/* ==========================================================================
   Ji Feng Research Group - Pure HTML/CSS Template
   Minimalistic academic website styling
   ========================================================================== */

/* CSS Variables */
:root {
  --color-bg: #0a0a0a;
  --color-bg-light: #111111;
  --color-bg-lighter: #1a1a1a;
  --color-text: #ffffff;
  --color-text-muted: #888888;
  --color-text-dim: #555555;
  --color-accent: #cccccc;
  --color-border: #333333;
  --font-heading: 'Georgia', 'Times New Roman', serif;
  --font-body: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  --max-width: 1200px;
  --spacing-xs: 0.5rem;
  --spacing-sm: 1rem;
  --spacing-md: 2rem;
  --spacing-lg: 4rem;
  --spacing-xl: 6rem;
}

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

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

body {
  font-family: var(--font-body);
  background-color: var(--color-bg);
  color: var(--color-text);
  line-height: 1.6;
  -webkit-font-smoothing: antialiased;
}

a {
  color: var(--color-text);
  text-decoration: none;
  transition: opacity 0.2s ease;
}

a:hover {
  opacity: 0.7;
}

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

/* Typography */
h1, h2, h3, h4, h5, h6 {
  font-family: var(--font-heading);
  font-weight: 400;
  line-height: 1.2;
  margin-bottom: var(--spacing-sm);
}

h1 { font-size: 3rem; }
h2 { font-size: 2.25rem; }
h3 { font-size: 1.5rem; }
h4 { font-size: 1.25rem; }

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

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

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

.section-alt {
  background-color: var(--color-bg-light);
}

/* Header & Navigation */
.header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: 100;
  background-color: rgba(10, 10, 10, 0.95);
  backdrop-filter: blur(10px);
  border-bottom: 1px solid var(--color-border);
}

.header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 70px;
}

.logo {
  font-family: var(--font-heading);
  font-size: 1.25rem;
  letter-spacing: 0.05em;
}

.nav {
  display: flex;
  gap: var(--spacing-md);
}

.nav a {
  font-size: 0.875rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--color-text-muted);
}

.nav a:hover,
.nav a.active {
  color: var(--color-text);
}

/* Mobile Navigation */
.nav-toggle {
  display: none;
  background: none;
  border: none;
  color: var(--color-text);
  font-size: 1.5rem;
  cursor: pointer;
}

@media (max-width: 768px) {
  .nav-toggle {
    display: block;
  }
  
  .nav {
    display: none;
    position: absolute;
    top: 70px;
    left: 0;
    right: 0;
    flex-direction: column;
    background-color: var(--color-bg);
    padding: var(--spacing-md);
    border-bottom: 1px solid var(--color-border);
  }
  
  .nav.active {
    display: flex;
  }
}

/* Hero Section */
.hero {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  text-align: center;
  padding-top: 70px;
  background: linear-gradient(180deg, var(--color-bg-light) 0%, var(--color-bg) 100%);
}

.hero-content {
  max-width: 800px;
  padding: var(--spacing-lg);
}

.hero-label {
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.3em;
  color: var(--color-text-dim);
  margin-bottom: var(--spacing-md);
}

.hero-title {
  font-size: clamp(2.5rem, 8vw, 5rem);
  margin-bottom: var(--spacing-md);
  line-height: 1.1;
}

.hero-subtitle {
  font-size: 1.125rem;
  color: var(--color-text-muted);
  margin-bottom: var(--spacing-lg);
  max-width: 600px;
  margin-left: auto;
  margin-right: auto;
}

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

/* Buttons */
.btn {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  padding: 0.875rem 1.75rem;
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.15em;
  border: 1px solid var(--color-border);
  background: transparent;
  color: var(--color-text);
  cursor: pointer;
  transition: all 0.2s ease;
}

.btn:hover {
  background: var(--color-text);
  color: var(--color-bg);
  opacity: 1;
}

.btn-primary {
  background: var(--color-text);
  color: var(--color-bg);
  border-color: var(--color-text);
}

.btn-primary:hover {
  background: var(--color-text-muted);
  border-color: var(--color-text-muted);
}

/* Section Headers */
.section-header {
  margin-bottom: var(--spacing-lg);
}

.section-label {
  font-size: 0.6875rem;
  text-transform: uppercase;
  letter-spacing: 0.2em;
  color: var(--color-text-dim);
  margin-bottom: var(--spacing-xs);
}

.section-title {
  font-size: clamp(2rem, 5vw, 3rem);
}

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

.card {
  padding: var(--spacing-md);
  border: 1px solid var(--color-border);
  background: var(--color-bg);
  transition: border-color 0.2s ease;
}

.card:hover {
  border-color: var(--color-text-muted);
}

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

.card-text {
  font-size: 0.9375rem;
  color: var(--color-text-muted);
  margin-bottom: var(--spacing-sm);
}

.card-meta {
  font-size: 0.6875rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--color-text-dim);
}

/* Stats */
.stats-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
  gap: var(--spacing-md);
  margin-top: var(--spacing-lg);
  padding-top: var(--spacing-lg);
  border-top: 1px solid var(--color-border);
}

.stat-value {
  font-family: var(--font-heading);
  font-size: 2.5rem;
  margin-bottom: var(--spacing-xs);
}

.stat-label {
  font-size: 0.6875rem;
  text-transform: uppercase;
  letter-spacing: 0.15em;
  color: var(--color-text-dim);
}

/* Profile Cards */
.profile {
  display: grid;
  grid-template-columns: 300px 1fr;
  gap: var(--spacing-lg);
  margin-bottom: var(--spacing-lg);
}

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

.profile-image {
  aspect-ratio: 3/4;
  background: var(--color-bg-lighter);
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--color-text-dim);
  font-size: 3rem;
}

.profile-content h2 {
  margin-bottom: var(--spacing-xs);
}

.profile-title {
  font-size: 0.875rem;
  color: var(--color-text-muted);
  text-transform: uppercase;
  letter-spacing: 0.1em;
  margin-bottom: var(--spacing-md);
}

.profile-bio {
  color: var(--color-text-muted);
  margin-bottom: var(--spacing-md);
}

.profile-links {
  display: flex;
  gap: var(--spacing-sm);
}

/* Team Grid */
.team-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(250px, 1fr));
  gap: var(--spacing-md);
}

.team-member {
  text-align: center;
}

.team-member-image {
  aspect-ratio: 1;
  background: var(--color-bg-lighter);
  margin-bottom: var(--spacing-sm);
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--color-text-dim);
  font-size: 2rem;
}

.team-member-role {
  font-size: 0.625rem;
  text-transform: uppercase;
  letter-spacing: 0.15em;
  color: var(--color-text-dim);
  margin-bottom: var(--spacing-xs);
}

.team-member-name {
  font-size: 1.125rem;
  margin-bottom: var(--spacing-xs);
}

.team-member-title {
  font-size: 0.8125rem;
  color: var(--color-text-muted);
}

/* Publications */
.publication-list {
  display: flex;
  flex-direction: column;
  gap: var(--spacing-md);
}

.publication {
  padding: var(--spacing-md) 0;
  border-bottom: 1px solid var(--color-border);
}

.publication-header {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: var(--spacing-sm);
  margin-bottom: var(--spacing-xs);
}

.publication-number {
  font-family: var(--font-heading);
  color: var(--color-text-dim);
  font-size: 0.875rem;
  min-width: 2rem;
}

.publication-title {
  font-size: 1.125rem;
  flex: 1;
  margin-bottom: var(--spacing-xs);
}

.publication-authors {
  font-size: 0.9375rem;
  color: var(--color-text-muted);
  margin-bottom: var(--spacing-xs);
}

.publication-journal {
  font-size: 0.8125rem;
  color: var(--color-text-dim);
  font-style: italic;
  margin-bottom: var(--spacing-sm);
}

.publication-meta {
  display: flex;
  gap: var(--spacing-sm);
  flex-wrap: wrap;
}

.publication-tag {
  font-size: 0.625rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--color-text-dim);
}

.publication-link {
  font-size: 0.6875rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--color-text-muted);
}

.publication-link:hover {
  color: var(--color-text);
}

/* PDF Icon */
.pdf-icon {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 20px;
  height: 20px;
  margin-left: 0.5rem;
  font-size: 0.75rem;
  color: var(--color-text-dim);
  border: 1px solid var(--color-border);
  border-radius: 2px;
  vertical-align: middle;
}

.pdf-icon:hover {
  color: var(--color-text);
  border-color: var(--color-text-muted);
}

/* Publication Title Link */
.publication-title a {
  color: var(--color-text);
}

.publication-title a:hover {
  opacity: 0.7;
}

/* Featured Publications */
.featured-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: var(--spacing-md);
  margin-bottom: var(--spacing-xl);
}

.featured-pub {
  padding: var(--spacing-md);
  border: 1px solid var(--color-border);
}

.featured-pub-header {
  display: flex;
  justify-content: space-between;
  margin-bottom: var(--spacing-sm);
}

.featured-pub-tag {
  font-size: 0.625rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--color-text-dim);
}

.featured-pub-year {
  font-size: 0.75rem;
  color: var(--color-text-dim);
}

.featured-pub-title {
  font-size: 1.125rem;
  margin-bottom: var(--spacing-xs);
}

.featured-pub-authors {
  font-size: 0.875rem;
  color: var(--color-text-muted);
  margin-bottom: var(--spacing-xs);
}

.featured-pub-journal {
  font-size: 0.75rem;
  color: var(--color-text-dim);
  font-style: italic;
  margin-bottom: var(--spacing-sm);
}

/* Filter Buttons */
.filter-bar {
  display: flex;
  gap: var(--spacing-xs);
  margin-bottom: var(--spacing-lg);
  flex-wrap: wrap;
}

.filter-btn {
  padding: 0.5rem 1rem;
  font-size: 0.6875rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  background: transparent;
  border: 1px solid var(--color-border);
  color: var(--color-text-muted);
  cursor: pointer;
  transition: all 0.2s ease;
}

.filter-btn:hover,
.filter-btn.active {
  background: var(--color-text);
  color: var(--color-bg);
  border-color: var(--color-text);
}

/* Courses */
.course-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(350px, 1fr));
  gap: var(--spacing-md);
}

.course {
  padding: var(--spacing-md);
  border: 1px solid var(--color-border);
}

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

.course-code {
  font-size: 0.75rem;
  color: var(--color-text-dim);
}

.course-level {
  font-size: 0.625rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  padding: 0.25rem 0.5rem;
  background: var(--color-bg-lighter);
  color: var(--color-text-muted);
}

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

.course-description {
  font-size: 0.9375rem;
  color: var(--color-text-muted);
  margin-bottom: var(--spacing-md);
}

.course-meta {
  display: flex;
  flex-direction: column;
  gap: var(--spacing-xs);
  margin-bottom: var(--spacing-md);
}

.course-meta-item {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-size: 0.8125rem;
  color: var(--color-text-dim);
}

.course-materials {
  padding-top: var(--spacing-sm);
  border-top: 1px solid var(--color-border);
}

.course-materials-label {
  font-size: 0.625rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--color-text-dim);
  margin-bottom: var(--spacing-xs);
}

.course-materials-list {
  display: flex;
  gap: var(--spacing-xs);
  flex-wrap: wrap;
}

.course-material {
  padding: 0.375rem 0.75rem;
  font-size: 0.625rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  background: var(--color-bg-lighter);
  color: var(--color-text-muted);
}

.course-material:hover {
  background: var(--color-text);
  color: var(--color-bg);
}

/* Past Courses */
.past-courses {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(250px, 1fr));
  gap: var(--spacing-sm);
}

.past-course {
  padding: var(--spacing-sm);
  border: 1px solid var(--color-border);
}

.past-course-header {
  display: flex;
  justify-content: space-between;
  margin-bottom: var(--spacing-xs);
}

.past-course-code {
  font-size: 0.6875rem;
  color: var(--color-text-dim);
}

.past-course-title {
  font-size: 1rem;
  margin-bottom: var(--spacing-xs);
}

.past-course-semester {
  font-size: 0.6875rem;
  color: var(--color-text-dim);
}

/* Footer */
.footer {
  background: var(--color-bg-light);
  padding: var(--spacing-lg) 0;
  border-top: 1px solid var(--color-border);
}

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

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

.footer-brand {
  font-family: var(--font-heading);
  font-size: 1.25rem;
  margin-bottom: var(--spacing-sm);
}

.footer-description {
  font-size: 0.9375rem;
  color: var(--color-text-muted);
}

.footer-title {
  font-size: 0.6875rem;
  text-transform: uppercase;
  letter-spacing: 0.15em;
  color: var(--color-text-dim);
  margin-bottom: var(--spacing-sm);
}

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

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

.footer-links a {
  font-size: 0.875rem;
  color: var(--color-text-muted);
}

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

.footer-contact {
  font-size: 0.875rem;
  color: var(--color-text-muted);
  line-height: 1.8;
}

.footer-bottom {
  padding-top: var(--spacing-md);
  border-top: 1px solid var(--color-border);
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  gap: var(--spacing-sm);
}

.footer-copyright {
  font-size: 0.75rem;
  color: var(--color-text-dim);
}

.footer-legal {
  display: flex;
  gap: var(--spacing-md);
}

.footer-legal a {
  font-size: 0.75rem;
  color: var(--color-text-dim);
}

/* Social Links */
.social-links {
  display: flex;
  gap: var(--spacing-sm);
  margin-top: var(--spacing-sm);
}

.social-link {
  width: 36px;
  height: 36px;
  display: flex;
  align-items: center;
  justify-content: center;
  border: 1px solid var(--color-border);
  color: var(--color-text-muted);
  font-size: 0.875rem;
}

.social-link:hover {
  border-color: var(--color-text);
  color: var(--color-text);
  opacity: 1;
}

/* Utility Classes */
.text-center { text-align: center; }
.mt-0 { margin-top: 0; }
.mb-0 { margin-bottom: 0; }

/* Two Column Layout */
.two-column {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--spacing-lg);
}

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

/* CTA Section */
.cta {
  text-align: center;
  padding: var(--spacing-xl) 0;
}

.cta-title {
  font-size: clamp(1.75rem, 4vw, 2.5rem);
  margin-bottom: var(--spacing-sm);
}

.cta-text {
  font-size: 1.0625rem;
  color: var(--color-text-muted);
  max-width: 600px;
  margin: 0 auto var(--spacing-md);
}

/* Icon placeholders (using text/emoji for no-JS) */
.icon::before {
  display: inline-block;
  width: 1em;
  text-align: center;
}

.icon-email::before { content: "✉"; }
.icon-linkedin::before { content: "in"; }
.icon-globe::before { content: "🌐"; }
.icon-clock::before { content: "◷"; }
.icon-location::before { content: "◎"; }
.icon-book::before { content: "📖"; }
.icon-flask::before { content: "⚗"; }
.icon-users::before { content: "👥"; }
.icon-graduation::before { content: "🎓"; }