ArtPlayer/packages/artplayer-plugin-multiple-subtitles/src/merge.ts

import type { Cue, WrappedNode } from './parser'
import type { Track, TrackOption } from './types'
import { WebVTTParser, WebVTTSerializer } from './parser'

export function parseTracks(vtts: readonly string[], subtitles: readonly TrackOption[]): Track[] {
  const parser = new WebVTTParser()
  return vtts.map((vtt, index) => {
    const tree = parser.parse(vtt, 'metadata')
    // Options and completed downloads share the same Promise.all index.
    return { ...tree, url: subtitles[index]!.url, name: subtitles[index]!.name }
  })
}

export function serializeTracks(trees: readonly (Track | undefined)[]): string {
  const cues: Cue<WrappedNode>[] = []
  for (const selected of trees) {
    // Unknown names retain the historical TypeError at the property access below.
    const tree = selected!
    for (const cue of tree.cues) {
      cues.push({
        ...cue,
        tree: {
          ...cue.tree,
          children: cue.tree.children.map(child => ({
            ...child,
            value: `<div class="art-subtitle-${tree.name}">${child.value}</div>`,
          })),
        },
      })
    }
  }
  return new WebVTTSerializer().serialize(cues)
}