/* ========== CSS Variables & Reset ========== */
:root {
  --bg-primary: #0f1117;
  --bg-secondary: #1a1d2e;
  --bg-card: #212438;
  --bg-code: #161825;
  --text-primary: #e4e7f1;
  --text-secondary: #9ca3bf;
  --text-muted: #6b7294;
  --accent-blue: #60a5fa;
  --accent-purple: #a78bfa;
  --accent-green: #34d399;
  --accent-orange: #fb923c;
  --accent-pink: #f472b6;
  --accent-cyan: #22d3ee;
  --accent-red: #f87171;
  --border-color: #2a2d42;
  --sidebar-width: 280px;
  --header-height: 64px;
  --font-mono: 'JetBrains Mono', 'Fira Code', 'Consolas', monospace;
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --radius: 12px;
  --radius-sm: 8px;
  --shadow: 0 4px 24px rgba(0,0,0,0.3);
  --shadow-lg: 0 8px 40px rgba(0,0,0,0.4);
}

* { margin: 0; padding: 0; box-sizing: border-box; }

html {
  scroll-behavior: smooth;
  scroll-padding-top: 80px;
}

body {
  font-family: var(--font-sans);
  background: var(--bg-primary);
  color: var(--text-primary);
  line-height: 1.7;
  overflow-x: hidden;
}

::-webkit-scrollbar { width: 6px; }
::-webkit-scrollbar-track { background: var(--bg-primary); }
::-webkit-scrollbar-thumb { background: var(--border-color); border-radius: 3px; }
::-webkit-scrollbar-thumb:hover { background: var(--text-muted); }

/* ========== Header ========== */
.header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--header-height);
  background: rgba(15, 17, 23, 0.85);
  backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--border-color);
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 24px;
  z-index: 1000;
}

.header-logo {
  display: flex;
  align-items: center;
  gap: 12px;
  font-size: 1.2rem;
  font-weight: 700;
  color: var(--text-primary);
}

.header-logo .logo-icon {
  width: 36px;
  height: 36px;
  background: linear-gradient(135deg, var(--accent-blue), var(--accent-purple));
  border-radius: 10px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.1rem;
}

.header-progress {
  display: flex;
  align-items: center;
  gap: 12px;
  font-size: 0.85rem;
  color: var(--text-secondary);
}

.progress-bar-container {
  width: 200px;
  height: 6px;
  background: var(--bg-card);
  border-radius: 3px;
  overflow: hidden;
}

.progress-bar-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--accent-blue), var(--accent-purple));
  border-radius: 3px;
  transition: width 0.5s ease;
  width: 0%;
}

.theme-toggle {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  color: var(--text-primary);
  padding: 8px 12px;
  border-radius: var(--radius-sm);
  cursor: pointer;
  font-size: 0.85rem;
  transition: all 0.2s;
}

.theme-toggle:hover {
  border-color: var(--accent-blue);
}

/* ========== Sidebar ========== */
.sidebar {
  position: fixed;
  top: var(--header-height);
  left: 0;
  width: var(--sidebar-width);
  height: calc(100vh - var(--header-height));
  background: var(--bg-secondary);
  border-right: 1px solid var(--border-color);
  overflow-y: auto;
  padding: 16px 0;
  z-index: 900;
  transition: transform 0.3s ease;
}

.sidebar-section {
  padding: 8px 16px;
  margin-bottom: 4px;
}

.sidebar-section-title {
  font-size: 0.7rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 1.2px;
  color: var(--text-muted);
  padding: 8px 0;
}

.sidebar-link {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 9px 16px;
  color: var(--text-secondary);
  text-decoration: none;
  font-size: 0.88rem;
  border-radius: var(--radius-sm);
  margin: 1px 8px;
  transition: all 0.2s;
  position: relative;
}

.sidebar-link:hover {
  background: rgba(96, 165, 250, 0.08);
  color: var(--text-primary);
}

.sidebar-link.active {
  background: rgba(96, 165, 250, 0.12);
  color: var(--accent-blue);
}

.sidebar-link.active::before {
  content: '';
  position: absolute;
  left: -8px;
  top: 50%;
  transform: translateY(-50%);
  width: 3px;
  height: 60%;
  background: var(--accent-blue);
  border-radius: 2px;
}

.sidebar-link .link-icon {
  width: 22px;
  height: 22px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: 6px;
  font-size: 0.75rem;
  flex-shrink: 0;
}

.sidebar-link .check-mark {
  margin-left: auto;
  color: var(--accent-green);
  font-size: 0.8rem;
  opacity: 0;
  transition: opacity 0.2s;
}

.sidebar-link.completed .check-mark {
  opacity: 1;
}

/* ========== Main Content ========== */
.main-content {
  margin-left: var(--sidebar-width);
  margin-top: var(--header-height);
  padding: 40px 48px;
  max-width: 960px;
}

/* ========== Sections ========== */
.section {
  display: none;
  animation: fadeIn 0.4s ease;
}

.section.active {
  display: block;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(12px); }
  to { opacity: 1; transform: translateY(0); }
}

/* ========== Typography ========== */
h1 {
  font-size: 2.2rem;
  font-weight: 800;
  margin-bottom: 12px;
  background: linear-gradient(135deg, var(--accent-blue), var(--accent-purple));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

h2 {
  font-size: 1.6rem;
  font-weight: 700;
  margin: 40px 0 16px;
  color: var(--text-primary);
  padding-bottom: 8px;
  border-bottom: 1px solid var(--border-color);
}

h3 {
  font-size: 1.25rem;
  font-weight: 600;
  margin: 28px 0 12px;
  color: var(--text-primary);
}

h4 {
  font-size: 1.05rem;
  font-weight: 600;
  margin: 20px 0 8px;
  color: var(--accent-blue);
}

p {
  color: var(--text-secondary);
  margin-bottom: 14px;
  font-size: 0.95rem;
}

.subtitle {
  font-size: 1.05rem;
  color: var(--text-muted);
  margin-bottom: 32px;
}

ul, ol {
  color: var(--text-secondary);
  padding-left: 24px;
  margin-bottom: 16px;
}

li {
  margin-bottom: 6px;
  font-size: 0.95rem;
}

strong { color: var(--text-primary); }

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

a:hover { text-decoration: underline; }

/* ========== Cards ========== */
.card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  padding: 24px;
  margin-bottom: 20px;
}

.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 16px;
  margin-bottom: 20px;
}

.feature-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  padding: 20px;
  transition: all 0.3s;
}

.feature-card:hover {
  border-color: var(--accent-blue);
  transform: translateY(-2px);
  box-shadow: var(--shadow);
}

.feature-card .card-icon {
  width: 44px;
  height: 44px;
  border-radius: 12px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.3rem;
  margin-bottom: 14px;
}

.feature-card h4 {
  margin: 0 0 8px;
  color: var(--text-primary);
}

.feature-card p {
  font-size: 0.88rem;
  margin: 0;
}

/* ========== Code Blocks ========== */
.code-block {
  background: var(--bg-code);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  margin: 16px 0;
  overflow: hidden;
}

.code-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 10px 16px;
  background: rgba(255,255,255,0.03);
  border-bottom: 1px solid var(--border-color);
}

.code-lang {
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.5px;
}

.code-copy {
  background: none;
  border: 1px solid var(--border-color);
  color: var(--text-muted);
  padding: 4px 10px;
  border-radius: 6px;
  cursor: pointer;
  font-size: 0.75rem;
  font-family: var(--font-sans);
  transition: all 0.2s;
}

.code-copy:hover {
  border-color: var(--accent-blue);
  color: var(--accent-blue);
}

.code-block pre {
  padding: 16px;
  overflow-x: auto;
  font-family: var(--font-mono);
  font-size: 0.85rem;
  line-height: 1.6;
  color: var(--text-primary);
}

.code-block code {
  font-family: var(--font-mono);
}

/* Inline code */
p code, li code, td code {
  background: var(--bg-code);
  border: 1px solid var(--border-color);
  padding: 2px 6px;
  border-radius: 4px;
  font-family: var(--font-mono);
  font-size: 0.85em;
  color: var(--accent-pink);
}

/* ========== Info Boxes ========== */
.info-box {
  border-radius: var(--radius);
  padding: 16px 20px;
  margin: 16px 0;
  border-left: 4px solid;
  font-size: 0.92rem;
}

.info-box.tip {
  background: rgba(52, 211, 153, 0.08);
  border-color: var(--accent-green);
}

.info-box.tip .info-title { color: var(--accent-green); }

.info-box.warning {
  background: rgba(251, 146, 60, 0.08);
  border-color: var(--accent-orange);
}

.info-box.warning .info-title { color: var(--accent-orange); }

.info-box.note {
  background: rgba(96, 165, 250, 0.08);
  border-color: var(--accent-blue);
}

.info-box.note .info-title { color: var(--accent-blue); }

.info-box.danger {
  background: rgba(248, 113, 113, 0.08);
  border-color: var(--accent-red);
}

.info-box.danger .info-title { color: var(--accent-red); }

.info-title {
  font-weight: 700;
  margin-bottom: 6px;
  font-size: 0.88rem;
}

.info-box p { margin: 0; color: var(--text-secondary); }

/* ========== Exercise ========== */
.exercise {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  margin: 20px 0;
  overflow: hidden;
}

.exercise-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 16px 20px;
  background: rgba(167, 139, 250, 0.06);
  border-bottom: 1px solid var(--border-color);
  cursor: pointer;
}

.exercise-header:hover {
  background: rgba(167, 139, 250, 0.1);
}

.exercise-title {
  display: flex;
  align-items: center;
  gap: 10px;
  font-weight: 600;
  font-size: 0.95rem;
  color: var(--accent-purple);
}

.exercise-badge {
  font-size: 0.7rem;
  padding: 3px 8px;
  border-radius: 4px;
  font-weight: 600;
}

.badge-easy {
  background: rgba(52, 211, 153, 0.15);
  color: var(--accent-green);
}

.badge-medium {
  background: rgba(251, 146, 60, 0.15);
  color: var(--accent-orange);
}

.badge-hard {
  background: rgba(248, 113, 113, 0.15);
  color: var(--accent-red);
}

.exercise-body {
  padding: 20px;
}

.exercise-question {
  margin-bottom: 16px;
  color: var(--text-secondary);
  font-size: 0.95rem;
}

.answer-toggle {
  background: linear-gradient(135deg, var(--accent-purple), var(--accent-blue));
  border: none;
  color: white;
  padding: 8px 20px;
  border-radius: var(--radius-sm);
  cursor: pointer;
  font-size: 0.88rem;
  font-weight: 600;
  font-family: var(--font-sans);
  transition: all 0.2s;
}

.answer-toggle:hover {
  opacity: 0.9;
  transform: translateY(-1px);
}

.answer-content {
  display: none;
  margin-top: 16px;
  padding-top: 16px;
  border-top: 1px solid var(--border-color);
}

.answer-content.show {
  display: block;
  animation: fadeIn 0.3s ease;
}

/* ========== Tables ========== */
table {
  width: 100%;
  border-collapse: collapse;
  margin: 16px 0;
  font-size: 0.9rem;
}

th {
  background: var(--bg-card);
  color: var(--text-primary);
  font-weight: 600;
  text-align: left;
  padding: 12px 16px;
  border-bottom: 2px solid var(--border-color);
}

td {
  padding: 10px 16px;
  border-bottom: 1px solid var(--border-color);
  color: var(--text-secondary);
}

tr:hover td {
  background: rgba(255,255,255,0.02);
}

/* ========== Math ========== */
.math-block {
  background: var(--bg-code);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  padding: 20px;
  margin: 16px 0;
  text-align: center;
  font-family: var(--font-mono);
  font-size: 0.95rem;
  color: var(--accent-cyan);
  overflow-x: auto;
}

.math-inline {
  font-family: var(--font-mono);
  color: var(--accent-cyan);
  background: var(--bg-code);
  padding: 1px 5px;
  border-radius: 3px;
  font-size: 0.9em;
}

/* ========== Roadmap / Steps ========== */
.roadmap {
  position: relative;
  padding-left: 36px;
  margin: 24px 0;
}

.roadmap::before {
  content: '';
  position: absolute;
  left: 14px;
  top: 0;
  bottom: 0;
  width: 2px;
  background: var(--border-color);
}

.roadmap-step {
  position: relative;
  margin-bottom: 24px;
}

.roadmap-step::before {
  content: '';
  position: absolute;
  left: -28px;
  top: 6px;
  width: 12px;
  height: 12px;
  border-radius: 50%;
  background: var(--bg-card);
  border: 2px solid var(--accent-blue);
}

.roadmap-step.completed::before {
  background: var(--accent-green);
  border-color: var(--accent-green);
}

.roadmap-step h4 {
  margin-top: 0;
  color: var(--text-primary);
}

.roadmap-step p {
  font-size: 0.9rem;
}

/* ========== Tags ========== */
.tag {
  display: inline-block;
  padding: 3px 10px;
  border-radius: 20px;
  font-size: 0.75rem;
  font-weight: 600;
  margin-right: 6px;
  margin-bottom: 4px;
}

.tag-blue { background: rgba(96,165,250,0.15); color: var(--accent-blue); }
.tag-purple { background: rgba(167,139,250,0.15); color: var(--accent-purple); }
.tag-green { background: rgba(52,211,153,0.15); color: var(--accent-green); }
.tag-orange { background: rgba(251,146,60,0.15); color: var(--accent-orange); }
.tag-pink { background: rgba(244,114,182,0.15); color: var(--accent-pink); }

/* ========== Navigation Buttons ========== */
.nav-buttons {
  display: flex;
  justify-content: space-between;
  margin-top: 48px;
  padding-top: 24px;
  border-top: 1px solid var(--border-color);
}

.nav-btn {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 12px 24px;
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  color: var(--text-primary);
  font-size: 0.9rem;
  font-weight: 500;
  cursor: pointer;
  transition: all 0.2s;
  text-decoration: none;
  font-family: var(--font-sans);
}

.nav-btn:hover {
  border-color: var(--accent-blue);
  transform: translateY(-1px);
  box-shadow: var(--shadow);
  text-decoration: none;
}

.nav-btn.primary {
  background: linear-gradient(135deg, var(--accent-blue), var(--accent-purple));
  border: none;
  color: white;
}

/* ========== Syntax Highlighting ========== */
.keyword { color: #c792ea; }
.string { color: #c3e88d; }
.comment { color: #546e7a; font-style: italic; }
.function { color: #82aaff; }
.number { color: #f78c6c; }
.decorator { color: #ffcb6b; }
.class-name { color: #ffcb6b; }
.param { color: #e4e7f1; }
.operator { color: #89ddff; }
.builtin { color: #82aaff; }
.self { color: #ff5370; }

/* ========== Responsive ========== */
.mobile-menu-btn {
  display: none;
  background: none;
  border: none;
  color: var(--text-primary);
  font-size: 1.3rem;
  cursor: pointer;
  padding: 8px;
}

@media (max-width: 900px) {
  .sidebar {
    transform: translateX(-100%);
  }

  .sidebar.open {
    transform: translateX(0);
  }

  .main-content {
    margin-left: 0;
    padding: 24px;
  }

  .mobile-menu-btn {
    display: block;
  }

  .progress-bar-container {
    width: 120px;
  }

  h1 { font-size: 1.6rem; }
  h2 { font-size: 1.3rem; }

  .card-grid {
    grid-template-columns: 1fr;
  }
}

/* ========== Diagram / Illustration ========== */
.diagram {
  background: var(--bg-code);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  padding: 24px;
  margin: 16px 0;
  font-family: var(--font-mono);
  font-size: 0.82rem;
  line-height: 1.5;
  color: var(--accent-cyan);
  overflow-x: auto;
  white-space: pre;
}

/* ========== Quiz / MC ========== */
.quiz {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  padding: 24px;
  margin: 20px 0;
}

.quiz-question {
  font-weight: 600;
  margin-bottom: 16px;
  color: var(--text-primary);
}

.quiz-options {
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.quiz-option {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 10px 16px;
  background: var(--bg-code);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-sm);
  cursor: pointer;
  transition: all 0.2s;
  font-size: 0.9rem;
  color: var(--text-secondary);
}

.quiz-option:hover {
  border-color: var(--accent-blue);
  background: rgba(96,165,250,0.06);
}

.quiz-option.correct {
  border-color: var(--accent-green);
  background: rgba(52,211,153,0.1);
  color: var(--accent-green);
}

.quiz-option.wrong {
  border-color: var(--accent-red);
  background: rgba(248,113,113,0.1);
  color: var(--accent-red);
}

.quiz-option .option-letter {
  width: 24px;
  height: 24px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: 50%;
  border: 1px solid var(--border-color);
  font-size: 0.8rem;
  font-weight: 600;
  flex-shrink: 0;
}

.quiz-explanation {
  display: none;
  margin-top: 12px;
  padding: 12px 16px;
  background: rgba(96,165,250,0.06);
  border-radius: var(--radius-sm);
  font-size: 0.88rem;
  color: var(--text-secondary);
}

.quiz-explanation.show {
  display: block;
  animation: fadeIn 0.3s ease;
}

/* ========== Tabs ========== */
.tabs {
  margin: 16px 0;
}

.tab-buttons {
  display: flex;
  gap: 4px;
  border-bottom: 1px solid var(--border-color);
  margin-bottom: 16px;
}

.tab-btn {
  padding: 8px 20px;
  background: none;
  border: none;
  border-bottom: 2px solid transparent;
  color: var(--text-muted);
  font-size: 0.9rem;
  font-weight: 500;
  cursor: pointer;
  transition: all 0.2s;
  font-family: var(--font-sans);
}

.tab-btn:hover {
  color: var(--text-secondary);
}

.tab-btn.active {
  color: var(--accent-blue);
  border-bottom-color: var(--accent-blue);
}

.tab-content {
  display: none;
}

.tab-content.active {
  display: block;
  animation: fadeIn 0.3s ease;
}

/* ========== Hero Section ========== */
.hero {
  text-align: center;
  padding: 40px 0 48px;
}

.hero h1 {
  font-size: 2.8rem;
  margin-bottom: 16px;
}

.hero .subtitle {
  font-size: 1.15rem;
  max-width: 640px;
  margin: 0 auto 32px;
}

.hero-stats {
  display: flex;
  justify-content: center;
  gap: 40px;
  margin-bottom: 32px;
}

.hero-stat {
  text-align: center;
}

.hero-stat .stat-number {
  font-size: 2rem;
  font-weight: 800;
  background: linear-gradient(135deg, var(--accent-blue), var(--accent-purple));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.hero-stat .stat-label {
  font-size: 0.8rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 1px;
}

.start-btn {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 14px 36px;
  background: linear-gradient(135deg, var(--accent-blue), var(--accent-purple));
  border: none;
  border-radius: var(--radius);
  color: white;
  font-size: 1rem;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.3s;
  font-family: var(--font-sans);
}

.start-btn:hover {
  transform: translateY(-2px);
  box-shadow: 0 8px 30px rgba(96, 165, 250, 0.3);
}

/* ========== Completion Checkbox ========== */
.section-complete {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 16px 20px;
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  margin: 32px 0 0;
  cursor: pointer;
  transition: all 0.2s;
}

.section-complete:hover {
  border-color: var(--accent-green);
}

.section-complete input[type="checkbox"] {
  width: 20px;
  height: 20px;
  accent-color: var(--accent-green);
  cursor: pointer;
}

.section-complete label {
  cursor: pointer;
  font-size: 0.95rem;
  color: var(--text-secondary);
}

.section-complete.checked {
  background: rgba(52, 211, 153, 0.06);
  border-color: var(--accent-green);
}