// server/supabase/functions/llm-proxy/provider-deadline.ts
// Deadlines for the Anthropic call, split by phase.
//
// A single AbortSignal.timeout(45 s) on fetch also errors the response body,
// so it used to cut off streamed answers that took longer than 45 s in total.
// The phases now have their own limits:
//   * time to first byte (response headers): ttfbMs — covers the provider call.
//     For non-stream requests the whole body is the answer, so the deadline
//     stays armed until the JSON has been read.
//   * streamed body: an idle limit between chunks and a total limit for the
//     stream, sized for a max_tokens=4096 answer.
// The client's request signal is tied in, so a client that disconnects cancels
// the upstream generation instead of letting it run to completion.

export interface LlmProviderTimeouts {
  /** Response headers must arrive within this time. */
  ttfbMs: number
  /** Longest gap allowed between two streamed chunks. */
  streamIdleMs: number
  /** Longest a streamed body may run after the headers arrived. */
  streamTotalMs: number
}

export const DEFAULT_LLM_PROVIDER_TIMEOUTS: Readonly<LlmProviderTimeouts> = Object.freeze({
  ttfbMs: 45_000,
  streamIdleMs: 30_000,
  streamTotalMs: 180_000,
})

export function providerTimeoutError(message: string): DOMException {
  return new DOMException(message, 'TimeoutError')
}

export interface ProviderCallDeadline {
  /** Pass to fetch; aborts on the first-byte deadline, a stream deadline or a client disconnect. */
  readonly signal: AbortSignal
  /** Stop the first-byte timer (the response headers arrived). */
  headersReceived(): void
  abort(reason: unknown): void
  /** Clear timers and listeners. Safe to call more than once. */
  dispose(): void
}

export function createProviderCallDeadline(
  ttfbMs: number,
  clientSignal: AbortSignal | null,
): ProviderCallDeadline {
  const controller = new AbortController()
  let ttfbTimer: ReturnType<typeof setTimeout> | null = setTimeout(() => {
    ttfbTimer = null
    controller.abort(providerTimeoutError('provider response timeout'))
  }, ttfbMs)

  const onClientAbort = (): void => {
    controller.abort(clientSignal?.reason ?? new DOMException('client disconnected', 'AbortError'))
  }
  if (clientSignal?.aborted) onClientAbort()
  else clientSignal?.addEventListener('abort', onClientAbort, { once: true })

  const clearTtfb = (): void => {
    if (ttfbTimer !== null) clearTimeout(ttfbTimer)
    ttfbTimer = null
  }

  return {
    signal: controller.signal,
    headersReceived: clearTtfb,
    abort(reason: unknown): void {
      controller.abort(reason)
    },
    dispose(): void {
      clearTtfb()
      clientSignal?.removeEventListener('abort', onClientAbort)
    },
  }
}

/**
 * Tracks whether an Anthropic SSE stream reached `message_stop`.
 * Chunks can split the marker, so a short tail of the previous text is kept.
 */
export function createSseCompletionTracker(): {
  push(chunk: Uint8Array): void
  readonly completed: boolean
} {
  const MARKER = 'message_stop'
  const decoder = new TextDecoder()
  let tail = ''
  let completed = false
  return {
    push(chunk: Uint8Array): void {
      if (completed) return
      const text = tail + decoder.decode(chunk, { stream: true })
      if (text.includes(MARKER)) {
        completed = true
        tail = ''
        return
      }
      tail = text.slice(-(MARKER.length - 1))
    },
    get completed(): boolean {
      return completed
    },
  }
}

/** How a relayed stream ended. */
export type StreamRelayOutcome =
  /** Upstream ended after message_stop: the client received the full answer. */
  | 'completed'
  /** Upstream errored, timed out, or ended without message_stop. */
  | 'failed'
  /** The client stopped reading (disconnect/cancel). */
  | 'cancelled'

export interface StreamRelayOptions {
  idleMs: number
  totalMs: number
  /** Abort the upstream request (fetch signal). */
  abortUpstream(reason: unknown): void
  /** Called exactly once when the relay ends. Awaited before the client stream closes or errors. */
  onSettled(outcome: StreamRelayOutcome): Promise<void>
}

/**
 * Relay a provider SSE body to the client with an idle deadline between
 * chunks and a total deadline for the whole stream.
 */
export function relayProviderStream(
  upstream: ReadableStream<Uint8Array>,
  options: StreamRelayOptions,
): ReadableStream<Uint8Array> {
  const reader = upstream.getReader()
  const tracker = createSseCompletionTracker()
  let settled = false
  let totalTimer: ReturnType<typeof setTimeout> | null = null
  let idleTimer: ReturnType<typeof setTimeout> | null = null
  let rejectIdle: ((reason: unknown) => void) | null = null
  let downstream: ReadableStreamDefaultController<Uint8Array> | null = null

  const clearTimers = (): void => {
    if (totalTimer !== null) clearTimeout(totalTimer)
    if (idleTimer !== null) clearTimeout(idleTimer)
    totalTimer = null
    idleTimer = null
  }

  const settle = async (outcome: StreamRelayOutcome): Promise<boolean> => {
    if (settled) return false
    settled = true
    clearTimers()
    try {
      await options.onSettled(outcome)
    } catch {
      // Settlement is best effort; the caller logs its own failures.
    }
    return true
  }

  const terminate = async (reason: unknown): Promise<void> => {
    if (settled) return
    options.abortUpstream(reason)
    reader.cancel(reason).catch(() => undefined)
    rejectIdle?.(reason)
    if (await settle('failed')) {
      try {
        downstream?.error(reason)
      } catch {
        // Already closed.
      }
    }
  }

  const readWithIdleDeadline = (): Promise<ReadableStreamReadResult<Uint8Array>> =>
    new Promise((resolve, reject) => {
      rejectIdle = reject
      idleTimer = setTimeout(() => {
        idleTimer = null
        void terminate(providerTimeoutError('provider stream idle timeout'))
      }, options.idleMs)
      reader.read().then(resolve, reject).finally(() => {
        if (idleTimer !== null) clearTimeout(idleTimer)
        idleTimer = null
        rejectIdle = null
      })
    })

  return new ReadableStream<Uint8Array>({
    start(controller) {
      downstream = controller
      // Also fires while the client applies backpressure and no read is pending.
      totalTimer = setTimeout(() => {
        totalTimer = null
        void terminate(providerTimeoutError('provider stream total timeout'))
      }, options.totalMs)
    },
    async pull(controller) {
      if (settled) return
      let result: ReadableStreamReadResult<Uint8Array>
      try {
        result = await readWithIdleDeadline()
      } catch (err) {
        if (settled) return
        options.abortUpstream(err)
        if (await settle('failed')) controller.error(err)
        return
      }
      if (settled) return
      if (result.done) {
        const outcome: StreamRelayOutcome = tracker.completed ? 'completed' : 'failed'
        // A stream that ended without message_stop is still closed cleanly: the
        // client already treats a missing message_stop as incomplete, and any
        // in-band `event: error` the provider sent stays readable.
        if (await settle(outcome)) controller.close()
        return
      }
      tracker.push(result.value)
      controller.enqueue(result.value)
    },
    async cancel(reason) {
      options.abortUpstream(reason)
      reader.cancel(reason).catch(() => undefined)
      await settle('cancelled')
    },
  })
}