(() => {
  const startBtn = document.getElementById('startBtn');
  const timeEl = document.getElementById('time');
  const progressEl = document.getElementById('progress');
  const problemEl = document.getElementById('problem');
  const optionsEl = document.getElementById('options');
  const gameEl = document.getElementById('game');
  const statusEl = document.getElementById('status');
  const resultEl = document.getElementById('result');
  const fxLayer = document.getElementById('fxLayer');

  let sessionId = null;
  let totalQuestions = 20;
  let remaining = 60;
  let countdown = null;
  let inFlight = false;

  function setStatus(msg) {
    statusEl.textContent = msg || '';
  }

  function showGame(show) {
    gameEl.classList.toggle('hidden', !show);
  }

  function showResult(score) {
    const { correct, total_answered, total } = score;
    resultEl.innerHTML = `
      <div class="score">Результат: <strong>${correct}</strong> из <strong>${total}</strong></div>
      <div class="small">Отвечено: ${total_answered}</div>
      <button class="again" id="againBtn">Сыграть ещё</button>
    `;
    resultEl.classList.remove('hidden');
    document.getElementById('againBtn')?.addEventListener('click', () => {
      resultEl.classList.add('hidden');
      start();
    });
  }

  function renderQuestion(q) {
    if (!q) return;
    problemEl.textContent = `${q.a} × ${q.b}`;
    optionsEl.innerHTML = '';
    q.options.forEach(value => {
      const btn = document.createElement('button');
      btn.className = 'option-btn';
      btn.textContent = String(value);
      btn.addEventListener('click', () => answer(value, btn));
      optionsEl.appendChild(btn);
    });
    progressEl.textContent = `${q.index}/${totalQuestions}`;
  }

  function stopTimer() {
    if (countdown) {
      clearInterval(countdown);
      countdown = null;
    }
  }

  function startTimer(seconds) {
    remaining = seconds;
    timeEl.textContent = String(remaining);
    stopTimer();
    countdown = setInterval(() => {
      remaining -= 1;
      if (remaining <= 0) {
        remaining = 0;
        timeEl.textContent = '0';
        stopTimer();
      } else {
        timeEl.textContent = String(remaining);
      }
    }, 1000);
  }

  async function start() {
    if (inFlight) return;
    inFlight = true;
    setStatus('');
    showGame(false);
    startBtn.disabled = true;
    resultEl.classList.add('hidden');
    try {
      const res = await fetch('/api/session/start', { method: 'POST' });
      if (!res.ok) throw new Error('Не удалось начать сессию');
      const data = await res.json();
      sessionId = data.session_id;
      totalQuestions = data.total_questions;
      startTimer(data.remaining_seconds);
      renderQuestion(data.question);
      showGame(true);
      setStatus('Отвечайте как можно быстрее!');
    } catch (e) {
      console.error(e);
      setStatus('Ошибка старта. Попробуйте ещё раз.');
    } finally {
      startBtn.disabled = false;
      inFlight = false;
    }
  }

  function celebrateFromElement(el) {
    if (!fxLayer) return;
    const rect = el.getBoundingClientRect();
    const x = rect.left + rect.width / 2;
    const y = rect.top + rect.height / 2;
    const count = 40;
    for (let i = 0; i < count; i++) {
      const p = document.createElement('span');
      p.className = 'confetti';
      // origin
      p.style.left = `${x}px`;
      p.style.top = `${y}px`;
      // random trajectory
      const angle = Math.random() * Math.PI * 2;
      const distance = 60 + Math.random() * 80;
      const tx = Math.cos(angle) * distance;
      const ty = Math.sin(angle) * distance + 40 + Math.random() * 60; // gravity pull
      const rot = (Math.random() * 720 - 360).toFixed(0) + 'deg';
      p.style.setProperty('--tx', `${tx.toFixed(0)}px`);
      p.style.setProperty('--ty', `${ty.toFixed(0)}px`);
      p.style.setProperty('--rot', rot);
      const hue = Math.floor(Math.random() * 360);
      p.style.backgroundColor = `hsl(${hue} 90% 55%)`;
      p.style.animation = `confetti ${650 + Math.random() * 400}ms ease-out forwards`;
      fxLayer.appendChild(p);
      setTimeout(() => p.remove(), 1200);
    }
  }

  async function answer(value, clickedBtn) {
    if (!sessionId || inFlight) return;
    inFlight = true;
    // Disable all buttons to prevent double clicks
    const buttons = Array.from(optionsEl.querySelectorAll('button'));
    buttons.forEach(b => b.disabled = true);
    try {
      const res = await fetch(`/api/session/${sessionId}/answer`, {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify({ answer: value }),
      });
      if (!res.ok) throw new Error('Не удалось отправить ответ');
      const data = await res.json();

      // feedback
      if (data.correct) {
        clickedBtn.classList.add('correct');
        celebrateFromElement(clickedBtn);
      } else {
        clickedBtn.classList.add('wrong');
      }

      // Small pause to show feedback
      await new Promise(r => setTimeout(r, data.correct ? 350 : 250));

      if (data.finished) {
        stopTimer();
        showGame(false);
        const reason = data.reason === 'timeout' ? 'Время вышло.' : 'Завершено!';
        setStatus(reason);
        showResult(data.score);
      } else {
        timeEl.textContent = String(data.remaining_seconds);
        renderQuestion(data.question);
      }
    } catch (e) {
      console.error(e);
      setStatus('Ошибка ответа. Перезапустите сессию.');
    } finally {
      inFlight = false;
    }
  }

  startBtn.addEventListener('click', start);
})();