/**
 * Measures and separates static and dynamic content in a response stream
 * by splitting on PPR boundary sentinel markers.
 *
 * This function is used in Partial Prerendering (PPR) tests to analyze the output
 * of Next.js pages and verify that static and dynamic content are properly separated.
 *
 * @param fn - A function that returns a Promise resolving to a readable stream
 *             containing HTML with PPR boundary markers
 * @returns An object containing the separated static and dynamic chunks
 *
 * @example
 * const [staticPart, dynamicPart] = await splitResponseWithPPRSentinel(async () => {
 *   return fetch('/some-ppr-page').then(res => res.body)
 * })
 * console.log(staticPart)  // Content before the boundary
 * console.log(dynamicPart) // Content after the boundary
 */
export async function splitResponseWithPPRSentinel(
  fn: () => Promise<NodeJS.ReadableStream>
): Promise<[staticPart: string, dynamicPart: string]> {
  const stream = await fn()
  const chunks: string[] = []

  await new Promise<void>((resolve, reject) => {
    stream.on('data', (chunk: Buffer | string) => {
      if (typeof chunk !== 'string') {
        chunk = chunk.toString('utf8')
      }

      chunks.push(chunk)
    })

    stream.on('end', resolve)
    stream.on('error', reject)
  })

  // Combine all chunks and split on the PPR boundary sentinel
  // The sentinel marks the boundary between static and dynamic content
  const parts = chunks.join('').split('<!-- PPR_BOUNDARY_SENTINEL -->', 2)
  return [parts[0] ?? '', parts[1] ?? '']
}