quiz/static/quiz.js

/* Student quiz SPA.
 *
 * Visit /?sid=<id>. If no cookie, render the join form. If cookie, open
 * the student WS and follow server messages through the lifecycle:
 *   lobby → question_open → submitted → question_closed → … → session_ended
 *
 * The server is authoritative for state transitions and scoring. The
 * client only animates the UI for whatever message the server sent.
 */

const app = document.querySelector("#app");
const params = new URLSearchParams(window.location.search);
const sid = params.get("sid");

const store = {
  me: null,
  ws: null,
  currentQuestion: null,
  submitted: null,
  pickedAnswer: null,
  deadlineMs: null,
};

// WS reconnect with exponential backoff. Total budget is ~27s across 8
// attempts (500ms, 1s, 2s, 4s, then 5s × 4), which covers typical mobile
// hand-off and Aliyun-edge TLS hiccups without giving up too quickly.
const RECONNECT = {
  attempt: 0,
  maxAttempts: 8,
  baseDelayMs: 500,
  maxDelayMs: 5000,
  timer: null,
};

let countdownTimer = null;

/* Tab-blur audit. We POST a server event whenever the student
 * backgrounds the page (visibilitychange) or moves focus away from the
 * window (blur). Both are debounced so a rapid alt-tab roundtrip
 * doesn't spam events. The server records each event in `student_events`
 * and surfaces a count to the instructor presence panel.
 *
 * We only ping during a question_open state — switching tabs between
 * questions is fine and we don't want to noise the audit. */
const FOCUS = {
  lastBlur: 0,
  lastHidden: 0,
  debounceMs: 1500,
};

function postEvent(kind) {
  if (!sid || !store.currentQuestion || store.submitted) return;
  // Use sendBeacon when leaving the page so the event survives the
  // navigation; otherwise fetch with credentials so the cookie rides.
  const body = JSON.stringify({ kind, question_idx: store.currentQuestion.question_idx });
  const url = `/api/session/${sid}/event`;
  if (kind === "visibility_hidden" && navigator.sendBeacon) {
    const blob = new Blob([body], { type: "application/json" });
    navigator.sendBeacon(url, blob);
    return;
  }
  fetch(url, {
    method: "POST",
    credentials: "same-origin",
    headers: { "Content-Type": "application/json" },
    body,
    keepalive: true,
  }).catch(() => {});
}

function onBlur() {
  const now = Date.now();
  if (now - FOCUS.lastBlur < FOCUS.debounceMs) return;
  FOCUS.lastBlur = now;
  postEvent("blur");
}

function onVisibility() {
  const now = Date.now();
  if (document.visibilityState === "hidden") {
    if (now - FOCUS.lastHidden < FOCUS.debounceMs) return;
    FOCUS.lastHidden = now;
    postEvent("visibility_hidden");
  } else if (document.visibilityState === "visible") {
    postEvent("visibility_visible");
  }
}

window.addEventListener("blur", onBlur);
document.addEventListener("visibilitychange", onVisibility);

function fmtScore(value) {
  // Scores are floats on a 0.05 grid in [0, 1]. Display as a fixed
  // two-decimal string so users see e.g. "0.85" instead of
  // "0.8500000000000001" when float math drifts in the leaderboard sum.
  const n = Number(value || 0);
  return n.toFixed(2);
}

function escapeText(value) {
  return String(value ?? "").replace(/[&<>"']/g, (c) => ({
    "&": "&amp;",
    "<": "&lt;",
    ">": "&gt;",
    '"': "&quot;",
    "'": "&#039;",
  })[c]);
}

function setView(markup) {
  app.innerHTML = `<section class="centered-shell">${markup}</section>`;
}

async function api(path, options = {}) {
  const headers = options.body ? { "Content-Type": "application/json", ...(options.headers || {}) } : options.headers;
  const response = await fetch(path, { credentials: "same-origin", ...options, headers });
  if (!response.ok) {
    const error = new Error(await response.text());
    error.status = response.status;
    throw error;
  }
  return response.json();
}

function showAskInstructor() {
  setView(`
    <div class="card narrow">
      <h1>Ask your instructor for the link</h1>
      <p class="muted">This quiz link is missing or no longer valid.</p>
    </div>
  `);
}

async function boot() {
  if (!sid) {
    showAskInstructor();
    return;
  }
  try {
    await api(`/api/session/${sid}`);
  } catch {
    showAskInstructor();
    return;
  }
  try {
    store.me = await api(`/api/session/${sid}/me`);
  } catch (err) {
    if (err.status === 401) {
      renderJoin();
      return;
    }
    showAskInstructor();
    return;
  }
  connect();
}

function renderJoin(error = null) {
  setView(`
    <form id="join-form" class="card narrow stack">
      <header class="card-header">
        <h1>Join the quiz</h1>
        <p class="muted">Enter your student ID and name. The cookie is per-device; clear it to switch.</p>
      </header>
      <label class="field">
        <span>Student ID</span>
        <input name="student_id" autocomplete="username" required autofocus>
      </label>
      <label class="field">
        <span>Name</span>
        <input name="name" autocomplete="name" required>
      </label>
      ${error ? `<p class="alert error">${escapeText(error)}</p>` : ""}
      <button type="submit" class="btn primary block">Join</button>
    </form>
  `);
  document.querySelector("#join-form").addEventListener("submit", async (event) => {
    event.preventDefault();
    const submit = event.submitter || event.currentTarget.querySelector("button");
    submit.disabled = true;
    const data = new FormData(event.currentTarget);
    try {
      await api(`/api/session/${sid}/join`, {
        method: "POST",
        body: JSON.stringify({
          student_id: data.get("student_id"),
          name: data.get("name"),
        }),
      });
      store.me = await api(`/api/session/${sid}/me`);
      connect();
    } catch (err) {
      submit.disabled = false;
      let msg = err.message || "Could not join.";
      // The /join endpoint returns the FastAPI default JSON error envelope
      // ({"detail": "..."}) — surface the human-readable detail rather
      // than the raw JSON blob in the alert.
      try {
        const parsed = JSON.parse(msg);
        if (parsed && parsed.detail) msg = parsed.detail;
      } catch {}
      renderJoin(msg);
    }
  });
}

function connect() {
  const protocol = window.location.protocol === "https:" ? "wss" : "ws";
  const ws = new WebSocket(`${protocol}://${window.location.host}/ws/student/${sid}`);
  store.ws = ws;
  ws.addEventListener("open", () => {
    clearReconnectState();
  });
  ws.addEventListener("message", (event) => {
    try { handleMessage(JSON.parse(event.data)); } catch (e) { console.warn("bad ws msg", e); }
  });
  ws.addEventListener("close", (event) => {
    // session_reset already drove a re-boot; suppress the reconnect path
    // so it doesn't fight with the "Re-joining…" interstitial.
    if (store.resetting) return;
    stopCountdown();
    // 1008 = policy violation (server rejected the cookie / session).
    // Retrying won't help; reload so /me re-checks auth and we land on
    // the join form (or "ask your instructor") cleanly.
    if (event.code === 1008) {
      window.location.reload();
      return;
    }
    scheduleReconnect();
  });
  ws.addEventListener("error", () => {
    // The "close" event will fire next; reconnect handling lives there.
  });
}

function scheduleReconnect() {
  if (store.resetting) return;
  if (RECONNECT.attempt >= RECONNECT.maxAttempts) {
    showReconnectFailed();
    return;
  }
  RECONNECT.attempt += 1;
  const delay = Math.min(
    RECONNECT.baseDelayMs * Math.pow(2, RECONNECT.attempt - 1),
    RECONNECT.maxDelayMs
  );
  showReconnectingBanner(`Reconnecting… (${RECONNECT.attempt}/${RECONNECT.maxAttempts})`);
  RECONNECT.timer = setTimeout(connect, delay);
}

function clearReconnectState() {
  if (RECONNECT.timer) {
    clearTimeout(RECONNECT.timer);
    RECONNECT.timer = null;
  }
  RECONNECT.attempt = 0;
  hideReconnectingBanner();
}

function showReconnectingBanner(text) {
  let el = document.querySelector("#reconnect-banner");
  if (!el) {
    el = document.createElement("div");
    el.id = "reconnect-banner";
    el.className = "reconnect-banner";
    el.setAttribute("role", "status");
    el.setAttribute("aria-live", "polite");
    document.body.appendChild(el);
  }
  el.textContent = text;
  el.hidden = false;
}

function hideReconnectingBanner() {
  const el = document.querySelector("#reconnect-banner");
  if (el) el.hidden = true;
}

function showReconnectFailed() {
  hideReconnectingBanner();
  setView(`
    <div class="card narrow center">
      <h1>Disconnected</h1>
      <p class="muted">We couldn't reconnect after several tries. Reload to try again.</p>
      <button class="btn primary block" onclick="window.location.reload()">Reload</button>
    </div>
  `);
}

function handleMessage(message) {
  switch (message.type) {
    case "state": return renderState(message);
    case "question_open": return renderQuestion(message);
    case "submit_ack": return renderSubmitted(message);
    case "question_closed": return renderReveal(message);
    case "between_questions": return renderBetween(message);
    case "session_ended": return renderFinished(message);
    case "session_reset": return handleSessionReset();
    case "error": return renderError(message);
  }
}

function handleSessionReset() {
  // Instructor cleared everyone out. Tear local state down and re-boot;
  // /api/session/<sid>/me will return 401 (with cookie cleared by the
  // server) and we'll land cleanly on the join form.
  store.resetting = true;
  stopCountdown();
  clearReconnectState();
  store.me = null;
  store.currentQuestion = null;
  store.submitted = null;
  store.pickedAnswer = null;
  if (store.ws) { try { store.ws.close(); } catch {} store.ws = null; }
  setView(`
    <div class="card narrow center">
      <h1>Session reset</h1>
      <p class="muted">Your instructor reset the session. Re-joining…</p>
      <div class="spinner" aria-hidden="true"></div>
    </div>
  `);
  setTimeout(() => { store.resetting = false; boot(); }, 600);
}

function renderState(message) {
  store.currentQuestion = null;
  store.submitted = null;
  store.pickedAnswer = null;
  stopCountdown();
  if (message.state === "lobby") {
    setView(`
      <div class="card narrow center">
        <p class="eyebrow">${escapeText(message.title || "Live quiz")}</p>
        <h1>You're in.</h1>
        <p class="muted">Hi <b>${escapeText(store.me?.name || "")}</b>. Waiting for your instructor to start.</p>
        <div class="spinner" aria-hidden="true"></div>
      </div>
    `);
  } else if (message.state === "finished") {
    // Edge case: rejoin after the quiz already ended. Render a friendly
    // placeholder and wait for a session_ended payload.
    setView(`
      <div class="card narrow center">
        <h1>Quiz finished</h1>
        <p class="muted">Final results coming through…</p>
      </div>
    `);
  }
}

function renderQuestion(message) {
  store.currentQuestion = message;
  store.submitted = null;
  store.pickedAnswer = null;
  store.deadlineMs = Date.now() + (message.remaining_ms ?? message.time_limit * 1000);
  setView(`
    <article class="card quiz-card">
      <div class="question-head">
        <span class="qnum">Question ${message.question_idx + 1}</span>
        <span id="countdown" class="countdown">—</span>
      </div>
      <div class="qbar"><span id="qbar-fill"></span></div>
      <h1 class="question-text">${escapeText(message.text)}</h1>
      <div class="answer-grid">
        ${["A","B","C","D"].map((k) => `
          <button class="answer-btn" data-answer="${k}">
            <span class="answer-key">${k}</span>
            <span class="answer-text">${escapeText(message.options[k] || "")}</span>
          </button>
        `).join("")}
      </div>
    </article>
  `);
  document.querySelectorAll("[data-answer]").forEach((btn) => {
    btn.addEventListener("click", () => submitAnswer(btn.dataset.answer));
  });
  startCountdown();
}

function submitAnswer(answer) {
  if (!store.currentQuestion || store.submitted || store.pickedAnswer) return;
  // Drop the click silently if the WS isn't open right now (mid-reconnect
  // or already torn down). On reconnect the server replays question_open
  // for the same qidx, which re-renders the card with buttons re-enabled,
  // so the student just clicks again.
  if (!store.ws || store.ws.readyState !== WebSocket.OPEN) return;
  store.pickedAnswer = answer;
  document.querySelectorAll("[data-answer]").forEach((btn) => {
    btn.disabled = true;
    if (btn.dataset.answer === answer) btn.classList.add("picked");
  });
  store.ws.send(JSON.stringify({
    type: "submit",
    question_idx: store.currentQuestion.question_idx,
    answer,
  }));
}

function renderSubmitted(message) {
  store.submitted = message;
  const seconds = (message.elapsed_ms / 1000).toFixed(1);
  setView(`
    <div class="card narrow center">
      <p class="eyebrow">Question ${message.question_idx + 1}</p>
      <h1 class="big-score">+${fmtScore(message.score)}</h1>
      <p class="muted">submitted in ${seconds}s</p>
      <p class="muted small">Waiting for the reveal…</p>
      <div class="spinner" aria-hidden="true"></div>
    </div>
  `);
}

function renderReveal(message) {
  stopCountdown();
  const q = store.currentQuestion;
  const yourAnswer = message.your_answer ?? null;
  const correct = message.correct;
  const won = yourAnswer === correct;
  const totalSubmitters = ["A","B","C","D"].reduce((a, k) => a + (message.histogram[k] || 0), 0) + (message.histogram.missed || 0);
  const denom = Math.max(1, totalSubmitters);
  setView(`
    <article class="card reveal-card">
      <div class="question-head">
        <span class="qnum">Q${message.question_idx + 1}</span>
        <span class="state-badge ${won ? "state-correct" : "state-wrong"}">${won ? "Correct" : (yourAnswer ? "Wrong" : "Missed")}</span>
      </div>
      ${q?.text ? `<h2 class="question-text small">${escapeText(q.text)}</h2>` : ""}
      <ol class="options reveal student-reveal">
        ${["A","B","C","D"].map((k) => {
          const isCorrect = k === correct;
          const isYours = k === yourAnswer;
          let cls = "";
          if (isCorrect) cls += " correct";
          if (isYours && !isCorrect) cls += " wrong-pick";
          if (isYours) cls += " yours";
          return `
            <li class="${cls}">
              <span class="key">${k}${isCorrect ? " ✓" : ""}${isYours && !isCorrect ? " ✗" : ""}</span>
              <span class="opt-text">${escapeText(q?.options?.[k] || "")}</span>
              <span class="opt-count muted">${message.histogram[k] || 0} (${Math.round(100 * (message.histogram[k] || 0) / denom)}%)</span>
            </li>
          `;
        }).join("")}
      </ol>
      ${message.explanation ? `<p class="explanation">${escapeText(message.explanation)}</p>` : ""}
      <div class="reveal-stats">
        <div class="stat"><span class="muted">Your score</span><b>+${fmtScore(message.your_score || 0)}</b></div>
        <div class="stat"><span class="muted">Total</span><b>${fmtScore(message.your_total)}</b></div>
        <div class="stat"><span class="muted">Rank</span><b>${message.your_rank ?? "—"}</b></div>
      </div>
      <h3>Top 5</h3>
      ${renderBoard(message.top5)}
    </article>
  `);
}

function renderBetween(message) {
  setView(`
    <div class="card narrow center">
      <p class="eyebrow">Up next</p>
      <h1>Question ${(message.next_idx ?? 0) + 1}</h1>
      <div class="reveal-stats">
        <div class="stat"><span class="muted">Total</span><b>${fmtScore(message.your_total)}</b></div>
        <div class="stat"><span class="muted">Rank</span><b>${message.your_rank ?? "—"}</b></div>
      </div>
      ${renderBoard(message.top5)}
      <div class="spinner" aria-hidden="true"></div>
    </div>
  `);
}

function renderFinished(message) {
  stopCountdown();
  setView(`
    <article class="card celebration-card">
      <div class="celebration-banner">Quiz complete</div>
      <div class="reveal-stats">
        <div class="stat big"><span class="muted">Your total</span><b>${fmtScore(message.your_total)}</b></div>
        <div class="stat"><span class="muted">Rank</span><b>${message.your_rank ?? "—"}</b></div>
        <div class="stat"><span class="muted">Correct</span><b>${message.questions_correct ?? 0} / ${message.questions_answered ?? 0}</b></div>
      </div>
      <h3>Final top 5</h3>
      ${renderBoard(message.final_top5)}
      <p class="muted small">Thanks for playing.</p>
    </article>
  `);
}

function renderBoard(rows = []) {
  if (!rows || !rows.length) return `<p class="muted small">No scores yet.</p>`;
  // The server marks the requesting student's row with `is_you: true` so
  // we can highlight by id without other students' ids ever crossing the
  // wire. Falls back to name match only if the server didn't mark anything
  // (older payloads pre-migration).
  const anyMarked = rows.some((r) => r.is_you);
  const myName = store.me?.name;
  return `
    <ol class="leaderboard">
      ${rows.map((r) => {
        const isYou = anyMarked
          ? !!r.is_you
          : (myName && r.name && r.name === myName);
        return `
        <li class="${isYou ? "is-you" : ""}">
          <span class="rank">${r.rank}</span>
          <span class="who"><b>${escapeText(r.name)}</b></span>
          <span class="score">${fmtScore(r.score)}</span>
        </li>
      `;
      }).join("")}
    </ol>
  `;
}

function renderError(message) {
  setView(`
    <div class="card narrow center">
      <h1>Server message</h1>
      <p class="muted">${escapeText(message.message || message.code || "Something went wrong.")}</p>
    </div>
  `);
}

function startCountdown() {
  stopCountdown();
  countdownTimer = setInterval(tickCountdown, 250);
  tickCountdown();
}

function stopCountdown() {
  if (countdownTimer) clearInterval(countdownTimer);
  countdownTimer = null;
}

function tickCountdown() {
  const el = document.querySelector("#countdown");
  const fill = document.querySelector("#qbar-fill");
  if (!el || !fill || !store.deadlineMs) return;
  const remaining = Math.max(0, store.deadlineMs - Date.now());
  const limit = (store.currentQuestion?.time_limit ?? 60) * 1000;
  el.textContent = `${Math.ceil(remaining / 1000)}s`;
  el.classList.toggle("urgent", remaining > 0 && remaining <= 10000);
  fill.style.width = `${Math.max(0, Math.min(100, remaining / limit * 100))}%`;
  if (remaining <= 0) stopCountdown();
}

boot();