import { getStateSnapshot, sseFrame } from '@/lib/game/state';
import { subscribe } from '@/lib/sse';

// ISS-129 (v0.57.0): Server-Sent Events feed of state snapshots — the current snapshot on connect,
// then one after every sim tick (broadcast from lib/sim/engine.ts). One-way push; player actions
// stay REST POSTs. Heartbeats keep idle proxies from dropping the connection. ISS-133 (v0.59.0):
// frames carry the sseFrame projection (terminal package history dropped) — same shape as the
// pushed frames, so a connect never costs one full-history frame.

const encoder = new TextEncoder();
const HEARTBEAT_MS = 15_000;

export async function GET() {
  let heartbeat: ReturnType<typeof setInterval> | null = null;
  let unsubscribe: (() => void) | null = null;
  const cleanup = () => {
    if (heartbeat !== null) clearInterval(heartbeat);
    if (unsubscribe !== null) unsubscribe();
  };

  const stream = new ReadableStream<Uint8Array>({
    start(controller) {
      // Snapshot FIRST, then subscribe — a broadcast can never land before the initial event, so
      // events are strictly ordered by sim clock. A tick in the gap just means the client waits ≤1 s
      // for the next push instead of seeing an out-of-order frame.
      getStateSnapshot()
        .then((snapshot) => {
          unsubscribe = subscribe((text) => {
            try {
              // broadcast carries the bare snapshot JSON — wrap it in an SSE frame here (the route owns
              // wire framing, matching the initial snapshot + heartbeat above; lib/sse stays generic).
              controller.enqueue(encoder.encode(`data: ${text}\n\n`));
            } catch {
              // stream already closed — cleanup runs on cancel
            }
          });
          heartbeat = setInterval(() => {
            try {
              controller.enqueue(encoder.encode(': ping\n\n'));
            } catch {
              // same as above
            }
          }, HEARTBEAT_MS);
          controller.enqueue(encoder.encode(`data: ${JSON.stringify(sseFrame(snapshot))}\n\n`));
        })
        .catch(() => {
          cleanup();
          try {
            controller.close();
          } catch {
            // already closed
          }
        });
    },
    cancel() {
      cleanup(); // client went away — drop the subscription + heartbeat
    },
  });

  return new Response(stream, {
    headers: {
      'Content-Type': 'text/event-stream',
      'Cache-Control': 'no-cache',
      Connection: 'keep-alive',
      'X-Accel-Buffering': 'no', // prod sits behind Traefik — don't buffer the stream
    },
  });
}