/**
 * Decode an SSE byte stream into event `data` payloads. Framing — chunk
 * reassembly, UTF-8/CRLF/BOM handling, comment and non-data field skipping,
 * multi-`data:` joining — is `eventsource-parser`'s. Comments are reported
 * only through an optional transport-activity callback. This module keeps the
 * DeepSeek protocol: the literal `[DONE]` is yielded so the caller owns final
 * flushing, and EOF before it raises {@link LlmError}. Framing is spec-strict:
 * an event dispatches only on its blank-line terminator, so an unterminated
 * tail at EOF is truncation, not a flushable payload.
 *
 * @module dsh-llm-deepseek/sse
 */

import { EventSourceParserStream } from 'eventsource-parser/stream'
import { LlmError } from '@deepseek-ai/dsh-llm'

/** The terminal payload DeepSeek (and OpenAI) send after the last chunk. */
export const DONE = '[DONE]'

/**
 * Parse an SSE byte stream into data payloads. Yields `[DONE]` as the final
 * value and returns; throws `LlmError('STREAM_CLOSED')` when the stream ends
 * without it (truncated response — the model call cannot be trusted).
 * @param stream - raw SSE bytes; reads may split anywhere, including mid-UTF-8 sequence.
 * @param onComment - optional transport-activity callback; comments never enter the yielded payload stream.
 * @returns each event's data payload in arrival order, the `[DONE]` sentinel last.
 */
export async function* parseSse(
  stream: ReadableStream<BufferSource>,
  onComment?: (comment: string) => void,
): AsyncGenerator<string> {
  const events = stream
    .pipeThrough(new TextDecoderStream())
    .pipeThrough(new EventSourceParserStream({ onComment }))
  for await (const { data } of events) {
    yield data
    if (data === DONE) return
  }
  throw new LlmError('SSE stream ended without [DONE]', 'STREAM_CLOSED')
}