/* The OpenAI-compatible backend, against a fake server that behaves the way
   real ones do — including the awkward ways.

   The whole point of this backend is that it is not written against an SDK,
   so the parsing is ours to get right. The cases that actually break a
   hand-rolled SSE reader are here: a JSON payload split across two writes,
   an event split across two writes, heartbeat comment lines, CRLF framing,
   and a stream that ends without ever sending [DONE]. */

import { describe, it, expect, beforeAll, afterAll } from "vitest";
import { createServer, type Server } from "node:http";
import { openaiBackend, makeThinkStripper } from "../../server/src/backends/openai.ts";
import type { TutorEvent } from "../../server/src/backends/types.ts";

/** What the next request should write, as a list of raw TCP writes. */
let script: string[] = [];
let lastBody: Record<string, unknown> = {};
let status = 200;
let server: Server;
let base = "";

beforeAll(async () => {
  server = createServer((req, res) => {
    let body = "";
    req.on("data", (c) => (body += c));
    req.on("end", async () => {
      lastBody = body ? (JSON.parse(body) as Record<string, unknown>) : {};
      if (status !== 200) {
        res.writeHead(status, { "content-type": "application/json" });
        res.end(JSON.stringify({ error: { message: "model not loaded" } }));
        return;
      }
      res.writeHead(200, { "content-type": "text/event-stream" });
      for (const piece of script) {
        res.write(piece);
        // Force separate reads, which is what splits a JSON payload.
        await new Promise((r) => setTimeout(r, 5));
      }
      res.end();
    });
  });
  await new Promise<void>((r) => server.listen(0, "127.0.0.1", r));
  const addr = server.address();
  base = `http://127.0.0.1:${typeof addr === "object" && addr ? addr.port : 0}/v1`;
});

afterAll(() => new Promise<void>((r) => server.close(() => r())));

const delta = (text: string) =>
  `data: ${JSON.stringify({ choices: [{ delta: { content: text } }] })}\n\n`;

async function collect(message = "hi"): Promise<TutorEvent[]> {
  const backend = openaiBackend({ baseUrl: base, model: "test-model" });
  const out: TutorEvent[] = [];
  for await (const e of backend.stream({
    system: "SYSTEM",
    history: [{ role: "user", content: "earlier" }],
    message,
    signal: new AbortController().signal,
  })) {
    out.push(e);
  }
  return out;
}

const textOf = (events: TutorEvent[]) =>
  events.filter((e) => e.type === "delta").map((e) => (e as { text: string }).text).join("");

describe("openai backend", () => {
  beforeAll(() => {
    status = 200;
  });

  it("streams deltas and finishes", async () => {
    script = [delta("안녕"), delta("하세요"), 'data: [DONE]\n\n'];
    const events = await collect();
    expect(textOf(events)).toBe("안녕하세요");
    expect(events[events.length - 1]!.type).toBe("done");
  });

  it("sends the system prompt as the first message, before the history", async () => {
    script = ['data: [DONE]\n\n'];
    await collect("what now");
    const msgs = lastBody.messages as { role: string; content: string }[];
    expect(msgs[0]).toEqual({ role: "system", content: "SYSTEM" });
    expect(msgs[1]).toEqual({ role: "user", content: "earlier" });
    expect(msgs[2]).toEqual({ role: "user", content: "what now" });
    expect(lastBody.stream).toBe(true);
    expect(lastBody.model).toBe("test-model");
  });

  it("reassembles a JSON payload split across two reads", async () => {
    const whole = delta("split me");
    script = [whole.slice(0, 30), whole.slice(30), 'data: [DONE]\n\n'];
    expect(textOf(await collect())).toBe("split me");
  });

  it("reassembles an event whose blank-line terminator lands in the next read", async () => {
    const a = delta("one");
    script = [a.slice(0, a.length - 1), a.slice(a.length - 1) + delta("two"), 'data: [DONE]\n\n'];
    expect(textOf(await collect())).toBe("onetwo");
  });

  it("ignores heartbeat comments and tolerates CRLF framing", async () => {
    script = [
      ": ping\n\n",
      `data: ${JSON.stringify({ choices: [{ delta: { content: "ok" } }] })}\r\n\r\n`,
      'data: [DONE]\n\n',
    ];
    expect(textOf(await collect())).toBe("ok");
  });

  it("ends cleanly when the server never sends [DONE]", async () => {
    script = [delta("abrupt")];
    const events = await collect();
    expect(textOf(events)).toBe("abrupt");
    expect(events[events.length - 1]!.type).toBe("done");
  });

  it("reports finish_reason", async () => {
    script = [
      `data: ${JSON.stringify({ choices: [{ delta: { content: "x" }, finish_reason: "length" }] })}\n\n`,
      'data: [DONE]\n\n',
    ];
    const done = (await collect()).at(-1) as { type: string; stopReason: string };
    expect(done.stopReason).toBe("length");
  });

  it("surfaces an HTTP error body rather than an empty reply", async () => {
    status = 400;
    const events = await collect();
    status = 200;
    const err = events[0] as { type: string; message: string };
    expect(err.type).toBe("error");
    // "model not loaded" is the most common local failure; it must reach the UI.
    expect(err.message).toContain("model not loaded");
  });

  it("reports an unreachable server instead of hanging", async () => {
    const backend = openaiBackend({ baseUrl: "http://127.0.0.1:1/v1" });
    const out: TutorEvent[] = [];
    for await (const e of backend.stream({
      system: "s",
      history: [],
      message: "m",
      signal: new AbortController().signal,
    })) {
      out.push(e);
    }
    const err = out[0] as { type: string; code: string };
    expect(err.type).toBe("error");
    expect(err.code).toBe("unreachable");
  });

  it("stops on abort without reporting a failure", async () => {
    script = [delta("a"), delta("b"), delta("c"), 'data: [DONE]\n\n'];
    const backend = openaiBackend({ baseUrl: base });
    const ctrl = new AbortController();
    const out: TutorEvent[] = [];
    for await (const e of backend.stream({
      system: "s",
      history: [],
      message: "m",
      signal: ctrl.signal,
    })) {
      out.push(e);
      if (e.type === "delta") ctrl.abort();
    }
    const last = out.at(-1) as { type: string; stopReason: string };
    expect(last.type).toBe("done");
    expect(last.stopReason).toBe("cancelled");
  });
});

describe("<think> stripping", () => {
  it("removes a reasoning block that arrives whole", () => {
    const strip = makeThinkStripper();
    expect(strip("<think>hmm</think>안녕")).toBe("안녕");
  });

  it("removes one split across many chunks, tags included", () => {
    const strip = makeThinkStripper();
    const out = ["<thi", "nk>let me ", "reason</thi", "nk>", "안녕", "하세요"]
      .map(strip)
      .join("");
    expect(out).toBe("안녕하세요");
  });

  it("never holds back ordinary text", () => {
    const strip = makeThinkStripper();
    expect(["a", "b", "c"].map(strip).join("")).toBe("abc");
  });

  it("keeps text on both sides of the block", () => {
    const strip = makeThinkStripper();
    expect(["before <think>x", "yz</think> after"].map(strip).join("")).toBe("before  after");
  });

  it("does not eat a lone angle bracket that is not a tag", () => {
    const strip = makeThinkStripper();
    expect(["1 < 2", " and 3 > 2"].map(strip).join("")).toBe("1 < 2 and 3 > 2");
  });
});