/* =====================================================
   DevNinja System Design Learning - Stylesheet
   ===================================================== */

:root {
  --bg: #0b0d12;
  --bg-elev: #11141b;
  --bg-elev-2: #161a23;
  --border: #1f2430;
  --border-strong: #2a3142;
  --text: #e7ecf3;
  --text-dim: #9aa3b2;
  --text-faint: #6b7280;
  --accent: #7c5cff;
  --accent-2: #22d3ee;
  --accent-3: #34d399;
  --warn: #fbbf24;
  --danger: #f87171;
  --code-bg: #0f1219;
  --shadow-sm: 0 2px 8px rgba(0,0,0,0.25);
  --shadow-md: 0 8px 32px rgba(0,0,0,0.35);
  --radius: 12px;
  --radius-sm: 8px;
  --font-sans: "Inter", "Segoe UI", system-ui, -apple-system, sans-serif;
  --font-mono: "JetBrains Mono", "Fira Code", ui-monospace, monospace;
  --max-w: 1200px;
  --content-w: 760px;
}

[data-theme="light"] {
  --bg: #fbfbfd;
  --bg-elev: #ffffff;
  --bg-elev-2: #f3f4f9;
  --border: #e5e7eb;
  --border-strong: #d1d5db;
  --text: #111827;
  --text-dim: #4b5563;
  --text-faint: #6b7280;
  --code-bg: #f3f4f9;
  --shadow-sm: 0 2px 8px rgba(0,0,0,0.04);
  --shadow-md: 0 8px 32px rgba(0,0,0,0.08);
}

* { box-sizing: border-box; }
html, body { margin: 0; padding: 0; }
body {
  font-family: var(--font-sans);
  background: var(--bg);
  color: var(--text);
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
  font-size: 16px;
}

a { color: var(--accent-2); text-decoration: none; }
a:hover { text-decoration: underline; }

/* ============ HEADER ============ */
.site-header {
  position: sticky; top: 0; z-index: 100;
  background: rgba(11,13,18,0.85);
  backdrop-filter: saturate(150%) blur(14px);
  -webkit-backdrop-filter: saturate(150%) blur(14px);
  border-bottom: 1px solid var(--border);
}
[data-theme="light"] .site-header { background: rgba(251,251,253,0.85); }

.site-header .nav {
  max-width: var(--max-w);
  margin: 0 auto;
  padding: 14px 24px;
  display: flex; align-items: center; gap: 24px;
}
.brand {
  display: flex; align-items: center; gap: 10px;
  font-weight: 700; font-size: 17px; color: var(--text);
}
.brand:hover { text-decoration: none; }
.brand-mark {
  width: 28px; height: 28px; border-radius: 8px;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  display: grid; place-items: center;
  color: white; font-weight: 800; font-size: 14px;
}
.nav-links { display: flex; gap: 6px; margin-left: auto; align-items: center; }
.nav-links a {
  color: var(--text-dim); padding: 8px 12px;
  border-radius: 8px; font-size: 14px; font-weight: 500;
}
.nav-links a:hover { color: var(--text); background: var(--bg-elev-2); text-decoration: none; }
.theme-toggle {
  background: var(--bg-elev-2); border: 1px solid var(--border);
  color: var(--text-dim); width: 36px; height: 36px;
  border-radius: 8px; cursor: pointer;
  display: grid; place-items: center;
}
.theme-toggle:hover { color: var(--text); }

/* ============ HOMEPAGE HERO ============ */
.hero {
  max-width: var(--max-w); margin: 0 auto;
  padding: 80px 24px 60px;
  text-align: center;
}
.hero-pill {
  display: inline-block;
  padding: 6px 14px;
  border-radius: 999px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  color: var(--accent-2);
  font-size: 12px; font-weight: 600;
  letter-spacing: 0.5px; text-transform: uppercase;
  margin-bottom: 24px;
}
.hero h1 {
  font-size: clamp(36px, 5vw, 56px);
  line-height: 1.1; letter-spacing: -0.02em;
  margin: 0 0 20px; font-weight: 800;
}
.hero h1 .grad {
  background: linear-gradient(120deg, var(--accent), var(--accent-2));
  -webkit-background-clip: text; background-clip: text;
  -webkit-text-fill-color: transparent;
}
.hero p.lede {
  font-size: 19px;
  color: var(--text-dim);
  max-width: 680px; margin: 0 auto 36px;
}
.hero-cta {
  display: flex; gap: 12px; justify-content: center; flex-wrap: wrap;
}
.btn {
  padding: 12px 22px; border-radius: 10px;
  font-weight: 600; font-size: 15px;
  border: 1px solid transparent; cursor: pointer;
  display: inline-flex; align-items: center; gap: 8px;
}
.btn-primary {
  background: linear-gradient(135deg, var(--accent), #5b3fd6);
  color: white;
}
.btn-primary:hover { text-decoration: none; transform: translateY(-1px); box-shadow: var(--shadow-md); }
.btn-ghost {
  background: var(--bg-elev); color: var(--text);
  border: 1px solid var(--border);
}
.btn-ghost:hover { background: var(--bg-elev-2); text-decoration: none; }

.hero-stats {
  display: flex; justify-content: center; gap: 48px;
  margin-top: 60px; flex-wrap: wrap;
}
.hero-stat .num {
  font-size: 32px; font-weight: 800;
  background: linear-gradient(120deg, var(--accent), var(--accent-2));
  -webkit-background-clip: text; -webkit-text-fill-color: transparent;
}
.hero-stat .lbl { color: var(--text-dim); font-size: 13px; }

/* ============ SEARCH ============ */
.search-wrap { max-width: 720px; margin: 0 auto 36px; padding: 0 24px; position: relative; }
.search-input {
  width: 100%; padding: 16px 18px 16px 48px;
  background: var(--bg-elev); border: 1px solid var(--border);
  border-radius: 12px; color: var(--text);
  font-size: 15px; font-family: var(--font-sans);
  outline: none; transition: border-color 0.15s;
}
.search-input:focus { border-color: var(--accent); }
.search-icon {
  position: absolute; left: 42px; top: 50%; transform: translateY(-50%);
  color: var(--text-faint); pointer-events: none;
}
.search-results {
  position: absolute; left: 24px; right: 24px;
  top: 100%; margin-top: 6px;
  background: var(--bg-elev); border: 1px solid var(--border);
  border-radius: 12px; box-shadow: var(--shadow-md);
  max-height: 380px; overflow-y: auto;
  display: none; z-index: 50;
}
.search-results.open { display: block; }
.search-results a {
  display: block; padding: 12px 16px;
  color: var(--text); border-bottom: 1px solid var(--border);
}
.search-results a:last-child { border-bottom: none; }
.search-results a:hover { background: var(--bg-elev-2); text-decoration: none; }
.search-results .res-title { font-weight: 600; font-size: 14px; }
.search-results .res-chapter { font-size: 12px; color: var(--text-faint); margin-top: 2px; }

/* ============ CHAPTERS GRID ============ */
.chapters {
  max-width: var(--max-w); margin: 0 auto;
  padding: 40px 24px 80px;
}
.section-title {
  font-size: 28px; font-weight: 700;
  margin: 0 0 8px; letter-spacing: -0.01em;
}
.section-sub { color: var(--text-dim); margin: 0 0 32px; }

.chapter-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
  gap: 18px;
}
.chapter-card {
  background: var(--bg-elev); border: 1px solid var(--border);
  border-radius: var(--radius); padding: 24px;
  transition: transform 0.15s, border-color 0.15s;
}
.chapter-card:hover { border-color: var(--border-strong); transform: translateY(-2px); }
.chapter-card .ch-num {
  font-family: var(--font-mono); font-size: 12px;
  color: var(--accent); font-weight: 700; letter-spacing: 1px;
}
.chapter-card h3 {
  margin: 8px 0 12px; font-size: 18px; font-weight: 700;
}
.chapter-card p { color: var(--text-dim); margin: 0 0 16px; font-size: 14px; }
.chapter-card ul { margin: 0; padding: 0; list-style: none; }
.chapter-card ul li { padding: 5px 0; font-size: 14px; }
.chapter-card ul li a { color: var(--text-dim); }
.chapter-card ul li a:hover { color: var(--accent-2); }
.chapter-card .more {
  display: inline-block; margin-top: 12px;
  font-size: 13px; color: var(--accent-2); font-weight: 600;
}

/* ============ FEATURES ============ */
.features {
  max-width: var(--max-w); margin: 0 auto;
  padding: 60px 24px;
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(260px, 1fr));
  gap: 24px;
}
.feature {
  text-align: center; padding: 24px;
}
.feature .icon-wrap {
  width: 56px; height: 56px; border-radius: 14px;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  display: grid; place-items: center; margin: 0 auto 16px;
  color: white;
}
.feature h4 { margin: 0 0 8px; font-size: 17px; }
.feature p { color: var(--text-dim); margin: 0; font-size: 14px; }

/* ============ TOPIC PAGE LAYOUT ============ */
.layout {
  display: grid;
  grid-template-columns: 280px 1fr 240px;
  max-width: 1400px; margin: 0 auto;
  gap: 0;
}
@media (max-width: 1100px) {
  .layout { grid-template-columns: 260px 1fr; }
  .toc-rail { display: none; }
}
@media (max-width: 768px) {
  .layout { grid-template-columns: 1fr; }
  .sidebar { display: none; }
  .sidebar.open { display: block; position: fixed; top: 60px; left: 0; bottom: 0; width: 280px; z-index: 90; background: var(--bg); }
}

.sidebar {
  border-right: 1px solid var(--border);
  padding: 24px 16px;
  height: calc(100vh - 60px);
  overflow-y: auto;
  position: sticky; top: 60px;
}
.sidebar-section { margin-bottom: 16px; }
.sidebar-title {
  font-size: 11px; font-weight: 700;
  text-transform: uppercase; letter-spacing: 1.5px;
  color: var(--text-faint);
  padding: 6px 12px; margin: 8px 0 4px;
}
.sidebar a {
  display: block; padding: 7px 12px;
  color: var(--text-dim); font-size: 14px;
  border-radius: 6px; margin: 1px 0;
}
.sidebar a:hover { color: var(--text); background: var(--bg-elev-2); text-decoration: none; }
.sidebar a.active { color: var(--accent-2); background: var(--bg-elev-2); font-weight: 600; }

/* ============ ARTICLE ============ */
.article {
  padding: 48px 56px;
  max-width: var(--content-w);
  margin: 0 auto;
}
@media (max-width: 768px) { .article { padding: 32px 20px; } }

.breadcrumb {
  font-size: 13px; color: var(--text-faint);
  margin-bottom: 16px;
}
.breadcrumb a { color: var(--text-dim); }
.breadcrumb .sep { margin: 0 6px; }

.article h1 {
  font-size: 38px; font-weight: 800; line-height: 1.15;
  letter-spacing: -0.02em; margin: 0 0 12px;
}
.article .article-meta {
  color: var(--text-faint); font-size: 14px; margin-bottom: 32px;
}
.article .lede {
  font-size: 19px; color: var(--text-dim);
  border-left: 3px solid var(--accent);
  padding: 4px 0 4px 16px; margin-bottom: 32px;
}
.article h2 {
  margin: 48px 0 16px; font-size: 26px;
  font-weight: 700; letter-spacing: -0.01em;
  padding-top: 12px;
}
.article h3 {
  margin: 32px 0 12px; font-size: 19px; font-weight: 700;
}
.article p { margin: 14px 0; }
.article ul, .article ol { padding-left: 22px; }
.article ul li, .article ol li { margin: 6px 0; }
.article strong { color: var(--text); font-weight: 700; }
.article em { color: var(--text-dim); }

.article code {
  background: var(--code-bg); padding: 2px 6px;
  border-radius: 4px; font-family: var(--font-mono);
  font-size: 0.9em; color: var(--accent-2);
}
.article pre {
  background: var(--code-bg); border: 1px solid var(--border);
  border-radius: var(--radius-sm); padding: 16px 18px;
  overflow-x: auto; font-size: 13.5px; line-height: 1.6;
}
.article pre code { background: none; padding: 0; color: var(--text); }

.callout {
  border: 1px solid var(--border-strong);
  border-left: 4px solid var(--accent);
  background: var(--bg-elev);
  padding: 16px 20px; border-radius: var(--radius-sm);
  margin: 20px 0;
}
.callout.warn { border-left-color: var(--warn); }
.callout.tip { border-left-color: var(--accent-3); }
.callout.danger { border-left-color: var(--danger); }
.callout strong { display: block; margin-bottom: 6px; }

.diagram {
  margin: 28px 0;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 24px;
  overflow-x: auto;
}
.diagram svg { display: block; max-width: 100%; height: auto; margin: 0 auto; }
.diagram .caption {
  text-align: center; color: var(--text-faint);
  font-size: 13px; margin-top: 12px;
}

table {
  width: 100%; border-collapse: collapse;
  margin: 20px 0; font-size: 14px;
}
th, td {
  text-align: left; padding: 10px 12px;
  border-bottom: 1px solid var(--border);
}
th { color: var(--text); font-weight: 700; background: var(--bg-elev); }
td { color: var(--text-dim); }

.prev-next {
  display: grid; grid-template-columns: 1fr 1fr;
  gap: 16px; margin: 56px 0 40px;
}
.prev-next a {
  border: 1px solid var(--border);
  background: var(--bg-elev);
  padding: 16px 20px;
  border-radius: var(--radius-sm);
  display: block;
}
.prev-next a:hover { border-color: var(--border-strong); text-decoration: none; }
.prev-next .label { color: var(--text-faint); font-size: 12px; text-transform: uppercase; letter-spacing: 1px; }
.prev-next .title { color: var(--text); font-weight: 600; margin-top: 4px; }
.prev-next .right { text-align: right; }

/* ============ TOC RAIL ============ */
.toc-rail {
  padding: 48px 24px 24px;
  position: sticky; top: 60px;
  max-height: calc(100vh - 60px);
  overflow-y: auto;
}
.toc-rail .toc-title {
  font-size: 11px; font-weight: 700;
  text-transform: uppercase; letter-spacing: 1.5px;
  color: var(--text-faint); margin-bottom: 12px;
}
.toc-rail ul { list-style: none; padding: 0; margin: 0; }
.toc-rail ul li { padding: 4px 0; }
.toc-rail ul a {
  color: var(--text-dim); font-size: 13px;
  display: block; padding: 4px 10px;
  border-left: 2px solid transparent;
}
.toc-rail ul a:hover { color: var(--text); text-decoration: none; }
.toc-rail ul a.active { border-left-color: var(--accent); color: var(--text); }
.toc-rail ul ul { padding-left: 12px; }

/* ============ FOOTER ============ */
.site-footer {
  border-top: 1px solid var(--border);
  background: var(--bg-elev);
  padding: 40px 24px;
  text-align: center;
  color: var(--text-dim);
  margin-top: 60px;
}
.site-footer .footer-brand { font-weight: 700; color: var(--text); margin-bottom: 8px; }
.site-footer a { color: var(--accent-2); }

/* ============ MOBILE MENU ============ */
.menu-btn {
  display: none;
  background: none; border: none; color: var(--text);
  cursor: pointer; padding: 8px;
}
@media (max-width: 768px) {
  .menu-btn { display: block; }
  .nav-links a:not(.theme-toggle) { display: none; }
}

/* ============ INTERACTIVE SIMULATIONS ============ */
.simulation {
  margin: 32px 0;
  padding: 20px;
  border: 1px solid var(--border-strong);
  border-radius: var(--radius);
  background: linear-gradient(180deg, var(--bg-elev) 0%, var(--bg-elev-2) 100%);
  position: relative;
  overflow: hidden;
}
.simulation::before {
  content: "INTERACTIVE";
  position: absolute;
  top: 12px; right: 12px;
  font-size: 10px;
  font-weight: 700;
  letter-spacing: 0.1em;
  padding: 4px 8px;
  border-radius: 4px;
  background: linear-gradient(90deg, var(--accent), var(--accent-2));
  color: #0b0d12;
}
.sim-title {
  font-size: 15px;
  font-weight: 700;
  color: var(--text);
  margin: 0 0 4px 0;
}
.sim-desc {
  font-size: 13px;
  color: var(--text-dim);
  margin: 0 0 16px 0;
}
.sim-canvas {
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 16px;
  min-height: 200px;
  position: relative;
}
.sim-controls {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  align-items: center;
  margin-top: 14px;
  padding-top: 14px;
  border-top: 1px solid var(--border);
}
.sim-btn {
  background: var(--bg-elev-2);
  color: var(--text);
  border: 1px solid var(--border-strong);
  padding: 8px 14px;
  border-radius: 6px;
  cursor: pointer;
  font-family: var(--font-sans);
  font-size: 13px;
  font-weight: 500;
  transition: all 0.15s;
}
.sim-btn:hover { background: var(--accent); color: #fff; border-color: var(--accent); }
.sim-btn.primary { background: var(--accent); color: #fff; border-color: var(--accent); }
.sim-btn.primary:hover { filter: brightness(1.1); }
.sim-btn.danger { background: rgba(239,68,68,0.15); color: var(--danger); border-color: rgba(239,68,68,0.4); }
.sim-btn.danger:hover { background: var(--danger); color: #fff; }
.sim-btn:disabled { opacity: 0.4; cursor: not-allowed; }
.sim-toggle {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  font-size: 13px;
  color: var(--text-dim);
  cursor: pointer;
  user-select: none;
}
.sim-toggle input { accent-color: var(--accent); }
.sim-slider {
  display: flex;
  flex-direction: column;
  gap: 4px;
  font-size: 12px;
  color: var(--text-dim);
}
.sim-slider input[type="range"] { width: 140px; accent-color: var(--accent); }
.sim-stat {
  display: inline-flex;
  flex-direction: column;
  padding: 6px 12px;
  border-radius: 6px;
  background: var(--bg);
  border: 1px solid var(--border);
  min-width: 70px;
}
.sim-stat .lbl { font-size: 10px; color: var(--text-faint); text-transform: uppercase; letter-spacing: 0.06em; }
.sim-stat .val { font-size: 16px; font-weight: 700; color: var(--text); font-family: var(--font-mono); }
.sim-stat.good .val { color: var(--accent-3); }
.sim-stat.bad .val { color: var(--danger); }
.sim-stat.warn .val { color: var(--warn); }
.sim-log {
  max-height: 120px;
  overflow-y: auto;
  font-family: var(--font-mono);
  font-size: 11px;
  background: var(--code-bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 8px 10px;
  margin-top: 10px;
  color: var(--text-dim);
}
.sim-log div { padding: 2px 0; }
.sim-log .ok { color: var(--accent-3); }
.sim-log .err { color: var(--danger); }
.sim-log .info { color: var(--accent-2); }
.sim-log .warn { color: var(--warn); }
.sim-pill {
  display: inline-block;
  padding: 3px 9px;
  border-radius: 999px;
  font-size: 11px;
  font-weight: 600;
  font-family: var(--font-mono);
}
.sim-pill.green { background: rgba(52,211,153,0.15); color: var(--accent-3); }
.sim-pill.red { background: rgba(248,113,113,0.15); color: var(--danger); }
.sim-pill.yellow { background: rgba(251,191,36,0.15); color: var(--warn); }
.sim-pill.blue { background: rgba(34,211,238,0.15); color: var(--accent-2); }