src/views/InstructorView.js

import { createRoom, subscribeToRoom, getChallenges, resetProgress, removeUser } from "../services/classroom.js";
import { generateMonsterSVG, getNextMonster, MONSTER_DEFS } from "../utils/monsterUtils.js";

// Load html-to-image dynamically (Better support than html2canvas)
const script = document.createElement('script');
script.src = "https://cdnjs.cloudflare.com/ajax/libs/html-to-image/1.11.11/html-to-image.js";
document.head.appendChild(script);

let cachedChallenges = [];
let currentStudents = [];

export async function renderInstructorView() {
    // Pre-fetch challenges for table headers
    try {
        cachedChallenges = await getChallenges();
    } catch (e) {
        console.error("Failed header load", e);
    }

    return `
    <div id="auth-modal" class="fixed inset-0 bg-black bg-opacity-90 backdrop-blur z-50 flex items-center justify-center">
        <div class="bg-gray-800 p-8 rounded-xl border border-gray-600 shadow-2xl max-w-sm w-full">
            <h2 class="text-xl font-bold text-center mb-6 text-white">🔒 講師身分驗證</h2>
            <input type="password" id="instructor-password" class="w-full bg-gray-900 border border-gray-700 rounded p-3 text-white text-center text-lg tracking-widest mb-4 focus:border-cyan-500 focus:outline-none" placeholder="輸入密碼">
                <button id="auth-btn" class="w-full bg-purple-600 hover:bg-purple-500 text-white font-bold py-3 rounded-lg transition-colors">確認進入</button>
        </div>
    </div>

    <!--Broadcast Modal(Hidden by default )-->
    <div id="broadcast-modal" class="fixed inset-0 bg-black/90 backdrop-blur z-50 hidden flex flex-col items-center justify-center p-8 transition-opacity duration-300">
        <button onclick="closeBroadcast()" class="absolute top-6 right-6 text-gray-400 hover:text-white text-2xl">✕</button>
        
        <div id="broadcast-content" class="bg-gray-800 border border-gray-600 rounded-2xl p-8 max-w-4xl w-full text-center shadow-2xl transform transition-transform scale-95 opacity-0">
            <div class="mb-4 flex flex-col items-center">
                <div class="w-16 h-16 rounded-full bg-cyan-600 flex items-center justify-center text-3xl font-bold text-white mb-2" id="broadcast-avatar">
                    D
                </div>
                <h3 class="text-xl text-cyan-300 font-bold" id="broadcast-author">Dave</h3>
                <span class="text-gray-500 text-sm" id="broadcast-challenge">Challenge Name</span>
            </div>
            
            <div class="bg-black/30 rounded-xl p-6 mb-8 text-left overflow-auto max-h-[50vh]">
                <pre class="text-green-400 font-mono text-lg whitespace-pre-wrap" id="broadcast-prompt">Loading...</pre>
            </div>

            <div class="flex justify-center space-x-4">
               <button id="btn-show-stage" class="bg-blue-600 hover:bg-blue-500 text-white font-bold py-3 px-8 rounded-xl flex items-center space-x-2 text-lg">
                    <span>🖥️ 投放到大螢幕 (本機)</span>
               </button>
               <button id="btn-reject-task" class="bg-red-600 hover:bg-red-500 text-white font-bold py-3 px-8 rounded-xl flex items-center space-x-2 text-lg shadow-lg">
                    <span>🛑 退回重做 (Reject)</span>
               </button>
               <!-- Future Feature: Send to Students -->
               <!-- 
               <button id="btn-broadcast-all" class="bg-purple-600 hover:bg-purple-500 text-white font-bold py-3 px-8 rounded-xl flex items-center space-x-2 text-lg opacity-50 cursor-not-allowed" title="此功能開發中">
                    <span>📡 推送給所有人</span>
               </button>
               -->
            </div>
        </div>
        
        <!-- Big Screen Mode (Initially hidden inside modal) -->
        <div id="stage-view" class="hidden absolute inset-0 bg-gray-900 flex flex-col items-center justify-center p-10">
             <button onclick="closeStage()" class="absolute top-6 right-6 text-gray-500 hover:text-white text-4xl">✕</button>
             <h1 class="text-4xl font-bold text-cyan-400 mb-8" id="stage-title">優秀作品展示</h1>
             <div class="bg-black border-2 border-cyan-500/50 rounded-2xl p-10 max-w-6xl w-full shadow-[0_0_50px_rgba(6,182,212,0.2)]">
                <pre class="text-3xl text-green-400 font-mono whitespace-pre-wrap leading-relaxed" id="stage-prompt">...</pre>
             </div>
             <div class="mt-8 text-2xl text-gray-400">
                Author: <span class="text-white font-bold" id="stage-author">Dave</span>
             </div>
        </div>
    </div>

    <!--Group Photo Modal-->
    <div id="group-photo-modal" class="fixed inset-0 bg-gray-900/95 backdrop-blur-md z-50 hidden flex flex-col items-center justify-center p-4 transition-opacity duration-300">
        <button onclick="document.getElementById('group-photo-modal').classList.add('hidden')" class="absolute top-6 right-6 text-gray-400 hover:text-white text-4xl z-50">✕</button>
        
        <div class="text-center mb-8 z-10">
            <h2 class="text-3xl md:text-5xl font-extrabold text-transparent bg-clip-text bg-gradient-to-r from-yellow-400 via-orange-500 to-red-500 tracking-wider drop-shadow-lg">
                大合照 CLASS PHOTO
            </h2>
            <p class="text-gray-400 mt-2 font-mono" id="photo-date">2026.01.27</p>
        </div>

        <div class="absolute top-6 left-6 z-50 flex space-x-4">
            <button id="snapshot-btn" class="bg-white/10 hover:bg-white/20 text-white border border-white/30 font-bold py-2 px-6 rounded-full backdrop-blur-md transition-all flex items-center space-x-2 shadow-lg group">
                <span class="text-2xl group-hover:scale-110 transition-transform">📸</span>
                <span>拍照 (Snapshot)</span>
            </button>
        </div>

        <div id="group-photo-container" class="w-full max-w-7xl flex flex-col items-center overflow-y-auto max-h-[80vh] custom-scrollbar relative">
            <!-- Dynamic Content -->
        </div>

        <!-- Countdown Overlay -->
        <div id="snapshot-overlay" class="absolute inset-0 z-[60] hidden flex-col items-center justify-center pointer-events-none">
             <div id="countdown-number" class="text-[150px] font-black text-white drop-shadow-[0_0_50px_rgba(0,0,0,0.8)] animate-pulse">3</div>
        </div>
    </div>

    <div class="min-h-screen p-6 pb-20 bg-gray-900 text-white">
        <!-- Header -->
        <header class="flex flex-col md:flex-row justify-between items-center mb-6 bg-gray-800 p-4 rounded-xl border border-gray-700 space-y-4 md:space-y-0 sticky top-0 z-30 shadow-lg">
            <div class="flex items-center space-x-4">
                <h1 class="text-2xl font-bold text-transparent bg-clip-text bg-gradient-to-r from-purple-400 to-pink-600">
                    儀表板 <span class="text-xs text-gray-600 font-mono ml-2">v26.01.27</span>
                </h1>
                <div id="room-info" class="hidden flex items-center space-x-2 bg-black/30 px-3 py-1 rounded-lg border border-gray-700">
                    <span class="text-xs text-gray-500 uppercase">Room</span>
                    <span id="display-room-code" class="text-xl font-mono font-bold text-cyan-400 tracking-widest"></span>
                </div>
            </div>
            
            <div class="flex space-x-3">
                <div class="flex items-center space-x-2 text-xs text-gray-400 mr-4 border-r border-gray-700 pr-4">
                    <div class="flex items-center"><div class="w-3 h-3 bg-gray-700 rounded-sm mr-1"></div> 未開始</div>
                    <div class="flex items-center"><div class="w-3 h-3 bg-blue-600 rounded-sm mr-1"></div> 進行中</div>
                    <div class="flex items-center"><div class="w-3 h-3 bg-green-500 rounded-sm mr-1"></div> 已完成</div>
                    <div class="flex items-center"><div class="w-3 h-3 bg-red-500 animate-pulse rounded-sm mr-1"></div> 卡關 (>5m)</div>
                </div>

                <button id="group-photo-btn" class="bg-gradient-to-r from-pink-600 to-purple-600 hover:from-pink-500 hover:to-purple-500 text-white font-bold py-2 px-4 rounded-lg transition-all shadow-lg border border-pink-400/30 flex items-center space-x-2">
                    <span>📸 大合照</span>
                </button>
                <button id="nav-admin-btn" class="bg-gray-700 hover:bg-gray-600 text-white font-bold py-2 px-4 rounded-lg transition-all border border-gray-600">
                    管理題目
                </button>
                <div id="create-room-container" class="flex items-center space-x-2">
                    <input type="text" id="rejoin-room-code" placeholder="代碼" class="bg-gray-900 border border-gray-700 text-white px-3 py-2 rounded-lg w-20 text-center focus:outline-none focus:border-cyan-500">
                    <button id="rejoin-room-btn" class="bg-gray-700 hover:bg-gray-600 text-white px-3 py-2 rounded-lg">重回</button>
                    <button id="create-room-btn" class="bg-purple-600 hover:bg-purple-500 text-white font-bold px-4 py-2 rounded-lg shadow-lg">開房</button>
                </div>
            </div>
        </header>

        <!-- Heatmap Content -->
        <div id="dashboard-content" class="hidden overflow-x-auto pb-10">
            <table class="w-full border-collapse">
                <thead>
                    <tr id="heatmap-header">
                        <th class="p-3 text-left sticky left-0 bg-gray-800 z-20 border-b border-gray-700 min-w-[150px]">學員 / 關卡</th>
                        <!-- Challenges headers generated dynamically -->
                    </tr>
                </thead>
                <tbody id="heatmap-body">
                    <!-- Rows generated dynamically -->
                    <tr><td colspan="100" class="text-center py-10 text-gray-500">等待資料載入...</td></tr>
                </tbody>
            </table>
        </div>
    </div>
`;
}

export function setupInstructorEvents() {
    // Auth Logic
    const authBtn = document.getElementById('auth-btn');
    const pwdInput = document.getElementById('instructor-password');
    const authModal = document.getElementById('auth-modal');

    // Default password check
    const checkPassword = async () => {
        const { verifyInstructorPassword } = await import("../services/classroom.js");

        authBtn.textContent = "驗證中...";
        authBtn.disabled = true;

        try {
            const isValid = await verifyInstructorPassword(pwdInput.value);
            if (isValid) {
                authModal.classList.add('hidden');
                // Store session to avoid re-login on reload
                sessionStorage.setItem('vibecoding_instructor_auth', 'true');
            } else {
                alert('密碼錯誤');
                pwdInput.value = '';
            }
        } catch (e) {
            console.error(e);
            alert("驗證出錯");
        } finally {
            authBtn.textContent = "確認進入";
            authBtn.disabled = false;
        }
    };

    authBtn.addEventListener('click', checkPassword);
    pwdInput.addEventListener('keypress', (e) => {
        if (e.key === 'Enter') checkPassword();
    });

    const createBtn = document.getElementById('create-room-btn');
    const roomInfo = document.getElementById('room-info');
    const createContainer = document.getElementById('create-room-container');
    const dashboardContent = document.getElementById('dashboard-content');
    const displayRoomCode = document.getElementById('display-room-code');
    const navAdminBtn = document.getElementById('nav-admin-btn');
    const groupPhotoBtn = document.getElementById('group-photo-btn');
    const snapshotBtn = document.getElementById('snapshot-btn');
    let isSnapshotting = false;

    // Snapshot Logic
    snapshotBtn.addEventListener('click', async () => {
        if (isSnapshotting || typeof htmlToImage === 'undefined') {
            if (typeof htmlToImage === 'undefined') alert("截圖元件尚未載入，請稍候再試");
            return;
        }
        isSnapshotting = true;

        const overlay = document.getElementById('snapshot-overlay');
        const countEl = document.getElementById('countdown-number');
        const container = document.getElementById('group-photo-container');
        const modal = document.getElementById('group-photo-modal');

        // Close button hide
        const closeBtn = modal.querySelector('button');
        if (closeBtn) closeBtn.style.opacity = '0';
        snapshotBtn.style.opacity = '0';

        overlay.classList.remove('hidden');
        overlay.classList.add('flex');

        // Countdown Sequence
        const runCountdown = (num) => new Promise(resolve => {
            countEl.textContent = num;
            countEl.style.transform = 'scale(1.5)';
            countEl.style.opacity = '1';

            // Animation reset
            requestAnimationFrame(() => {
                countEl.style.transition = 'all 0.5s ease-out';
                countEl.style.transform = 'scale(1)';
                countEl.style.opacity = '0.5';
                setTimeout(resolve, 1000);
            });
        });

        await runCountdown(3);
        await runCountdown(2);
        await runCountdown(1);

        // Action!
        countEl.textContent = '';
        overlay.classList.add('hidden');

        // 1. Emojis Explosion
        const emojis = ['🤘', '✌️', '👍', '🫶', '😎', '🔥'];
        const cards = container.querySelectorAll('.group\\/card');

        cards.forEach(card => {
            // Find the monster image container
            const imgContainer = card.querySelector('.monster-img-container');
            if (!imgContainer) return;

            // Random Emoji
            const emoji = emojis[Math.floor(Math.random() * emojis.length)];
            const emojiEl = document.createElement('div');
            emojiEl.textContent = emoji;
            // Position: Top-Right of the *Image*, slightly overlapping
            emojiEl.className = 'absolute -top-2 -right-2 text-2xl animate-bounce z-50 drop-shadow-md transform rotate-12';
            emojiEl.style.animationDuration = '0.6s';
            imgContainer.appendChild(emojiEl);

            // Remove after 3s
            setTimeout(() => emojiEl.remove(), 3000);
        });

        // 2. Capture using html-to-image
        setTimeout(async () => {
            try {
                // Flash Effect
                const flash = document.createElement('div');
                flash.className = 'fixed inset-0 bg-white z-[100] transition-opacity duration-300 pointer-events-none';
                document.body.appendChild(flash);
                setTimeout(() => flash.style.opacity = '0', 50);
                setTimeout(() => flash.remove(), 300);

                // Use htmlToImage.toPng
                const dataUrl = await htmlToImage.toPng(container, {
                    backgroundColor: '#111827',
                    pixelRatio: 2,
                    cacheBust: true,
                });

                // Download
                const link = document.createElement('a');
                const dateStr = new Date().toISOString().slice(0, 10);
                link.download = `VIBE_Class_Photo_${dateStr}.png`;
                link.href = dataUrl;
                link.click();

            } catch (e) {
                console.error("Snapshot failed:", e);
                alert("截圖失敗 (請嘗試手動截圖/PrtSc)\n原因: " + e.message);
            } finally {
                // Restore UI
                if (closeBtn) closeBtn.style.opacity = '1';
                snapshotBtn.style.opacity = '1';
                isSnapshotting = false;
            }
        }, 600); // Slight delay for emojis to appear
    });

    // Group Photo Logic
    groupPhotoBtn.addEventListener('click', () => {
        const modal = document.getElementById('group-photo-modal');
        const container = document.getElementById('group-photo-container');
        const dateEl = document.getElementById('photo-date');

        // Update Date
        const now = new Date();
        dateEl.textContent = `${now.getFullYear()}.${String(now.getMonth() + 1).padStart(2, '0')}.${String(now.getDate()).padStart(2, '0')} `;

        // Get saved name
        const savedName = localStorage.getItem('vibecoding_instructor_name') || '講師 (Instructor)';

        container.innerHTML = '';

        // 1. Container for Relative Positioning with Custom Background
        const relativeContainer = document.createElement('div');
        relativeContainer.className = 'relative w-full h-[600px] md:h-[700px] overflow-hidden rounded-3xl border border-gray-700/30 shadow-2xl flex items-center justify-center bg-cover bg-center';
        relativeContainer.style.backgroundImage = "url('assets/photobg.png')";
        container.appendChild(relativeContainer);

        // Watermark (Bottom Right, High Z-Index, Gradient Text, Dark Backdrop)
        const watermark = document.createElement('div');
        watermark.className = 'absolute bottom-2 right-2 md:bottom-4 md:right-4 z-[60] bg-black/70 backdrop-blur-sm rounded-lg px-4 py-2 pointer-events-none select-none border border-white/10 shadow-lg';

        const d = new Date();
        const dateStr = `${d.getFullYear()}.${String(d.getMonth() + 1).padStart(2, '0')}.${String(d.getDate()).padStart(2, '0')} `;

        watermark.innerHTML = `
    <span class="text-lg md:text-2xl font-black font-mono bg-clip-text text-transparent bg-gradient-to-r from-cyan-400 to-purple-400 tracking-wider">
        ${dateStr} VibeCoding 怪獸成長營
    </span>
    `;
        relativeContainer.appendChild(watermark);

        // 2. Instructor Section (Absolute Center)
        const instructorSection = document.createElement('div');
        instructorSection.className = 'absolute top-1/2 left-1/2 transform -translate-x-1/2 -translate-y-1/2 flex flex-col items-center justify-center z-20 group cursor-pointer';
        instructorSection.innerHTML = `
    <div class="relative">
                <div class="absolute inset-0 bg-yellow-500/20 blur-3xl rounded-full animate-pulse"></div>
                <!--Pixel Art Avatar-->
    <img src="assets/instructor_avatar.png" class="relative w-48 h-48 md:w-64 md:h-64 object-contain pixel-art drop-shadow-[0_10px_30px_rgba(0,0,0,0.6)] z-10 hover:scale-105 transition-transform duration-300" alt="Instructor">

        <!-- Editable Name Tag -->
        <div class="absolute -bottom-8 left-1/2 transform -translate-x-1/2 bg-black/80 backdrop-blur text-yellow-400 px-6 py-2 rounded-full border border-yellow-500/30 shadow-2xl flex items-center justify-center space-x-2 z-30 whitespace-nowrap group-hover:bg-black transition-colors min-w-[150px] max-w-[300px]">
            <span class="text-xl">👑</span>
            <input type="text" id="instructor-name-input"
                value="${savedName}"
                class="bg-transparent border-b border-transparent hover:border-yellow-500/50 focus:border-yellow-500 text-lg font-bold text-yellow-400 text-center focus:outline-none transition-all placeholder-yellow-700"
                style="width: ${Math.max(savedName.length * 20, 100)}px;"
                onclick="this.select()"
                oninput="this.style.width = Math.max(this.value.length * 20, 100) + 'px'"
            >
        </div>
    </div>
`;
        relativeContainer.appendChild(instructorSection);

        // Save name on change
        setTimeout(() => {
            const input = document.getElementById('instructor-name-input');
            if (input) {
                input.addEventListener('input', (e) => {
                    localStorage.setItem('vibecoding_instructor_name', e.target.value);
                });
            }
        }, 100);

        // 3. Students Scatter
        if (currentStudents.length > 0) {
            // Randomize array to prevent fixed order bias
            const students = [...currentStudents].sort(() => Math.random() - 0.5);
            const total = students.length;

            // --- Dynamic Sizing Logic ---
            let sizeClass = 'w-20 h-20 md:w-24 md:h-24'; // Default (Size 100%)
            let scaleFactor = 1.0;

            if (total >= 40) {
                sizeClass = 'w-12 h-12 md:w-14 md:h-14'; // Size 60%
                scaleFactor = 0.6;
            } else if (total >= 20) {
                sizeClass = 'w-16 h-16 md:w-20 md:h-20'; // Size 80%
                scaleFactor = 0.8;
            }

            students.forEach((s, index) => {
                const progressMap = s.progress || {};
                const totalLikes = Object.values(progressMap).reduce((acc, p) => acc + (p.likes || 0), 0);
                const totalCompleted = Object.values(progressMap).filter(p => p.status === 'completed').length;

                // FIXED: Prioritize stored ID if valid (same as StudentView logic)
                let monster;
                if (s.monster_id && s.monster_id !== 'Egg' && s.monster_id !== 'Unknown') {
                    const stored = MONSTER_DEFS?.find(m => m.id === s.monster_id);
                    if (stored) {
                        monster = stored;
                    } else {
                        // Fallback if ID invalid
                        monster = getNextMonster(s.monster_stage || 0, totalLikes, total, s.monster_id);
                    }
                } else {
                    monster = getNextMonster(s.monster_stage || 0, totalLikes, total, s.monster_id);
                }

                // --- FIXED: Even Arc Distribution (Safe Zone: 135 deg to 405 deg) ---
                // Avoids Bottom Right (0-90) and Bottom Center (90-120) entirely
                const minR = 220;
                // Avoids Bottom Right (0-90) and Bottom Center (90-120) entirely

                // Safe Arc Range: Starts at 135 deg (Bottom Left) -> Goes CW -> Ends at 405 deg (45 deg, Bottom Right)
                // Total Span = 270 degrees
                // If many students, use double ring

                const safeStartAngle = 135 * (Math.PI / 180);
                const safeSpan = 270 * (Math.PI / 180);

                // Distribute evenly
                // If only 1 student, put at top (270 deg / 4.71 rad)
                let finalAngle;

                if (total === 1) {
                    finalAngle = 270 * (Math.PI / 180);
                } else {
                    const step = safeSpan / (total - 1);
                    finalAngle = safeStartAngle + (step * index);
                }

                // Radius: Fixed base + slight variation for "natural" look (but not overlap causing)
                // Double ring logic if crowded
                let radius = minR + (index % 2) * 40; // Zigzag radius (220, 260, 220...) to minimize overlap

                // Reduce zigzag if few students
                if (total < 10) radius = minR + (index % 2) * 20;

                const xOff = Math.cos(finalAngle) * radius;
                const yOff = Math.sin(finalAngle) * radius * 0.8;

                const card = document.createElement('div');
                card.className = 'absolute flex flex-col items-center group/card z-10 hover:z-50 transition-all duration-500 cursor-move';

                card.style.left = `calc(50% + ${xOff}px)`;
                card.style.top = `calc(50% + ${yOff}px)`;
                card.style.transform = 'translate(-50%, -50%)';

                const floatDelay = Math.random() * 2;

                card.innerHTML = `
    <!--Top Info: Monster Stats-->
                    <div class="mb-1 text-center bg-gray-900/60 backdrop-blur-sm rounded-lg px-2 py-1 border border-gray-600/30 group-hover/card:bg-gray-800 group-hover/card:border-cyan-500/50 transition-all opacity-80 group-hover/card:opacity-100 transform translate-y-2 group-hover/card:translate-y-0 duration-300">
                         <div class="text-[10px] text-gray-300 mb-0.5 whitespace-nowrap">${monster.name.split(' ')[1] || monster.name}</div>
                         <div class="flex items-center justify-center space-x-2">
                            <span class="text-[10px] bg-blue-900/50 text-blue-300 px-1.5 rounded border border-blue-500/30">Lv.${totalCompleted + 1}</span>
                            <div class="flex items-center text-[10px] text-pink-400 font-bold">
                                <span>♥</span>
                                <span class="ml-0.5">${totalLikes}</span>
                            </div>
                        </div>
                    </div>

                    <!--Monster Image-->
                    <div class="monster-img-container relative ${sizeClass} flex items-center justify-center transform group-hover/card:scale-125 transition-transform duration-300" style="animation: float 3s ease-in-out infinite; animation-delay: -${floatDelay}s;">
                        <div class="w-full h-full pixel-art drop-shadow-md filter group-hover/card:brightness-110 transition-all">
                            ${generateMonsterSVG(monster)}
                        </div>
                    </div>
                    
                    <!--Bottom Info: User Nickname-->
    <div class="mt-1 text-center bg-black/60 backdrop-blur-sm rounded-full px-3 py-0.5 border border-gray-600/50 group-hover/card:bg-cyan-900/80 group-hover/card:border-cyan-400 transition-all">
        <div class="text-xs font-bold text-white shadow-black drop-shadow-md whitespace-nowrap tracking-wide">${s.nickname}</div>
    </div>
`;
                relativeContainer.appendChild(card);

                // Enable Drag & Drop
                setupDraggable(card, relativeContainer);
            });
        }

        modal.classList.remove('hidden');
    });

    // Helper: Drag & Drop Logic
    function setupDraggable(el, container) {
        let isDragging = false;
        let startX, startY, initialLeft, initialTop;

        el.addEventListener('mousedown', (e) => {
            isDragging = true;
            startX = e.clientX;
            startY = e.clientY;

            // Disable transition during drag for responsiveness
            el.style.transition = 'none';
            el.style.zIndex = 100; // Bring to front

            // Convert current computed position to fixed pixels if relying on calc
            const rect = el.getBoundingClientRect();
            const containerRect = container.getBoundingClientRect();

            // Calculate position relative to container
            // The current transform is translate(-50%, -50%). 
            // We want to set left/top such that the center remains under the mouse offset, 
            // but for simplicity, let's just use current offsetLeft/Top if possible, 
            // OR robustly recalculate from rects.

            // Current center point relative to container:
            const centerX = rect.left - containerRect.left + rect.width / 2;
            const centerY = rect.top - containerRect.top + rect.height / 2;

            // Set explicit pixel values replacing calc()
            el.style.left = `${centerX}px`;
            el.style.top = `${centerY}px`;

            initialLeft = centerX;
            initialTop = centerY;
        });

        window.addEventListener('mousemove', (e) => {
            if (!isDragging) return;
            e.preventDefault();

            const dx = e.clientX - startX;
            const dy = e.clientY - startY;

            el.style.left = `${initialLeft + dx}px`;
            el.style.top = `${initialTop + dy}px`;
        });

        window.addEventListener('mouseup', () => {
            if (isDragging) {
                isDragging = false;
                el.style.transition = ''; // Re-enable hover effects
                el.style.zIndex = ''; // Restore z-index rule (or let hover take over)
            }
        });
    }

    // Add float animation style if not exists
    if (!document.getElementById('anim-float')) {
        const style = document.createElement('style');
        style.id = 'anim-float';
        style.innerHTML = `
@keyframes float {

    0%, 100% { transform: translateY(0) scale(1); }
    50% { transform: translateY(-5px) scale(1.02); }
}
}
`;
        document.head.appendChild(style);
    }

    navAdminBtn.addEventListener('click', () => {
        // Save current room to return later
        const currentRoom = localStorage.getItem('vibecoding_instructor_room');
        localStorage.setItem('vibecoding_admin_referer', 'instructor'); // track entry source
        window.location.hash = 'admin';
    });

    // Auto-fill code
    const savedRoomCode = localStorage.getItem('vibecoding_instructor_room');
    if (savedRoomCode) {
        document.getElementById('rejoin-room-code').value = savedRoomCode;
    }

    const rejoinBtn = document.getElementById('rejoin-room-btn');
    rejoinBtn.addEventListener('click', () => {
        const code = document.getElementById('rejoin-room-code').value.trim();
        if (!code) return alert('請輸入教室代碼');
        enterRoom(code);
    });

    createBtn.addEventListener('click', async () => {
        try {
            createBtn.disabled = true;
            createBtn.textContent = "...";
            const roomCode = await createRoom();
            enterRoom(roomCode);
        } catch (error) {
            console.error(error);
            alert("建立失敗");
            createBtn.disabled = false;
        }
    });

    // Check Previous Session
    if (sessionStorage.getItem('vibecoding_instructor_auth') === 'true') {
        authModal.classList.add('hidden');
    }

    // Check Active Room State
    const activeRoom = sessionStorage.getItem('vibecoding_instructor_in_room');
    if (activeRoom === 'true' && savedRoomCode) {
        enterRoom(savedRoomCode);
    }

    function enterRoom(roomCode) {
        createContainer.classList.add('hidden');
        roomInfo.classList.remove('hidden');
        dashboardContent.classList.remove('hidden');
        displayRoomCode.textContent = roomCode;
        localStorage.setItem('vibecoding_instructor_room', roomCode);
        sessionStorage.setItem('vibecoding_instructor_in_room', 'true');

        // Subscribe to updates
        subscribeToRoom(roomCode, (students) => {
            currentStudents = students;
            renderTransposedHeatmap(students);
        });
    }

    // Modal Events
    window.closeBroadcast = () => {
        const modal = document.getElementById('broadcast-modal');
        const content = document.getElementById('broadcast-content');
        content.classList.remove('opacity-100', 'scale-100');
        content.classList.add('scale-95', 'opacity-0');
        setTimeout(() => modal.classList.add('hidden'), 300);
    };

    window.openStage = (prompt, author) => {
        document.getElementById('broadcast-content').classList.add('hidden');
        const stage = document.getElementById('stage-view');
        stage.classList.remove('hidden');
        document.getElementById('stage-prompt').textContent = prompt;
        document.getElementById('stage-author').textContent = author;
    };

    window.closeStage = () => {
        document.getElementById('stage-view').classList.add('hidden');
        document.getElementById('broadcast-content').classList.remove('hidden');
    };

    document.getElementById('btn-show-stage').addEventListener('click', () => {
        const prompt = document.getElementById('broadcast-prompt').textContent;
        const author = document.getElementById('broadcast-author').textContent;
        window.openStage(prompt, author);
    });

    // Reject Logic
    document.getElementById('btn-reject-task').addEventListener('click', async () => {
        if (!confirm('確定要退回此題目讓學員重做嗎？')) return;

        // We need student ID (userId) and Challenge ID.
        // Currently showBroadcastModal only receives nickname, title, prompt.
        // We need to attach data-userid and data-challengeid to the modal.
        const modal = document.getElementById('broadcast-modal');
        const userId = modal.dataset.userId;
        const challengeId = modal.dataset.challengeId;
        const roomCode = localStorage.getItem('vibecoding_instructor_room');

        if (userId && challengeId && roomCode) {
            try {
                await resetProgress(userId, roomCode, challengeId);
                // Close modal
                window.closeBroadcast();
            } catch (e) {
                console.error(e);
                alert('退回失敗');
            }
        }
    });
}

/**
 * Renders the Transposed Heatmap (Rows=Challenges, Cols=Students)
 */
function renderTransposedHeatmap(students) {
    const thead = document.getElementById('heatmap-header');
    const tbody = document.getElementById('heatmap-body');

    if (students.length === 0) {
        thead.innerHTML = '<th class="p-4 text-left">等待資料...</th>';
        tbody.innerHTML = '<tr><td class="p-10 text-center text-gray-500">尚無學員加入</td></tr>';
        return;
    }

    // 1. Render Header (Students)
    // Sticky Top for Header Row
    // Sticky Left for the first cell ("Challenge/Student")
    let headerHtml = `
    <th class="p-3 text-left sticky left-0 top-0 bg-gray-800 z-30 border-b border-gray-600 min-w-[200px] border-r border-gray-700 shadow-md">
        <div class="flex justify-between items-end">
            <span class="text-sm text-gray-400">題目</span>
            <span class="text-sm text-cyan-400">學員 (${students.length})</span>
        </div>
        </th>
    `;

    students.forEach(student => {
        headerHtml += `
    <th class="p-2 text-center sticky top-0 bg-gray-800 z-20 border-b border-gray-700 min-w-[80px] group">
        <div class="flex flex-col items-center space-y-2 py-2">
            <div class="w-8 h-8 rounded-full bg-gradient-to-br from-gray-700 to-gray-600 flex items-center justify-center text-xs font-bold text-white uppercase border border-gray-500 shadow-sm relative">
                ${student.nickname[0]}
                <!-- Online Indicator (Simulated) -->
                <div class="absolute -bottom-1 -right-1 w-3 h-3 bg-green-500 border-2 border-gray-800 rounded-full"></div>
            </div>
            <div class="flex items-center justify-center space-x-1">
                <span class="text-xs text-gray-300 font-medium truncate max-w-[80px] writing-vertical-lr" style="writing-mode: vertical-rl; text-orientation: mixed;">
                    ${student.nickname}
                </span>
                <button onclick="window.confirmKick('${student.id}', '${student.nickname}')" class="text-gray-600 hover:text-red-500 opacity-0 group-hover:opacity-100 transition-opacity" title="踢出學員">
                    🗑️
                </button>
            </div>
        </div>
            </th>
    `;
    });
    thead.innerHTML = headerHtml;

    // 2. Render Body (Challenges as Rows)
    if (cachedChallenges.length === 0) {
        tbody.innerHTML = '<tr><td colspan="100" class="text-center py-4">沒有題目資料</td></tr>';
        return;
    }

    tbody.innerHTML = cachedChallenges.map((c, index) => {
        const colors = { beginner: 'cyan', intermediate: 'blue', advanced: 'purple' };
        const color = colors[c.level] || 'gray';

        // Build Row Cells (One per student)
        const rowCells = students.map(student => {
            const p = student.progress?.[c.id];
            let statusClass = 'bg-gray-800/30 border-gray-800'; // Default
            let content = '';
            let action = '';

            if (p) {
                if (p.status === 'completed') {
                    statusClass = 'bg-green-500/20 border-green-500/50 hover:bg-green-500/40 cursor-pointer shadow-[0_0_10px_rgba(34,197,94,0.1)]';
                    content = '✅';
                    const safePrompt = p.prompt.replace(/"/g, '&quot;').replace(/'/g, "\\'");
                    action = `onclick = "window.showBroadcastModal('${student.id}', '${c.id}', '${student.nickname}', '${c.title}', '${safePrompt}')"`;
                } else if (p.status === 'started') {
                    // Check stuck
                    const startedAt = p.timestamp ? p.timestamp.toDate() : new Date();
                    const now = new Date();
                    const diffMins = (now - startedAt) / 1000 / 60;

                    if (diffMins > 5) {
                        statusClass = 'bg-red-900/50 border-red-500 animate-pulse cursor-help';
                        content = '🆘';
                    } else {
                        statusClass = 'bg-blue-600/20 border-blue-500';
                        content = '🔵';
                    }
                }
            }

            return `
    <td class="p-1 border border-gray-800/50 text-center align-middle h-14 hover:bg-white/5 transition-colors">
        <div class="mx-auto w-10 h-10 rounded-lg border flex items-center justify-center ${statusClass} transition-all duration-300" ${action}>
            ${content}
        </div>
                </td>
    `;
        }).join('');

        // Row Header (Challenge Title)
        return `
    <tr class="hover:bg-gray-800/50 transition-colors">
        <td class="p-3 sticky left-0 bg-gray-900 z-10 border-r border-b border-gray-700 shadow-md">
            <div class="flex items-center justify-between">
                <div class="flex flex-col">
                    <span class="text-xs text-${color}-400 font-bold uppercase tracking-wider mb-0.5">${c.level}</span>
                    <span class="font-bold text-white text-sm truncate max-w-[180px]" title="${c.title}">${index + 1}. ${c.title}</span>
                </div>
                <!-- Stats (Optional) -->
                <!-- <span class="text-xs text-gray-500">0%</span> -->
            </div>
        </td>
                ${rowCells}
            </tr>
    `;
    }).join('');
}

// Global scope for HTML access
window.showBroadcastModal = (userId, challengeId, nickname, title, prompt) => {
    const modal = document.getElementById('broadcast-modal');
    const content = document.getElementById('broadcast-content');

    document.getElementById('broadcast-avatar').textContent = nickname[0];
    document.getElementById('broadcast-author').textContent = nickname;
    document.getElementById('broadcast-challenge').textContent = title;
    document.getElementById('broadcast-prompt').textContent = prompt;

    // Store IDs for actions
    modal.dataset.userId = userId;
    modal.dataset.challengeId = challengeId;

    modal.classList.remove('hidden');
    // Animation trigger
    setTimeout(() => {
        content.classList.remove('scale-95', 'opacity-0');
        content.classList.add('opacity-100', 'scale-100');
    }, 10);
};