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

:root {
    --color-primary: #4f46e5;
    --color-primary-light: #6366f1;
    --color-primary-bg: #eef2ff;
    --color-bg: #ffffff;
    --color-bg-soft: #f8fafc;
    --color-bg-mute: #f1f5f9;
    --color-text: #1e293b;
    --color-text-2: #475569;
    --color-text-3: #94a3b8;
    --color-border: #e2e8f0;
    --color-border-light: #f1f5f9;
    --color-code-bg: #f8fafc;
    --sidebar-w: 280px;
    --header-h: 56px;
    --content-max-w: 860px;
    --radius: 8px;
    --radius-sm: 6px;
    --shadow-sm: 0 1px 3px rgba(0,0,0,.06);
    --shadow-md: 0 4px 12px rgba(0,0,0,.08);
    --transition: .2s ease;
    --font-sans: 'Noto Sans SC', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
    --font-mono: 'JetBrains Mono', 'SF Mono', 'Fira Code', monospace;
}

html { scroll-behavior: smooth; }
body {
    font-family: var(--font-sans);
    font-size: 15px;
    line-height: 1.7;
    color: var(--color-text);
    background: var(--color-bg);
    -webkit-font-smoothing: antialiased;
}

/* ===== Header ===== */
.header {
    position: fixed; top: 0; left: 0; right: 0; z-index: 100;
    height: var(--header-h);
    background: rgba(255,255,255,.85);
    backdrop-filter: blur(12px);
    border-bottom: 1px solid var(--color-border);
    display: flex; align-items: center; justify-content: space-between;
    padding: 0 20px;
}
.header-left { display: flex; align-items: center; gap: 12px; }
.header-right { display: flex; align-items: center; gap: 8px; }

.menu-toggle {
    display: none;
    background: none; border: none; cursor: pointer;
    color: var(--color-text-2); padding: 4px; border-radius: 6px;
}
.menu-toggle:hover { background: var(--color-bg-mute); }

.header-brand {
    display: flex; align-items: center; gap: 10px;
    text-decoration: none; color: inherit;
}
.header-logo { color: var(--color-primary); flex-shrink: 0; }
.brand-title {
    font-size: 17px; font-weight: 700; letter-spacing: -0.3px;
    color: var(--color-text);
}
.brand-sub {
    display: block; font-size: 11px; font-weight: 400;
    color: var(--color-text-3); letter-spacing: 0.5px; margin-top: -2px;
}

.header-link {
    display: flex; align-items: center; justify-content: center;
    width: 36px; height: 36px; border-radius: 8px;
    color: var(--color-text-2); transition: var(--transition);
}
.header-link:hover { background: var(--color-bg-mute); color: var(--color-text); }

/* ===== Layout ===== */
.layout {
    display: flex;
    padding-top: var(--header-h);
    min-height: 100vh;
}

/* ===== Sidebar ===== */
.sidebar {
    position: fixed; top: var(--header-h); left: 0; bottom: 0;
    width: var(--sidebar-w);
    background: var(--color-bg);
    border-right: 1px solid var(--color-border);
    overflow-y: auto; overflow-x: hidden;
    padding: 12px 0;
    z-index: 90;
    scrollbar-width: thin;
    scrollbar-color: var(--color-border) transparent;
}
.sidebar::-webkit-scrollbar { width: 4px; }
.sidebar::-webkit-scrollbar-thumb { background: var(--color-border); border-radius: 4px; }

.sidebar-search {
    display: flex; align-items: center; gap: 8px;
    margin: 4px 14px 12px; padding: 8px 12px;
    border: 1px solid var(--color-border);
    border-radius: var(--radius);
    background: var(--color-bg-soft);
    transition: var(--transition);
}
.sidebar-search:focus-within {
    border-color: var(--color-primary-light);
    box-shadow: 0 0 0 3px rgba(99,102,241,.12);
    background: var(--color-bg);
}
.sidebar-search svg { color: var(--color-text-3); flex-shrink: 0; }
.sidebar-search input {
    border: none; background: none; outline: none;
    font: inherit; font-size: 13px; width: 100%;
    color: var(--color-text);
}
.sidebar-search input::placeholder { color: var(--color-text-3); }

.sidebar-nav { padding: 0 8px; }

/* Nav chapter */
.nav-chapter { margin-bottom: 2px; }
.nav-chapter-header {
    display: flex; align-items: center; gap: 8px;
    padding: 8px 10px; cursor: pointer; border-radius: var(--radius-sm);
    font-size: 13px; font-weight: 600; color: var(--color-text);
    transition: var(--transition); user-select: none;
}
.nav-chapter-header:hover { background: var(--color-bg-mute); }
.nav-chapter-header.active { color: var(--color-primary); }

.nav-chapter-num {
    display: inline-flex; align-items: center; justify-content: center;
    width: 22px; height: 22px; border-radius: 5px;
    background: var(--color-bg-mute); font-size: 11px; font-weight: 600;
    color: var(--color-text-2); flex-shrink: 0;
}
.nav-chapter-header.active .nav-chapter-num {
    background: var(--color-primary-bg); color: var(--color-primary);
}

.nav-arrow {
    margin-left: auto; flex-shrink: 0; transition: transform .2s;
    color: var(--color-text-3);
}
.nav-chapter.open > .nav-chapter-header .nav-arrow { transform: rotate(90deg); }

.nav-children {
    display: none; padding-left: 16px;
}
.nav-chapter.open > .nav-children { display: block; }

/* Nav items */
.nav-item {
    display: block; padding: 5px 10px 5px 22px;
    font-size: 13px; color: var(--color-text-2);
    text-decoration: none; border-radius: var(--radius-sm);
    transition: var(--transition);
    white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
}
.nav-item:hover { background: var(--color-bg-mute); color: var(--color-text); }
.nav-item.active {
    background: var(--color-primary-bg); color: var(--color-primary);
    font-weight: 500;
}

/* Nav groups (nested sub-dirs) */
.nav-group { margin: 2px 0; }
.nav-group-title {
    display: flex; align-items: center; gap: 6px;
    padding: 5px 10px 5px 18px;
    font-size: 12px; font-weight: 600; color: var(--color-text-3);
    text-transform: uppercase; letter-spacing: 0.5px;
    cursor: pointer; user-select: none; border-radius: var(--radius-sm);
}
.nav-group-title:hover { color: var(--color-text-2); }
.nav-group-children { display: none; }
.nav-group.open > .nav-group-children { display: block; }
.nav-group.open > .nav-group-title { color: var(--color-text-2); }

.sidebar-overlay {
    display: none; position: fixed; inset: 0; z-index: 80;
    background: rgba(0,0,0,.3);
}

/* ===== Content Area ===== */
.content {
    flex: 1;
    margin-left: var(--sidebar-w);
    padding: 32px 40px 80px;
    min-width: 0;
    overflow-y: auto;
}

.loading-init, .loading {
    display: flex; align-items: center; justify-content: center;
    height: 200px; color: var(--color-text-3); font-size: 15px;
}
.error-msg {
    text-align: center; padding: 60px 20px;
    color: var(--color-text-3); font-size: 15px;
}

/* ===== Welcome Page ===== */
.welcome-hero {
    text-align: center; padding: 48px 20px 40px;
}
.welcome-banner {
    width: 100%; max-width: 720px; border-radius: 12px;
    box-shadow: 0 4px 24px rgba(0,0,0,.12);
    margin-bottom: 28px;
}
.welcome-hero h1 {
    font-size: 40px; font-weight: 800; letter-spacing: -1px;
    background: linear-gradient(135deg, var(--color-primary) 0%, #a855f7 100%);
    -webkit-background-clip: text; -webkit-text-fill-color: transparent;
    background-clip: text;
}
.welcome-hero p {
    margin-top: 12px; font-size: 17px; color: var(--color-text-2);
    max-width: 480px; margin-inline: auto;
}
.welcome-stats {
    display: flex; justify-content: center; gap: 32px;
    margin-top: 20px; font-size: 13px; color: var(--color-text-3);
}
.welcome-stats span { font-weight: 600; color: var(--color-primary); }

.chapter-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
    gap: 16px; max-width: 960px; margin: 0 auto; padding: 0 8px;
}
.chapter-card {
    position: relative; padding: 20px;
    background: var(--color-bg);
    border: 1px solid var(--color-border);
    border-radius: 12px;
    cursor: pointer; transition: all .25s;
    overflow: hidden;
}
.chapter-card::before {
    content: ''; position: absolute; top: 0; left: 0; right: 0;
    height: 3px; background: linear-gradient(90deg, var(--color-primary), #a855f7);
    opacity: 0; transition: opacity .25s;
}
.chapter-card:hover {
    border-color: var(--color-primary-light);
    box-shadow: var(--shadow-md);
    transform: translateY(-2px);
}
.chapter-card:hover::before { opacity: 1; }
.chapter-card-num {
    font-size: 28px; font-weight: 800; color: var(--color-primary-bg);
    line-height: 1; margin-bottom: 8px;
}
.chapter-card:hover .chapter-card-num { color: var(--color-primary-light); }
.chapter-card-title {
    font-size: 15px; font-weight: 600; color: var(--color-text);
    line-height: 1.4;
}
.chapter-card-count {
    margin-top: 8px; font-size: 12px; color: var(--color-text-3);
}

/* ===== Breadcrumb ===== */
.breadcrumb {
    display: flex; align-items: center; gap: 6px;
    font-size: 13px; color: var(--color-text-3);
    margin-bottom: 20px; flex-wrap: wrap;
}
.breadcrumb a { color: var(--color-primary); text-decoration: none; }
.breadcrumb a:hover { text-decoration: underline; }
.breadcrumb-sep { font-size: 11px; }

/* ===== Article ===== */
.markdown-body {
    max-width: var(--content-max-w);
    font-size: 15.5px; line-height: 1.8;
    color: var(--color-text);
}

.markdown-body h1 {
    font-size: 32px; font-weight: 800; letter-spacing: -0.5px;
    margin: 0 0 24px; padding-bottom: 16px;
    border-bottom: 2px solid var(--color-border-light);
    line-height: 1.3;
}
.markdown-body h2 {
    font-size: 22px; font-weight: 700; letter-spacing: -0.3px;
    margin: 40px 0 16px; padding-bottom: 8px;
    border-bottom: 1px solid var(--color-border-light);
    line-height: 1.4;
}
.markdown-body h3 {
    font-size: 18px; font-weight: 600; margin: 32px 0 12px; line-height: 1.4;
}
.markdown-body h4 {
    font-size: 16px; font-weight: 600; margin: 24px 0 8px;
}
.markdown-body p { margin: 0 0 16px; }
.markdown-body a {
    color: var(--color-primary); text-decoration: none;
    border-bottom: 1px solid transparent; transition: var(--transition);
}
.markdown-body a:hover { border-bottom-color: var(--color-primary); }

.markdown-body ul, .markdown-body ol { margin: 0 0 16px; padding-left: 24px; }
.markdown-body li { margin-bottom: 4px; }
.markdown-body li > ul, .markdown-body li > ol { margin: 4px 0 0; }

.markdown-body blockquote {
    margin: 0 0 16px; padding: 12px 20px;
    border-left: 4px solid var(--color-primary-light);
    background: var(--color-primary-bg);
    border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
    color: var(--color-text-2);
}
.markdown-body blockquote p:last-child { margin-bottom: 0; }

.markdown-body img {
    max-width: 100%; height: auto; border-radius: var(--radius);
    margin: 8px 0; box-shadow: var(--shadow-sm);
}

/* Code */
.markdown-body code {
    font-family: var(--font-mono); font-size: 0.88em;
    padding: 2px 6px; border-radius: 4px;
    background: var(--color-bg-mute); color: #e11d48;
}
.markdown-body pre {
    margin: 0 0 16px; border-radius: var(--radius);
    border: 1px solid var(--color-border);
    overflow-x: auto; position: relative;
}
.markdown-body pre code {
    display: block; padding: 16px 20px;
    background: var(--color-code-bg);
    color: var(--color-text); font-size: 13.5px;
    line-height: 1.65; border-radius: 0;
}

/* Tables */
.markdown-body table {
    width: 100%; border-collapse: collapse;
    margin: 0 0 16px; font-size: 14px;
    border-radius: var(--radius); overflow: hidden;
    border: 1px solid var(--color-border);
}
.markdown-body th {
    background: var(--color-bg-mute); font-weight: 600;
    text-align: left; padding: 10px 14px;
    border-bottom: 2px solid var(--color-border);
}
.markdown-body td {
    padding: 10px 14px;
    border-bottom: 1px solid var(--color-border-light);
}
.markdown-body tr:last-child td { border-bottom: none; }
.markdown-body tr:hover td { background: var(--color-bg-soft); }

/* Horizontal rule */
.markdown-body hr {
    border: none; height: 1px; background: var(--color-border);
    margin: 32px 0;
}

/* Strong & Em */
.markdown-body strong { font-weight: 600; }

/* Mermaid */
.markdown-body .mermaid {
    margin: 16px 0; text-align: center;
    background: var(--color-bg-soft); border-radius: var(--radius);
    padding: 20px; border: 1px solid var(--color-border-light);
}

/* Math */
.markdown-body .katex-display {
    margin: 16px 0; overflow-x: auto; overflow-y: hidden;
    padding: 8px 0;
}

/* ===== Article Navigation ===== */
.article-nav {
    display: flex; gap: 16px; margin-top: 48px;
    padding-top: 24px; border-top: 1px solid var(--color-border);
}
.article-nav a {
    flex: 1; display: block; padding: 16px;
    border: 1px solid var(--color-border); border-radius: var(--radius);
    text-decoration: none; transition: var(--transition);
}
.article-nav a:hover {
    border-color: var(--color-primary-light); box-shadow: var(--shadow-sm);
}
.article-nav-label {
    font-size: 12px; color: var(--color-text-3); margin-bottom: 4px;
}
.article-nav-title {
    font-size: 14px; font-weight: 500; color: var(--color-primary);
}
.article-nav a.next { text-align: right; }

/* ===== Back to Top ===== */
.back-to-top {
    position: fixed; bottom: 28px; right: 28px;
    width: 40px; height: 40px; border-radius: 50%;
    background: var(--color-bg); border: 1px solid var(--color-border);
    box-shadow: var(--shadow-md); cursor: pointer;
    display: flex; align-items: center; justify-content: center;
    color: var(--color-text-2); opacity: 0; pointer-events: none;
    transition: all .3s;
    z-index: 50;
}
.back-to-top.visible { opacity: 1; pointer-events: auto; }
.back-to-top:hover { background: var(--color-primary); color: #fff; border-color: var(--color-primary); }

/* ===== Responsive ===== */
@media (max-width: 860px) {
    .menu-toggle { display: flex; }
    .sidebar {
        transform: translateX(-100%);
        transition: transform .3s;
        box-shadow: none;
    }
    .sidebar.open {
        transform: translateX(0);
        box-shadow: var(--shadow-md);
    }
    .sidebar-overlay.open { display: block; }
    .content { margin-left: 0; padding: 24px 20px 60px; }
    .welcome-hero h1 { font-size: 28px; }
    .chapter-grid { grid-template-columns: 1fr; }
    .article-nav { flex-direction: column; }
}

@media (max-width: 480px) {
    .brand-sub { display: none; }
    .content { padding: 16px 14px 60px; }
    .markdown-body pre code { font-size: 12.5px; padding: 12px 14px; }
}

/* ===== Animations ===== */
@keyframes fadeIn {
    from { opacity: 0; transform: translateY(8px); }
    to { opacity: 1; transform: translateY(0); }
}
.markdown-body { animation: fadeIn .3s ease; }
.welcome-hero { animation: fadeIn .4s ease; }
.chapter-card { animation: fadeIn .4s ease backwards; }
.chapter-card:nth-child(2) { animation-delay: .05s; }
.chapter-card:nth-child(3) { animation-delay: .1s; }
.chapter-card:nth-child(4) { animation-delay: .15s; }
.chapter-card:nth-child(5) { animation-delay: .2s; }
.chapter-card:nth-child(6) { animation-delay: .25s; }
.chapter-card:nth-child(7) { animation-delay: .3s; }
.chapter-card:nth-child(8) { animation-delay: .35s; }