/* The tutor endpoint's wire behaviour, against a mock backend.

   Testable with no API key, which matters: the framing and the abort path
   are where this breaks, and neither needs a real model to exercise. A
   raw newline inside a `data:` field would silently truncate every Korean
   reply — that is the bug this file exists to catch. */

import { describe, it, expect } from "vitest";
import { tutorRoute } from "../../server/src/tutor.ts";
import type { TutorBackend, TutorEvent } from "../../server/src/backends/types.ts";

/** Emits the chunks it is given, then done. */
function mockBackend(chunks: string[], opts: { hang?: boolean } = {}): TutorBackend {
  return {
    name: "mock",
    async *stream(req): AsyncIterable<TutorEvent> {
      for (const text of chunks) {
        if (req.signal.aborted) return;
        yield { type: "delta", text };
      }
      if (opts.hang) {
        // Stay open until aborted, so the abort path can be observed.
        await new Promise<void>((resolve) => {
          if (req.signal.aborted) return resolve();
          req.signal.addEventListener("abort", () => resolve(), { once: true });
        });
        return;
      }
      yield { type: "done", stopReason: "end_turn" };
    },
  };
}

const post = (app: ReturnType<typeof tutorRoute>, body: unknown) =>
  app.request("/", {
    method: "POST",
    headers: { "content-type": "application/json" },
    body: JSON.stringify(body),
  });

/** Parse an SSE body into its events, the way the client does. */
function parseSSE(text: string): { event: string; data: string }[] {
  return text
    .split("\n\n")
    .filter((f) => f.trim())
    .map((frame) => {
      const lines = frame.split("\n");
      return {
        event: lines.find((l) => l.startsWith("event:"))?.slice(6).trim() ?? "",
        data: lines
          .filter((l) => l.startsWith("data:"))
          .map((l) => l.slice(5).trim())
          .join(""),
      };
    });
}

describe("tutor endpoint", () => {
  it("rejects a request with no system prompt or message", async () => {
    const app = tutorRoute(mockBackend(["hi"]));
    expect((await post(app, { message: "안녕" })).status).toBe(400);
    expect((await post(app, { system: "S" })).status).toBe(400);
  });

  it("streams deltas as SSE and closes with done", async () => {
    const app = tutorRoute(mockBackend(["안녕", "하세요"]));
    const res = await post(app, { system: "S", history: [], message: "안녕" });

    expect(res.status).toBe(200);
    expect(res.headers.get("content-type")).toContain("text/event-stream");
    // Compression or a caching proxy would break the stream; both are
    // refused explicitly.
    expect(res.headers.get("cache-control")).toContain("no-transform");

    const events = parseSSE(await res.text());
    const deltas = events.filter((e) => e.event === "delta").map((e) => JSON.parse(e.data).text);
    expect(deltas).toEqual(["안녕", "하세요"]);
    expect(events.at(-1)?.event).toBe("done");
  });

  /* The framing bug this guards against: a tutor reply is full of newlines
     — every ::words row, every task line — and a raw newline inside a
     `data:` field ends the event early, truncating the turn. */
  it("survives newlines in the payload, which every real reply has", async () => {
    const reply = "좋아.\n\n::task translate\n밥 먹어\n::\n::words\n밥 | rice\n::";
    const app = tutorRoute(mockBackend([reply]));
    const res = await post(app, { system: "S", history: [], message: "네" });

    const events = parseSSE(await res.text());
    const delta = events.find((e) => e.event === "delta");
    expect(JSON.parse(delta!.data).text).toBe(reply);
  });

  it("passes the transcript through and caps its length", async () => {
    let seen: { history: unknown[]; system: string } | null = null;
    const spy: TutorBackend = {
      name: "spy",
      async *stream(req) {
        seen = { history: req.history, system: req.system };
        yield { type: "done", stopReason: "end_turn" };
      },
    };
    const app = tutorRoute(spy);
    const history: { role: "user" | "assistant"; content: string }[] = Array.from(
      { length: 60 },
      (_, i) => ({ role: i % 2 ? "assistant" : "user", content: `turn ${i}` }),
    );

    const res = await post(app, { system: "GATE", history, message: "다음" });
    await res.text();

    expect(seen!.system).toBe("GATE");
    expect(seen!.history.length).toBe(40); // MAX_HISTORY
    expect((seen!.history.at(-1) as { content: string }).content).toBe("turn 59");
  });

  it("reports a backend error as an error event rather than a dead stream", async () => {
    const failing: TutorBackend = {
      name: "failing",
      // eslint-disable-next-line require-yield
      async *stream() {
        throw new Error("upstream exploded");
      },
    };
    const res = await post(tutorRoute(failing), { system: "S", history: [], message: "x" });
    const events = parseSSE(await res.text());
    expect(events.at(-1)?.event).toBe("error");
    expect(JSON.parse(events.at(-1)!.data).message).toContain("upstream exploded");
  });

  /* The abort path is what makes "Stop" stop billing: the client going away
     has to reach the backend's signal, not just close the socket. */
  it("aborts the backend when the client goes away", async () => {
    let seen: AbortSignal | null = null;
    const holding: TutorBackend = {
      name: "holding",
      async *stream(req) {
        seen = req.signal;
        yield { type: "delta", text: "부" };
        await new Promise<void>((resolve) =>
          req.signal.addEventListener("abort", () => resolve(), { once: true }),
        );
      },
    };

    const res = await post(tutorRoute(holding), { system: "S", history: [], message: "x" });
    const reader = res.body!.getReader();
    await reader.read(); // the turn is live and the backend holds the signal

    expect(seen).not.toBeNull();
    expect(seen!.aborted).toBe(false);

    await reader.cancel(); // the client goes away

    for (let i = 0; i < 50 && !seen!.aborted; i++) {
      await new Promise((r) => setTimeout(r, 20));
    }
    expect(seen!.aborted, "the backend must see the client disconnect").toBe(true);
  });
});