/* ==========================================================================
   Semester III Exam Preparation Portal - Academic Modern Stylesheet
   ========================================================================== */

:root {
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
  --font-mono: 'JetBrains Mono', 'Fira Code', Consolas, Monaco, monospace;
  --font-serif: 'Merriweather', Georgia, serif;

  /* Light Theme (Default) */
  --bg-primary: #f8fafc;
  --bg-secondary: #ffffff;
  --bg-tertiary: #f1f5f9;
  --bg-card: #ffffff;
  --bg-card-hover: #f8fafc;
  --bg-accent: #eff6ff;
  --bg-code: #1e293b;
  --text-code: #e2e8f0;

  --text-primary: #0f172a;
  --text-secondary: #334155;
  --text-muted: #64748b;
  --text-accent: #1d4ed8;

  --border-color: #e2e8f0;
  --border-focus: #3b82f6;

  --primary: #2563eb;
  --primary-hover: #1d4ed8;
  --primary-light: #dbeafe;
  --primary-dark: #1e40af;

  --success: #10b981;
  --success-bg: #ecfdf5;
  --warning: #f59e0b;
  --warning-bg: #fffbeb;
  --danger: #ef4444;
  --danger-bg: #fef2f2;
  --info: #06b6d4;
  --info-bg: #ecfeff;

  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
  --shadow-xl: 0 20px 25px -5px rgba(0, 0, 0, 0.1), 0 10px 10px -5px rgba(0, 0, 0, 0.04);

  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --radius-full: 9999px;

  --transition-fast: 150ms ease;
  --transition-normal: 250ms ease;
}

[data-theme="dark"] {
  --bg-primary: #0b0f19;
  --bg-secondary: #111827;
  --bg-tertiary: #1f2937;
  --bg-card: #151e2e;
  --bg-card-hover: #1c283c;
  --bg-accent: #1e293b;
  --bg-code: #090d16;
  --text-code: #cbd5e1;

  --text-primary: #f8fafc;
  --text-secondary: #cbd5e1;
  --text-muted: #94a3b8;
  --text-accent: #60a5fa;

  --border-color: #273549;
  --border-focus: #60a5fa;

  --primary: #3b82f6;
  --primary-hover: #60a5fa;
  --primary-light: rgba(59, 130, 246, 0.15);
  --primary-dark: #1d4ed8;

  --success-bg: rgba(16, 185, 129, 0.15);
  --warning-bg: rgba(245, 158, 11, 0.15);
  --danger-bg: rgba(239, 68, 68, 0.15);
  --info-bg: rgba(6, 182, 212, 0.15);

  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.3);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.4);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.5);
  --shadow-xl: 0 20px 25px -5px rgba(0, 0, 0, 0.6);
}

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

html {
  font-size: 16px;
  scroll-behavior: smooth;
  color-scheme: light dark;
}

body {
  font-family: var(--font-sans);
  background-color: var(--bg-primary);
  color: var(--text-primary);
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

/* Reading Progress Bar */
#reading-progress {
  position: fixed;
  top: 0;
  left: 0;
  height: 4px;
  background: linear-gradient(90deg, #3b82f6, #8b5cf6, #ec4899);
  width: 0%;
  z-index: 9999;
  transition: width 0.1s ease;
}

/* Header & Nav */
.header-nav {
  position: sticky;
  top: 0;
  z-index: 1000;
  background-color: var(--bg-secondary);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border-color);
}

.nav-container {
  max-width: 1280px;
  margin: 0 auto;
  padding: 0.85rem 1.5rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
}

.nav-brand {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  text-decoration: none;
  color: var(--text-primary);
  font-weight: 700;
  font-size: 1.15rem;
}

.nav-brand .badge {
  background: var(--primary);
  color: #ffffff;
  font-size: 0.75rem;
  padding: 0.2rem 0.6rem;
  border-radius: var(--radius-full);
  font-weight: 600;
  letter-spacing: 0.5px;
}

.nav-controls {
  display: flex;
  align-items: center;
  gap: 0.75rem;
}

.theme-toggle-btn, .nav-btn {
  background: var(--bg-tertiary);
  border: 1px solid var(--border-color);
  color: var(--text-primary);
  padding: 0.5rem 0.85rem;
  border-radius: var(--radius-md);
  cursor: pointer;
  font-size: 0.9rem;
  font-weight: 500;
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  transition: all var(--transition-fast);
  text-decoration: none;
}

.theme-toggle-btn:hover, .nav-btn:hover {
  background: var(--primary-light);
  border-color: var(--primary);
  color: var(--primary);
}

/* Breadcrumbs */
.breadcrumb-container {
  background: var(--bg-secondary);
  border-bottom: 1px solid var(--border-color);
  padding: 0.65rem 1.5rem;
}

.breadcrumbs {
  max-width: 1280px;
  margin: 0 auto;
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 0.5rem;
  font-size: 0.875rem;
  color: var(--text-muted);
}

.breadcrumbs a {
  color: var(--text-secondary);
  text-decoration: none;
  font-weight: 500;
  transition: color var(--transition-fast);
}

.breadcrumbs a:hover {
  color: var(--primary);
  text-decoration: underline;
}

.breadcrumbs .separator {
  color: var(--text-muted);
  user-select: none;
}

.breadcrumbs .current {
  color: var(--primary);
  font-weight: 600;
}

/* Container */
.main-wrapper {
  flex: 1;
  max-width: 1280px;
  width: 100%;
  margin: 0 auto;
  padding: 2rem 1.5rem 4rem 1.5rem;
}

/* Hero Section */
.hero-section {
  text-align: center;
  padding: 3rem 1.5rem 2.5rem;
  background: linear-gradient(135deg, var(--bg-secondary) 0%, var(--bg-accent) 100%);
  border-radius: var(--radius-lg);
  border: 1px solid var(--border-color);
  margin-bottom: 2.5rem;
  box-shadow: var(--shadow-sm);
  position: relative;
  overflow: hidden;
}

.hero-tag {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  background: var(--primary-light);
  color: var(--primary-dark);
  padding: 0.35rem 0.9rem;
  border-radius: var(--radius-full);
  font-size: 0.85rem;
  font-weight: 600;
  margin-bottom: 1.25rem;
  text-transform: uppercase;
  letter-spacing: 0.75px;
}

[data-theme="dark"] .hero-tag {
  color: #93c5fd;
}

.hero-title {
  font-size: 2.3rem;
  font-weight: 800;
  color: var(--text-primary);
  line-height: 1.25;
  margin-bottom: 1rem;
}

.hero-subtitle {
  font-size: 1.1rem;
  color: var(--text-secondary);
  max-width: 780px;
  margin: 0 auto 1.5rem;
}

.hero-stats {
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
  gap: 2rem;
  margin-top: 1.5rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border-color);
}

.stat-item {
  display: flex;
  flex-direction: column;
  align-items: center;
}

.stat-value {
  font-size: 1.75rem;
  font-weight: 800;
  color: var(--primary);
}

.stat-label {
  font-size: 0.85rem;
  color: var(--text-muted);
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.5px;
}

/* Search Bar */
.search-container {
  margin: 0 auto 2.5rem;
  max-width: 680px;
  position: relative;
}

.search-input-box {
  width: 100%;
  padding: 0.9rem 1.25rem 0.9rem 3.25rem;
  background: var(--bg-card);
  border: 2px solid var(--border-color);
  border-radius: var(--radius-lg);
  font-size: 1.05rem;
  color: var(--text-primary);
  box-shadow: var(--shadow-sm);
  outline: none;
  transition: all var(--transition-fast);
}

.search-input-box:focus {
  border-color: var(--border-focus);
  box-shadow: 0 0 0 4px var(--primary-light);
}

.search-icon {
  position: absolute;
  left: 1.2rem;
  top: 50%;
  transform: translateY(-50%);
  color: var(--text-muted);
  font-size: 1.2rem;
  pointer-events: none;
}

/* Grid Layouts */
.cards-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(340px, 1fr));
  gap: 1.75rem;
  margin-bottom: 3rem;
}

.card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 1.75rem;
  box-shadow: var(--shadow-sm);
  transition: all var(--transition-normal);
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  position: relative;
  text-decoration: none;
  color: inherit;
}

.card:hover {
  transform: translateY(-4px);
  box-shadow: var(--shadow-lg);
  border-color: var(--primary);
}

.card-header {
  margin-bottom: 1rem;
}

.card-code {
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.75px;
  color: var(--primary);
  background: var(--primary-light);
  padding: 0.2rem 0.6rem;
  border-radius: var(--radius-sm);
  display: inline-block;
  margin-bottom: 0.75rem;
}

.card-title {
  font-size: 1.3rem;
  font-weight: 700;
  color: var(--text-primary);
  margin-bottom: 0.5rem;
  line-height: 1.35;
}

.card-description {
  color: var(--text-secondary);
  font-size: 0.95rem;
  line-height: 1.55;
  margin-bottom: 1.25rem;
}

.card-meta {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding-top: 1rem;
  border-top: 1px solid var(--border-color);
  font-size: 0.875rem;
  color: var(--text-muted);
}

.card-btn {
  background: var(--primary);
  color: #ffffff;
  padding: 0.6rem 1.2rem;
  border-radius: var(--radius-md);
  font-weight: 600;
  font-size: 0.9rem;
  text-align: center;
  text-decoration: none;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 0.5rem;
  transition: background var(--transition-fast);
}

.card-btn:hover {
  background: var(--primary-hover);
}

/* Quick Nav / Table of Contents */
.toc-box {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 1.5rem;
  margin-bottom: 2rem;
  box-shadow: var(--shadow-sm);
}

.toc-title {
  font-size: 1.15rem;
  font-weight: 700;
  margin-bottom: 1rem;
  display: flex;
  align-items: center;
  gap: 0.5rem;
  color: var(--text-primary);
}

.toc-list {
  list-style: none;
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 0.5rem;
}

.toc-list a {
  color: var(--text-secondary);
  text-decoration: none;
  padding: 0.4rem 0.75rem;
  border-radius: var(--radius-sm);
  display: block;
  font-size: 0.9rem;
  transition: all var(--transition-fast);
}

.toc-list a:hover {
  background: var(--primary-light);
  color: var(--primary);
  font-weight: 600;
}

/* Content Sections */
.study-section {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 2.25rem;
  margin-bottom: 2.5rem;
  box-shadow: var(--shadow-sm);
}

.section-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  border-bottom: 2px solid var(--border-color);
  padding-bottom: 1rem;
  margin-bottom: 1.75rem;
  flex-wrap: wrap;
  gap: 1rem;
}

.section-title {
  font-size: 1.55rem;
  font-weight: 800;
  color: var(--text-primary);
  display: flex;
  align-items: center;
  gap: 0.75rem;
}

.section-badge {
  font-size: 0.8rem;
  background: var(--primary-light);
  color: var(--primary);
  padding: 0.25rem 0.75rem;
  border-radius: var(--radius-full);
  font-weight: 600;
}

/* Topic Notes Content */
.topic-block {
  margin-bottom: 2.5rem;
  padding-bottom: 2rem;
  border-bottom: 1px dashed var(--border-color);
}

.topic-block:last-child {
  border-bottom: none;
  margin-bottom: 0;
  padding-bottom: 0;
}

.topic-heading {
  font-size: 1.3rem;
  font-weight: 700;
  color: var(--text-primary);
  margin-bottom: 1rem;
  padding-left: 0.75rem;
  border-left: 4px solid var(--primary);
}

.topic-body p {
  margin-bottom: 1rem;
  color: var(--text-secondary);
  font-size: 1.02rem;
  line-height: 1.75;
}

.topic-body ul, .topic-body ol {
  margin: 1rem 0 1.25rem 1.5rem;
  color: var(--text-secondary);
}

.topic-body li {
  margin-bottom: 0.6rem;
  line-height: 1.65;
}

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

/* Code & Pre */
pre, code {
  font-family: var(--font-mono);
}

code {
  background: var(--bg-tertiary);
  padding: 0.2rem 0.45rem;
  border-radius: var(--radius-sm);
  font-size: 0.88em;
  color: var(--text-accent);
}

pre {
  background: var(--bg-code);
  color: var(--text-code);
  padding: 1.25rem;
  border-radius: var(--radius-md);
  overflow-x: auto;
  margin: 1.25rem 0;
  font-size: 0.9rem;
  line-height: 1.5;
  border: 1px solid var(--border-color);
}

pre code {
  background: transparent;
  padding: 0;
  color: inherit;
}

/* Callout Boxes */
.callout {
  padding: 1.25rem;
  border-radius: var(--radius-md);
  margin: 1.25rem 0;
  border-left: 4px solid;
  background: var(--bg-tertiary);
}

.callout-info {
  border-color: var(--info);
  background: var(--info-bg);
}

.callout-tip {
  border-color: var(--success);
  background: var(--success-bg);
}

.callout-warning {
  border-color: var(--warning);
  background: var(--warning-bg);
}

.callout-title {
  font-weight: 700;
  margin-bottom: 0.5rem;
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

/* Question & Answer Accordion */
.qa-controls {
  display: flex;
  gap: 0.75rem;
  margin-bottom: 1.25rem;
}

.qa-control-btn {
  background: var(--bg-tertiary);
  border: 1px solid var(--border-color);
  padding: 0.45rem 0.9rem;
  border-radius: var(--radius-md);
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  color: var(--text-secondary);
  transition: all var(--transition-fast);
}

.qa-control-btn:hover {
  background: var(--primary);
  color: #ffffff;
  border-color: var(--primary);
}

.qa-card {
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  margin-bottom: 1.25rem;
  background: var(--bg-card);
  overflow: hidden;
  box-shadow: var(--shadow-sm);
  transition: border-color var(--transition-fast);
}

.qa-card.open {
  border-color: var(--primary);
}

.qa-header {
  padding: 1.25rem 1.5rem;
  cursor: pointer;
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 1rem;
  background: var(--bg-card);
  user-select: none;
  transition: background var(--transition-fast);
}

.qa-header:hover {
  background: var(--bg-card-hover);
}

.qa-question-title {
  font-size: 1.1rem;
  font-weight: 700;
  color: var(--text-primary);
  line-height: 1.4;
  flex: 1;
}

.qa-tags {
  display: flex;
  gap: 0.5rem;
  margin-top: 0.4rem;
}

.qa-tag {
  font-size: 0.75rem;
  font-weight: 600;
  padding: 0.15rem 0.5rem;
  border-radius: var(--radius-sm);
  background: var(--primary-light);
  color: var(--primary-dark);
}

.qa-icon {
  font-size: 1.25rem;
  color: var(--text-muted);
  transition: transform var(--transition-normal);
  font-weight: bold;
}

.qa-card.open .qa-icon {
  transform: rotate(180deg);
  color: var(--primary);
}

.qa-body {
  display: none;
  padding: 1.5rem;
  border-top: 1px solid var(--border-color);
  background: var(--bg-primary);
}

.qa-card.open .qa-body {
  display: block;
}

.qa-points-list {
  list-style: none;
  counter-reset: qa-counter;
  padding: 0;
}

.qa-point-item {
  position: relative;
  padding-left: 2.75rem;
  margin-bottom: 1.25rem;
  color: var(--text-secondary);
  font-size: 1rem;
  line-height: 1.65;
}

.qa-point-item::before {
  counter-increment: qa-counter;
  content: counter(qa-counter);
  position: absolute;
  left: 0;
  top: 0;
  width: 1.85rem;
  height: 1.85rem;
  background: var(--primary);
  color: #ffffff;
  font-size: 0.85rem;
  font-weight: 700;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
}

.qa-point-title {
  font-weight: 700;
  color: var(--text-primary);
  display: inline;
  margin-right: 0.35rem;
}

/* Comparison Tables */
.table-responsive {
  width: 100%;
  overflow-x: auto;
  margin: 1.5rem 0;
  border-radius: var(--radius-md);
  border: 1px solid var(--border-color);
  box-shadow: var(--shadow-sm);
}

.comparison-table {
  width: 100%;
  border-collapse: collapse;
  text-align: left;
  font-size: 0.95rem;
  background: var(--bg-card);
}

.comparison-table th {
  background: var(--bg-tertiary);
  color: var(--text-primary);
  font-weight: 700;
  padding: 1rem 1.25rem;
  border-bottom: 2px solid var(--border-color);
  white-space: nowrap;
}

.comparison-table td {
  padding: 0.9rem 1.25rem;
  border-bottom: 1px solid var(--border-color);
  color: var(--text-secondary);
  vertical-align: top;
  line-height: 1.55;
}

.comparison-table tr:last-child td {
  border-bottom: none;
}

.comparison-table tr:hover td {
  background: var(--bg-card-hover);
}

.comparison-table td:first-child {
  font-weight: 700;
  color: var(--text-primary);
  background: var(--bg-accent);
  width: 22%;
}

/* Quick Revision Key Concepts */
.revision-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 1.25rem;
  margin: 1.5rem 0;
}

.revision-card {
  background: var(--bg-tertiary);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  padding: 1.25rem;
}

.revision-card-title {
  font-weight: 700;
  color: var(--primary);
  margin-bottom: 0.5rem;
  font-size: 1rem;
}

.revision-card-body {
  font-size: 0.9rem;
  color: var(--text-secondary);
  line-height: 1.55;
}

/* Navigation Buttons at bottom */
.unit-nav-footer {
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  gap: 1rem;
  margin-top: 3rem;
  padding-top: 2rem;
  border-top: 1px solid var(--border-color);
}

.unit-nav-btn {
  padding: 0.75rem 1.5rem;
  border-radius: var(--radius-md);
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  color: var(--text-primary);
  text-decoration: none;
  font-weight: 600;
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  transition: all var(--transition-fast);
}

.unit-nav-btn:hover {
  background: var(--primary);
  color: #ffffff;
  border-color: var(--primary);
}

/* Floating Back to Top Button */
#back-to-top {
  position: fixed;
  bottom: 2rem;
  right: 2rem;
  width: 46px;
  height: 46px;
  border-radius: 50%;
  background: var(--primary);
  color: #ffffff;
  border: none;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.25rem;
  box-shadow: var(--shadow-lg);
  opacity: 0;
  visibility: hidden;
  transition: all var(--transition-normal);
  z-index: 999;
}

#back-to-top.visible {
  opacity: 1;
  visibility: visible;
}

#back-to-top:hover {
  background: var(--primary-hover);
  transform: translateY(-3px);
}

/* Footer */
footer {
  background: var(--bg-secondary);
  border-top: 1px solid var(--border-color);
  padding: 2.5rem 1.5rem;
  text-align: center;
  color: var(--text-muted);
  font-size: 0.9rem;
  margin-top: auto;
}

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

/* Print Styles */
@media print {
  body {
    background: #ffffff !important;
    color: #000000 !important;
    font-size: 11pt;
  }
  .header-nav, .breadcrumb-container, .qa-controls, #back-to-top, .unit-nav-footer, #reading-progress, .theme-toggle-btn {
    display: none !important;
  }
  .main-wrapper {
    max-width: 100% !important;
    padding: 0 !important;
  }
  .qa-body {
    display: block !important;
  }
  .qa-card, .study-section, .card {
    box-shadow: none !important;
    border: 1px solid #ccc !important;
    break-inside: avoid;
    page-break-inside: avoid;
  }
}

/* Responsive adjustments */
@media (max-width: 768px) {
  .hero-title {
    font-size: 1.85rem;
  }
  .cards-grid {
    grid-template-columns: 1fr;
  }
  .study-section {
    padding: 1.25rem;
  }
  .qa-header {
    padding: 1rem;
  }
}