<?php
/**
 * ============================================================
 *  ON THI THPT QUOC GIA - APP GOM 1 FILE
 * ============================================================
 * Upload file này lên cPanel (public_html hoặc thư mục con),
 * sửa phần CẤU HÌNH bên dưới rồi mở bằng trình duyệt.
 * Hướng dẫn cài đặt chi tiết: xem file HUONG_DAN.txt đi kèm.
 * ============================================================
 */
 
// ==================== CẤU HÌNH (SỬA CHỖ NÀY) ====================
define('DB_HOST', 'localhost');
define('DB_NAME', 'xewiovzx_gacon');     // VD: user_onthi
define('DB_USER', 'xewiovzx_gacon');     // VD: user_onthiadmin
define('DB_PASS', 'xewiovzx_gacon');
define('ADMIN_PASSWORD', 'hack');  // Mật khẩu trang quản trị
define('CRON_SECRET_KEY', 'doi_chuoi_bi_mat_nay'); // Dùng cho cron tự động crawl
// ==================================================================
 
session_start();
error_reporting(E_ALL & ~E_DEPRECATED & ~E_WARNING);
 
try {
    $pdo = new PDO(
        "mysql:host=" . DB_HOST . ";dbname=" . DB_NAME . ";charset=utf8mb4",
        DB_USER, DB_PASS,
        [PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION]
    );
} catch (Exception $e) {
    die("Không kết nối được database. Kiểm tra lại DB_HOST/DB_NAME/DB_USER/DB_PASS trong file. Lỗi: " . $e->getMessage());
}
 
// ==================== TỰ ĐỘNG TẠO BẢNG (chạy 1 lần, sau đó bỏ qua) ====================
$pdo->exec("CREATE TABLE IF NOT EXISTS subjects (
    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(100) NOT NULL,
    slug VARCHAR(100) NOT NULL UNIQUE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4");
 
$pdo->exec("CREATE TABLE IF NOT EXISTS topics (
    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    subject_id INT UNSIGNED NOT NULL,
    name VARCHAR(150) NOT NULL,
    FOREIGN KEY (subject_id) REFERENCES subjects(id) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4");
 
$pdo->exec("CREATE TABLE IF NOT EXISTS sources (
    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(200) NOT NULL,
    type ENUM('official_exam','crawled','ai_generated','manual') NOT NULL DEFAULT 'manual',
    url VARCHAR(500) NULL,
    auto_recrawl BOOLEAN NOT NULL DEFAULT FALSE,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4");
 
$pdo->exec("CREATE TABLE IF NOT EXISTS questions (
    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    subject_id INT UNSIGNED NOT NULL,
    topic_id INT UNSIGNED NULL,
    source_id INT UNSIGNED NULL,
    content TEXT NOT NULL,
    difficulty ENUM('nhan_biet','thong_hieu','van_dung','van_dung_cao') NOT NULL DEFAULT 'thong_hieu',
    explanation TEXT NULL,
    status ENUM('draft','published') NOT NULL DEFAULT 'draft',
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    FOREIGN KEY (subject_id) REFERENCES subjects(id) ON DELETE CASCADE,
    FOREIGN KEY (topic_id) REFERENCES topics(id) ON DELETE SET NULL,
    FOREIGN KEY (source_id) REFERENCES sources(id) ON DELETE SET NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4");
 
$pdo->exec("CREATE TABLE IF NOT EXISTS answer_choices (
    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    question_id INT UNSIGNED NOT NULL,
    label VARCHAR(5) NOT NULL,
    content TEXT NOT NULL,
    is_correct BOOLEAN NOT NULL DEFAULT FALSE,
    FOREIGN KEY (question_id) REFERENCES questions(id) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4");
 
$pdo->exec("CREATE TABLE IF NOT EXISTS exams (
    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    subject_id INT UNSIGNED NOT NULL,
    title VARCHAR(255) NOT NULL,
    is_published BOOLEAN NOT NULL DEFAULT FALSE,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    FOREIGN KEY (subject_id) REFERENCES subjects(id) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4");
 
$pdo->exec("CREATE TABLE IF NOT EXISTS exam_questions (
    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    exam_id INT UNSIGNED NOT NULL,
    question_id INT UNSIGNED NOT NULL,
    order_no SMALLINT UNSIGNED NOT NULL DEFAULT 1,
    FOREIGN KEY (exam_id) REFERENCES exams(id) ON DELETE CASCADE,
    FOREIGN KEY (question_id) REFERENCES questions(id) ON DELETE CASCADE,
    UNIQUE KEY uniq_eq (exam_id, question_id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4");
 
// Seed môn Toán nếu bảng subjects đang trống
$count = $pdo->query("SELECT COUNT(*) FROM subjects")->fetchColumn();
if ($count == 0) {
    $pdo->exec("INSERT INTO subjects (name, slug) VALUES ('Toán', 'toan')");
    $sid = $pdo->lastInsertId();
    foreach (['Hàm số','Mũ và Logarit','Nguyên hàm - Tích phân','Số phức','Hình học không gian','Hình học tọa độ Oxyz','Tổ hợp - Xác suất'] as $t) {
        $pdo->prepare("INSERT INTO topics (subject_id, name) VALUES (?, ?)")->execute([$sid, $t]);
    }
}
 
// ==================== HÀM TIỆN ÍCH ====================
function h($s) { return htmlspecialchars((string)$s, ENT_QUOTES, 'UTF-8'); }
function isAdmin() { return !empty($_SESSION['is_admin']); }
function redirect($r) { header("Location: ?r=" . $r); exit; }
 
// Bóc tách câu hỏi từ HTML thô theo mẫu phổ biến "Câu 1:" ... "A." "B." "C." "D."
// Đây là bóc tách TỔNG QUÁT (heuristic) - áp dụng tốt với các trang trình bày
// đề thi dạng văn bản thuần. Với trang có cấu trúc đặc biệt, bạn nên chỉnh
// lại hàm này hoặc dùng chức năng "Import theo định dạng chuẩn" (chính xác hơn).
function genericParseExam(string $html): array {
    $text = strip_tags(str_ireplace(['<br>', '<br/>', '<br />', '</p>', '</div>'], "\n", $html));
    $text = html_entity_decode($text, ENT_QUOTES, 'UTF-8');
    $text = preg_replace("/\n{2,}/", "\n", $text);
 
    $blocks = preg_split('/(?=Câu\s*\d+[:.\)])/u', $text);
    $questions = [];
 
    foreach ($blocks as $block) {
        $block = trim($block);
        if (!preg_match('/^Câu\s*\d+[:.\)]/u', $block)) continue;
 
        if (!preg_match_all('/^([A-D])[.\)]\s*(.+)$/mu', $block, $m, PREG_SET_ORDER)) continue;
        if (count($m) < 2) continue;
 
        $firstChoicePos = mb_strpos($block, $m[0][0]);
        $content = trim(mb_substr($block, 0, $firstChoicePos));
        $content = preg_replace('/^Câu\s*\d+[:.\)]\s*/u', '', $content);
        if (mb_strlen($content) < 3) continue;
 
        $choices = [];
        $correctLabel = null;
        if (preg_match('/Đáp\s*án\s*[:\-]?\s*([A-D])/iu', $block, $am)) {
            $correctLabel = strtoupper($am[1]);
        }
        foreach ($m as $choiceMatch) {
            $label = strtoupper($choiceMatch[1]);
            $choices[] = [
                'label' => $label,
                'content' => trim($choiceMatch[2]),
                'is_correct' => ($correctLabel !== null && $label === $correctLabel),
            ];
        }
 
        $questions[] = ['content' => $content, 'choices' => $choices];
    }
    return $questions;
}
 
function saveQuestions(PDO $pdo, int $subjectId, int $sourceId, array $questions): int {
    $saved = 0;
    foreach ($questions as $q) {
        $check = $pdo->prepare("SELECT id FROM questions WHERE subject_id=? AND content=? LIMIT 1");
        $check->execute([$subjectId, $q['content']]);
        if ($check->fetch()) continue;
 
        $stmt = $pdo->prepare("INSERT INTO questions (subject_id, source_id, content, status) VALUES (?, ?, ?, 'draft')");
        $stmt->execute([$subjectId, $sourceId, $q['content']]);
        $qid = $pdo->lastInsertId();
 
        $cs = $pdo->prepare("INSERT INTO answer_choices (question_id, label, content, is_correct) VALUES (?, ?, ?, ?)");
        foreach ($q['choices'] as $c) {
            $cs->execute([$qid, $c['label'], $c['content'], $c['is_correct'] ? 1 : 0]);
        }
        $saved++;
    }
    return $saved;
}
 
// ==================== ROUTER ====================
$r = $_GET['r'] ?? 'home';
 
// --- Cron endpoint (gọi từ cPanel Cron Job qua wget/curl, không cần đăng nhập, bảo vệ bằng key) ---
if ($r === 'cron_crawl') {
    if (($_GET['key'] ?? '') !== CRON_SECRET_KEY) { http_response_code(403); die('Sai key'); }
    $sources = $pdo->query("SELECT * FROM sources WHERE auto_recrawl = 1 AND url IS NOT NULL")->fetchAll(PDO::FETCH_ASSOC);
    $total = 0;
    foreach ($sources as $src) {
        $html = @file_get_contents($src['url']);
        if (!$html) continue;
        $qs = genericParseExam($html);
        $subjectRow = $pdo->query("SELECT id FROM subjects LIMIT 1")->fetch();
        $total += saveQuestions($pdo, (int)$subjectRow['id'], (int)$src['id'], $qs);
    }
    die("Cron OK - đã lưu $total câu hỏi mới.");
}
 
// --- Đăng nhập / đăng xuất admin ---
if ($r === 'login' && $_SERVER['REQUEST_METHOD'] === 'POST') {
    if (($_POST['password'] ?? '') === ADMIN_PASSWORD) {
        $_SESSION['is_admin'] = true;
        redirect('admin');
    }
    $loginError = 'Sai mật khẩu';
}
if ($r === 'logout') { unset($_SESSION['is_admin']); redirect('home'); }
 
// --- Xử lý form admin (POST) ---
if ($r === 'admin_crawl' && isAdmin() && $_SERVER['REQUEST_METHOD'] === 'POST') {
    $url = trim($_POST['url']);
    $subjectId = (int)$_POST['subject_id'];
    $autoRecrawl = isset($_POST['auto_recrawl']) ? 1 : 0;
 
    $html = @file_get_contents($url);
    if ($html === false) {
        $crawlMsg = "Không tải được trang. Kiểm tra lại URL hoặc trang có chặn truy cập tự động.";
    } else {
        $srcStmt = $pdo->prepare("INSERT INTO sources (name, type, url, auto_recrawl) VALUES (?, 'crawled', ?, ?)");
        $srcStmt->execute(["Crawl: $url", $url, $autoRecrawl]);
        $sourceId = $pdo->lastInsertId();
 
        $qs = genericParseExam($html);
        $saved = saveQuestions($pdo, $subjectId, $sourceId, $qs);
        $crawlMsg = "Quét xong: tìm thấy " . count($qs) . " câu, lưu mới $saved câu (trạng thái draft, cần duyệt).";
    }
}
 
if ($r === 'admin_import_text' && isAdmin() && $_SERVER['REQUEST_METHOD'] === 'POST') {
    $subjectId = (int)$_POST['subject_id'];
    $srcStmt = $pdo->prepare("INSERT INTO sources (name, type) VALUES ('Nhập tay theo định dạng chuẩn', 'manual')");
    $srcStmt->execute();
    $sourceId = $pdo->lastInsertId();
    $qs = genericParseExam($_POST['raw_text']);
    $saved = saveQuestions($pdo, $subjectId, $sourceId, $qs);
    $crawlMsg = "Import xong: tìm thấy " . count($qs) . " câu, lưu mới $saved câu.";
}
 
if ($r === 'admin_publish' && isAdmin()) {
    $pdo->prepare("UPDATE questions SET status='published' WHERE id=?")->execute([(int)$_GET['id']]);
    redirect('admin');
}
if ($r === 'admin_delete_q' && isAdmin()) {
    $pdo->prepare("DELETE FROM questions WHERE id=?")->execute([(int)$_GET['id']]);
    redirect('admin');
}
if ($r === 'admin_create_exam' && isAdmin() && $_SERVER['REQUEST_METHOD'] === 'POST') {
    $subjectId = (int)$_POST['subject_id'];
    $title = trim($_POST['title']);
    $qIds = $_POST['question_ids'] ?? [];
    $pdo->prepare("INSERT INTO exams (subject_id, title, is_published) VALUES (?, ?, 1)")->execute([$subjectId, $title]);
    $examId = $pdo->lastInsertId();
    $order = 1;
    $insQ = $pdo->prepare("INSERT INTO exam_questions (exam_id, question_id, order_no) VALUES (?, ?, ?)");
    foreach ($qIds as $qid) { $insQ->execute([$examId, (int)$qid, $order++]); }
    redirect('admin');
}
 
// --- Nộp bài thi (học sinh) ---
if ($r === 'submit_exam' && $_SERVER['REQUEST_METHOD'] === 'POST') {
    $examId = (int)$_POST['exam_id'];
    $answers = $_POST['answers'] ?? []; // question_id => choice_id
    $questions = $pdo->prepare("
        SELECT q.id, q.content, q.explanation FROM exam_questions eq
        JOIN questions q ON q.id = eq.question_id WHERE eq.exam_id = ? ORDER BY eq.order_no");
    $questions->execute([$examId]);
    $questions = $questions->fetchAll(PDO::FETCH_ASSOC);
 
    $correctCount = 0;
    $results = [];
    foreach ($questions as $q) {
        $choices = $pdo->prepare("SELECT * FROM answer_choices WHERE question_id=?");
        $choices->execute([$q['id']]);
        $choices = $choices->fetchAll(PDO::FETCH_ASSOC);
        $chosenId = $answers[$q['id']] ?? null;
        $correctChoice = null;
        foreach ($choices as $c) { if ($c['is_correct']) $correctChoice = $c; }
        $isCorrect = $chosenId && $correctChoice && (int)$chosenId === (int)$correctChoice['id'];
        if ($isCorrect) $correctCount++;
        $results[] = ['question' => $q, 'choices' => $choices, 'chosen_id' => $chosenId, 'is_correct' => $isCorrect, 'correct_choice' => $correctChoice];
    }
    $total = count($questions);
    $score = $total > 0 ? round(($correctCount / $total) * 10, 2) : 0;
}
 
// ==================== GIAO DIỆN (HTML) ====================
function layout(string $title, string $body): void {
    global $r;
    echo "<!DOCTYPE html><html lang='vi'><head><meta charset='UTF-8'>";
    echo "<meta name='viewport' content='width=device-width, initial-scale=1'>";
    echo "<title>" . h($title) . "</title><style>
        body{font-family:system-ui,Arial,sans-serif;background:#f4f6f8;margin:0;color:#222}
        header{background:#1e3a5f;color:#fff;padding:14px 20px;display:flex;justify-content:space-between;align-items:center}
        header a{color:#fff;text-decoration:none;margin-left:14px}
        .wrap{max-width:800px;margin:20px auto;padding:0 16px}
        .card{background:#fff;border-radius:8px;padding:18px;margin-bottom:14px;box-shadow:0 1px 3px rgba(0,0,0,.1)}
        .btn{display:inline-block;background:#1e3a5f;color:#fff;padding:8px 16px;border-radius:6px;text-decoration:none;border:none;cursor:pointer;font-size:14px}
        .btn.red{background:#c0392b} .btn.green{background:#27ae60}
        input,textarea,select{width:100%;padding:8px;margin:6px 0 12px;border:1px solid #ccc;border-radius:6px;box-sizing:border-box}
        .choice{padding:6px 0} .correct{color:#27ae60;font-weight:bold} .wrong{color:#c0392b}
        .badge{display:inline-block;padding:2px 8px;border-radius:10px;font-size:12px;background:#eee}
        .msg{background:#eaf7ea;border:1px solid #b6e3b6;padding:10px;border-radius:6px;margin-bottom:14px}
    </style></head><body>";
    echo "<header><strong>Ôn Thi THPT QG</strong><div><a href='?r=home'>Trang chủ</a>";
    echo isAdmin() ? "<a href='?r=admin'>Quản trị</a><a href='?r=logout'>Đăng xuất</a>" : "<a href='?r=login'>Quản trị</a>";
    echo "</div></header><div class='wrap'>" . $body . "</div></body></html>";
}
 
ob_start();
 
if ($r === 'login') {
    ?>
    <div class="card">
        <h2>Đăng nhập quản trị</h2>
        <?php if (!empty($loginError)): ?><p style="color:red"><?= h($loginError) ?></p><?php endif; ?>
        <form method="post" action="?r=login">
            <label>Mật khẩu</label>
            <input type="password" name="password" required>
            <button class="btn" type="submit">Đăng nhập</button>
        </form>
    </div>
    <?php
}
 
elseif ($r === 'admin' && isAdmin()) {
    $subjects = $pdo->query("SELECT * FROM subjects")->fetchAll(PDO::FETCH_ASSOC);
    $drafts = $pdo->query("SELECT * FROM questions WHERE status='draft' ORDER BY id DESC LIMIT 50")->fetchAll(PDO::FETCH_ASSOC);
    $published = $pdo->query("SELECT * FROM questions WHERE status='published' ORDER BY id DESC")->fetchAll(PDO::FETCH_ASSOC);
    ?>
    <?php if (!empty($crawlMsg)): ?><div class="msg"><?= h($crawlMsg) ?></div><?php endif; ?>
 
    <div class="card">
        <h3>1. Quét đề từ URL website</h3>
        <form method="post" action="?r=admin_crawl">
            <label>Môn học</label>
            <select name="subject_id">
                <?php foreach ($subjects as $s): ?><option value="<?= $s['id'] ?>"><?= h($s['name']) ?></option><?php endforeach; ?>
            </select>
            <label>URL trang đề thi</label>
            <input type="url" name="url" placeholder="https://..." required>
            <label><input type="checkbox" name="auto_recrawl" style="width:auto;display:inline-block"> Tự động quét lại định kỳ (qua Cron Job)</label>
            <button class="btn" type="submit">Quét ngay</button>
        </form>
        <p style="font-size:13px;color:#666">Lưu ý: chỉ hoạt động tốt với trang trình bày đề dạng "Câu 1: ... A. ... B. ... C. ... D. ...". Trang có cấu trúc phức tạp (JS render, ảnh) có thể không quét được — khi đó dùng cách nhập tay bên dưới.</p>
    </div>
 
    <div class="card">
        <h3>2. Import theo định dạng chuẩn (dán văn bản)</h3>
        <form method="post" action="?r=admin_import_text">
            <label>Môn học</label>
            <select name="subject_id">
                <?php foreach ($subjects as $s): ?><option value="<?= $s['id'] ?>"><?= h($s['name']) ?></option><?php endforeach; ?>
            </select>
            <label>Dán đề thi (mỗi câu theo mẫu: Câu 1: ... / A. .../ B. .../ C. .../ D. .../ Đáp án: B)</label>
            <textarea name="raw_text" rows="8" required></textarea>
            <button class="btn" type="submit">Import</button>
        </form>
    </div>
 
    <div class="card">
        <h3>3. Câu hỏi chờ duyệt (draft) — <?= count($drafts) ?></h3>
        <?php foreach ($drafts as $q): ?>
            <div style="border-bottom:1px solid #eee;padding:10px 0">
                <p><?= h($q['content']) ?></p>
                <a class="btn green" href="?r=admin_publish&id=<?= $q['id'] ?>">Duyệt & Xuất bản</a>
                <a class="btn red" href="?r=admin_delete_q&id=<?= $q['id'] ?>" onclick="return confirm('Xóa câu này?')">Xóa</a>
            </div>
        <?php endforeach; ?>
        <?php if (empty($drafts)): ?><p>Không có câu nào đang chờ.</p><?php endif; ?>
    </div>
 
    <div class="card">
        <h3>4. Tạo đề thi từ câu hỏi đã xuất bản (<?= count($published) ?> câu)</h3>
        <form method="post" action="?r=admin_create_exam">
            <label>Môn học</label>
            <select name="subject_id">
                <?php foreach ($subjects as $s): ?><option value="<?= $s['id'] ?>"><?= h($s['name']) ?></option><?php endforeach; ?>
            </select>
            <label>Tên đề thi</label>
            <input type="text" name="title" placeholder="VD: Đề luyện tập số 1" required>
            <label>Chọn câu hỏi</label>
            <div style="max-height:250px;overflow:auto;border:1px solid #ddd;padding:8px;border-radius:6px">
                <?php foreach ($published as $q): ?>
                    <div class="choice"><label><input type="checkbox" name="question_ids[]" value="<?= $q['id'] ?>" style="width:auto;display:inline-block"> <?= h(mb_substr($q['content'],0,80)) ?></label></div>
                <?php endforeach; ?>
            </div>
            <button class="btn" type="submit" style="margin-top:10px">Tạo đề</button>
        </form>
    </div>
    <?php
}
 
elseif ($r === 'admin') {
    redirect('login');
}
 
elseif ($r === 'take_exam') {
    $examId = (int)$_GET['id'];
    $exam = $pdo->prepare("SELECT * FROM exams WHERE id=?"); $exam->execute([$examId]); $exam = $exam->fetch(PDO::FETCH_ASSOC);
    $qs = $pdo->prepare("SELECT q.* FROM exam_questions eq JOIN questions q ON q.id=eq.question_id WHERE eq.exam_id=? ORDER BY eq.order_no");
    $qs->execute([$examId]); $qs = $qs->fetchAll(PDO::FETCH_ASSOC);
    ?>
    <div class="card">
        <h2><?= h($exam['title'] ?? 'Đề thi') ?></h2>
        <form method="post" action="?r=submit_exam">
            <input type="hidden" name="exam_id" value="<?= $examId ?>">
            <?php foreach ($qs as $i => $q):
                $choices = $pdo->prepare("SELECT * FROM answer_choices WHERE question_id=?");
                $choices->execute([$q['id']]); $choices = $choices->fetchAll(PDO::FETCH_ASSOC);
            ?>
                <div style="margin-bottom:16px">
                    <p><strong>Câu <?= $i+1 ?>:</strong> <?= h($q['content']) ?></p>
                    <?php foreach ($choices as $c): ?>
                        <div class="choice"><label><input type="radio" name="answers[<?= $q['id'] ?>]" value="<?= $c['id'] ?>" style="width:auto;display:inline-block"> <?= h($c['label']) ?>. <?= h($c['content']) ?></label></div>
                    <?php endforeach; ?>
                </div>
            <?php endforeach; ?>
            <button class="btn" type="submit">Nộp bài</button>
        </form>
    </div>
    <?php
}
 
elseif ($r === 'submit_exam') {
    ?>
    <div class="card">
        <h2>Kết quả: <?= $score ?>/10 (<?= $correctCount ?>/<?= $total ?> câu đúng)</h2>
    </div>
    <?php foreach ($results as $i => $res): ?>
        <div class="card">
            <p><strong>Câu <?= $i+1 ?>:</strong> <?= h($res['question']['content']) ?></p>
            <?php foreach ($res['choices'] as $c):
                $cls = $c['is_correct'] ? 'correct' : (($res['chosen_id'] == $c['id']) ? 'wrong' : '');
            ?>
                <div class="choice <?= $cls ?>"><?= h($c['label']) ?>. <?= h($c['content']) ?> <?= $c['is_correct'] ? '✓' : '' ?></div>
            <?php endforeach; ?>
            <?php if (!empty($res['question']['explanation'])): ?>
                <p style="color:#555"><em>Giải thích: <?= h($res['question']['explanation']) ?></em></p>
            <?php endif; ?>
        </div>
    <?php endforeach; ?>
    <?php
}
 
else { // home
    $subjects = $pdo->query("SELECT * FROM subjects")->fetchAll(PDO::FETCH_ASSOC);
    $exams = $pdo->query("SELECT e.*, s.name as subject_name FROM exams e JOIN subjects s ON s.id=e.subject_id WHERE e.is_published=1 ORDER BY e.id DESC")->fetchAll(PDO::FETCH_ASSOC);
    ?>
    <div class="card">
        <h2>Danh sách đề thi</h2>
        <?php foreach ($exams as $e): ?>
            <p><span class="badge"><?= h($e['subject_name']) ?></span> <a href="?r=take_exam&id=<?= $e['id'] ?>"><?= h($e['title']) ?></a></p>
        <?php endforeach; ?>
        <?php if (empty($exams)): ?><p>Chưa có đề thi nào được xuất bản. Vào trang <a href="?r=login">Quản trị</a> để tạo đề.</p><?php endif; ?>
    </div>
    <?php
}
 
$body = ob_get_clean();
layout('Ôn Thi THPT Quốc Gia', $body);