const db = require('../db/db');
const { stripTags } = require('../utils/sanitize');

/* ── SM-2 algorithm ───────────────────────────────────────────────────────
   quality: 0 = blackout, 1 = wrong, 2 = wrong but familiar,
            3 = correct with difficulty, 4 = correct, 5 = perfect
   ─────────────────────────────────────────────────────────────────────── */
function sm2(easeFactor, intervalDays, repetitions, quality) {
  let ef  = easeFactor;
  let n   = repetitions;
  let iv  = intervalDays;

  if (quality < 3) {
    n  = 0;
    iv = 1;
  } else {
    if (n === 0)      iv = 1;
    else if (n === 1) iv = 6;
    else              iv = Math.round(iv * ef);
    n++;
  }
  ef = Math.max(1.3, ef + 0.1 - (5 - quality) * (0.08 + (5 - quality) * 0.02));
  const due = new Date(Date.now() + iv * 86400000).toISOString();
  return { easeFactor: ef, intervalDays: iv, repetitions: n, dueAt: due };
}

/* ── GET /api/flashcards/decks ─────────────────────────────────────────── */
function listDecks(req, res) {
  const uid = req.user.id;
  const decks = db.prepare(`
    SELECT d.*,
      (SELECT COUNT(*) FROM flashcard_cards c WHERE c.deck_id = d.id) AS card_count,
      (SELECT COUNT(*) FROM flashcard_cards c
       LEFT JOIN flashcard_reviews r ON r.card_id = c.id AND r.user_id = ?
       WHERE c.deck_id = d.id AND (r.id IS NULL OR r.due_at <= datetime('now'))) AS due_count
    FROM flashcard_decks d
    WHERE d.user_id = ?
    ORDER BY d.created_at DESC
  `).all(uid, uid);
  res.json({ decks });
}

/* ── POST /api/flashcards/decks ────────────────────────────────────────── */
function createDeck(req, res) {
  const uid  = req.user.id;
  const { title, description = '', color = '#9B5DE5' } = req.body;
  if (!title?.trim()) return res.status(400).json({ error: 'Title required' });
  const r = db.prepare(
    `INSERT INTO flashcard_decks (user_id, title, description, color) VALUES (?,?,?,?)`
  ).run(uid, title.trim(), description, color);
  res.json({ id: r.lastInsertRowid, title: title.trim(), description, color, card_count: 0, due_count: 0 });
}

/* ── PUT /api/flashcards/decks/:id ─────────────────────────────────────── */
function updateDeck(req, res) {
  const uid = req.user.id;
  const { title, description, color } = req.body;
  const deck = db.prepare(`SELECT * FROM flashcard_decks WHERE id = ? AND user_id = ?`)
                  .get(req.params.id, uid);
  if (!deck) return res.status(404).json({ error: 'Not found' });
  db.prepare(`UPDATE flashcard_decks SET title=?, description=?, color=? WHERE id=?`)
    .run(title ?? deck.title, description ?? deck.description, color ?? deck.color, deck.id);
  res.json({ ok: true });
}

/* ── DELETE /api/flashcards/decks/:id ──────────────────────────────────── */
function deleteDeck(req, res) {
  const uid = req.user.id;
  const deck = db.prepare(`SELECT id FROM flashcard_decks WHERE id = ? AND user_id = ?`)
                  .get(req.params.id, uid);
  if (!deck) return res.status(404).json({ error: 'Not found' });
  db.prepare(`DELETE FROM flashcard_decks WHERE id = ?`).run(deck.id);
  res.json({ ok: true });
}

/* ── GET /api/flashcards/decks/:id/cards ───────────────────────────────── */
function getCards(req, res) {
  const uid = req.user.id;
  const deck = db.prepare(`SELECT id FROM flashcard_decks WHERE id = ? AND user_id = ?`)
                  .get(req.params.id, uid);
  if (!deck) return res.status(404).json({ error: 'Not found' });
  const cards = db.prepare(`
    SELECT c.*, r.ease_factor, r.interval_days, r.repetitions, r.due_at, r.last_reviewed
    FROM flashcard_cards c
    LEFT JOIN flashcard_reviews r ON r.card_id = c.id AND r.user_id = ?
    WHERE c.deck_id = ?
    ORDER BY c.order_idx, c.id
  `).all(uid, deck.id);
  res.json({ cards });
}

/* ── POST /api/flashcards/decks/:id/cards ──────────────────────────────── */
function addCard(req, res) {
  const uid = req.user.id;
  const deck = db.prepare(`SELECT id FROM flashcard_decks WHERE id = ? AND user_id = ?`)
                  .get(req.params.id, uid);
  if (!deck) return res.status(404).json({ error: 'Not found' });
  const front = stripTags((req.body.front || '').slice(0, 5000));
  const back  = stripTags((req.body.back  || '').slice(0, 5000));
  const maxIdx = db.prepare(`SELECT MAX(order_idx) AS m FROM flashcard_cards WHERE deck_id = ?`)
                    .get(deck.id)?.m ?? -1;
  const r = db.prepare(`INSERT INTO flashcard_cards (deck_id, front, back, order_idx) VALUES (?,?,?,?)`)
               .run(deck.id, front, back, maxIdx + 1);
  res.json({ id: r.lastInsertRowid, deck_id: deck.id, front, back, order_idx: maxIdx + 1 });
}

/* ── POST /api/flashcards/decks/:id/cards/bulk ──────────────────────────── */
function addCardsBulk(req, res) {
  const uid = req.user.id;
  const deck = db.prepare(`SELECT id FROM flashcard_decks WHERE id = ? AND user_id = ?`)
                  .get(req.params.id, uid);
  if (!deck) return res.status(404).json({ error: 'Not found' });
  const { cards } = req.body;
  if (!Array.isArray(cards) || !cards.length) return res.status(400).json({ error: 'cards[] required' });
  const maxIdx = db.prepare(`SELECT MAX(order_idx) AS m FROM flashcard_cards WHERE deck_id = ?`)
                    .get(deck.id)?.m ?? -1;
  const stmt = db.prepare(`INSERT INTO flashcard_cards (deck_id, front, back, order_idx) VALUES (?,?,?,?)`);
  const inserted = [];
  const ins = db.transaction(() => {
    cards.forEach((c, i) => {
      const r = stmt.run(deck.id, c.front || '', c.back || '', maxIdx + 1 + i);
      inserted.push({ id: r.lastInsertRowid, front: c.front, back: c.back });
    });
  });
  ins();
  res.json({ inserted });
}

/* ── PUT /api/flashcards/cards/:id ─────────────────────────────────────── */
function updateCard(req, res) {
  const uid  = req.user.id;
  const card = db.prepare(`
    SELECT c.* FROM flashcard_cards c
    JOIN flashcard_decks d ON d.id = c.deck_id
    WHERE c.id = ? AND d.user_id = ?
  `).get(req.params.id, uid);
  if (!card) return res.status(404).json({ error: 'Not found' });
  const { front, back } = req.body;
  db.prepare(`UPDATE flashcard_cards SET front=?, back=? WHERE id=?`)
    .run(front ?? card.front, back ?? card.back, card.id);
  res.json({ ok: true });
}

/* ── DELETE /api/flashcards/cards/:id ──────────────────────────────────── */
function deleteCard(req, res) {
  const uid  = req.user.id;
  const card = db.prepare(`
    SELECT c.id FROM flashcard_cards c
    JOIN flashcard_decks d ON d.id = c.deck_id
    WHERE c.id = ? AND d.user_id = ?
  `).get(req.params.id, uid);
  if (!card) return res.status(404).json({ error: 'Not found' });
  db.prepare(`DELETE FROM flashcard_cards WHERE id = ?`).run(card.id);
  res.json({ ok: true });
}

/* ── GET /api/flashcards/decks/:id/study ───────────────────────────────── */
function getStudySession(req, res) {
  const uid = req.user.id;
  const deck = db.prepare(`SELECT id FROM flashcard_decks WHERE id = ? AND user_id = ?`)
                  .get(req.params.id, uid);
  if (!deck) return res.status(404).json({ error: 'Not found' });
  // due cards first, then new cards (no review yet), limit 20
  const cards = db.prepare(`
    SELECT c.id, c.front, c.back,
      COALESCE(r.ease_factor, 2.5) AS ease_factor,
      COALESCE(r.interval_days, 1) AS interval_days,
      COALESCE(r.repetitions, 0)   AS repetitions,
      COALESCE(r.due_at, datetime('now')) AS due_at,
      r.last_reviewed,
      CASE WHEN r.id IS NULL THEN 0 ELSE 1 END AS seen
    FROM flashcard_cards c
    LEFT JOIN flashcard_reviews r ON r.card_id = c.id AND r.user_id = ?
    WHERE c.deck_id = ?
      AND (r.id IS NULL OR r.due_at <= datetime('now'))
    ORDER BY seen ASC, r.due_at ASC
    LIMIT 20
  `).all(uid, deck.id);
  const total_due = db.prepare(`
    SELECT COUNT(*) AS n FROM flashcard_cards c
    LEFT JOIN flashcard_reviews r ON r.card_id = c.id AND r.user_id = ?
    WHERE c.deck_id = ? AND (r.id IS NULL OR r.due_at <= datetime('now'))
  `).get(uid, deck.id).n;
  res.json({ cards, total_due });
}

/* ── POST /api/flashcards/cards/:id/review ─────────────────────────────── */
function submitReview(req, res) {
  const uid     = req.user.id;
  const { quality } = req.body;   // 0..5
  if (quality === undefined || quality < 0 || quality > 5)
    return res.status(400).json({ error: 'quality 0-5 required' });

  const card = db.prepare(`
    SELECT c.id FROM flashcard_cards c
    JOIN flashcard_decks d ON d.id = c.deck_id
    WHERE c.id = ? AND d.user_id = ?
  `).get(req.params.id, uid);
  if (!card) return res.status(404).json({ error: 'Not found' });

  const existing = db.prepare(
    `SELECT * FROM flashcard_reviews WHERE user_id = ? AND card_id = ?`
  ).get(uid, card.id);

  const prev = existing || { ease_factor: 2.5, interval_days: 1, repetitions: 0 };
  const next = sm2(prev.ease_factor, prev.interval_days, prev.repetitions, quality);

  if (existing) {
    db.prepare(`
      UPDATE flashcard_reviews
      SET ease_factor=?, interval_days=?, repetitions=?, due_at=?, last_reviewed=datetime('now')
      WHERE user_id=? AND card_id=?
    `).run(next.easeFactor, next.intervalDays, next.repetitions, next.dueAt, uid, card.id);
  } else {
    db.prepare(`
      INSERT INTO flashcard_reviews (user_id, card_id, ease_factor, interval_days, repetitions, due_at, last_reviewed)
      VALUES (?,?,?,?,?,?,datetime('now'))
    `).run(uid, card.id, next.easeFactor, next.intervalDays, next.repetitions, next.dueAt);
  }
  res.json({ ok: true, next_review: next.dueAt, interval_days: next.intervalDays });
}

/* ── GET /api/flashcards/stats ─────────────────────────────────────────── */
function getStats(req, res) {
  const uid = req.user.id;
  const decks_count = db.prepare(`SELECT COUNT(*) AS n FROM flashcard_decks WHERE user_id=?`).get(uid).n;
  const cards_count = db.prepare(`
    SELECT COUNT(*) AS n FROM flashcard_cards c
    JOIN flashcard_decks d ON d.id = c.deck_id WHERE d.user_id=?
  `).get(uid).n;
  const due_count = db.prepare(`
    SELECT COUNT(*) AS n FROM flashcard_cards c
    JOIN flashcard_decks d ON d.id = c.deck_id
    LEFT JOIN flashcard_reviews r ON r.card_id = c.id AND r.user_id = ?
    WHERE d.user_id = ? AND (r.id IS NULL OR r.due_at <= datetime('now'))
  `).get(uid, uid).n;
  const reviewed_today = db.prepare(`
    SELECT COUNT(*) AS n FROM flashcard_reviews
    WHERE user_id = ? AND date(last_reviewed) = date('now')
  `).get(uid).n;
  res.json({ decks_count, cards_count, due_count, reviewed_today });
}

module.exports = {
  listDecks, createDeck, updateDeck, deleteDeck,
  getCards, addCard, addCardsBulk, updateCard, deleteCard,
  getStudySession, submitReview, getStats,
};