/* ========================================
   INTEGRAL CALCULUS WEBSITE - DESIGN SYSTEM
   Premium, Modern, AdSense-Ready
   ======================================== */

/* ===== CSS Variables ===== */
:root {
  /* Primary Colors */
  --primary: #6366f1;
  --primary-light: #818cf8;
  --primary-dark: #4f46e5;
  
  /* Accent Colors */
  --accent: #22d3ee;
  --accent-light: #67e8f9;
  --accent-dark: #06b6d4;
  
  /* Neutral Colors */
  --bg-dark: #0f0f1a;
  --bg-card: #1a1a2e;
  --bg-card-hover: #252542;
  --text-primary: #f8fafc;
  --text-secondary: #94a3b8;
  --text-muted: #64748b;
  
  /* Gradients */
  --gradient-primary: linear-gradient(135deg, #6366f1 0%, #8b5cf6 50%, #a855f7 100%);
  --gradient-accent: linear-gradient(135deg, #06b6d4 0%, #22d3ee 100%);
  --gradient-dark: linear-gradient(180deg, #0f0f1a 0%, #1a1a2e 100%);
  
  /* Shadows */
  --shadow-sm: 0 2px 8px rgba(0, 0, 0, 0.3);
  --shadow-md: 0 4px 20px rgba(0, 0, 0, 0.4);
  --shadow-lg: 0 8px 40px rgba(0, 0, 0, 0.5);
  --shadow-glow: 0 0 30px rgba(99, 102, 241, 0.3);
  
  /* Borders */
  --border-color: rgba(255, 255, 255, 0.1);
  --border-radius: 12px;
  --border-radius-lg: 20px;
  
  /* Typography */
  --font-main: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --font-heading: 'Outfit', 'Inter', sans-serif;
  --font-mono: 'Fira Code', 'Consolas', monospace;
  
  /* Spacing */
  --spacing-xs: 0.5rem;
  --spacing-sm: 1rem;
  --spacing-md: 1.5rem;
  --spacing-lg: 2rem;
  --spacing-xl: 3rem;
  --spacing-2xl: 5rem;
  
  /* Transitions */
  --transition-fast: 0.15s ease;
  --transition-normal: 0.3s ease;
  --transition-slow: 0.5s ease;
}

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

html {
  scroll-behavior: smooth;
  font-size: 16px;
}

body {
  font-family: var(--font-main);
  background: var(--bg-dark);
  color: var(--text-primary);
  line-height: 1.7;
  min-height: 100vh;
  overflow-x: hidden;
}

/* Background Pattern */
body::before {
  content: '';
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: 
    radial-gradient(ellipse at 20% 20%, rgba(99, 102, 241, 0.15) 0%, transparent 50%),
    radial-gradient(ellipse at 80% 80%, rgba(139, 92, 246, 0.1) 0%, transparent 50%),
    radial-gradient(ellipse at 50% 50%, rgba(34, 211, 238, 0.05) 0%, transparent 70%);
  pointer-events: none;
  z-index: -1;
}

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

a:hover {
  color: var(--accent-light);
}

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

/* ===== Typography ===== */
h1, h2, h3, h4, h5, h6 {
  font-family: var(--font-heading);
  font-weight: 700;
  line-height: 1.2;
  margin-bottom: var(--spacing-sm);
}

h1 {
  font-size: clamp(2.5rem, 5vw, 4rem);
  background: var(--gradient-primary);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

h2 {
  font-size: clamp(2rem, 4vw, 3rem);
  color: var(--text-primary);
}

h3 {
  font-size: clamp(1.5rem, 3vw, 2rem);
  color: var(--text-primary);
}

h4 {
  font-size: 1.25rem;
  color: var(--text-secondary);
}

p {
  margin-bottom: var(--spacing-sm);
  color: var(--text-secondary);
}

.lead {
  font-size: 1.25rem;
  color: var(--text-secondary);
  max-width: 800px;
}

/* ===== Layout ===== */
.container {
  width: 100%;
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 var(--spacing-md);
}

.section {
  padding: var(--spacing-2xl) 0;
}

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

/* ===== Header & Navigation ===== */
.header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: 1000;
  background: rgba(15, 15, 26, 0.8);
  backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--border-color);
  transition: var(--transition-normal);
}

.header.scrolled {
  background: rgba(15, 15, 26, 0.95);
  box-shadow: var(--shadow-md);
}

.nav {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--spacing-sm) 0;
}

.logo {
  display: flex;
  align-items: center;
  gap: var(--spacing-xs);
  font-family: var(--font-heading);
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--text-primary);
}

.logo-icon {
  width: 40px;
  height: 40px;
  background: var(--gradient-primary);
  border-radius: 10px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.25rem;
}

.nav-menu {
  display: flex;
  align-items: center;
  gap: var(--spacing-md);
  list-style: none;
}

.nav-link {
  color: var(--text-secondary);
  font-weight: 500;
  padding: var(--spacing-xs) var(--spacing-sm);
  border-radius: 8px;
  transition: var(--transition-fast);
  position: relative;
}

.nav-link:hover,
.nav-link.active {
  color: var(--text-primary);
  background: rgba(99, 102, 241, 0.1);
}

.nav-link.active::after {
  content: '';
  position: absolute;
  bottom: -2px;
  left: 50%;
  transform: translateX(-50%);
  width: 20px;
  height: 3px;
  background: var(--gradient-primary);
  border-radius: 2px;
}

/* Mobile Menu Toggle */
.nav-toggle {
  display: none;
  flex-direction: column;
  gap: 5px;
  background: none;
  border: none;
  cursor: pointer;
  padding: var(--spacing-xs);
}

.nav-toggle span {
  width: 25px;
  height: 2px;
  background: var(--text-primary);
  border-radius: 2px;
  transition: var(--transition-fast);
}

/* ===== Hero Section ===== */
.hero {
  min-height: 100vh;
  display: flex;
  align-items: center;
  padding-top: 80px;
  position: relative;
  overflow: hidden;
}

.hero-content {
  max-width: 700px;
}

.hero-badge {
  display: inline-flex;
  align-items: center;
  gap: var(--spacing-xs);
  padding: var(--spacing-xs) var(--spacing-sm);
  background: rgba(99, 102, 241, 0.1);
  border: 1px solid rgba(99, 102, 241, 0.3);
  border-radius: 50px;
  font-size: 0.875rem;
  color: var(--primary-light);
  margin-bottom: var(--spacing-md);
}

.hero-title {
  margin-bottom: var(--spacing-md);
}

.hero-description {
  font-size: 1.25rem;
  margin-bottom: var(--spacing-lg);
}

.hero-buttons {
  display: flex;
  gap: var(--spacing-sm);
  flex-wrap: wrap;
}

.hero-visual {
  position: absolute;
  right: -10%;
  top: 50%;
  transform: translateY(-50%);
  width: 600px;
  height: 600px;
  opacity: 0.5;
}

/* Animated Integral Symbol */
.integral-animation {
  font-size: 400px;
  font-family: serif;
  background: var(--gradient-primary);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
  animation: float 6s ease-in-out infinite;
  filter: blur(1px);
}

@keyframes float {
  0%, 100% { transform: translateY(0) rotate(-5deg); }
  50% { transform: translateY(-30px) rotate(5deg); }
}

/* ===== Buttons ===== */
.btn {
  display: inline-flex;
  align-items: center;
  gap: var(--spacing-xs);
  padding: var(--spacing-sm) var(--spacing-lg);
  font-family: var(--font-main);
  font-size: 1rem;
  font-weight: 600;
  border: none;
  border-radius: var(--border-radius);
  cursor: pointer;
  transition: var(--transition-normal);
  text-decoration: none;
}

.btn-primary {
  background: var(--gradient-primary);
  color: white;
  box-shadow: var(--shadow-glow);
}

.btn-primary:hover {
  transform: translateY(-2px);
  box-shadow: 0 0 40px rgba(99, 102, 241, 0.5);
  color: white;
}

.btn-secondary {
  background: transparent;
  color: var(--text-primary);
  border: 1px solid var(--border-color);
}

.btn-secondary:hover {
  background: rgba(255, 255, 255, 0.05);
  border-color: var(--primary);
  color: var(--text-primary);
}

/* ===== Cards ===== */
.card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--border-radius-lg);
  padding: var(--spacing-lg);
  transition: var(--transition-normal);
}

.card:hover {
  background: var(--bg-card-hover);
  transform: translateY(-5px);
  box-shadow: var(--shadow-lg);
  border-color: rgba(99, 102, 241, 0.3);
}

.card-icon {
  width: 60px;
  height: 60px;
  background: var(--gradient-primary);
  border-radius: var(--border-radius);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.5rem;
  margin-bottom: var(--spacing-md);
}

.card-title {
  font-size: 1.25rem;
  margin-bottom: var(--spacing-xs);
}

.card-description {
  color: var(--text-muted);
  font-size: 0.95rem;
}

/* Card Grid */
.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: var(--spacing-md);
}

/* ===== Feature Cards ===== */
.feature-card {
  position: relative;
  overflow: hidden;
}

.feature-card::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  background: var(--gradient-primary);
  transform: scaleX(0);
  transition: var(--transition-normal);
}

.feature-card:hover::before {
  transform: scaleX(1);
}

/* ===== Math Content ===== */
.math-block {
  background: rgba(99, 102, 241, 0.05);
  border: 1px solid rgba(99, 102, 241, 0.2);
  border-radius: var(--border-radius);
  padding: var(--spacing-md);
  margin: var(--spacing-md) 0;
  font-family: var(--font-mono);
  font-size: 1.1rem;
  text-align: center;
  overflow-x: auto;
}

.math-inline {
  font-family: var(--font-mono);
  background: rgba(99, 102, 241, 0.1);
  padding: 2px 8px;
  border-radius: 4px;
  font-size: 0.95em;
}

/* Formula Cards */
.formula-card {
  background: linear-gradient(135deg, rgba(99, 102, 241, 0.1) 0%, rgba(139, 92, 246, 0.05) 100%);
  border: 1px solid rgba(99, 102, 241, 0.2);
}

.formula-card .math-block {
  background: transparent;
  border: none;
  font-size: 1.3rem;
}

/* ===== Content Sections ===== */
.content-wrapper {
  display: grid;
  grid-template-columns: 1fr 300px;
  gap: var(--spacing-xl);
  padding-top: 100px;
}

.content-main {
  min-width: 0;
}

.content-sidebar {
  position: sticky;
  top: 100px;
  height: fit-content;
}

/* Article Content */
.article-content {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--border-radius-lg);
  padding: var(--spacing-xl);
}

.article-content h2 {
  margin-top: var(--spacing-xl);
  padding-top: var(--spacing-lg);
  border-top: 1px solid var(--border-color);
}

.article-content h2:first-child {
  margin-top: 0;
  padding-top: 0;
  border-top: none;
}

.article-content ul,
.article-content ol {
  margin: var(--spacing-sm) 0;
  padding-left: var(--spacing-lg);
  color: var(--text-secondary);
}

.article-content li {
  margin-bottom: var(--spacing-xs);
}

/* Sidebar TOC */
.toc {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--border-radius);
  padding: var(--spacing-md);
}

.toc-title {
  font-size: 0.875rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--text-muted);
  margin-bottom: var(--spacing-sm);
}

.toc-list {
  list-style: none;
}

.toc-link {
  display: block;
  padding: var(--spacing-xs) 0;
  color: var(--text-secondary);
  font-size: 0.9rem;
  border-left: 2px solid transparent;
  padding-left: var(--spacing-sm);
  transition: var(--transition-fast);
}

.toc-link:hover,
.toc-link.active {
  color: var(--primary-light);
  border-left-color: var(--primary);
}

/* ===== Practice Problems ===== */
.problem-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--border-radius);
  padding: var(--spacing-lg);
  margin-bottom: var(--spacing-md);
}

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

.problem-number {
  font-weight: 600;
  color: var(--primary-light);
}

.problem-difficulty {
  font-size: 0.75rem;
  padding: 4px 12px;
  border-radius: 50px;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.difficulty-easy {
  background: rgba(34, 197, 94, 0.1);
  color: #22c55e;
}

.difficulty-medium {
  background: rgba(234, 179, 8, 0.1);
  color: #eab308;
}

.difficulty-hard {
  background: rgba(239, 68, 68, 0.1);
  color: #ef4444;
}

.solution-toggle {
  margin-top: var(--spacing-md);
}

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

.solution-content.show {
  display: block;
}

/* ===== Footer ===== */
.footer {
  background: rgba(15, 15, 26, 0.95);
  border-top: 1px solid var(--border-color);
  padding: var(--spacing-2xl) 0 var(--spacing-lg);
}

.footer-grid {
  display: grid;
  grid-template-columns: 2fr 1fr 1fr 1fr;
  gap: var(--spacing-xl);
  margin-bottom: var(--spacing-xl);
}

.footer-brand {
  max-width: 300px;
}

.footer-brand .logo {
  margin-bottom: var(--spacing-sm);
}

.footer-brand p {
  font-size: 0.9rem;
  color: var(--text-muted);
}

.footer-title {
  font-size: 0.875rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--text-primary);
  margin-bottom: var(--spacing-md);
}

.footer-links {
  list-style: none;
}

.footer-links li {
  margin-bottom: var(--spacing-xs);
}

.footer-links a {
  color: var(--text-muted);
  font-size: 0.9rem;
  transition: var(--transition-fast);
}

.footer-links a:hover {
  color: var(--primary-light);
}

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

.footer-social {
  display: flex;
  gap: var(--spacing-sm);
}

.footer-social a {
  width: 40px;
  height: 40px;
  display: flex;
  align-items: center;
  justify-content: center;
  background: rgba(255, 255, 255, 0.05);
  border-radius: 8px;
  color: var(--text-secondary);
  transition: var(--transition-fast);
}

.footer-social a:hover {
  background: var(--primary);
  color: white;
}

/* ===== Responsive Design ===== */
@media (max-width: 1024px) {
  .content-wrapper {
    grid-template-columns: 1fr;
  }
  
  .content-sidebar {
    display: none;
  }
  
  .hero-visual {
    display: none;
  }
  
  .footer-grid {
    grid-template-columns: 1fr 1fr;
  }
}

@media (max-width: 768px) {
  html {
    font-size: 14px;
  }
  
  .nav-menu {
    position: fixed;
    top: 70px;
    left: 0;
    right: 0;
    background: rgba(15, 15, 26, 0.98);
    backdrop-filter: blur(20px);
    flex-direction: column;
    padding: var(--spacing-md);
    gap: var(--spacing-xs);
    transform: translateY(-100%);
    opacity: 0;
    visibility: hidden;
    transition: var(--transition-normal);
    border-bottom: 1px solid var(--border-color);
  }
  
  .nav-menu.active {
    transform: translateY(0);
    opacity: 1;
    visibility: visible;
  }
  
  .nav-toggle {
    display: flex;
  }
  
  .nav-toggle.active span:nth-child(1) {
    transform: rotate(45deg) translate(5px, 5px);
  }
  
  .nav-toggle.active span:nth-child(2) {
    opacity: 0;
  }
  
  .nav-toggle.active span:nth-child(3) {
    transform: rotate(-45deg) translate(5px, -5px);
  }
  
  .hero-buttons {
    flex-direction: column;
  }
  
  .btn {
    width: 100%;
    justify-content: center;
  }
  
  .footer-grid {
    grid-template-columns: 1fr;
    text-align: center;
  }
  
  .footer-brand {
    max-width: none;
  }
  
  .footer-bottom {
    flex-direction: column;
    gap: var(--spacing-md);
  }
  
  .article-content {
    padding: var(--spacing-md);
  }
}

/* ===== Utility Classes ===== */
.text-center { text-align: center; }
.text-gradient {
  background: var(--gradient-primary);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.mt-1 { margin-top: var(--spacing-sm); }
.mt-2 { margin-top: var(--spacing-md); }
.mt-3 { margin-top: var(--spacing-lg); }
.mt-4 { margin-top: var(--spacing-xl); }

.mb-1 { margin-bottom: var(--spacing-sm); }
.mb-2 { margin-bottom: var(--spacing-md); }
.mb-3 { margin-bottom: var(--spacing-lg); }
.mb-4 { margin-bottom: var(--spacing-xl); }

/* ===== Animations ===== */
@keyframes fadeInUp {
  from {
    opacity: 0;
    transform: translateY(30px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.animate-fadeInUp {
  animation: fadeInUp 0.6s ease forwards;
}

.delay-1 { animation-delay: 0.1s; }
.delay-2 { animation-delay: 0.2s; }
.delay-3 { animation-delay: 0.3s; }
.delay-4 { animation-delay: 0.4s; }

/* Scroll animations - initially hidden */
[data-animate] {
  opacity: 0;
  transform: translateY(30px);
  transition: opacity 0.6s ease, transform 0.6s ease;
}

[data-animate].visible {
  opacity: 1;
  transform: translateY(0);
}