/**
 * 极简 SSE（text/event-stream）解析器。
 *
 * 只依赖 Web 标准的 ReadableStream，可直接消费 fetch 响应体。
 * 上游既可能是标准 SSE，也可能在末尾缺少空行，因此以「块边界」为准做收尾。
 */

export interface SseEvent {
  event?: string
  data: string
}

export async function* parseSse(
  stream: ReadableStream<Uint8Array>
): AsyncGenerator<SseEvent, void, void> {
  const reader = stream.getReader()
  const decoder = new TextDecoder()
  let buffer = ''

  try {
    for (;;) {
      const { done, value } = await reader.read()
      if (done) break
      buffer += decoder.decode(value, { stream: true })

      let index: number
      // 事件之间以空行分隔，兼容 \r\n 与 \n
      while ((index = findBoundary(buffer)) !== -1) {
        const raw = buffer.slice(0, index)
        buffer = buffer.slice(index).replace(/^(\r?\n){2}/, '')
        const parsed = parseBlock(raw)
        if (parsed) yield parsed
      }
    }

    // 收尾：流结束时可能残留最后一个事件
    buffer += decoder.decode()
    const tail = parseBlock(buffer)
    if (tail) yield tail
  } finally {
    reader.releaseLock()
  }
}

function findBoundary(buffer: string): number {
  const a = buffer.indexOf('\n\n')
  const b = buffer.indexOf('\r\n\r\n')
  if (a === -1) return b
  if (b === -1) return a
  return Math.min(a, b)
}

function parseBlock(block: string): SseEvent | null {
  const lines = block.split(/\r?\n/)
  const dataLines: string[] = []
  let event: string | undefined

  for (const line of lines) {
    if (!line || line.startsWith(':')) continue
    if (line.startsWith('event:')) {
      event = line.slice(6).trim()
    } else if (line.startsWith('data:')) {
      dataLines.push(line.slice(5).replace(/^ /, ''))
    }
  }

  if (dataLines.length === 0) return null
  return { event, data: dataLines.join('\n') }
}