export type SseEvent<T> = {
  id?: string;
  eventType: string;
  data: T;
};

export class EventStreamDecoder {
  private buffer = '';

  static async *decode<T = any>(body: ReadableStream<Uint8Array>): AsyncGenerator<SseEvent<T>> {
    const decoder = new EventStreamDecoder();
    for await (const chunk of body) {
      for (const { id, eventType, data } of decoder.push(Buffer.from(chunk))) {
        try {
          yield { id, eventType, data: JSON.parse(data) };
        } catch {
          // skip malformed events — non-fatal
        }
      }
    }
  }

  push(chunk: Buffer | string): SseEvent<string>[] {
    const text = typeof chunk === 'string' ? chunk : chunk.toString('utf8');
    this.buffer += text.replace(/\r\n/g, '\n').replace(/\r/g, '\n');

    const events: SseEvent<string>[] = [];
    let boundaryIdx: number;

    while ((boundaryIdx = this.buffer.indexOf('\n\n')) !== -1) {
      const eventText = this.buffer.slice(0, boundaryIdx);
      this.buffer = this.buffer.slice(boundaryIdx + 2);

      if (!eventText.trim()) continue;

      let id: string | undefined;
      let eventType = '';
      const dataLines: string[] = [];
      for (const line of eventText.split('\n')) {
        if (line.startsWith('id:')) {
          id = line.slice(3).trim();
        } else if (line.startsWith('event:')) {
          eventType = line.slice(6).trim();
        } else if (line.startsWith('data:')) {
          dataLines.push(line.slice(5).trim());
        }
      }

      const data = dataLines.join('\n');
      if (eventType && data) {
        events.push({ id, eventType, data });
      }
    }

    return events;
  }
}