import { Readable } from 'node:stream'

export interface NodeReadableToWebStreamOptions {
  onCancel?: (reason?: unknown) => void | Promise<void>
}

/**
 * 将 Node.js Readable 转换为受 Web Stream 消费速度约束的流。
 *
 * 每次 pull 只向下游交付一个分块，避免 data 事件持续读取大文件、
 * 而浏览器或代理来不及接收时在 Node 进程内堆积大量内存。
 */
export function nodeReadableToWebStream(
  source: Readable,
  options: NodeReadableToWebStreamOptions = {},
): ReadableStream<Uint8Array> {
  const iterator = source[Symbol.asyncIterator]()
  let finished = false

  return new ReadableStream<Uint8Array>(
    {
      async pull(controller) {
        if (finished) {
          return
        }

        try {
          const next = await iterator.next()
          if (next.done) {
            finished = true
            controller.close()
            return
          }

          const chunk = next.value
          controller.enqueue(chunk instanceof Uint8Array ? chunk : Buffer.from(chunk))
        } catch (error) {
          finished = true
          if (!source.destroyed) {
            source.destroy()
          }
          controller.error(error)
        }
      },
      async cancel(reason) {
        finished = true
        try {
          await options.onCancel?.(reason)
        } finally {
          if (!source.destroyed) {
            source.destroy()
          }
        }
      },
    },
    {
      highWaterMark: 1,
      size: () => 1,
    },
  )
}