.replace(/(<([^>]+)>)/ig, ""); // remove all html tag.
    var start = text[i].getAttribute('start');
    var end = '';

    if (i + 1 >= len) {
      end = parseFloat(text[i].getAttribute('start')) + parseFloat(text[i].getAttribute('dur'));
    } else {
      end = text[i + 1].getAttribute('start');
    }

    // we want SRT format:
    /*
        1
        00:00:01,939 --> 00:00:04,350
        everybody Craig Adams here I'm a

        2
        00:00:04,350 --> 00:00:06,720
        filmmaker on YouTube who's digging
    */
    var new_line = "\n";
    result = result + index + new_line;
    // 1

    var start_time = process_time(parseFloat(start));
    var end_time = process_time(parseFloat(end));
    result = result + start_time;
    result = result + ' --> ';
    result = result + end_time + new_line;
    // 00:00:01,939 --> 00:00:04,350

    content = htmlDecode(content);
    // turn HTML entity back to text. example: &#39; back to apostrophe (')

    result = result + content + new_line + new_line;
  }
  return result;
}

// return "English (auto-generated)" or a default name;
function get_auto_subtitle_name() {
  try {
    var raw_string = ytplayer.config.args.player_response;
    var json = JSON.parse(raw_string);

    if (typeof json.captions !== "undefined") {
      var captionTracks = json.captions.playerCaptionsTracklistRenderer.captionTracks;
      for (var index in captionTracks) {
        var caption = captionTracks[index];
        if (typeof caption.kind === 'string' && caption.kind == 'asr') {
          return captionTracks[index].name.simpleText;
        }
      }
    }
  } catch (error) {
    return 'Auto Subtitle';
  }
}