/*
 * book.css — 《深度学习入门》Web 书的框架与阅读排版
 * 纯静态零依赖。配色沿用项目讲解页的深色科技语言, 但针对"长时间阅读"做了优化:
 * 更大的正文字号、更松的行距、克制的阅读栏宽度。
 */

:root {
  --bg: #08111f;
  --bg-soft: #0f1a2e;
  --panel: rgba(18, 29, 49, 0.86);
  --panel-strong: rgba(23, 38, 63, 0.94);
  --text: #eef4ff;
  --text-soft: #b6c5e1;
  --text-dim: #8499bd;
  --line: rgba(149, 176, 227, 0.18);
  --line-strong: rgba(149, 176, 227, 0.32);
  --primary: #6ac3ff;
  --primary-strong: #8b7bff;
  --accent: #8ef0d1;
  --warn: #ffcf72;
  --danger: #ff8fa3;
  --shadow: 0 20px 60px rgba(0, 0, 0, 0.32);
  --radius: 20px;
  --reading-width: 47rem;
  --header-height: 56px;
  --font-sans: "Inter", ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont,
    "Segoe UI", "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", sans-serif;
  --font-mono: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono",
    monospace;
}

* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
  scroll-padding-top: calc(var(--header-height) + 16px);
}

body {
  margin: 0;
  font-family: var(--font-sans);
  background:
    radial-gradient(circle at top left, rgba(72, 124, 255, 0.16), transparent 30%),
    radial-gradient(circle at top right, rgba(73, 255, 189, 0.1), transparent 22%),
    linear-gradient(180deg, #07101d 0%, #08111f 100%);
  background-attachment: fixed;
  color: var(--text);
  line-height: 1.85;
  font-size: 1.0625rem;
  -webkit-font-smoothing: antialiased;
}

a {
  color: var(--primary);
  text-decoration: none;
}

a:hover {
  text-decoration: underline;
}

/* ============================================================= */
/* 顶部固定 header + 阅读进度条 (由 book.js 注入)                  */
/* ============================================================= */

.book-header {
  position: fixed;
  inset: 0 0 auto 0;
  z-index: 50;
  height: var(--header-height);
  display: flex;
  align-items: center;
  gap: 0.8rem;
  padding: 0 1rem;
  background: rgba(7, 15, 28, 0.82);
  border-bottom: 1px solid var(--line);
  backdrop-filter: blur(14px);
}

.book-header__menu {
  display: inline-grid;
  place-items: center;
  width: 40px;
  height: 40px;
  flex: 0 0 auto;
  border-radius: 12px;
  border: 1px solid var(--line);
  background: rgba(255, 255, 255, 0.04);
  color: var(--text);
  cursor: pointer;
  font-size: 1.1rem;
  transition: background 160ms ease, border-color 160ms ease;
}

.book-header__menu:hover {
  background: rgba(106, 195, 255, 0.14);
  border-color: var(--line-strong);
}

.book-header__home {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  flex: 0 0 auto;
  max-width: min(12.5rem, 34vw);
  padding: 0.38rem 0.72rem 0.38rem 0.55rem;
  border-radius: 999px;
  border: 1px solid rgba(255, 255, 255, 0.1);
  background: linear-gradient(
    135deg,
    rgba(255, 255, 255, 0.06) 0%,
    rgba(106, 195, 255, 0.08) 100%
  );
  box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.05);
  font-weight: 800;
  letter-spacing: 0.01em;
  color: var(--text);
  white-space: nowrap;
  transition:
    background 160ms ease,
    border-color 160ms ease,
    color 160ms ease,
    box-shadow 160ms ease,
    transform 160ms ease;
}

.book-header__home-icon {
  display: inline-grid;
  place-items: center;
  width: 1.55rem;
  height: 1.55rem;
  border-radius: 999px;
  background: rgba(246, 248, 250, 0.1);
  color: #f0f3f6;
  flex: 0 0 auto;
}

.book-header__home-icon svg {
  display: block;
  width: 0.95rem;
  height: 0.95rem;
}

.book-header__home-text {
  overflow: hidden;
  text-overflow: ellipsis;
}

.book-header__home:hover {
  text-decoration: none;
  color: #f6f8fa;
  border-color: rgba(246, 248, 250, 0.28);
  background: linear-gradient(
    135deg,
    rgba(246, 248, 250, 0.12) 0%,
    rgba(106, 195, 255, 0.16) 100%
  );
  box-shadow:
    inset 0 1px 0 rgba(255, 255, 255, 0.08),
    0 0 0 1px rgba(106, 195, 255, 0.08);
  transform: translateY(-1px);
}

.book-header__home:hover .book-header__home-icon {
  background: rgba(246, 248, 250, 0.18);
  color: #ffffff;
}

.book-header__home:focus-visible {
  outline: 2px solid rgba(106, 195, 255, 0.65);
  outline-offset: 2px;
}

.book-header__current {
  flex: 1 1 auto;
  min-width: 0;
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
  color: var(--text-dim);
  font-size: 0.95rem;
}

.book-header__pct {
  flex: 0 0 auto;
  font-variant-numeric: tabular-nums;
  color: var(--text-soft);
  font-weight: 700;
  font-size: 0.9rem;
}

.book-progress {
  position: fixed;
  top: var(--header-height);
  left: 0;
  z-index: 50;
  height: 3px;
  width: 0%;
  background: linear-gradient(90deg, var(--primary), var(--accent));
  box-shadow: 0 0 12px rgba(106, 195, 255, 0.6);
  transition: width 120ms linear;
}

/* ============================================================= */
/* 侧边目录抽屉 (由 book.js 注入)                                  */
/* ============================================================= */

.book-toc-backdrop {
  position: fixed;
  inset: 0;
  z-index: 60;
  background: rgba(3, 8, 16, 0.55);
  backdrop-filter: blur(2px);
  opacity: 0;
  pointer-events: none;
  transition: opacity 220ms ease;
}

.book-toc-backdrop.is-open {
  opacity: 1;
  pointer-events: auto;
}

.book-toc {
  position: fixed;
  top: 0;
  left: 0;
  z-index: 70;
  height: 100%;
  width: min(340px, 86vw);
  padding: 1.1rem 1rem 2rem;
  background: var(--panel-strong);
  border-right: 1px solid var(--line);
  box-shadow: var(--shadow);
  transform: translateX(-104%);
  transition: transform 260ms cubic-bezier(0.22, 1, 0.36, 1);
  overflow-y: auto;
}

.book-toc.is-open {
  transform: translateX(0);
}

.book-toc__title {
  margin: 0.2rem 0 1rem;
  font-size: 0.8rem;
  letter-spacing: 0.18em;
  text-transform: uppercase;
  color: var(--accent);
}

.book-toc__list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: grid;
  gap: 0.25rem;
}

.book-toc__link {
  display: flex;
  align-items: baseline;
  gap: 0.6rem;
  padding: 0.55rem 0.7rem;
  border-radius: 12px;
  color: var(--text-soft);
  border: 1px solid transparent;
  transition: background 150ms ease, color 150ms ease, border-color 150ms ease;
}

.book-toc__link:hover {
  text-decoration: none;
  background: rgba(255, 255, 255, 0.04);
  color: var(--text);
}

.book-toc__link.is-current {
  background: rgba(106, 195, 255, 0.14);
  border-color: rgba(106, 195, 255, 0.4);
  color: var(--text);
}

.book-toc__num {
  flex: 0 0 auto;
  width: 1.6rem;
  font-variant-numeric: tabular-nums;
  font-weight: 800;
  color: var(--primary);
  font-size: 0.9rem;
}

.book-toc__name {
  font-size: 0.95rem;
  line-height: 1.4;
}

/* ============================================================= */
/* 正文章节排版                                                    */
/* ============================================================= */

.chapter {
  padding: calc(var(--header-height) + 2.4rem) 1.2rem 2rem;
}

.chapter__inner {
  max-width: var(--reading-width);
  margin: 0 auto;
}

.chapter__eyebrow {
  margin: 0 0 0.5rem;
  text-transform: uppercase;
  letter-spacing: 0.16em;
  font-size: 0.78rem;
  color: var(--accent);
}

.chapter h1 {
  font-size: clamp(2rem, 5vw, 2.9rem);
  line-height: 1.14;
  margin: 0 0 1rem;
}

.chapter h2 {
  font-size: clamp(1.5rem, 3.4vw, 2rem);
  line-height: 1.2;
  margin: 3rem 0 1rem;
  padding-top: 0.6rem;
  border-top: 1px solid var(--line);
}

.chapter h3 {
  font-size: 1.28rem;
  margin: 2.2rem 0 0.8rem;
  color: var(--text);
}

.chapter h4 {
  font-size: 1.08rem;
  margin: 1.6rem 0 0.6rem;
  color: var(--text);
}

.chapter p {
  margin: 1rem 0;
  color: var(--text-soft);
}

.chapter strong {
  color: var(--text);
  font-weight: 700;
}

.chapter ul,
.chapter ol {
  margin: 1rem 0;
  padding-left: 1.4rem;
  color: var(--text-soft);
}

.chapter li {
  margin: 0.45rem 0;
}

.chapter li::marker {
  color: var(--primary);
}

.chapter .lead {
  font-size: 1.18rem;
  line-height: 1.8;
  color: var(--text);
}

/* 跨章引用链接 */
a.xref {
  color: var(--primary);
  text-decoration: none;
  border-bottom: 1px solid rgba(106, 195, 255, 0.45);
  transition: color 0.15s ease, border-color 0.15s ease;
}

a.xref:hover {
  color: #9dd8ff;
  border-bottom-color: rgba(157, 216, 255, 0.85);
}

/* 行内代码与术语 */
.chapter code {
  padding: 0.1rem 0.4rem;
  border-radius: 7px;
  background: rgba(139, 123, 255, 0.16);
  color: #ddd6ff;
  font-family: var(--font-mono);
  font-size: 0.9em;
}

.term {
  color: var(--accent);
  font-weight: 700;
  border-bottom: 1px dotted rgba(142, 240, 209, 0.5);
}

/* 术语表悬浮卡:正文里可点击的 glossary 词(蓝色,与 .term 绿色区分) */
.term--glossary {
  cursor: pointer;
  padding: 0;
  margin: 0;
  border: none;
  background: none;
  font: inherit;
  color: var(--primary);
  font-weight: 600;
  border-bottom: 1px dotted rgba(106, 195, 255, 0.55);
  text-decoration: none;
  border-radius: 2px;
  transition: color 0.15s ease, background 0.15s ease;
}

.term.term--glossary {
  color: var(--primary);
  border-bottom-color: rgba(106, 195, 255, 0.55);
}

.term--glossary:hover,
.term--glossary:focus-visible {
  color: #a8dcff;
  background: rgba(106, 195, 255, 0.12);
  outline: none;
}

.term--glossary.is-active {
  color: #fff;
  background: rgba(106, 195, 255, 0.22);
  border-bottom-style: solid;
}

code .term--glossary,
pre .term--glossary {
  font-family: var(--font-mono);
  font-size: 0.95em;
  font-weight: 600;
}

.glossary-popover {
  position: fixed;
  z-index: 1200;
  width: min(22rem, calc(100vw - 2rem));
  padding: 0;
  border: 1px solid var(--line-strong);
  border-radius: 14px;
  background: var(--panel-strong);
  box-shadow: var(--shadow);
  color: var(--text);
  animation: glossary-pop-in 0.14s ease-out;
}

@keyframes glossary-pop-in {
  from { opacity: 0; transform: translateY(4px) scale(0.98); }
  to { opacity: 1; transform: translateY(0) scale(1); }
}

.glossary-popover__inner {
  padding: 0.85rem 1rem 0.75rem;
}

.glossary-popover__title {
  margin: 0 0 0.45rem;
  font-size: 0.95rem;
  font-weight: 700;
  color: var(--accent);
  line-height: 1.4;
}

.glossary-popover__body {
  margin: 0;
  font-size: 0.88rem;
  line-height: 1.65;
  color: var(--text-soft);
}

.glossary-popover__foot {
  display: flex;
  gap: 0.75rem;
  margin-top: 0.7rem;
  padding-top: 0.55rem;
  border-top: 1px solid var(--line);
  font-size: 0.8rem;
}

.glossary-popover__link {
  color: var(--primary);
  text-decoration: none;
}

.glossary-popover__link:hover {
  text-decoration: underline;
}

.glossary-popover__hint {
  margin-left: auto;
  color: var(--text-dim);
}

.glossary-popover__arrow {
  position: absolute;
  width: 10px;
  height: 10px;
  background: var(--panel-strong);
  border-left: 1px solid var(--line-strong);
  border-top: 1px solid var(--line-strong);
  transform: rotate(45deg);
}

.glossary-popover.is-below .glossary-popover__arrow {
  top: -6px;
  left: var(--arrow-x, 1.5rem);
}

.glossary-popover.is-above .glossary-popover__arrow {
  bottom: -6px;
  left: var(--arrow-x, 1.5rem);
  transform: rotate(225deg);
}

.glossary-backdrop {
  position: fixed;
  inset: 0;
  z-index: 1199;
  background: transparent;
}

/* 引用 / 旁白 */
.chapter blockquote {
  margin: 1.4rem 0;
  padding: 0.4rem 1.2rem;
  border-left: 3px solid var(--primary);
  color: var(--text-soft);
  font-style: italic;
}

/* ============================================================= */
/* 教学组件: 学习目标 / 提示框 / 类比 / 小结 / 自测                 */
/* ============================================================= */

.objectives {
  margin: 1.6rem 0 2rem;
  padding: 1.1rem 1.3rem;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--panel);
  box-shadow: var(--shadow);
}

.objectives h2,
.objectives h3 {
  margin: 0 0 0.6rem;
  border: 0;
  padding: 0;
  font-size: 1rem;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--accent);
}

.objectives ul {
  margin: 0;
}

.callout {
  margin: 1.6rem 0;
  padding: 1rem 1.2rem 1rem 1.1rem;
  border: 1px solid var(--line);
  border-left-width: 4px;
  border-radius: 14px;
  background: var(--panel);
}

.callout > :first-child {
  margin-top: 0;
}

.callout > :last-child {
  margin-bottom: 0;
}

.callout__title {
  display: block;
  margin-bottom: 0.3rem;
  font-weight: 800;
  color: var(--text);
}

.callout--tip {
  border-left-color: var(--accent);
}
.callout--tip .callout__title {
  color: var(--accent);
}

.callout--note {
  border-left-color: var(--primary);
}
.callout--note .callout__title {
  color: var(--primary);
}

.callout--warn {
  border-left-color: var(--warn);
}
.callout--warn .callout__title {
  color: var(--warn);
}

.callout--analogy {
  border-left-color: var(--primary-strong);
  background: rgba(139, 123, 255, 0.08);
}
.callout--analogy .callout__title {
  color: #c9bdff;
}

.summary {
  margin: 2.6rem 0 1.6rem;
  padding: 1.3rem 1.4rem;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: linear-gradient(160deg, rgba(106, 195, 255, 0.08), rgba(142, 240, 209, 0.05));
}

.summary h2,
.summary h3 {
  margin-top: 0;
  border: 0;
  padding: 0;
}

.quiz {
  margin: 1.6rem 0;
  display: grid;
  gap: 0.8rem;
}

.quiz__item {
  border: 1px solid var(--line);
  border-radius: 14px;
  background: var(--panel);
  overflow: hidden;
}

.quiz__q {
  margin: 0;
  padding: 0.9rem 1.1rem;
  font-weight: 700;
  color: var(--text);
  cursor: pointer;
  list-style: none;
  position: relative;
}

.quiz__q::-webkit-details-marker {
  display: none;
}

.quiz__q::after {
  content: "显示答案";
  position: absolute;
  right: 1.1rem;
  top: 50%;
  transform: translateY(-50%);
  font-size: 0.8rem;
  font-weight: 600;
  color: var(--primary);
}

details[open] .quiz__q::after {
  content: "收起";
}

.quiz__a {
  padding: 0 1.1rem 1rem;
  color: var(--text-soft);
  border-top: 1px solid var(--line);
}

.quiz__a > :first-child {
  margin-top: 0.8rem;
}

/* 关键词高亮的小药丸 */
.pill {
  display: inline-block;
  padding: 0.1rem 0.55rem;
  border-radius: 999px;
  background: rgba(106, 195, 255, 0.16);
  border: 1px solid rgba(106, 195, 255, 0.3);
  color: var(--text);
  font-size: 0.82rem;
  font-weight: 600;
}

/* ============================================================= */
/* 章节底部导航: 上一章 / 下一章 (由 book.js 注入)                 */
/* ============================================================= */

.chapter-nav {
  max-width: var(--reading-width);
  margin: 3.2rem auto 1rem;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1rem;
}

.chapter-nav__link {
  display: grid;
  gap: 0.25rem;
  padding: 1rem 1.2rem;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--panel);
  transition: transform 160ms ease, border-color 160ms ease, background 160ms ease;
}

.chapter-nav__link:hover {
  text-decoration: none;
  transform: translateY(-2px);
  border-color: var(--line-strong);
  background: var(--panel-strong);
}

.chapter-nav__link--next {
  text-align: right;
}

.chapter-nav__dir {
  font-size: 0.78rem;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--text-dim);
}

.chapter-nav__title {
  font-weight: 700;
  color: var(--text);
}

.chapter-nav__link--placeholder {
  opacity: 0.4;
  pointer-events: none;
}

/* ============================================================= */
/* 桌面端双栏导航: 左章节 / 右本章目录 (由 book.js 注入)          */
/* ============================================================= */

.book-layout {
  display: contents;
}

.book-layout__main {
  min-width: 0;
}

.book-rail {
  display: none;
}

.book-rail__title {
  margin: 0 0 0.85rem;
  font-size: 0.76rem;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: var(--accent);
}

.book-rail__list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: grid;
  gap: 0.2rem;
}

.book-rail__part {
  margin: 0.85rem 0 0.35rem;
  font-size: 0.72rem;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--text-dim);
  line-height: 1.35;
}

.book-rail__part:first-child {
  margin-top: 0;
}

.book-rail__link,
.book-rail__outline-link {
  display: block;
  padding: 0.45rem 0.55rem;
  border-radius: 10px;
  color: var(--text-soft);
  border: 1px solid transparent;
  line-height: 1.4;
  font-size: 0.88rem;
  transition: background 150ms ease, color 150ms ease, border-color 150ms ease;
}

.book-rail__link {
  display: flex;
  align-items: baseline;
  gap: 0.45rem;
}

.book-rail__link:hover,
.book-rail__outline-link:hover {
  text-decoration: none;
  background: rgba(255, 255, 255, 0.04);
  color: var(--text);
}

.book-rail__link.is-current,
.book-rail__outline-link.is-current {
  background: rgba(106, 195, 255, 0.14);
  border-color: rgba(106, 195, 255, 0.35);
  color: var(--text);
}

.book-rail__num {
  flex: 0 0 auto;
  width: 1.2rem;
  font-variant-numeric: tabular-nums;
  font-weight: 800;
  color: var(--primary);
  font-size: 0.82rem;
}

.book-rail__name {
  min-width: 0;
}

.book-rail__outline-item--sub {
  padding-left: 0.65rem;
}

.book-rail__outline-item--sub .book-rail__outline-link {
  font-size: 0.82rem;
  color: var(--text-dim);
}

.book-rail__outline-item--sub .book-rail__outline-link.is-current {
  color: var(--text);
}

.book-rail__empty {
  margin: 0;
  color: var(--text-dim);
  font-size: 0.85rem;
  line-height: 1.5;
}

@media (min-width: 1120px) {
  .book-header__menu {
    display: none;
  }

  .chapter {
    padding-left: 1.5rem;
    padding-right: 1.5rem;
  }

  .book-layout {
    display: grid;
    grid-template-columns: 220px minmax(0, var(--reading-width)) minmax(180px, 220px);
    gap: 2rem;
    max-width: 1280px;
    margin: 0 auto;
    align-items: start;
  }

  .book-rail {
    display: block;
    position: sticky;
    top: calc(var(--header-height) + 1.25rem);
    max-height: calc(100vh - var(--header-height) - 2rem);
    overflow-y: auto;
    padding: 0.2rem 0.2rem 1rem 0;
    scrollbar-width: none; /* Firefox: 隐藏滚动条 */
    -ms-overflow-style: none; /* 旧 Edge/IE */
  }

  /* Chrome/Safari: 隐藏侧栏滚动条 (仍可滚动) */
  .book-rail::-webkit-scrollbar {
    width: 0;
    height: 0;
    display: none;
  }

  .book-rail--chapters {
    padding-right: 0.35rem;
  }

  .book-rail--outline {
    padding-left: 0.35rem;
    border-left: 1px solid var(--line);
  }

  .chapter__inner {
    max-width: none;
    margin: 0;
  }

  .chapter-nav {
    max-width: none;
    margin-left: 0;
    margin-right: 0;
  }
}

@media (min-width: 1320px) {
  .book-layout {
    max-width: 1380px;
    grid-template-columns: 240px minmax(0, 50rem) 240px;
    gap: 2.4rem;
  }
}

/* ============================================================= */
/* 封面 / 目录页 (index.html)                                     */
/* ============================================================= */

.cover {
  position: relative;
  overflow: hidden;
  padding: 4.5rem 1.4rem 3rem;
  text-align: center;
}

.cover__grid {
  position: absolute;
  inset: 0;
  background-image:
    linear-gradient(rgba(255, 255, 255, 0.05) 1px, transparent 1px),
    linear-gradient(90deg, rgba(255, 255, 255, 0.05) 1px, transparent 1px);
  background-size: 42px 42px;
  mask-image: linear-gradient(180deg, rgba(0, 0, 0, 0.8), transparent 70%);
  pointer-events: none;
}

.cover__inner {
  position: relative;
  max-width: 52rem;
  margin: 0 auto;
}

.cover__eyebrow {
  text-transform: uppercase;
  letter-spacing: 0.2em;
  color: var(--accent);
  font-size: 0.85rem;
  margin: 0 0 1rem;
}

.cover h1 {
  font-size: clamp(2.6rem, 7vw, 4.6rem);
  line-height: 1.05;
  margin: 0 0 1.2rem;
}

.cover__lead {
  max-width: 40rem;
  margin: 0 auto 1.8rem;
  font-size: 1.15rem;
  color: var(--text-soft);
}

.cover__actions {
  display: flex;
  flex-wrap: wrap;
  gap: 0.8rem;
  justify-content: center;
  margin-bottom: 0.5rem;
}

/* GitHub 右上角角标 (封面页, tholman/github-corners 官方路径) */
.github-corner {
  position: fixed;
  top: 0;
  right: 0;
  z-index: 200;
  width: 80px;
  height: 80px;
  border: 0;
}

.github-corner svg {
  display: block;
  width: 80px;
  height: 80px;
}

.github-corner__ribbon {
  fill: #f6f8fa;
}

.github-corner__arm,
.github-corner__body {
  fill: #24292f;
}

.github-corner__arm {
  transform-origin: 130px 106px;
}

.github-corner:hover .github-corner__arm {
  animation: github-corner-wave 560ms ease-in-out;
}

@keyframes github-corner-wave {
  0%,
  100% {
    transform: rotate(0);
  }
  20%,
  60% {
    transform: rotate(-20deg);
  }
  40%,
  80% {
    transform: rotate(8deg);
  }
}

@media (prefers-reduced-motion: reduce) {
  .github-corner:hover .github-corner__arm {
    animation: none;
  }
}

@media (max-width: 640px) {
  .github-corner,
  .github-corner svg {
    width: 64px;
    height: 64px;
  }
}

.button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 48px;
  padding: 0.8rem 1.4rem;
  border-radius: 999px;
  font-weight: 700;
  cursor: pointer;
  transition: transform 160ms ease, border-color 160ms ease, background 160ms ease;
}

.button:hover {
  text-decoration: none;
  transform: translateY(-1px);
}

.button--primary {
  background: linear-gradient(120deg, var(--primary), var(--primary-strong));
  color: #06101f;
}

.button--ghost {
  border: 1px solid var(--line-strong);
  background: rgba(255, 255, 255, 0.04);
  color: var(--text);
}

.toc-section {
  max-width: 64rem;
  margin: 0 auto;
  padding: 1rem 1.4rem 5rem;
}

.toc-section h2 {
  text-align: center;
  font-size: clamp(1.6rem, 3.4vw, 2.2rem);
  margin: 1rem 0 0.4rem;
}

.toc-section__hint {
  text-align: center;
  color: var(--text-dim);
  margin: 0 0 2rem;
}

.toc-part {
  margin: 2.4rem 0 1rem;
  font-size: 0.85rem;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: var(--accent);
}

.toc-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 1rem;
}

.toc-card {
  position: relative;
  display: grid;
  gap: 0.4rem;
  align-content: start;
  height: auto;
  min-height: 0;
  padding: 1.2rem 1.2rem 1.3rem;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--panel);
  box-shadow: var(--shadow);
  transition: transform 180ms ease, border-color 180ms ease, background 180ms ease;
}

.toc-card:hover {
  text-decoration: none;
  transform: translateY(-3px);
  border-color: var(--line-strong);
  background: var(--panel-strong);
}

.toc-card__num {
  font-variant-numeric: tabular-nums;
  font-weight: 800;
  font-size: 0.85rem;
  color: var(--primary);
}

.toc-card__title {
  font-size: 1.15rem;
  font-weight: 700;
  color: var(--text);
}

.toc-card__desc {
  color: var(--text-soft);
  font-size: 0.92rem;
  line-height: 1.55;
  overflow-wrap: anywhere;
}

.toc-card--appendix {
  border-style: dashed;
  background: rgba(142, 240, 209, 0.04);
}

.toc-card--appendix .toc-card__num {
  color: var(--accent);
}

.toc-card--appendix:hover {
  border-color: rgba(142, 240, 209, 0.35);
  background: rgba(142, 240, 209, 0.08);
}

/* ============================================================= */
/* 通用工具                                                       */
/* ============================================================= */

.center {
  text-align: center;
}

.muted {
  color: var(--text-dim);
}

hr {
  border: 0;
  height: 1px;
  background: var(--line);
  margin: 2.4rem 0;
}

/* 进入视口淡入 (book.js 加 is-visible) */
.reveal {
  opacity: 0;
  transform: translateY(18px);
  transition: opacity 520ms ease, transform 520ms ease;
}

.reveal.is-visible {
  opacity: 1;
  transform: translateY(0);
}

@media (prefers-reduced-motion: reduce) {
  * {
    scroll-behavior: auto !important;
  }
  .reveal {
    opacity: 1;
    transform: none;
    transition: none;
  }
}

/* ============================================================= */
/* 全书搜索 (由 book.js 注入)                                      */
/* ============================================================= */

.book-header__search {
  display: inline-grid;
  place-items: center;
  width: 40px;
  height: 40px;
  flex: 0 0 auto;
  border-radius: 12px;
  border: 1px solid var(--line);
  background: rgba(255, 255, 255, 0.04);
  color: var(--text);
  cursor: pointer;
  transition: background 160ms ease, border-color 160ms ease;
}

.book-header__search:hover {
  background: rgba(106, 195, 255, 0.14);
  border-color: var(--line-strong);
}

.book-header__search svg,
.book-search__icon svg {
  width: 18px;
  height: 18px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2;
  stroke-linecap: round;
}

.book-search {
  position: fixed;
  inset: 0;
  z-index: 80;
  display: none;
}

.book-search.is-open {
  display: block;
}

.book-search__backdrop {
  position: absolute;
  inset: 0;
  background: rgba(3, 8, 16, 0.62);
  backdrop-filter: blur(3px);
}

.book-search__panel {
  position: relative;
  max-width: 720px;
  margin: 7vh auto 0;
  max-height: 82vh;
  display: flex;
  flex-direction: column;
  background: var(--panel-strong);
  border: 1px solid var(--line-strong);
  border-radius: 16px;
  box-shadow: var(--shadow);
  overflow: hidden;
  animation: book-search-pop 180ms ease;
}

@keyframes book-search-pop {
  from { opacity: 0; transform: translateY(-8px); }
  to { opacity: 1; transform: translateY(0); }
}

.book-search__bar {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  padding: 0.7rem 0.8rem;
  border-bottom: 1px solid var(--line);
}

.book-search__icon {
  display: inline-grid;
  place-items: center;
  flex: 0 0 auto;
  color: var(--text-dim);
}

.book-search__input {
  flex: 1 1 auto;
  min-width: 0;
  background: transparent;
  border: 0;
  outline: none;
  color: var(--text);
  font-size: 1.05rem;
  font-family: inherit;
  padding: 0.4rem 0.2rem;
}

.book-search__input::placeholder {
  color: var(--text-dim);
}

.book-search__close {
  flex: 0 0 auto;
  padding: 0.3rem 0.6rem;
  border-radius: 8px;
  border: 1px solid var(--line);
  background: rgba(255, 255, 255, 0.04);
  color: var(--text-dim);
  font-size: 0.78rem;
  font-weight: 700;
  cursor: pointer;
}

.book-search__close:hover {
  color: var(--text);
  border-color: var(--line-strong);
}

.book-search__status {
  padding: 0.5rem 0.9rem;
  color: var(--text-dim);
  font-size: 0.82rem;
  border-bottom: 1px solid var(--line);
}

.book-search__results {
  overflow-y: auto;
  padding: 0.4rem;
  display: grid;
  gap: 0.15rem;
}

.book-search__result {
  display: block;
  padding: 0.6rem 0.7rem;
  border-radius: 10px;
  border: 1px solid transparent;
  color: var(--text-soft);
}

.book-search__result:hover,
.book-search__result.is-active {
  text-decoration: none;
  background: rgba(106, 195, 255, 0.12);
  border-color: rgba(106, 195, 255, 0.3);
}

.book-search__result-chapter {
  display: block;
  font-size: 0.72rem;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--primary);
}

.book-search__result-heading {
  display: block;
  font-weight: 700;
  color: var(--text);
  margin: 0.12rem 0;
}

.book-search__result-snippet {
  display: block;
  font-size: 0.9rem;
  line-height: 1.55;
  color: var(--text-soft);
}

.book-search mark {
  background: rgba(255, 207, 114, 0.3);
  color: var(--text);
  border-radius: 3px;
  padding: 0 0.12em;
}

.book-search__empty {
  padding: 1.4rem;
  text-align: center;
  color: var(--text-dim);
}

/* ============================================================= */
/* 响应式                                                         */
/* ============================================================= */

@media (max-width: 720px) {
  body {
    font-size: 1rem;
  }

  .chapter {
    padding: calc(var(--header-height) + 1.6rem) 1rem 1.5rem;
  }

  .chapter-nav {
    grid-template-columns: 1fr;
  }

  .book-header__current {
    display: none;
  }

  .book-search__panel {
    margin: 0;
    height: 100%;
    max-height: 100vh;
    border-radius: 0;
  }
}