import type { NextApiRequest, NextApiResponse } from 'next'

// Adapter that lets Next.js pages-router API handlers run under TanStack
// Start server routes without rewriting them. The adapter builds a
// NextApiRequest-shaped object from the Web `Request`, plus a
// NextApiResponse proxy that captures `status` / `setHeader` / `json` /
// `send` / `write` / `end` / `redirect` and produces a Web `Response`.
//
// Limitations (acceptable for the bulk of studio API routes):
// - Body is buffered: streaming handlers that want to flush to the client
//   before finishing will still buffer in memory. Studio has 2 such routes
//   (functions body.ts, mcp/index.ts) that need bespoke handling.
// - Body parsing covers JSON and urlencoded. Multipart/raw binary inbound
//   is not implemented — no studio route reads multipart in.

type NextHandler = (req: NextApiRequest, res: NextApiResponse) => unknown | Promise<unknown>

interface RouteCtx {
  request: Request
  params?: Record<string, string | undefined>
}

export function toWebHandler(handler: NextHandler) {
  return async ({ request, params = {} }: RouteCtx): Promise<Response> => {
    const req = await buildRequest(request, params)
    const { res, finalize } = buildResponse()

    const result = await handler(req, res)

    // Handlers like /api/ai/docs.ts already return a Web Response directly
    // (they were written for the edge runtime). Pass it through.
    if (result instanceof Response) return result

    return finalize()
  }
}

async function buildRequest(
  request: Request,
  params: Record<string, string | undefined>
): Promise<NextApiRequest> {
  const url = new URL(request.url)
  const method = request.method.toUpperCase()

  const headers: Record<string, string | string[]> = {}
  request.headers.forEach((value, key) => {
    const k = key.toLowerCase()
    const existing = headers[k]
    if (existing === undefined) {
      headers[k] = value
    } else if (Array.isArray(existing)) {
      existing.push(value)
    } else {
      headers[k] = [existing, value]
    }
  })

  const cookies: Record<string, string> = {}
  const cookieHeader = request.headers.get('cookie')
  if (cookieHeader) {
    for (const part of cookieHeader.split(';')) {
      const trimmed = part.trim()
      if (!trimmed) continue
      const eq = trimmed.indexOf('=')
      const name = eq >= 0 ? trimmed.slice(0, eq) : trimmed
      const value = eq >= 0 ? trimmed.slice(eq + 1) : ''
      cookies[name] = decodeURIComponent(value)
    }
  }

  // Next merges route params and URL search params into `req.query`.
  // Duplicate keys become arrays (URLSearchParams.getAll). Route params
  // take precedence over search params of the same name, so they're
  // applied last.
  const query: Record<string, string | string[]> = {}
  for (const key of new Set(url.searchParams.keys())) {
    const values = url.searchParams.getAll(key)
    query[key] = values.length === 1 ? values[0] : values
  }
  for (const [k, v] of Object.entries(params)) {
    if (v !== undefined) query[k] = v
  }

  let body: unknown = undefined
  if (method !== 'GET' && method !== 'HEAD' && request.body) {
    const contentType = request.headers.get('content-type') ?? ''
    if (contentType.includes('application/json')) {
      const text = await request.text()
      body = text ? JSON.parse(text) : undefined
    } else if (contentType.includes('application/x-www-form-urlencoded')) {
      const text = await request.text()
      body = Object.fromEntries(new URLSearchParams(text))
    } else {
      body = await request.text()
    }
  }

  // Surface a tiny subset of Node's `IncomingMessage` EventEmitter API so
  // handlers that wire client-abort via `req.on('close', …)` /
  // `req.on('aborted', …)` (e.g. AI streaming routes) can still hook into
  // the Web `Request.signal`. We don't model arbitrary event types — only
  // `close` and `aborted`, both of which fire when the signal aborts.
  const listeners: Record<string, Set<(...args: unknown[]) => void>> = {}
  const fireAbort = () => {
    for (const event of ['close', 'aborted'] as const) {
      const set = listeners[event]
      if (!set) continue
      for (const fn of set) {
        try {
          fn()
        } catch {
          // Swallow — Node would emit `error` on the request, but we
          // have nothing meaningful to do with it here.
        }
      }
    }
  }
  if (request.signal.aborted) {
    queueMicrotask(fireAbort)
  } else {
    request.signal.addEventListener('abort', fireAbort, { once: true })
  }

  return {
    method,
    url: url.pathname + url.search,
    headers,
    query,
    body,
    cookies,
    on(event: string, fn: (...args: unknown[]) => void) {
      ;(listeners[event] ??= new Set()).add(fn)
      return this
    },
    off(event: string, fn: (...args: unknown[]) => void) {
      listeners[event]?.delete(fn)
      return this
    },
    once(event: string, fn: (...args: unknown[]) => void) {
      const wrapper = (...args: unknown[]) => {
        listeners[event]?.delete(wrapper)
        fn(...args)
      }
      ;(listeners[event] ??= new Set()).add(wrapper)
      return this
    },
    removeListener(event: string, fn: (...args: unknown[]) => void) {
      listeners[event]?.delete(fn)
      return this
    },
    removeAllListeners(event?: string) {
      if (event) listeners[event]?.clear()
      else for (const set of Object.values(listeners)) set.clear()
      return this
    },
    emit(event: string, ...args: unknown[]) {
      const set = listeners[event]
      if (!set) return false
      for (const fn of set) fn(...args)
      return true
    },
  } as unknown as NextApiRequest
}

function buildResponse() {
  const responseHeaders = new Headers()
  const chunks: Uint8Array[] = []
  const encoder = new TextEncoder()

  const encode = (data: unknown): Uint8Array => {
    if (data instanceof Uint8Array) return data
    if (typeof data === 'string') return encoder.encode(data)
    if (data === undefined || data === null) return new Uint8Array(0)
    if (typeof Buffer !== 'undefined' && Buffer.isBuffer(data)) {
      return new Uint8Array(data)
    }
    return encoder.encode(String(data))
  }

  // Streaming mode is entered as soon as the handler calls
  // `res.writeHead(...)` — that's the Node `ServerResponse` signal that
  // headers are committed and chunks should flush as they're written
  // (matches what `result.pipeUIMessageStreamToResponse(res, …)` from
  // the AI SDK does for token-by-token streaming).
  let streamMode = false
  let streamController: ReadableStreamDefaultController<Uint8Array> | null = null
  let streamClosed = false
  // Backpressure for upstream writers: drain when the controller's
  // desired-size goes negative. We don't get a true "drain" event
  // from a Web ReadableStream, but most handlers just check the
  // return value of `res.write()`.
  const stream = new ReadableStream<Uint8Array>({
    start(controller) {
      streamController = controller
    },
    cancel() {
      streamClosed = true
    },
  })
  const enterStreamMode = () => {
    if (streamMode) return
    streamMode = true
    // Flush anything that was buffered before the streaming switch.
    for (const c of chunks) streamController?.enqueue(c)
    chunks.length = 0
  }

  const res = {} as NextApiResponse & Record<string, unknown>

  // `res.statusCode` is the single source of truth so handlers that write
  // it directly (a standard Node/Next idiom) are honoured at finalize.
  res.statusCode = 200
  res.status = (code: number) => {
    res.statusCode = code
    return res
  }
  res.setHeader = (name: string, value: number | string | readonly string[]) => {
    const key = name.toLowerCase()
    if (Array.isArray(value)) {
      responseHeaders.delete(key)
      for (const v of value) responseHeaders.append(key, String(v))
    } else {
      responseHeaders.set(key, String(value))
    }
    return res
  }
  res.getHeader = (name: string) => responseHeaders.get(name.toLowerCase()) ?? undefined
  res.getHeaders = () => {
    const out: Record<string, string> = {}
    responseHeaders.forEach((v, k) => {
      out[k] = v
    })
    return out
  }
  res.hasHeader = (name: string) => responseHeaders.has(name.toLowerCase())
  res.removeHeader = (name: string) => {
    responseHeaders.delete(name.toLowerCase())
  }
  // Node `ServerResponse.writeHead(statusCode, statusMessage?, headers?)`.
  // We honour `statusCode` and `headers`; `statusMessage` is dropped
  // (Fetch `Response` doesn't preserve a custom HTTP/1 reason phrase
  // when running through TanStack's runtime).
  type WriteHead = (
    code: number,
    headersOrMessage?: string | Record<string, number | string | readonly string[]>,
    maybeHeaders?: Record<string, number | string | readonly string[]>
  ) => unknown
  ;(res as unknown as { writeHead: WriteHead }).writeHead = (
    code: number,
    headersOrMessage?: string | Record<string, number | string | readonly string[]>,
    maybeHeaders?: Record<string, number | string | readonly string[]>
  ) => {
    res.statusCode = code
    const headers =
      typeof headersOrMessage === 'object' && headersOrMessage !== null
        ? headersOrMessage
        : maybeHeaders
    if (headers) {
      for (const [name, value] of Object.entries(headers)) {
        const key = name.toLowerCase()
        if (Array.isArray(value)) {
          responseHeaders.delete(key)
          for (const v of value) responseHeaders.append(key, String(v))
        } else if (value !== undefined && value !== null) {
          responseHeaders.set(key, String(value))
        }
      }
    }
    enterStreamMode()
    return res
  }
  ;(res as unknown as { flushHeaders: () => unknown }).flushHeaders = () => {
    enterStreamMode()
    return res
  }
  res.json = (data: unknown) => {
    if (!responseHeaders.has('content-type')) {
      responseHeaders.set('content-type', 'application/json')
    }
    const payload = encode(JSON.stringify(data))
    if (streamMode) streamController?.enqueue(payload)
    else chunks.push(payload)
    return res
  }
  res.send = (data: unknown) => {
    if (data === undefined || data === null) return res
    if (
      typeof data === 'object' &&
      !(data instanceof Uint8Array) &&
      !(typeof Buffer !== 'undefined' && Buffer.isBuffer(data))
    ) {
      return res.json(data)
    }
    const payload = encode(data)
    if (streamMode) streamController?.enqueue(payload)
    else chunks.push(payload)
    return res
  }
  res.write = (chunk: unknown) => {
    const payload = encode(chunk)
    if (streamMode) streamController?.enqueue(payload)
    else chunks.push(payload)
    return true
  }
  res.end = (data?: unknown) => {
    if (streamMode) {
      if (data !== undefined && data !== null) streamController?.enqueue(encode(data))
      if (!streamClosed) {
        try {
          streamController?.close()
        } catch {
          // Already closed (or never opened): nothing to do.
        }
        streamClosed = true
      }
    } else if (data !== undefined) {
      chunks.push(encode(data))
    }
    return res
  }
  res.redirect = (...args: unknown[]) => {
    const [status, location] =
      typeof args[0] === 'number' ? [args[0], args[1] as string] : [302, args[0] as string]
    res.statusCode = status
    responseHeaders.set('location', location)
    return res
  }

  // Node `ServerResponse` is a Writable EventEmitter. AI SDK / other
  // pipe-style helpers may attach listeners (drain/close/error) for
  // backpressure or cleanup. Accept and no-op them so they don't
  // crash; abort is already handled at the `req` level via the
  // Request.signal subscription.
  const noopEE = (...args: unknown[]) => {
    void args
    return res
  }
  ;(res as unknown as Record<string, unknown>).on = noopEE
  ;(res as unknown as Record<string, unknown>).once = noopEE
  ;(res as unknown as Record<string, unknown>).off = noopEE
  ;(res as unknown as Record<string, unknown>).removeListener = noopEE
  ;(res as unknown as Record<string, unknown>).removeAllListeners = noopEE
  ;(res as unknown as Record<string, unknown>).emit = () => false
  ;(res as unknown as Record<string, unknown>).addListener = noopEE
  ;(res as unknown as Record<string, unknown>).prependListener = noopEE
  ;(res as unknown as { headersSent: boolean }).headersSent = false
  ;(res as unknown as { writableEnded: boolean }).writableEnded = false

  const finalize = (): Response => {
    if (streamMode) {
      // The handler entered streaming mode (writeHead/flushHeaders) and
      // may still be pushing chunks asynchronously after this return —
      // the stream stays open until res.end() runs.
      return new Response(stream, { status: res.statusCode, headers: responseHeaders })
    }
    const totalLen = chunks.reduce((n, c) => n + c.length, 0)
    const payload = new Uint8Array(totalLen)
    let offset = 0
    for (const c of chunks) {
      payload.set(c, offset)
      offset += c.length
    }
    return new Response(payload.length === 0 ? null : payload, {
      status: res.statusCode,
      headers: responseHeaders,
    })
  }

  return { res, finalize }
}