/**
 * Parse SRT subtitle format to WebVTT
 */
export const parseSRT = (srtContent: string): string => {
  const normalised = srtContent.replace(/\r\n/g, '\n').replace(/\r/g, '\n').trim()

  if (!normalised) {
    return 'WEBVTT\n\n'
  }

  const cues = normalised
    .split(/\n{2,}/)
    .map((cueBlock) => {
      const lines = cueBlock
        .split('\n')
        .map((line) => line.replace(/^\ufeff/, '').trimEnd())
        .filter((line, index, arr) => !(line === '' && index === arr.length - 1))

      if (lines.length === 0) {
        return null
      }

      if (/^\d+$/.test(lines[0].trim())) {
        lines.shift()
      }

      if (lines.length === 0) {
        return null
      }

      const timeLine = lines.shift()
      if (!timeLine || !timeLine.includes('-->')) {
        return null
      }

      const vttTimeLine = timeLine
        .split('-->')
        .map((part) => part.trim().replace(/,/g, '.'))
        .join(' --> ')

      if (!vttTimeLine.includes('-->')) {
        return null
      }

      const text = lines.join('\n')
      return `${vttTimeLine}\n${text}`.trim()
    })
    .filter((cueBlock): cueBlock is string => Boolean(cueBlock))

  const header = 'WEBVTT\n\n'
  if (cues.length === 0) {
    return header
  }

  return header + cues.join('\n\n') + '\n'
}

/**
 * Create a blob URL from subtitle content
 */
export const createSubtitleBlobURL = (content: string, format: 'vtt' | 'srt'): string => {
  const vttContent = format === 'srt' ? parseSRT(content) : content
  const blob = new Blob([vttContent], { type: 'text/vtt;charset=utf-8' })
  return URL.createObjectURL(blob)
}

/**
 * Fetch and parse subtitle file
 */
export const fetchSubtitle = async (url: string): Promise<string> => {
  const response = await fetch(url)
  const content = await response.text()

  // Detect format
  if (url.endsWith('.srt')) {
    return parseSRT(content)
  }

  return content
}