Curio/src/app/api/lessons/stream/route.ts

import { type NextRequest } from 'next/server';
import { getStreamSegments, isStreamDone } from '@/lib/cache/stream';
import { DEFAULT_DEPTH, type LessonDepth } from '@/lib/generation/depth';
import type { AgeGroup } from '@/schemas/preferences';

export const runtime = 'nodejs';
export const dynamic = 'force-dynamic';

const POLL_INTERVAL_MS = 500;
const SEGMENT_EMIT_DELAY_MS = 300; // paced delivery between segments
const MAX_DURATION_MS = 28_000; // leave headroom before 30s Vercel limit

const sleep = (ms: number) => new Promise<void>((resolve) => setTimeout(resolve, ms));

function sseEvent(event: string, data: string): string {
  return `event: ${event}\ndata: ${data}\n\n`;
}

export async function GET(request: NextRequest): Promise<Response> {
  const { searchParams } = new URL(request.url);
  const key = searchParams.get('key') ?? '';
  const locale = searchParams.get('locale') ?? 'en';
  const depth = (searchParams.get('depth') ?? DEFAULT_DEPTH) as LessonDepth;
  const ageGroup = (searchParams.get('ageGroup') ?? 'adult') as AgeGroup;
  const position = parseInt(searchParams.get('position') ?? '0', 10);

  const encoder = new TextEncoder();
  const abortSignal = request.signal;

  const stream = new ReadableStream({
    async start(controller) {
      const enqueue = (event: string, data: string) => {
        try {
          controller.enqueue(encoder.encode(sseEvent(event, data)));
        } catch {
          // Controller may be closed if client disconnected
        }
      };

      let emittedCount = 0;
      const deadline = Date.now() + MAX_DURATION_MS;

      try {
        while (Date.now() < deadline && !abortSignal.aborted) {
          const segments = await getStreamSegments(key, locale, depth, ageGroup, position);

          // Emit newly-arrived segments one-by-one with pacing
          while (emittedCount < segments.length && !abortSignal.aborted) {
            enqueue('segment', JSON.stringify(segments[emittedCount]));
            emittedCount++;
            if (emittedCount < segments.length) {
              await sleep(SEGMENT_EMIT_DELAY_MS);
            }
          }

          const done = await isStreamDone(key, locale, depth, ageGroup, position);
          if (done && emittedCount >= segments.length) {
            enqueue('ready', '{}');
            break;
          }

          await sleep(POLL_INTERVAL_MS);
        }
      } catch {
        // Swallow — client falls back to the 4s poll
      } finally {
        try { controller.close(); } catch { /* already closed */ }
      }
    },
  });

  return new Response(stream, {
    headers: {
      'Content-Type': 'text/event-stream; charset=utf-8',
      'Cache-Control': 'no-cache, no-transform',
      'Connection': 'keep-alive',
      'X-Accel-Buffering': 'no',
    },
  });
}