/* ============================================================
   TYPOGRAPHY
   ============================================================ */
@import url('https://fonts.googleapis.com/css2?family=Noto+Serif+SC:wght@400;500;600;700&family=DM+Mono:wght@400;500&family=DM+Sans:opsz@9..40&display=swap');

:root {
  /* Palette */
  --paper:      #fdfbf7;
  --paper-warm: #faf7f1;
  --ink:        #1c1c1c;
  --ink-soft:   #4a4540;
  --muted:      #8c8580;
  --faint:      #c4bfb8;
  --vermilion:  #b5343a;
  --vermilion-dim: #d4747a;
  --gold:       #c4a44a;
  --border:     #e8e3db;
  --card-bg:    #faf8f3;
  --code-bg:    #f2efe8;

  /* Typography */
  --font-serif: 'Noto Serif SC', 'Source Han Serif SC', 'STSong', 'SimSun', 'Songti SC', serif;
  --font-mono:  'DM Mono', 'JetBrains Mono', 'Cascadia Code', 'Fira Code', monospace;
  --font-ui:    'DM Sans', 'PingFang SC', 'Hiragino Sans GB', 'Microsoft YaHei', system-ui, sans-serif;

  /* Spacing scale */
  --gap-xs:  0.375rem;
  --gap-sm:  0.75rem;
  --gap-md:  1.25rem;
  --gap-lg:  2rem;
  --gap-xl:  3rem;
  --gap-2xl: 5rem;

  /* Layout */
  --content-width: 780px;
  --home-width:    860px;
  --toc-width:     220px;
}

/* ============================================================
   RESET & BASE
   ============================================================ */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

html {
  font-size: 16px;
  scroll-behavior: smooth;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

body {
  font-family: var(--font-serif);
  color: var(--ink);
  background: var(--paper);
  line-height: 1.75;
  min-height: 100vh;
}

/* Subtle paper texture overlay */
body::before {
  content: '';
  position: fixed;
  inset: 0;
  z-index: 9999;
  pointer-events: none;
  opacity: 0.025;
  background-image: url("data:image/svg+xml,%3Csvg viewBox='0 0 256 256' xmlns='http://www.w3.org/2000/svg'%3E%3Cfilter id='noise'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.9' numOctaves='4' stitchTiles='stitch'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23noise)'/%3E%3C/svg%3E");
}

a { color: var(--vermilion); text-decoration: none; transition: color 0.2s; }
a:hover { color: var(--vermilion-dim); }

img { max-width: 100%; height: auto; display: block; }

/* ============================================================
   HEADER
   ============================================================ */
.site-header {
  position: static;
  background: var(--paper);
  padding: var(--gap-lg) var(--gap-lg) var(--gap-md);
  text-align: center;
  max-width: var(--home-width);
  margin: 0 auto;
}

.header-inner {
  margin-bottom: var(--gap-md);
}

.site-title {
  font-family: var(--font-serif);
  font-size: 2.4rem;
  font-weight: 700;
  letter-spacing: 0.06em;
  color: var(--ink);
  display: inline-block;
  transition: color 0.2s;
}

.site-title a {
  color: inherit;
}

.site-title:hover {
  color: var(--ink);
}

.site-subtitle {
  font-family: var(--font-ui);
  font-size: 0.8rem;
  font-weight: 400;
  color: var(--muted);
  letter-spacing: 0.12em;
  text-transform: uppercase;
}

/* Decorative vermillion rule */
.header-rule {
  width: 40px;
  height: 2px;
  background: var(--vermilion);
  margin: var(--gap-md) auto 0;
  border: none;
  opacity: 0.5;
}

.header-nav {
  display: flex;
  justify-content: center;
  gap: var(--gap-lg);
  flex-wrap: wrap;
}

.header-nav a {
  font-family: var(--font-ui);
  font-size: 0.82rem;
  font-weight: 500;
  color: var(--ink-soft);
  letter-spacing: 0.04em;
  padding-bottom: 2px;
  border-bottom: 1px solid transparent;
  transition: border-color 0.2s, color 0.2s;
}

.header-nav a:hover {
  color: var(--ink);
  border-bottom-color: var(--vermilion);
}

/* ============================================================
   CATEGORY NAV (home page)
   ============================================================ */
.category-nav {
  display: flex;
  justify-content: center;
  gap: var(--gap-lg);
  padding: 0 var(--gap-lg) var(--gap-xl);
  flex-wrap: wrap;
  max-width: var(--home-width);
  margin: 0 auto;
}

.category-nav a {
  font-family: var(--font-ui);
  font-size: 0.82rem;
  font-weight: 500;
  color: var(--ink-soft);
  letter-spacing: 0.04em;
  padding-bottom: 2px;
  border-bottom: 1px solid transparent;
  transition: border-color 0.2s, color 0.2s;
  scroll-behavior: smooth;
}

.category-nav a:hover {
  color: var(--ink);
  border-bottom-color: var(--vermilion);
}

.category-nav .nav-count {
  font-family: var(--font-mono);
  font-size: 0.65rem;
  color: var(--muted);
  margin-left: 2px;
}

/* ============================================================
   MAIN CONTENT
   ============================================================ */
.main-content {
  max-width: var(--home-width);
  margin: 0 auto;
  padding: 0 var(--gap-lg) var(--gap-2xl);
}

/* ============================================================
   CATEGORY SECTION
   ============================================================ */
.category-section {
  margin-bottom: var(--gap-2xl);
}

.category-heading {
  display: flex;
  align-items: baseline;
  gap: var(--gap-sm);
  margin-bottom: var(--gap-lg);
}

.category-heading h2 {
  font-family: var(--font-serif);
  font-size: 1.25rem;
  font-weight: 600;
  color: var(--ink);
  letter-spacing: 0.03em;
  flex-shrink: 0;
}
.category-heading h2 a {
  color: inherit;
  text-decoration: none;
}
.category-heading h2 a:hover {
  text-decoration: underline;
  text-underline-offset: 0.2em;
}

.category-heading .cat-count {
  font-family: var(--font-mono);
  font-size: 0.7rem;
  color: var(--muted);
  flex-shrink: 0;
}

/* Decorative horizontal line extending right from heading */
.category-heading::after {
  content: '';
  flex: 1;
  height: 1px;
  background: var(--border);
  margin-left: var(--gap-sm);
}

/* ============================================================
   POST CARDS
   ============================================================ */
.post-list {
  list-style: none;
  display: flex;
  flex-direction: column;
}

.post-card {
  padding: var(--gap-md) 0;
  border-bottom: 1px solid var(--border);
  transition: padding-left 0.25s ease;
}

.post-card:first-child {
  padding-top: 0;
}

.post-card:hover {
  padding-left: var(--gap-sm);
}

.post-card .post-date {
  font-family: var(--font-mono);
  font-size: 0.72rem;
  color: var(--muted);
  letter-spacing: 0.02em;
  margin-bottom: 2px;
}

.post-card .post-title {
  font-family: var(--font-serif);
  font-size: 1.05rem;
  font-weight: 500;
  color: var(--ink);
  line-height: 1.5;
  display: inline-block;
  transition: color 0.2s;
}

.post-card:hover .post-title { color: var(--vermilion); }

.post-card .post-description {
  font-size: 0.85rem;
  color: var(--ink-soft);
  margin-top: var(--gap-xs);
  line-height: 1.6;
  display: -webkit-box;
  -webkit-line-clamp: 2;
  -webkit-box-orient: vertical;
  overflow: hidden;
}

.post-card .post-tags {
  display: flex;
  gap: var(--gap-sm);
  margin-top: var(--gap-xs);
  flex-wrap: wrap;
}

.post-card .post-tag {
  font-family: var(--font-ui);
  font-size: 0.68rem;
  color: var(--muted);
  letter-spacing: 0.03em;
}

.post-card .post-tag::before { content: '#'; opacity: 0.4; margin-right: 1px; }

/* Math indicator badge */
.post-card .math-badge {
  display: inline-block;
  font-family: var(--font-mono);
  font-size: 0.6rem;
  color: var(--vermilion-dim);
  background: transparent;
  border: 1px solid var(--vermilion-dim);
  padding: 0px 5px;
  border-radius: 3px;
  margin-left: var(--gap-xs);
  vertical-align: middle;
  opacity: 0.6;
}

/* ============================================================
   MORE TOGGLE BUTTON
   ============================================================ */
.more-toggle {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  margin-top: var(--gap-md);
  font-family: var(--font-ui);
  font-size: 0.75rem;
  font-weight: 500;
  color: var(--muted);
  letter-spacing: 0.04em;
  cursor: pointer;
  background: none;
  border: none;
  padding: 4px 0;
  transition: color 0.2s;
}

.more-toggle:hover { color: var(--ink); }

.more-toggle .arrow {
  display: inline-block;
  transition: transform 0.3s ease;
  font-size: 0.65rem;
}

.more-toggle.expanded .arrow { transform: rotate(180deg); }

/* Hidden posts */
.post-card.hidden-card {
  display: none;
}

.post-card.hidden-card.visible {
  display: block;
  animation: fadeSlide 0.4s ease both;
}

@keyframes fadeSlide {
  from { opacity: 0; transform: translateY(-6px); }
  to   { opacity: 1; transform: translateY(0); }
}

/* ============================================================
   POST PAGE — TWO-COLUMN LAYOUT WITH TOC SIDEBAR
   ============================================================ */
.post-layout {
  display: flex;
  max-width: calc(var(--content-width) + var(--toc-width) + var(--gap-2xl));
  margin: 0 auto;
  padding: 0 var(--gap-lg);
  gap: var(--gap-2xl);
}

/* ---- TOC Sidebar ---- */
.post-toc-sidebar {
  width: var(--toc-width);
  flex-shrink: 0;
  position: sticky;
  top: 0;
  max-height: 100vh;
  overflow-y: auto;
  padding-top: var(--gap-2xl);
  padding-bottom: var(--gap-2xl);
}

.toc-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: var(--gap-md);
}

.toc-title {
  font-family: var(--font-ui);
  font-size: 0.72rem;
  font-weight: 500;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: 0.08em;
}

.toc-close-btn {
  display: none; /* hidden on desktop, visible in drawer */
  background: none;
  border: none;
  cursor: pointer;
  color: var(--muted);
  padding: 4px;
  transition: color 0.2s;
}

.toc-close-btn:hover {
  color: var(--vermilion);
}

.toc-nav {
  font-family: var(--font-ui);
  font-size: 0.78rem;
  line-height: 1.7;
}

/* Reset python-markdown TOC structure */
.toc-nav .toc {
  padding: 0;
}

.toc-nav ul {
  list-style: none;
  padding: 0;
  margin: 0;
}

.toc-nav li {
  position: relative;
  padding-left: 0;
}

/* Multi-level indent with left vertical line */
.toc-nav > ul,
.toc-nav .toc > ul {
  border-left: 1px solid var(--border);
  padding-left: var(--gap-sm);
}

.toc-nav > ul > li {
  padding-left: 0;
}

.toc-nav ul ul {
  border-left: 1px solid var(--border);
  padding-left: var(--gap-sm);
  margin-left: 0;
}

.toc-nav ul ul ul {
  padding-left: var(--gap-sm);
}

.toc-nav a {
  display: block;
  color: var(--muted);
  text-decoration: none;
  padding: 2px 0;
  transition: color 0.2s;
  font-weight: 400;
}

.toc-nav a:hover {
  color: var(--ink);
}

/* Active TOC item — highlighted in vermillion */
.toc-nav a.active {
  color: var(--vermilion);
  font-weight: 500;
}

/* ---- Post Main Content ---- */
.post-main {
  flex: 1;
  min-width: 0;
  max-width: var(--content-width);
  padding: var(--gap-2xl) 0 var(--gap-2xl);
}

.post-back {
  font-family: var(--font-ui);
  font-size: 0.78rem;
  color: var(--muted);
  display: inline-flex;
  align-items: center;
  gap: 4px;
  margin-bottom: var(--gap-xl);
  cursor: pointer;
  background: none;
  border: none;
  padding: 0;
  transition: color 0.2s;
}

.post-back:hover { color: var(--ink); }

.post-back::before { content: '\2190'; font-size: 0.85rem; }

.post-header {
  margin-bottom: var(--gap-md);
}

.post-category-label {
  font-family: var(--font-ui);
  font-size: 0.72rem;
  font-weight: 500;
  color: var(--vermilion);
  text-transform: uppercase;
  letter-spacing: 0.08em;
  margin-bottom: var(--gap-sm);
}

.post-title-main {
  font-family: var(--font-serif);
  font-size: 1.8rem;
  font-weight: 700;
  line-height: 1.4;
  color: var(--ink);
  margin-bottom: var(--gap-sm);
}

.post-meta {
  font-family: var(--font-mono);
  font-size: 0.72rem;
  color: var(--muted);
}

/* ---- Post Body Typography ---- */
.post-body {
  font-family: var(--font-serif);
  font-size: 1rem;
  line-height: 1.85;
  color: var(--ink);
}

.post-body h2, .post-body h3, .post-body h4 {
  scroll-margin-top: 180px;
}

.post-body h2 {
  font-size: 1.25rem;
  font-weight: 600;
  margin: var(--gap-lg) 0 var(--gap-xs);
  color: var(--ink);
}

.post-body h3 {
  font-size: 1.05rem;
  font-weight: 600;
  margin: var(--gap-md) 0 var(--gap-xs);
  color: var(--ink-soft);
}

.post-body h4 {
  font-size: 0.95rem;
  font-weight: 600;
  margin: var(--gap-md) 0 var(--gap-xs);
  color: var(--ink-soft);
}

.post-body p {
  margin-bottom: var(--gap-sm);
}

.post-body a {
  color: var(--vermilion);
  text-decoration: none;
}

.post-body a:hover {
  text-decoration: underline;
}

.post-body ul, .post-body ol {
  margin-bottom: var(--gap-sm);
  padding-left: var(--gap-lg);
}

.post-body li {
  margin-bottom: var(--gap-xs);
}

.post-body code {
  font-family: var(--font-mono);
  font-size: 0.82em;
  background: var(--code-bg);
  padding: 1px 5px;
  border-radius: 3px;
}

.post-body pre {
  background: var(--code-bg);
  padding: var(--gap-md);
  border-radius: 6px;
  overflow-x: auto;
  margin-bottom: var(--gap-sm);
  font-size: 0.82rem;
  line-height: 1.6;
}

.post-body pre code {
  background: none;
  padding: 0;
}

.post-body blockquote {
  border-left: 2px solid var(--vermilion);
  margin: var(--gap-sm) 0;
  padding: var(--gap-xs) var(--gap-md);
  color: var(--ink-soft);
  font-style: italic;
}

.post-body hr {
  border: none;
  border-top: 1px solid var(--border);
  margin: var(--gap-md) 0;
}

.post-body img {
  border-radius: 4px;
  margin: var(--gap-sm) auto;
}

/* KaTeX display math */
.post-body .katex-display {
  margin: var(--gap-md) 0;
  overflow-x: auto;
  overflow-y: hidden;
}

.post-body .katex {
  font-size: 1.05em;
  user-select: none;
}

.post-body table {
  width: 100%;
  border-collapse: collapse;
  margin-bottom: var(--gap-sm);
}

.post-body th,
.post-body td {
  border: 1px solid var(--border);
  padding: var(--gap-xs) var(--gap-sm);
  text-align: left;
}

.post-body th {
  background: var(--card-bg);
  font-weight: 600;
  font-size: 0.9rem;
}

/* ============================================================
   TOC FAB & OVERLAY (mobile drawer)
   ============================================================ */
.toc-fab {
  display: none; /* hidden on desktop */
}

.toc-overlay {
  display: none;
  position: fixed;
  inset: 0;
  z-index: 1001;
  background: rgba(0,0,0,0.25);
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.3s ease;
}

.toc-drawer-open .toc-overlay {
  opacity: 1;
  pointer-events: auto;
  cursor: pointer;
}

/* ============================================================
   FRIENDS PAGE
   ============================================================ */
.friends-main, .about-main {
  max-width: var(--content-width);
  margin: 0 auto;
  padding: var(--gap-2xl) var(--gap-lg) var(--gap-2xl);
}

.page-header {
  text-align: center;
  margin-bottom: var(--gap-xl);
}

.page-title {
  font-family: var(--font-serif);
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--ink);
  letter-spacing: 0.04em;
  margin-bottom: 0;
}

.page-header .header-rule {
  margin-top: var(--gap-sm);
}

.friends-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: var(--gap-md);
}

.friend-card {
  display: flex;
  align-items: center;
  gap: var(--gap-md);
  padding: var(--gap-md);
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: 8px;
  transition: border-color 0.2s, box-shadow 0.2s;
}

.friend-card:hover {
  border-color: var(--faint);
  box-shadow: 0 2px 12px rgba(0, 0, 0, 0.04);
}

.friend-avatar {
  width: 56px;
  height: 56px;
  border-radius: 50%;
  flex-shrink: 0;
  object-fit: cover;
  border: 1px solid var(--border);
}

.friend-info {
  min-width: 0;
}

.friend-name {
  font-family: var(--font-serif);
  font-size: 1rem;
  font-weight: 600;
  color: var(--ink);
  transition: color 0.2s;
}

.friend-name:hover {
  color: var(--vermilion);
}

.friend-bio {
  font-family: var(--font-ui);
  font-size: 0.8rem;
  color: var(--ink-soft);
  margin-top: 2px;
  line-height: 1.5;
}

/* ============================================================
   ABOUT PAGE
   ============================================================ */
.about-bio {
  margin-bottom: var(--gap-xl);
}

.about-bio p {
  font-family: var(--font-serif);
  font-size: 1rem;
  line-height: 1.85;
  color: var(--ink-soft);
  margin-bottom: var(--gap-sm);
}

.about-links {
  display: flex;
  gap: var(--gap-lg);
  flex-wrap: wrap;
}

.about-link {
  display: inline-flex;
  align-items: center;
  gap: var(--gap-xs);
  font-family: var(--font-ui);
  font-size: 0.85rem;
  color: var(--ink-soft);
  padding: var(--gap-xs) var(--gap-sm);
  border: 1px solid var(--border);
  border-radius: 6px;
  transition: color 0.2s, border-color 0.2s, background 0.2s;
}

.about-link:hover {
  color: var(--vermilion);
  border-color: var(--vermilion-dim);
  background: var(--card-bg);
}

.about-link-icon {
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--muted);
  transition: color 0.2s;
}

.about-link:hover .about-link-icon {
  color: var(--vermilion);
}

/* ============================================================
   FOOTER
   ============================================================ */
.site-footer {
  text-align: center;
  padding: var(--gap-2xl) var(--gap-lg);
  max-width: var(--home-width);
  margin: 0 auto;
}

.site-footer .footer-text {
  font-family: var(--font-ui);
  font-size: 0.7rem;
  color: var(--faint);
  letter-spacing: 0.06em;
}

/* ============================================================
   RESPONSIVE — TABLET (max-width: 768px)
   ============================================================ */
@media (max-width: 768px) {
  html { font-size: 16px; }

  .site-title { font-size: 2rem; }
  .site-header { padding: var(--gap-xl) var(--gap-md) var(--gap-lg); }

  .category-nav { gap: var(--gap-md); padding-bottom: var(--gap-lg); }
  .main-content { padding: 0 var(--gap-md) var(--gap-xl); }
  .category-section { margin-bottom: var(--gap-xl); }

  /* Post layout: stack, sidebar becomes fixed drawer */
  .post-layout {
    flex-direction: column;
    padding: 0 var(--gap-md);
  }

  .post-toc-sidebar {
    position: fixed;
    top: 0;
    right: 0;
    width: 280px;
    max-width: 80vw;
    height: 100vh;
    max-height: 100vh;
    z-index: 1002;
    background: var(--paper);
    box-shadow: -4px 0 20px rgba(0,0,0,0.1);
    padding: var(--gap-lg);
    overflow-y: auto;
    transform: translateX(100%);
    transition: transform 0.3s ease;
  }

  .toc-drawer-open .post-toc-sidebar {
    transform: translateX(0);
  }

  .toc-header {
    padding: 0 0 var(--gap-sm);
    margin-bottom: 0;
  }

  .toc-close-btn {
    display: flex;
    align-items: center;
    justify-content: center;
  }

  .toc-nav {
    display: block;
    padding-top: var(--gap-sm);
  }

  .toc-fab {
    display: flex;
    align-items: center;
    gap: 6px;
    position: fixed;
    bottom: 24px;
    right: 24px;
    z-index: 1000;
    padding: 10px 16px;
    background: var(--paper);
    border: 1px solid var(--border);
    border-radius: 24px;
    box-shadow: 0 2px 12px rgba(0,0,0,0.08);
    font-family: var(--font-ui);
    font-size: 0.82rem;
    font-weight: 500;
    color: var(--ink-soft);
    cursor: pointer;
    transition: color 0.2s, box-shadow 0.2s;
  }

  .toc-fab:hover {
    color: var(--vermilion);
    box-shadow: 0 4px 16px rgba(0,0,0,0.12);
  }

  .toc-overlay {
    display: block;
  }

  body.toc-drawer-open {
    overflow: hidden;
  }

  .post-main {
    padding: var(--gap-lg) 0 var(--gap-xl);
  }

  .post-title-main { font-size: 1.4rem; }

  .friends-main, .about-main {
    padding: var(--gap-xl) var(--gap-md) var(--gap-xl);
  }
}

/* ============================================================
   RESPONSIVE — MOBILE (max-width: 480px)
   ============================================================ */
@media (max-width: 480px) {
  html { font-size: 15px; }

  .site-title { font-size: 1.7rem; }

  .post-card .post-title { font-size: 0.95rem; }

  .category-nav { gap: var(--gap-sm); }
  .category-nav a { font-size: 0.75rem; }

  .header-nav { gap: var(--gap-md); }

  .post-title-main { font-size: 1.3rem; }

  .friends-grid {
    gap: var(--gap-sm);
  }

  .friend-card {
    padding: var(--gap-sm);
  }

  .friend-avatar {
    width: 44px;
    height: 44px;
  }

  .about-links {
    flex-direction: column;
    gap: var(--gap-sm);
  }

  .about-link {
    width: 100%;
    justify-content: center;
  }
}

/* ============================================================
   PRINT STYLES
   ============================================================ */
@media print {
  body::before { display: none; }

  .site-header,
  .category-nav,
  .site-footer,
  .post-back,
  .post-toc-sidebar {
    display: none;
  }

  .post-layout {
    display: block;
    padding: 0;
  }

  .post-main {
    padding: 0;
    max-width: 100%;
  }

  .post-body {
    font-size: 11pt;
    line-height: 1.6;
  }

  a { color: var(--ink); }
}

/* ============================================================
   UTILITY
   ============================================================ */
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

.empty-state {
  text-align: center;
  color: var(--muted);
  font-family: var(--font-ui);
  font-size: 0.85rem;
  padding: var(--gap-xl) 0;
}