/* ==========================================================================
   Design System - CSS Variables
   ========================================================================== */

:root {
  /* Light mode (default) */
  --bg-primary: #ffffff;
  --bg-secondary: #f8fafc;
  --bg-tertiary: #f1f5f9;
  --text-primary: #1e293b;
  --text-secondary: #64748b;
  --text-muted: #94a3b8;
  /* Teal (active) */
  --accent: #0d9488;
  --accent-hover: #0f766e;
  --accent-light: rgba(13, 148, 136, 0.1);
  /* Orange option */
  /* --accent: #ea580c; */
  /* --accent-hover: #c2410c; */
  /* --accent-light: rgba(234, 88, 12, 0.1); */
  /* Sky Blue option */
  /* --accent: #0ea5e9; */
  /* --accent-hover: #0284c7; */
  /* --accent-light: rgba(14, 165, 233, 0.1); */
  --border: #e2e8f0;
  --shadow: rgba(0, 0, 0, 0.05);
  --shadow-lg: rgba(0, 0, 0, 0.1);

  /* Spacing */
  --space-xs: 0.25rem;
  --space-sm: 0.5rem;
  --space-md: 1rem;
  --space-lg: 1.5rem;
  --space-xl: 2rem;
  --space-2xl: 3rem;
  --space-3xl: 4rem;

  /* Typography */
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  --font-mono: 'JetBrains Mono', 'Fira Code', monospace;

  /* Sizing */
  --max-width: 900px;
  --border-radius: 8px;
  --border-radius-lg: 12px;

  /* Transitions */
  --transition-fast: 150ms ease;
  --transition-base: 200ms ease;
  --transition-slow: 300ms ease;
}

[data-theme="dark"] {
  --bg-primary: #0f172a;
  --bg-secondary: #1e293b;
  --bg-tertiary: #334155;
  --text-primary: #f1f5f9;
  --text-secondary: #94a3b8;
  --text-muted: #64748b;
  /* Teal (active) */
  --accent: #2dd4bf;
  --accent-hover: #5eead4;
  --accent-light: rgba(45, 212, 191, 0.1);
  /* Orange option */
  /* --accent: #fb923c; */
  /* --accent-hover: #fdba74; */
  /* --accent-light: rgba(251, 146, 60, 0.1); */
  /* Sky Blue option */
  /* --accent: #38bdf8; */
  /* --accent-hover: #7dd3fc; */
  /* --accent-light: rgba(56, 189, 248, 0.1); */
  --border: #334155;
  --shadow: rgba(0, 0, 0, 0.3);
  --shadow-lg: rgba(0, 0, 0, 0.5);
}

/* ==========================================================================
   Base Styles
   ========================================================================== */

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

html {
  scroll-behavior: smooth;
}

body {
  font-family: var(--font-sans);
  font-size: 16px;
  line-height: 1.7;
  color: var(--text-primary);
  background-color: var(--bg-primary);
  transition: background-color var(--transition-slow), color var(--transition-slow);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

/* ==========================================================================
   Typography
   ========================================================================== */

h1, h2, h3, h4, h5, h6 {
  font-weight: 700;
  line-height: 1.3;
  color: var(--text-primary);
}

h1 { font-size: 2.5rem; }
h2 { font-size: 1.75rem; }
h3 { font-size: 1.25rem; }
h4 { font-size: 1.1rem; }

p {
  margin-bottom: var(--space-md);
}

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

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

/* ==========================================================================
   Layout
   ========================================================================== */

.container {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: var(--space-lg);
}

@media (min-width: 768px) {
  .container {
    padding: var(--space-2xl);
  }
}

section {
  margin-bottom: var(--space-3xl);
}

/* ==========================================================================
   Header & Navigation
   ========================================================================== */

.header {
  position: sticky;
  top: 0;
  z-index: 100;
  background-color: var(--bg-primary);
  border-bottom: 1px solid var(--border);
  transition: background-color var(--transition-slow);
}

.header-inner {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: var(--space-md) var(--space-lg);
  display: flex;
  justify-content: space-between;
  align-items: center;
}

@media (min-width: 768px) {
  .header-inner {
    padding: var(--space-md) var(--space-2xl);
  }
}

.logo {
  font-size: 1.25rem;
  font-weight: 700;
  color: var(--text-primary);
}

.logo:hover {
  color: var(--accent);
}

.nav-links {
  display: flex;
  align-items: center;
  gap: var(--space-lg);
}

.nav-links a {
  color: var(--text-secondary);
  font-size: 0.9rem;
  font-weight: 500;
  transition: color var(--transition-fast);
}

.nav-links a:hover {
  color: var(--accent);
}

/* Theme Toggle */
.theme-toggle {
  background: none;
  border: none;
  cursor: pointer;
  padding: var(--space-sm);
  border-radius: var(--border-radius);
  color: var(--text-secondary);
  transition: background-color var(--transition-fast), color var(--transition-fast);
  display: flex;
  align-items: center;
  justify-content: center;
}

.theme-toggle:hover {
  background-color: var(--bg-secondary);
  color: var(--accent);
}

.theme-toggle svg {
  width: 20px;
  height: 20px;
}

.theme-toggle .sun-icon { display: none; }
.theme-toggle .moon-icon { display: block; }

[data-theme="dark"] .theme-toggle .sun-icon { display: block; }
[data-theme="dark"] .theme-toggle .moon-icon { display: none; }

/* ==========================================================================
   Hero Section
   ========================================================================== */

.hero {
  display: grid;
  grid-template-columns: 1fr;
  gap: var(--space-2xl);
  align-items: center;
  padding: var(--space-2xl) 0 var(--space-lg) 0;
  margin-bottom: var(--space-lg);
}

@media (min-width: 768px) {
  .hero {
    grid-template-columns: 280px 1fr;
    gap: var(--space-3xl);
  }
}

.hero-image {
  width: 100%;
  max-width: 280px;
  margin: 0 auto;
}

.hero-image img {
  width: 100%;
  height: auto;
  border-radius: var(--border-radius-lg);
  box-shadow: 0 4px 20px var(--shadow-lg);
}

.hero-content h1 {
  margin-bottom: var(--space-sm);
}

.hero-title {
  color: var(--text-secondary);
  font-size: 1.15rem;
  font-weight: 400;
  margin-bottom: var(--space-lg);
}

.hero-title a {
  font-weight: 500;
}

/* Research Tags */
.research-tags {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-sm);
  margin-bottom: var(--space-lg);
}

.tag {
  display: inline-block;
  padding: var(--space-xs) var(--space-md);
  background-color: var(--accent-light);
  color: var(--accent);
  border-radius: 20px;
  font-size: 0.85rem;
  font-weight: 500;
  transition: background-color var(--transition-fast);
}

/* Social Links */
.social-links {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-md);
}

.social-link {
  display: inline-flex;
  align-items: center;
  gap: var(--space-sm);
  padding: var(--space-sm) var(--space-md);
  background-color: var(--bg-secondary);
  border-radius: var(--border-radius);
  color: var(--text-secondary);
  font-size: 0.9rem;
  font-weight: 500;
  transition: all var(--transition-fast);
}

.social-link:hover {
  background-color: var(--accent-light);
  color: var(--accent);
}

.social-link svg {
  width: 18px;
  height: 18px;
}

/* ==========================================================================
   About Section
   ========================================================================== */

.about-content {
  font-size: 1.05rem;
  color: var(--text-secondary);
  line-height: 1.8;
}

.about-content p {
  margin-bottom: var(--space-lg);
}

.about-content a {
  font-weight: 500;
}

/* ==========================================================================
   Section Titles
   ========================================================================== */

.section-title {
  font-size: 1.5rem;
  margin-bottom: var(--space-xl);
  padding-bottom: var(--space-md);
  border-bottom: 2px solid var(--border);
  display: flex;
  align-items: center;
  gap: var(--space-md);
}

.section-title-icon {
  color: var(--accent);
}

/* ==========================================================================
   Papers Section
   ========================================================================== */

.paper-card {
  display: grid;
  grid-template-columns: 1fr;
  gap: var(--space-lg);
  padding: var(--space-lg);
  background-color: var(--bg-secondary);
  border-radius: var(--border-radius-lg);
  margin-bottom: var(--space-lg);
  transition: transform var(--transition-fast), box-shadow var(--transition-fast);
  border: 1px solid var(--border);
}

@media (min-width: 640px) {
  .paper-card {
    grid-template-columns: 180px 1fr;
    gap: var(--space-xl);
    align-items: center;
  }
}

.paper-card:hover {
  transform: translateY(-2px);
  box-shadow: 0 8px 24px var(--shadow);
}

.paper-image {
  width: 100%;
  border-radius: var(--border-radius);
  background-color: var(--bg-tertiary);
  display: flex;
  align-items: center;
  justify-content: center;
}

.paper-image img,
.paper-image video {
  width: 100%;
  height: auto;
  object-fit: contain;
  border-radius: var(--border-radius);
  transition: transform var(--transition-slow);
}

.paper-card:hover .paper-image img,
.paper-card:hover .paper-image video {
  transform: scale(1.05);
}

.paper-content {
  display: flex;
  flex-direction: column;
  justify-content: center;
}

.paper-title {
  font-size: 1.05rem;
  font-weight: 600;
  margin-bottom: var(--space-sm);
  color: var(--text-primary);
  line-height: 1.4;
}

.paper-authors {
  font-size: 0.9rem;
  color: var(--text-secondary);
  margin-bottom: var(--space-sm);
}

.paper-authors .highlight {
  color: var(--accent);
  font-weight: 600;
}

.paper-venue {
  font-size: 0.85rem;
  color: var(--text-secondary);
  font-weight: 500;
  margin-bottom: var(--space-sm);
}

.paper-links {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-sm);
}

.paper-link {
  display: inline-flex;
  align-items: center;
  gap: var(--space-xs);
  padding: var(--space-xs) var(--space-sm);
  background-color: var(--bg-tertiary);
  border-radius: var(--border-radius);
  font-size: 0.8rem;
  color: var(--text-secondary);
  transition: all var(--transition-fast);
}

.paper-link:hover {
  background-color: var(--accent-light);
  color: var(--accent);
}

.see-all-link {
  display: inline-flex;
  align-items: center;
  gap: var(--space-sm);
  margin-top: var(--space-lg);
  font-weight: 500;
}

.see-all-link svg {
  width: 16px;
  height: 16px;
  transition: transform var(--transition-fast);
}

.see-all-link:hover svg {
  transform: translateX(4px);
}

/* ==========================================================================
   Mentoring Section
   ========================================================================== */

.mentoring-intro {
  color: var(--text-secondary);
  margin-bottom: var(--space-xl);
  font-size: 1.05rem;
}

.mentoring-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: var(--space-xl);
}

@media (min-width: 768px) {
  .mentoring-grid {
    grid-template-columns: repeat(2, 1fr);
  }
}

.mentoring-column {
  background-color: var(--bg-secondary);
  border-radius: var(--border-radius-lg);
  padding: var(--space-lg);
  border: 1px solid var(--border);
}

.mentoring-column h3 {
  font-size: 1rem;
  color: var(--accent);
  margin-bottom: var(--space-md);
  padding-bottom: var(--space-sm);
  border-bottom: 1px solid var(--border);
}

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

.mentoring-list li {
  padding: var(--space-sm) 0;
  font-size: 0.95rem;
  color: var(--text-secondary);
  border-bottom: 1px solid var(--border);
  white-space: nowrap;
}

.mentoring-list li:last-child {
  border-bottom: none;
}

.mentoring-list a {
  font-weight: 500;
}

.role-arrow {
  color: var(--text-muted);
  margin: 0 var(--space-xs);
}

/* ==========================================================================
   Teaching Section
   ========================================================================== */

.course-card {
  padding: var(--space-lg);
  background-color: var(--bg-secondary);
  border-radius: var(--border-radius-lg);
  margin-bottom: var(--space-md);
  border: 1px solid var(--border);
}

.course-card h3 {
  margin-bottom: var(--space-sm);
}

.course-card h3 a {
  color: var(--text-primary);
  font-weight: 600;
}

.course-card h3 a:hover {
  color: var(--accent);
}

.course-role {
  display: inline-block;
  padding: var(--space-xs) var(--space-sm);
  background-color: var(--accent-light);
  color: var(--accent);
  border-radius: var(--border-radius);
  font-size: 0.8rem;
  font-weight: 500;
  margin-left: var(--space-sm);
}

.course-description {
  color: var(--text-secondary);
  font-size: 0.95rem;
  margin: 0;
}

/* ==========================================================================
   Footer
   ========================================================================== */

.footer {
  padding: var(--space-2xl) 0;
  border-top: 1px solid var(--border);
  text-align: center;
}

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

.footer-links {
  display: flex;
  justify-content: center;
  gap: var(--space-lg);
  margin-bottom: var(--space-md);
}

.footer-links a {
  color: var(--text-secondary);
}

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

/* ==========================================================================
   Animations
   ========================================================================== */

.fade-in {
  opacity: 0;
  transform: translateY(20px);
  transition: opacity 0.6s ease, transform 0.6s ease;
}

.fade-in.visible {
  opacity: 1;
  transform: translateY(0);
}

/* Reduced motion */
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }

  html {
    scroll-behavior: auto;
  }

  .fade-in {
    opacity: 1;
    transform: none;
  }
}

/* ==========================================================================
   Utilities
   ========================================================================== */

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}