/* 變數定義 */
:root {
    --book-cover: #2c3e50;
    --book-page: #fdfdfd;
    --text-gold: #e6be8a;
    --flip-speed: 0.6s;
}

body {
    margin: 0;
    padding: 0;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: radial-gradient(circle, #222 0%, #000 100%);
    font-family: 'Noto Serif TC', "Microsoft JhengHei", serif;
    perspective: 2000px;
    overflow: hidden;
}

/* UI 層 */
.ui-layer {
    position: absolute;
    top: 40px;
    text-align: center;
    z-index: 100;
    pointer-events: none;
}

.ui-layer h1 {
    color: var(--text-gold);
    font-size: 2rem;
    letter-spacing: 8px;
    margin-bottom: 5px;
    text-shadow: 0 0 10px rgba(230, 190, 138, 0.5);
}

/* 書本容器 */
.book {
    width: 320px;
    height: 450px;
    position: relative;
    transform-style: preserve-3d;
    transition: transform 1s ease-in-out;
    cursor: pointer;
}

/* 翻開時的位移 */
.book.flipping {
    transform: rotateY(-20deg) translateX(50px);
}

/* 通用頁面設定 */
.page {
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
    transform-origin: left center;
    transition: transform var(--flip-speed) ease-in-out;
    transform-style: preserve-3d;
    border-radius: 2px 8px 8px 2px;
}

/* 封面樣式 */
.cover {
    background: var(--book-cover);
    z-index: 10;
    display: flex;
    justify-content: center;
    align-items: center;
    border: 1px solid rgba(255,255,255,0.1);
}

.cover-content {
    border: 2px solid var(--text-gold);
    padding: 30px 15px;
    text-align: center;
    color: var(--text-gold);
}

/* 裝飾頁面 - 製造厚度與翻動感 */
.page-deco {
    background-color: var(--book-page);
    z-index: 5;
    border-left: 2px solid #ddd;
    box-shadow: inset 10px 0 20px rgba(0,0,0,0.05);
}

/* 最底層解答頁 */
.page-answer {
    background-color: var(--book-page);
    z-index: 1;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 40px;
    box-sizing: border-box;
    text-align: center;
    color: #2c3e50;
    font-size: 1.5rem;
    font-weight: bold;
    box-shadow: 5px 10px 30px rgba(0,0,0,0.5);
}

/* 翻頁動畫細節：遞增延遲 */
.book.flipping .cover {
    transform: rotateY(-170deg);
}

/* 利用 nth-child 讓每一頁翻開的速度有微小落差 */
.book.flipping .page-deco:nth-child(2) { transform: rotateY(-165deg); transition-delay: 0.1s; }
.book.flipping .page-deco:nth-child(3) { transform: rotateY(-160deg); transition-delay: 0.2s; }
.book.flipping .page-deco:nth-child(4) { transform: rotateY(-155deg); transition-delay: 0.3s; }
.book.flipping .page-deco:nth-child(5) { transform: rotateY(-150deg); transition-delay: 0.4s; }
.book.flipping .page-deco:nth-child(6) { transform: rotateY(-145deg); transition-delay: 0.5s; }

/* 答案文字動畫 */
.answer-text {
    opacity: 0;
    transform: translateY(10px);
    transition: all 0.5s ease;
}

.book.flipping .answer-text {
    opacity: 1;
    transform: translateY(0);
    transition-delay: 0.8s;
}

/* 按鈕樣式 */
.controls {
    margin-top: 50px;
    z-index: 100;
}

button {
    background: transparent;
    border: 1px solid var(--text-gold);
    color: var(--text-gold);
    padding: 12px 30px;
    font-size: 1rem;
    cursor: pointer;
    transition: 0.3s;
    letter-spacing: 2px;
}

button:hover {
    background: var(--text-gold);
    color: #000;
}