/* 
 * Academic Personal Website - Main Stylesheet
 * Author: Chenhao Zhou
 * Version: 11.0 - Refined Academic Professional Theme
 * Last Updated: 2025
 * 
 * Design Philosophy:
 * - Elegant academic credibility with refined sophistication
 * - Clean, minimal clutter with clear visual hierarchy
 * - Modern, contemporary design with classic academic touches
 * - Accessible (WCAG 2.1 AA compliant)
 * - Performant and maintainable
 */

/* ============================================
   GOOGLE FONTS - Professional Academic Fonts
   ============================================ */
@import url('https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700;800&family=Crimson+Pro:ital,wght@0,400;0,500;0,600;1,400&family=JetBrains+Mono:wght@400;500&display=swap');

/* ============================================
   CSS CUSTOM PROPERTIES (Design Tokens)
   ============================================ */
:root {
  /* Academic Color Palette - Refined & Sophisticated */
  --color-primary: #1a365d;
  --color-primary-light: #2b4c7e;
  --color-primary-weak: #ebf4ff;
  --color-primary-dark: #0f2442;
  --color-secondary: #234e70;
  --color-accent: #7c3aed;
  --color-success: #047857;
  --color-warning: #b45309;
  --color-error: #b91c1c;
  --color-info: #0369a1;
  
  /* Gold/Bronze Academic Accent */
  --color-gold: #b8860b;
  --color-gold-light: #daa520;
  
  /* Neutral Colors - Warmer Academic Tones */
  --color-text: #1e293b;
  --color-text-muted: #64748b;
  --color-text-light: #94a3b8;
  --color-bg: #f8fafc;
  --color-bg-alt: #ffffff;
  --color-surface: #ffffff;
  --color-border: #e2e8f0;
  --color-border-dark: #cbd5e1;
  
  /* Typography - Academic & Professional */
  --font-sans: 'Inter', ui-sans-serif, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  --font-serif: 'Crimson Pro', Georgia, "Times New Roman", Times, serif;
  --font-mono: 'JetBrains Mono', ui-monospace, "Courier New", monospace;
  
  --font-size-xs: 0.75rem;
  --font-size-sm: 0.875rem;
  --font-size-base: 0.9375rem;
  --font-size-md: 1rem;
  --font-size-lg: 1.125rem;
  --font-size-xl: 1.25rem;
  --font-size-2xl: 1.5rem;
  --font-size-3xl: 1.875rem;
  --font-size-4xl: 2.25rem;
  --font-size-5xl: 3rem;
  
  --line-height-tight: 1.2;
  --line-height-snug: 1.375;
  --line-height-normal: 1.5;
  --line-height-relaxed: 1.7;
  
  /* Spacing Scale */
  --space-1: 0.25rem;
  --space-2: 0.5rem;
  --space-3: 0.75rem;
  --space-4: 1rem;
  --space-5: 1.25rem;
  --space-6: 1.5rem;
  --space-8: 2rem;
  --space-10: 2.5rem;
  --space-12: 3rem;
  --space-16: 4rem;
  
  /* Layout */
  --container-max-width: 960px;
  --radius-sm: 0.375rem;
  --radius-md: 0.5rem;
  --radius-lg: 0.75rem;
  --radius-xl: 1rem;
  --radius-2xl: 1.25rem;
  --radius-full: 9999px;
  
  /* Refined Shadows */
  --shadow-xs: 0 1px 2px rgba(0, 0, 0, 0.04);
  --shadow-sm: 0 2px 4px rgba(0, 0, 0, 0.06);
  --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.08);
  --shadow-lg: 0 8px 24px rgba(0, 0, 0, 0.1);
  --shadow-xl: 0 16px 48px rgba(0, 0, 0, 0.12);
  --shadow-card: 0 4px 20px rgba(26, 54, 93, 0.08);
  --shadow-card-hover: 0 12px 40px rgba(26, 54, 93, 0.12);
  
  /* Transitions */
  --transition-fast: 150ms cubic-bezier(0.4, 0, 0.2, 1);
  --transition-base: 250ms cubic-bezier(0.4, 0, 0.2, 1);
  --transition-slow: 350ms cubic-bezier(0.4, 0, 0.2, 1);
}

/* ============================================
   CSS RESET & BASE STYLES
   ============================================ */
*, *::before, *::after { box-sizing: border-box; }

html {
  height: 100%;
  scroll-behavior: smooth;
  font-size: 16px;
}

body {
  margin: 0;
  padding: 0;
  min-height: 100%;
  font-family: var(--font-sans);
  font-size: var(--font-size-base);
  font-weight: 400;
  line-height: var(--line-height-relaxed);
  color: var(--color-text);
  background: 
    linear-gradient(135deg, rgba(26, 54, 93, 0.02) 0%, transparent 50%),
    linear-gradient(225deg, rgba(124, 58, 237, 0.015) 0%, transparent 50%),
    var(--color-bg);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-rendering: optimizeLegibility;
}

/* ============================================
   ACCESSIBILITY - Skip Link
   ============================================ */
.skip-to-main {
  position: absolute;
  top: -60px;
  left: var(--space-4);
  background: var(--color-primary);
  color: white;
  padding: var(--space-3) var(--space-6);
  text-decoration: none;
  border-radius: 0 0 var(--radius-lg) var(--radius-lg);
  z-index: 1000;
  font-weight: 600;
  font-size: var(--font-size-sm);
  transition: top var(--transition-fast);
}
.skip-to-main:focus {
  top: 0;
  outline: 3px solid var(--color-accent);
  outline-offset: 2px;
}

/* ============================================
   TYPOGRAPHY - Academic & Professional
   ============================================ */
h1, h2, h3, h4, h5, h6 {
  margin-top: 0;
  margin-bottom: var(--space-4);
  font-family: var(--font-sans);
  font-weight: 700;
  line-height: var(--line-height-tight);
  color: var(--color-primary-dark);
  letter-spacing: -0.025em;
}

h1 {
  font-size: clamp(var(--font-size-2xl), 4vw, var(--font-size-5xl));
  font-weight: 800;
  background: linear-gradient(135deg, var(--color-primary-dark) 0%, var(--color-primary) 100%);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

h2 {
  font-size: clamp(var(--font-size-xl), 2.5vw, var(--font-size-3xl));
  color: var(--color-primary);
  display: flex;
  align-items: center;
  gap: var(--space-3);
}

h2::before {
  content: '';
  display: inline-block;
  width: 4px;
  height: 1.2em;
  background: linear-gradient(180deg, var(--color-primary) 0%, var(--color-accent) 100%);
  border-radius: var(--radius-full);
}

h3 {
  font-size: clamp(var(--font-size-lg), 2vw, var(--font-size-2xl));
  color: var(--color-primary-dark);
}

p {
  margin: 0 0 var(--space-4);
}

/* Academic emphasis for important text */
.lead {
  font-size: var(--font-size-lg);
  color: var(--color-text-muted);
  line-height: var(--line-height-relaxed);
}

/* ============================================
   LINKS
   ============================================ */
a {
  color: var(--color-primary);
  text-decoration: none;
  transition: all var(--transition-fast);
  position: relative;
}

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

/* Underline animation for content links */
.content a:not(.btn):not(.nav-link) {
  text-decoration: underline;
  text-decoration-color: rgba(26, 54, 93, 0.3);
  text-underline-offset: 3px;
  transition: text-decoration-color var(--transition-fast);
}

.content a:not(.btn):not(.nav-link):hover {
  text-decoration-color: var(--color-accent);
}

a:focus-visible {
  outline: 2px solid var(--color-accent);
  outline-offset: 3px;
  border-radius: var(--radius-sm);
}

/* ============================================
   LAYOUT - Container
   ============================================ */
.container {
  max-width: var(--container-max-width);
  margin: 0 auto;
  padding: var(--space-6) var(--space-5) var(--space-16);
}

section {
  margin-top: var(--space-10);
}

/* ============================================
   HEADER & NAVIGATION - Refined Academic Style
   ============================================ */
header {
  position: sticky;
  top: 0;
  z-index: 100;
  backdrop-filter: saturate(180%) blur(20px);
  -webkit-backdrop-filter: saturate(180%) blur(20px);
  border-bottom: 1px solid rgba(226, 232, 240, 0.8);
  background: rgba(255, 255, 255, 0.85);
}

nav {
  max-width: var(--container-max-width);
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-4) var(--space-5);
  gap: var(--space-4);
  flex-wrap: wrap;
}

.nav-brand {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  text-decoration: none;
  transition: transform var(--transition-fast);
}

.nav-brand:hover {
  transform: translateY(-1px);
}

.nav-brand img {
  width: 42px;
  height: 42px;
  border-radius: var(--radius-full);
  border: 2px solid var(--color-primary-weak);
  object-fit: cover;
  transition: all var(--transition-base);
  box-shadow: var(--shadow-sm);
}

.nav-brand:hover img {
  border-color: var(--color-primary-light);
  box-shadow: var(--shadow-md);
}

.nav-brand-text {
  font-weight: 800;
  font-size: var(--font-size-md);
  color: var(--color-primary-dark);
  letter-spacing: -0.02em;
}

.nav-menu {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-1);
  list-style: none;
  margin: 0;
  padding: 0;
}

.nav-menu a {
  display: inline-flex;
  align-items: center;
  padding: var(--space-2) var(--space-4);
  font-weight: 600;
  font-size: var(--font-size-sm);
  color: var(--color-text-muted);
  border-radius: var(--radius-lg);
  transition: all var(--transition-fast);
  text-decoration: none;
}

.nav-menu a:hover {
  color: var(--color-primary);
  background: var(--color-primary-weak);
}

.nav-menu a.active {
  color: var(--color-primary);
  background: var(--color-primary-weak);
  font-weight: 700;
}

/* ============================================
   CARDS - Elegant Academic Style
   ============================================ */
.card {
  background: var(--color-surface);
  border-radius: var(--radius-2xl);
  box-shadow: var(--shadow-card);
  border: 1px solid var(--color-border);
  padding: var(--space-6) var(--space-8);
  margin-bottom: var(--space-6);
  transition: all var(--transition-base);
  position: relative;
  overflow: hidden;
}

.card::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  background: linear-gradient(90deg, var(--color-primary) 0%, var(--color-accent) 100%);
  opacity: 0;
  transition: opacity var(--transition-base);
}

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

.card:hover::before {
  opacity: 1;
}

/* Card variants */
.card-highlight {
  border-left: 4px solid var(--color-primary);
  padding-left: var(--space-6);
}

.card-success {
  border-left: 4px solid var(--color-success);
  background: linear-gradient(90deg, rgba(4, 120, 87, 0.04) 0%, transparent 100%);
}

/* ============================================
   BADGES - Professional & Academic
   ============================================ */
.badge {
  display: inline-flex;
  align-items: center;
  gap: var(--space-1);
  padding: var(--space-1) var(--space-3);
  font-size: var(--font-size-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  border-radius: var(--radius-full);
  background: var(--color-primary);
  color: white;
}

.badge-secondary {
  background: var(--color-secondary);
}

.badge-success {
  background: var(--color-success);
}

.badge-warning {
  background: var(--color-warning);
}

.badge-accent {
  background: var(--color-accent);
}

.badge-outline {
  background: transparent;
  border: 1.5px solid var(--color-primary);
  color: var(--color-primary);
}

/* Publication status badges */
.badge-r1 {
  background: linear-gradient(135deg, #0891b2 0%, #06b6d4 100%);
}

.badge-r2 {
  background: linear-gradient(135deg, #047857 0%, #059669 100%);
}

.badge-r3 {
  background: linear-gradient(135deg, #7c3aed 0%, #8b5cf6 100%);
}

/* ============================================
   BUTTONS - Academic Professional Style
   ============================================ */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  padding: var(--space-3) var(--space-5);
  font-family: var(--font-sans);
  font-size: var(--font-size-sm);
  font-weight: 600;
  border: 2px solid var(--color-primary);
  border-radius: var(--radius-lg);
  background: transparent;
  color: var(--color-primary);
  cursor: pointer;
  transition: all var(--transition-base);
  text-decoration: none;
  white-space: nowrap;
}

.btn:hover {
  background: var(--color-primary);
  color: white;
  box-shadow: 0 8px 24px rgba(26, 54, 93, 0.25);
  transform: translateY(-2px);
  text-decoration: none;
}

.btn:active {
  transform: translateY(0);
}

.btn:focus-visible {
  outline: 2px solid var(--color-accent);
  outline-offset: 2px;
}

.btn-primary {
  background: linear-gradient(135deg, var(--color-primary) 0%, var(--color-primary-light) 100%);
  border-color: transparent;
  color: white;
}

.btn-primary:hover {
  background: linear-gradient(135deg, var(--color-primary-light) 0%, var(--color-primary) 100%);
  box-shadow: 0 12px 32px rgba(26, 54, 93, 0.3);
}

.btn-sm {
  padding: var(--space-2) var(--space-4);
  font-size: var(--font-size-xs);
}

.btn-lg {
  padding: var(--space-4) var(--space-8);
  font-size: var(--font-size-md);
}

/* ============================================
   TABLES - Clean Academic Style
   ============================================ */
table {
  width: 100%;
  border-collapse: collapse;
  margin: var(--space-6) 0;
  font-size: var(--font-size-sm);
}

th, td {
  padding: var(--space-4) var(--space-5);
  border: 1px solid var(--color-border);
  text-align: left;
  vertical-align: top;
}

th {
  background: linear-gradient(180deg, #f8fafc 0%, #f1f5f9 100%);
  font-weight: 700;
  color: var(--color-primary-dark);
  text-transform: uppercase;
  font-size: var(--font-size-xs);
  letter-spacing: 0.05em;
}

tbody tr {
  transition: background var(--transition-fast);
}

tbody tr:hover {
  background: var(--color-primary-weak);
}

/* ============================================
   LISTS - Academic Style
   ============================================ */
ul, ol {
  margin: 0 0 var(--space-4);
  padding-left: var(--space-6);
}

li {
  margin-bottom: var(--space-2);
  line-height: var(--line-height-relaxed);
}

ul.clean {
  list-style: none;
  padding-left: 0;
}

ul.clean li {
  position: relative;
  padding-left: var(--space-6);
}

ul.clean li::before {
  content: '';
  position: absolute;
  left: 0;
  top: 0.6em;
  width: 6px;
  height: 6px;
  background: var(--color-primary);
  border-radius: var(--radius-full);
}

/* ============================================
   INFO GRID - Academic Profile Layout
   ============================================ */
.info-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
  gap: var(--space-4);
  margin: var(--space-6) 0;
}

.info-item {
  padding: var(--space-4) var(--space-5);
  border-radius: var(--radius-lg);
  background: linear-gradient(135deg, var(--color-primary-weak) 0%, rgba(235, 244, 255, 0.5) 100%);
  border: 1px solid rgba(26, 54, 93, 0.08);
  transition: all var(--transition-fast);
}

.info-item:hover {
  transform: translateY(-2px);
  box-shadow: var(--shadow-sm);
}

.info-item .label {
  font-size: var(--font-size-xs);
  font-weight: 700;
  color: var(--color-primary);
  text-transform: uppercase;
  letter-spacing: 0.1em;
  margin-bottom: var(--space-1);
}

.info-item .value {
  font-weight: 600;
  color: var(--color-primary-dark);
  font-size: var(--font-size-base);
}

/* ============================================
   PUBLICATION ITEMS - Academic Research Style
   ============================================ */
.pub-item {
  margin-bottom: var(--space-5);
  padding-bottom: var(--space-5);
  border-bottom: 1px solid var(--color-border);
}

.pub-item:last-child {
  border-bottom: none;
  margin-bottom: 0;
  padding-bottom: 0;
}

.pub-title {
  font-weight: 600;
  color: var(--color-primary-dark);
  font-size: var(--font-size-md);
  margin-bottom: var(--space-2);
  line-height: var(--line-height-snug);
}

.pub-meta {
  font-size: var(--font-size-sm);
  color: var(--color-text-muted);
  line-height: var(--line-height-normal);
}

.pub-meta em {
  color: var(--color-text);
  font-style: italic;
}

/* ============================================
   SKILLS & TAGS
   ============================================ */
.skills-grid {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-2);
  margin: var(--space-4) 0;
}

.skill-tag {
  display: inline-flex;
  align-items: center;
  padding: var(--space-2) var(--space-4);
  background: var(--color-primary-weak);
  color: var(--color-primary);
  border-radius: var(--radius-lg);
  font-size: var(--font-size-sm);
  font-weight: 500;
  transition: all var(--transition-fast);
  border: 1px solid transparent;
}

.skill-tag:hover {
  background: var(--color-primary);
  color: white;
  transform: translateY(-1px);
}

/* ============================================
   HERO SECTION - Academic Profile
   ============================================ */
.hero {
  display: grid;
  grid-template-columns: 1fr 240px;
  gap: var(--space-8);
  align-items: start;
  margin-top: var(--space-6);
}

@media (max-width: 768px) {
  .hero {
    grid-template-columns: 1fr;
  }
}

.profile-card {
  text-align: center;
  padding: var(--space-8);
}

.profile-card img {
  width: 180px;
  height: 180px;
  border-radius: var(--radius-full);
  object-fit: cover;
  border: 4px solid var(--color-primary-weak);
  margin-bottom: var(--space-5);
  box-shadow: var(--shadow-lg);
  transition: all var(--transition-base);
}

.profile-card:hover img {
  border-color: var(--color-primary-light);
  transform: scale(1.02);
}

.contact-info {
  font-size: var(--font-size-sm);
  color: var(--color-text-muted);
  line-height: 1.8;
}

.contact-info a {
  color: var(--color-primary);
  font-weight: 500;
}

/* ============================================
   FOOTER - Refined Academic Style
   ============================================ */
footer {
  margin-top: var(--space-16);
  padding: var(--space-8) 0;
  text-align: center;
  color: var(--color-text-muted);
  font-size: var(--font-size-sm);
  border-top: 1px solid var(--color-border);
  background: linear-gradient(180deg, transparent 0%, rgba(248, 250, 252, 0.8) 100%);
}

footer a {
  color: var(--color-text-muted);
  text-decoration: none;
  transition: color var(--transition-fast);
  margin: 0 var(--space-2);
}

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

/* ============================================
   BACK TO TOP BUTTON
   ============================================ */
.back-to-top {
  position: fixed;
  bottom: var(--space-8);
  right: var(--space-8);
  width: 52px;
  height: 52px;
  background: linear-gradient(135deg, var(--color-primary) 0%, var(--color-primary-light) 100%);
  color: white;
  border: none;
  border-radius: var(--radius-full);
  box-shadow: var(--shadow-lg);
  cursor: pointer;
  opacity: 0;
  visibility: hidden;
  transition: all var(--transition-base);
  z-index: 50;
  display: flex;
  align-items: center;
  justify-content: center;
}

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

.back-to-top:hover {
  background: linear-gradient(135deg, var(--color-primary-light) 0%, var(--color-accent) 100%);
  transform: translateY(-4px);
  box-shadow: var(--shadow-xl);
}

.back-to-top:focus-visible {
  outline: 2px solid var(--color-accent);
  outline-offset: 2px;
}

/* ============================================
   UTILITY CLASSES
   ============================================ */
.text-center { text-align: center; }
.text-muted { color: var(--color-text-muted); }
.text-primary { color: var(--color-primary); }
.text-sm { font-size: var(--font-size-sm); }
.text-xs { font-size: var(--font-size-xs); }

.mt-4 { margin-top: var(--space-4); }
.mt-6 { margin-top: var(--space-6); }
.mt-8 { margin-top: var(--space-8); }
.mb-4 { margin-bottom: var(--space-4); }
.mb-6 { margin-bottom: var(--space-6); }

.font-semibold { font-weight: 600; }
.font-bold { font-weight: 700; }

/* ============================================
   NOTE & CALLOUT BOXES
   ============================================ */
.note {
  font-size: var(--font-size-sm);
  color: var(--color-text-muted);
}

.callout {
  padding: var(--space-4) var(--space-5);
  border-radius: var(--radius-lg);
  margin: var(--space-4) 0;
}

.callout-success {
  background: linear-gradient(90deg, rgba(4, 120, 87, 0.08) 0%, rgba(4, 120, 87, 0.02) 100%);
  border-left: 4px solid var(--color-success);
}

.callout-info {
  background: linear-gradient(90deg, rgba(3, 105, 161, 0.08) 0%, rgba(3, 105, 161, 0.02) 100%);
  border-left: 4px solid var(--color-info);
}

/* ============================================
   RESPONSIVE DESIGN
   ============================================ */
@media (max-width: 860px) {
  nav {
    padding: var(--space-3) var(--space-4);
  }
  
  .nav-menu {
    gap: var(--space-1);
  }
  
  .nav-menu a {
    padding: var(--space-2) var(--space-3);
  }
  
  .card {
    padding: var(--space-5) var(--space-5);
  }
  
  th, td {
    padding: var(--space-3);
    font-size: var(--font-size-xs);
  }
}

@media (max-width: 640px) {
  .container {
    padding: var(--space-4) var(--space-4) var(--space-12);
  }
  
  h2::before {
    display: none;
  }
  
  .hero {
    gap: var(--space-6);
  }
  
  .profile-card img {
    width: 140px;
    height: 140px;
  }
  
  .back-to-top {
    bottom: var(--space-4);
    right: var(--space-4);
    width: 44px;
    height: 44px;
  }
}

/* ============================================
   PRINT STYLES
   ============================================ */
@media print {
  header, footer, .back-to-top {
    display: none;
  }
  
  body {
    background: white;
    color: black;
  }
  
  .card {
    box-shadow: none;
    border: 1px solid #ddd;
    page-break-inside: avoid;
  }
  
  a {
    color: black;
    text-decoration: underline;
  }
  
  h1, h2, h3 {
    color: black;
    background: none;
    -webkit-text-fill-color: black;
  }
}

/* ============================================
   ANIMATION KEYFRAMES
   ============================================ */
@keyframes fadeInUp {
  from {
    opacity: 0;
    transform: translateY(20px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

@keyframes pulse {
  0%, 100% {
    opacity: 1;
  }
  50% {
    opacity: 0.6;
  }
}

.animate-fade-in {
  animation: fadeInUp 0.5s ease-out forwards;
}