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src/srt.js (2758 bytes)

 /* Subtitles as data: read an .srt (or .vtt), and find the cue for a moment of
  * the video. No browser APIs here, so Node can test it.
  *
  * The video on YouTube is often not the same file as the audio the subtitles
  * were made from: an intro was added, or the speed differs a little. So the
  * time of the video is first moved onto the clock of the subtitles:
  *
  *     subtitle time = (video time - offset) * rate
  *
  * The transcript panel needs the other way round, from a cue's time back to a
  * video time to seek to:
  *
  *     video time = subtitle time / rate + offset
  */
 (function (root) {
   'use strict';
 
   const STAMP = /(?:(\d+):)?(\d{1,2}):(\d{2})[.,](\d{1,3})/;
 
   function seconds(stamp) {
     const m = STAMP.exec(stamp);
     if (!m) return NaN;
     return (+(m[1] || 0)) * 3600 + (+m[2]) * 60 + (+m[3]) + (+m[4].padEnd(3, '0')) / 1000;
   }
 
   /** Cues in order of start: [{ start, end, text }]. Blocks that are not cues are passed over. */
   function parse(source) {
     const cues = [];
     const blocks = String(source).replace(/^/, '').replace(/\r\n?/g, '\n').split(/\n{2,}/);
     for (const block of blocks) {
       const lines = block.split('\n');
       const at = lines.findIndex((l) => l.includes('-->'));
       if (at < 0) continue;
       const [from, to] = lines[at].split('-->');
       const start = seconds(from), end = seconds(to);
       const text = lines.slice(at + 1).join('\n').replace(/<[^>]*>/g, '').trim();
       if (Number.isFinite(start) && Number.isFinite(end) && end > start && text) cues.push({ start, end, text });
     }
     return cues.sort((a, b) => a.start - b.start);
   }
 
   /** The cue that is on at `t` (seconds on the clock of the subtitles), or null. */
   function cueAt(cues, t) {
     let lo = 0, hi = cues.length - 1, found = -1;
     while (lo <= hi) {                       // the last cue that starts at or before t
       const mid = (lo + hi) >> 1;
       if (cues[mid].start <= t) { found = mid; lo = mid + 1; } else hi = mid - 1;
     }
     // Cues can overlap a little; the latest one that is still on wins.
     for (let i = found; i >= 0 && i > found - 4; i--) if (t < cues[i].end) return cues[i];
     return null;
   }
 
   const subtitleTime = (videoT, offset, rate) => (videoT - offset) * rate;
 
   // The inverse: a moment on the clock of the subtitles, back to a moment of the video.
   // Used by the transcript panel to seek: videoTime(subtitleTime(t, offset, rate), offset, rate) is t.
   const videoTime = (subTime, offset, rate) => subTime / rate + offset;
 
   const api = { parse, cueAt, subtitleTime, videoTime, seconds };
   if (typeof module !== 'undefined' && module.exports) module.exports = api;
   else root.SubReadSrt = api;
 })(typeof globalThis !== 'undefined' ? globalThis : this);