import assert from "node:assert/strict";
import test from "node:test";

function createRegistrationApi() {
  const seats = new Map([["CTDL01", 1], ["KTPM01", 0], ["PTTK01", 2]]);
  const completedPrerequisite = new Set(["SV01", "SV02"]);
  const enrolled = new Set();

  return async function handle(request) {
    const path = new URL(request.url).pathname;
    if (path !== "/registrations") return Response.json({ code: "NOT_FOUND" }, { status: 404 });
    if (request.method !== "POST") {
      return Response.json({ code: "METHOD_NOT_ALLOWED" }, { status: 405, headers: { Allow: "POST" } });
    }

    let body;
    try {
      body = await request.json();
    } catch {
      return Response.json({ code: "INVALID_JSON" }, { status: 400 });
    }
    const { studentId, courseId } = body ?? {};
    if (typeof studentId !== "string" || !studentId.trim() ||
        typeof courseId !== "string" || !courseId.trim()) {
      return Response.json({ code: "INVALID_INPUT" }, { status: 400 });
    }
    if (!seats.has(courseId)) {
      return Response.json({ code: "COURSE_NOT_FOUND" }, { status: 404 });
    }

    const key = JSON.stringify([studentId, courseId]);
    if (enrolled.has(key)) {
      return Response.json({ code: "ALREADY_ENROLLED" }, { status: 409 });
    }
    if (!completedPrerequisite.has(studentId)) {
      return Response.json({ code: "MISSING_PREREQUISITE" }, { status: 409 });
    }
    const remaining = seats.get(courseId);
    if (remaining === 0) {
      return Response.json({ code: "COURSE_FULL" }, { status: 409 });
    }

    enrolled.add(key);
    seats.set(courseId, remaining - 1);
    return Response.json({ studentId, courseId, remainingSeats: remaining - 1 }, { status: 201 });
  };
}

function post(body) {
  return new Request("https://example.test/registrations", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify(body)
  });
}

test("201 trả dữ liệu đăng ký và số chỗ còn lại", async () => {
  const api = createRegistrationApi();
  const response = await api(post({ studentId: "SV01", courseId: "CTDL01" }));
  assert.equal(response.status, 201);
  assert.match(response.headers.get("Content-Type"), /^application\/json/);
  assert.deepEqual(await response.json(), {
    studentId: "SV01", courseId: "CTDL01", remainingSeats: 0
  });
});

test("400 cho dữ liệu thiếu và 404 cho mã lớp sai", async () => {
  const api = createRegistrationApi();
  for (const [body, status, code] of [
    [{ courseId: "CTDL01" }, 400, "INVALID_INPUT"],
    [{ studentId: "SV01", courseId: "SAI" }, 404, "COURSE_NOT_FOUND"]
  ]) {
    const response = await api(post(body));
    assert.equal(response.status, status);
    assert.deepEqual(await response.json(), { code });
  }
});

test("409 trả đúng lý do và không làm mất chỗ", async () => {
  const api = createRegistrationApi();
  const denied = await api(post({ studentId: "SV03", courseId: "CTDL01" }));
  assert.equal(denied.status, 409);
  assert.deepEqual(await denied.json(), { code: "MISSING_PREREQUISITE" });

  const accepted = await api(post({ studentId: "SV01", courseId: "CTDL01" }));
  assert.equal(accepted.status, 201);
  assert.equal((await accepted.json()).remainingSeats, 0);
});

test("phân biệt đăng ký trùng với lớp đầy", async () => {
  const api = createRegistrationApi();
  assert.equal((await api(post({ studentId: "SV01", courseId: "CTDL01" }))).status, 201);

  const duplicate = await api(post({ studentId: "SV01", courseId: "CTDL01" }));
  assert.equal(duplicate.status, 409);
  assert.deepEqual(await duplicate.json(), { code: "ALREADY_ENROLLED" });

  const full = await api(post({ studentId: "SV02", courseId: "CTDL01" }));
  assert.equal(full.status, 409);
  assert.deepEqual(await full.json(), { code: "COURSE_FULL" });
});