discovery-by-llm/frontend/src/lib/recommendationStream.ts

import { formatMessage, messages } from './messages'
import type { RecommendationRequest, StreamEvent } from './types'
import type { TransportFailureKind } from './models'
import { parseStreamEvent, StreamParseError } from './streamParser'

type StreamPhase = 'metadata' | 'events' | 'terminal'

export class StreamTransportError extends Error {
  readonly kind: TransportFailureKind

  constructor(kind: TransportFailureKind, message: string) {
    super(message)
    this.name = 'StreamTransportError'
    this.kind = kind
  }
}

function parseLine(line: string): StreamEvent {
  let value: unknown
  try {
    value = JSON.parse(line)
  } catch {
    throw new StreamTransportError('parse', messages.recommendationStreamInvalidJson)
  }

  try {
    return parseStreamEvent(value)
  } catch (error) {
    if (error instanceof StreamParseError) {
      throw new StreamTransportError('parse', error.message)
    }
    throw error
  }
}

function advancePhase(phase: StreamPhase, event: StreamEvent): StreamPhase {
  if (phase === 'metadata') {
    if (event.type === 'metadata') return 'events'
    if (event.type === 'error') return 'terminal'
    throw new StreamTransportError('protocol', messages.recommendationStreamMissingMetadata)
  }
  if (phase === 'events') {
    if (event.type === 'track' || event.type === 'warning') return 'events'
    if (event.type === 'done' || event.type === 'error') return 'terminal'
    throw new StreamTransportError('protocol', messages.recommendationStreamDuplicateMetadata)
  }
  throw new StreamTransportError('protocol', messages.recommendationStreamAfterFinalEvent)
}

async function cancelBody(body: ReadableStream<Uint8Array> | null): Promise<void> {
  try {
    await body?.cancel()
  } catch {
    return
  }
}

async function cancelReader(reader: ReadableStreamDefaultReader<Uint8Array>): Promise<void> {
  try {
    await reader.cancel()
  } catch {
    return
  }
}

function releaseReader(reader: ReadableStreamDefaultReader<Uint8Array>): void {
  try {
    reader.releaseLock()
  } catch {
    return
  }
}

/** Post a recommendation request and consume its NDJSON event stream. */
export async function streamRecommendations(
  request: RecommendationRequest,
  signal: AbortSignal,
  onEvent: (event: StreamEvent) => void,
): Promise<void> {
  let response: Response
  try {
    response = await fetch('/api/recommendations', {
      method: 'POST',
      credentials: 'same-origin',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(request),
      signal,
    })
  } catch (error) {
    if (signal.aborted) throw error
    throw new StreamTransportError('network', messages.recommendationStreamUnavailable)
  }

  if (!response.ok) {
    await cancelBody(response.body)
    throw new StreamTransportError(
      'http',
      formatMessage('recommendationStreamHttpFailure', { status: response.status }),
    )
  }
  const contentType = response.headers.get('content-type')?.split(';')[0].trim()
  if (contentType !== 'application/x-ndjson') {
    await cancelBody(response.body)
    throw new StreamTransportError('protocol', messages.recommendationStreamInvalidContentType)
  }
  if (!response.body) {
    throw new StreamTransportError('protocol', messages.recommendationStreamEmpty)
  }

  const reader = response.body.getReader()
  const decoder = new TextDecoder()
  let buffer = ''
  let phase: StreamPhase = 'metadata'

  try {
    while (true) {
      const { done, value } = await reader.read()
      buffer += decoder.decode(value, { stream: !done })
      const lines = buffer.split('\n')
      buffer = lines.pop() ?? ''

      for (const rawLine of lines) {
        const line = rawLine.trim()
        if (!line) continue
        const event = parseLine(line)
        phase = advancePhase(phase, event)
        onEvent(event)
        if (phase === 'terminal') {
          await cancelReader(reader)
          return
        }
      }

      if (done) break
    }

    const finalLine = buffer.trim()
    if (finalLine) {
      const event = parseLine(finalLine)
      phase = advancePhase(phase, event)
      onEvent(event)
      if (phase === 'terminal') return
    }

    throw new StreamTransportError('unexpected_eof', messages.recommendationStreamUnexpectedEnd)
  } catch (error) {
    await cancelReader(reader)
    if (signal.aborted || error instanceof StreamTransportError) throw error
    throw new StreamTransportError('network', messages.recommendationStreamInterrupted)
  } finally {
    releaseReader(reader)
  }
}

/** Return whether a rejected stream operation was intentionally aborted. */
export function isAbortError(error: unknown, signal: AbortSignal): boolean {
  return signal.aborted || (error instanceof DOMException && error.name === 'AbortError')
}