/* ============================================
   CSS Reset & Normalization
   ============================================ */

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

html, body {
  height: 100%;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
  line-height: 1.6;
  color: #333;
  background-color: #ffffff;
}

/* ============================================
   Root Variables
   ============================================ */

:root {
  --primary-color: #0066cc;
  --primary-dark: #0052a3;
  --primary-light: #e6f0ff;
  --secondary-color: #666666;
  --text-color: #333333;
  --text-light: #666666;
  --text-lighter: #999999;
  --bg-color: #ffffff;
  --bg-light: #f5f5f5;
  --bg-lighter: #fafafa;
  --border-color: #ddd;
  --border-light: #eeeeee;
  
  --spacing-xs: 0.5rem;
  --spacing-sm: 1rem;
  --spacing-md: 1.5rem;
  --spacing-lg: 2rem;
  --spacing-xl: 3rem;
  
  --border-radius: 8px;
  --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.1);
  --shadow-md: 0 4px 6px rgba(0, 0, 0, 0.1);
  --shadow-lg: 0 10px 15px rgba(0, 0, 0, 0.1);
  
  --transition: all 0.3s ease;
  
  --max-width: 1200px;
}

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

h1, h2, h3, h4, h5, h6 {
  font-weight: 600;
  line-height: 1.2;
  margin-bottom: var(--spacing-md);
  color: var(--text-color);
}

h1 {
  font-size: 2.5rem;
  margin-bottom: var(--spacing-lg);
}

h2 {
  font-size: 1.875rem;
  margin-bottom: var(--spacing-md);
}

h3 {
  font-size: 1.5rem;
  margin-bottom: var(--spacing-sm);
}

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

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

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

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

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

main {
  min-height: calc(100vh - 200px);
}

/* ============================================
   Navigation Bar
   ============================================ */

.navbar {
  background-color: var(--bg-color);
  border-bottom: 1px solid var(--border-light);
  position: sticky;
  top: 0;
  z-index: 100;
  box-shadow: var(--shadow-sm);
}

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

.nav-logo {
  font-size: 1.25rem;
  font-weight: 700;
  color: var(--primary-color);
  display: flex;
  align-items: center;
}

.logo-text {
  color: var(--text-color);
}

.nav-menu {
  display: flex;
  list-style: none;
  gap: var(--spacing-lg);
}

.nav-item {
  display: inline-block;
}

.nav-link {
  padding: var(--spacing-xs) var(--spacing-sm);
  color: var(--text-color);
  font-weight: 500;
  border-bottom: 2px solid transparent;
  transition: var(--transition);
}

.nav-link:hover {
  color: var(--primary-color);
  text-decoration: none;
  border-bottom-color: var(--primary-color);
}

.nav-link.active {
  color: var(--primary-color);
  border-bottom-color: var(--primary-color);
}

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

.hero {
  text-align: center;
  padding: var(--spacing-xl) 0;
  background: linear-gradient(135deg, var(--primary-light) 0%, var(--bg-light) 100%);
  border-radius: var(--border-radius);
  margin-bottom: var(--spacing-xl);
}

.hero h1 {
  margin-bottom: var(--spacing-md);
  color: var(--text-color);
}

.subtitle {
  font-size: 1.25rem;
  color: var(--text-light);
  margin-bottom: 0;
}

/* ============================================
   Features Section
   ============================================ */

.features {
  margin-bottom: var(--spacing-xl);
}

.feature-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: var(--spacing-lg);
}

.feature-card {
  padding: var(--spacing-lg);
  background-color: var(--bg-light);
  border-radius: var(--border-radius);
  border: 1px solid var(--border-light);
  transition: var(--transition);
}

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

.feature-card h2 {
  color: var(--text-color);
  margin-bottom: var(--spacing-md);
}

/* ============================================
   Button Styles
   ============================================ */

.btn {
  display: inline-block;
  padding: var(--spacing-sm) var(--spacing-lg);
  background-color: var(--primary-color);
  color: white;
  border-radius: var(--border-radius);
  font-weight: 500;
  text-decoration: none;
  transition: var(--transition);
  border: none;
  cursor: pointer;
}

.btn:hover {
  background-color: var(--primary-dark);
  text-decoration: none;
}

/* ============================================
   Page Header
   ============================================ */

.page-header {
  text-align: center;
  margin-bottom: var(--spacing-xl);
  padding-bottom: var(--spacing-lg);
  border-bottom: 1px solid var(--border-light);
}

.page-header h1 {
  margin-bottom: var(--spacing-md);
}

.updated-at {
  font-size: 0.875rem;
  color: var(--text-lighter);
}

/* ============================================
   Papers Grid & Cards
   ============================================ */

.papers-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(500px, 1fr));
  gap: var(--spacing-lg);
  margin-bottom: var(--spacing-xl);
}

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

.paper-card {
  background-color: var(--bg-light);
  border: 1px solid var(--border-light);
  border-radius: var(--border-radius);
  overflow: hidden;
  transition: var(--transition);
  display: flex;
  flex-direction: column;
}

.paper-card:hover {
  box-shadow: var(--shadow-md);
  border-color: var(--primary-color);
}

.paper-header {
  padding: var(--spacing-lg);
  border-bottom: 1px solid var(--border-light);
}

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

.paper-title a {
  color: var(--primary-color);
}

.paper-title a:hover {
  text-decoration: underline;
}

.paper-meta {
  display: flex;
  flex-direction: column;
  gap: var(--spacing-xs);
  font-size: 0.875rem;
  color: var(--text-lighter);
}

.paper-authors {
  font-weight: 500;
}

.paper-body {
  padding: var(--spacing-lg);
  flex-grow: 1;
}

.paper-summary {
  margin-bottom: var(--spacing-md);
  font-size: 0.95rem;
  line-height: 1.6;
}

.paper-key-points {
  list-style: none;
  padding: 0;
  margin-bottom: var(--spacing-md);
}

.paper-key-points li {
  padding-left: var(--spacing-md);
  margin-bottom: var(--spacing-sm);
  position: relative;
  font-size: 0.9rem;
  color: var(--text-light);
}

.paper-key-points li:before {
  content: "▸";
  position: absolute;
  left: 0;
  color: var(--primary-color);
  font-weight: bold;
}

.paper-footer {
  padding: var(--spacing-lg);
  border-top: 1px solid var(--border-light);
  background-color: var(--bg-lighter);
}

.arxiv-link {
  display: inline-flex;
  align-items: center;
  color: var(--primary-color);
  font-weight: 500;
  text-decoration: none;
}

.arxiv-link:hover {
  color: var(--primary-dark);
  text-decoration: underline;
}

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

.papers-footer {
  text-align: center;
  padding: var(--spacing-lg);
  background-color: var(--bg-light);
  border-radius: var(--border-radius);
  color: var(--text-light);
}

.papers-footer p {
  margin-bottom: var(--spacing-sm);
}

.papers-footer p:last-child {
  margin-bottom: 0;
}

/* ============================================
   Placeholder Content
   ============================================ */

.placeholder-card {
  text-align: center;
  padding: var(--spacing-xl);
  background-color: var(--bg-light);
  border-radius: var(--border-radius);
  border: 2px dashed var(--border-color);
}

.placeholder-card h2 {
  color: var(--text-color);
}

.placeholder-card p {
  color: var(--text-light);
  margin-bottom: 0;
}

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

.footer {
  background-color: var(--bg-light);
  border-top: 1px solid var(--border-light);
  margin-top: var(--spacing-xl);
}

.footer-content {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: var(--spacing-lg);
  text-align: center;
  color: var(--text-light);
}

.footer-content p {
  margin-bottom: var(--spacing-sm);
  font-size: 0.9rem;
}

.footer-content p:last-of-type {
  margin-bottom: 0;
}

.footer-links {
  list-style: none;
  display: flex;
  justify-content: center;
  gap: var(--spacing-lg);
  margin-top: var(--spacing-md);
}

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

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

/* ============================================
   Responsive Design
   ============================================ */

@media (max-width: 768px) {
  h1 {
    font-size: 1.875rem;
  }
  
  h2 {
    font-size: 1.5rem;
  }
  
  h3 {
    font-size: 1.25rem;
  }
  
  .container {
    padding: var(--spacing-md);
  }
  
  .nav-container {
    padding: var(--spacing-md);
  }
  
  .nav-menu {
    gap: var(--spacing-md);
  }
  
  .hero {
    padding: var(--spacing-lg) var(--spacing-md);
  }
  
  .feature-grid {
    grid-template-columns: 1fr;
  }
  
  .papers-grid {
    grid-template-columns: 1fr;
    gap: var(--spacing-md);
  }
  
  .page-header {
    padding-bottom: var(--spacing-md);
  }
}

@media (max-width: 480px) {
  h1 {
    font-size: 1.5rem;
  }
  
  h2 {
    font-size: 1.25rem;
  }
  
  h3 {
    font-size: 1.125rem;
  }
  
  .container {
    padding: var(--spacing-md);
  }
  
  .nav-menu {
    gap: var(--spacing-sm);
  }
  
  .nav-link {
    padding: var(--spacing-xs);
    font-size: 0.9rem;
  }
  
  .subtitle {
    font-size: 1rem;
  }
  
  .btn {
    padding: var(--spacing-xs) var(--spacing-md);
    font-size: 0.9rem;
  }
  
  .paper-card {
    border-radius: 4px;
  }
  
  .paper-header,
  .paper-body,
  .paper-footer {
    padding: var(--spacing-md);
  }
  
  .footer-links {
    gap: var(--spacing-md);
  }
}

/* ============================================
   Accessibility & Print
   ============================================ */

@media print {
  .navbar,
  .footer {
    display: none;
  }
  
  body {
    color: #000;
    background: #fff;
  }
  
  a {
    text-decoration: underline;
  }
}

/* Focus styles for accessibility */
a:focus,
.btn:focus,
.nav-link:focus {
  outline: 2px solid var(--primary-color);
  outline-offset: 2px;
}

/* Reduced motion support */
@media (prefers-reduced-motion: reduce) {
  * {
    animation: none !important;
    transition: none !important;
  }
}