/* ===================================
   CSS Variables & Reset
   =================================== */

:root {
  /* Colors */
  --primary-blue: #2563eb;
  --secondary-cyan: #0891b2;
  --text-primary: #374151;
  --text-secondary: #6b7280;
  --bg-white: #ffffff;
  --bg-light: #f9fafb;
  --border-gray: #e5e7eb;
  --border-light: #f3f4f6;

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

  /* Typography */
  --font-base: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --font-mono: "SF Mono", Monaco, "Cascadia Code", "Roboto Mono", Consolas, "Courier New", monospace;

  /* Transitions */
  --transition-fast: 150ms ease;
  --transition-base: 250ms ease;

  /* Shadows */
  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
}

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

html {
  scroll-behavior: smooth;
}

body {
  font-family: var(--font-base);
  font-size: 16px;
  line-height: 1.6;
  color: var(--text-primary);
  background-color: var(--bg-white);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

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

h1, h2, h3, h4, h5, h6 {
  line-height: 1.2;
  font-weight: 700;
  margin-bottom: var(--spacing-md);
  color: var(--text-primary);
}

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

h2 {
  font-size: 2rem;
  margin-bottom: var(--spacing-md);
}

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

h4 {
  font-size: 1.25rem;
  margin-bottom: var(--spacing-sm);
}

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

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

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

strong {
  font-weight: 600;
}

code {
  font-family: var(--font-mono);
  font-size: 0.9em;
  background-color: var(--bg-light);
  padding: 0.2em 0.4em;
  border-radius: 3px;
  color: var(--text-primary);
}

pre {
  background-color: var(--bg-light);
  border: 1px solid var(--border-gray);
  border-radius: 6px;
  padding: var(--spacing-md);
  overflow-x: auto;
  margin-bottom: var(--spacing-md);
}

pre code {
  background-color: transparent;
  padding: 0;
}

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

.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 var(--spacing-md);
}

.container-narrow {
  max-width: 800px;
  margin: 0 auto;
  padding: 0 var(--spacing-md);
}

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

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

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

.navbar {
  background-color: var(--bg-white);
  border-bottom: 1px solid var(--border-gray);
  position: sticky;
  top: 0;
  z-index: 1000;
  box-shadow: var(--shadow-sm);
}

.nav-container {
  max-width: 1200px;
  margin: 0 auto;
  padding: var(--spacing-sm) var(--spacing-md);
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.nav-brand {
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--primary-blue);
  text-decoration: none;
}

.nav-brand:hover {
  text-decoration: none;
  color: var(--secondary-cyan);
}

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

.nav-link {
  color: var(--text-primary);
  font-weight: 500;
  padding: var(--spacing-xs) var(--spacing-sm);
  border-radius: 4px;
  transition: all var(--transition-fast);
}

.nav-link:hover {
  color: var(--primary-blue);
  background-color: var(--bg-light);
  text-decoration: none;
}

.nav-link.active {
  color: var(--primary-blue);
  background-color: var(--bg-light);
}

/* Mobile Menu Toggle (Pure CSS) */
.nav-toggle {
  display: none;
  flex-direction: column;
  cursor: pointer;
  gap: 4px;
}

.nav-toggle span {
  display: block;
  width: 25px;
  height: 3px;
  background-color: var(--text-primary);
  transition: all var(--transition-fast);
}

@media (max-width: 768px) {
  .nav-toggle {
    display: flex;
  }

  .nav-menu {
    position: absolute;
    top: 100%;
    left: 0;
    right: 0;
    background-color: var(--bg-white);
    flex-direction: column;
    gap: 0;
    padding: var(--spacing-md);
    border-bottom: 1px solid var(--border-gray);
    box-shadow: var(--shadow-md);
    display: none;
  }

  .nav-menu.active {
    display: flex;
  }

  .nav-link {
    width: 100%;
    padding: var(--spacing-sm);
  }
}

/* ===================================
   Buttons
   =================================== */

.btn {
  display: inline-block;
  padding: 0.75rem 1.5rem;
  font-size: 1rem;
  font-weight: 600;
  text-align: center;
  text-decoration: none;
  border-radius: 6px;
  transition: all var(--transition-base);
  border: none;
  cursor: pointer;
  font-family: var(--font-base);
}

.btn-primary {
  background-color: var(--primary-blue);
  color: white;
}

.btn-primary:hover {
  background-color: #1d4ed8;
  text-decoration: none;
  transform: translateY(-2px);
  box-shadow: var(--shadow-md);
}

.btn-secondary {
  background-color: transparent;
  color: var(--primary-blue);
  border: 2px solid var(--primary-blue);
}

.btn-secondary:hover {
  background-color: var(--primary-blue);
  color: white;
  text-decoration: none;
  transform: translateY(-2px);
}

.btn-group {
  display: flex;
  gap: var(--spacing-md);
  flex-wrap: wrap;
}

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

.hero {
  padding: var(--spacing-2xl) 0;
  text-align: center;
  background: linear-gradient(135deg, #f9fafb 0%, #ffffff 100%);
}

.hero h1 {
  font-size: 3rem;
  margin-bottom: var(--spacing-sm);
  color: var(--primary-blue);
}

.hero .subtitle {
  font-size: 1.5rem;
  color: var(--text-secondary);
  margin-bottom: var(--spacing-md);
  font-weight: 400;
}

.hero .tagline {
  font-size: 1.125rem;
  color: var(--text-primary);
  max-width: 800px;
  margin: 0 auto var(--spacing-xl) auto;
  line-height: 1.8;
}

@media (max-width: 768px) {
  .hero h1 {
    font-size: 2rem;
  }

  .hero .subtitle {
    font-size: 1.25rem;
  }
}

/* ===================================
   Cards
   =================================== */

.card {
  background-color: var(--bg-white);
  border: 1px solid var(--border-gray);
  border-radius: 8px;
  padding: var(--spacing-lg);
  margin-bottom: var(--spacing-md);
  transition: all var(--transition-base);
}

.card:hover {
  box-shadow: var(--shadow-lg);
  transform: translateY(-4px);
}

.card h3 {
  color: var(--primary-blue);
  margin-bottom: var(--spacing-sm);
}

.card-title {
  font-size: 1.5rem;
  margin-bottom: var(--spacing-xs);
}

.card-subtitle {
  color: var(--text-secondary);
  font-size: 0.95rem;
  margin-bottom: var(--spacing-sm);
}

.card-meta {
  color: var(--text-secondary);
  font-size: 0.9rem;
  margin-bottom: var(--spacing-sm);
}

/* ===================================
   Grid Layouts
   =================================== */

.grid {
  display: grid;
  gap: var(--spacing-lg);
}

.grid-2 {
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
}

.grid-3 {
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
}

/* ===================================
   Skills & Tags
   =================================== */

.skills-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: var(--spacing-lg);
  margin-bottom: var(--spacing-lg);
}

.skill-category h3 {
  color: var(--primary-blue);
  margin-bottom: var(--spacing-sm);
  font-size: 1.25rem;
}

.skill-category ul {
  list-style: none;
  padding: 0;
}

.skill-category li {
  padding: var(--spacing-xs) 0;
  color: var(--text-primary);
}

.tags {
  display: flex;
  flex-wrap: wrap;
  gap: var(--spacing-xs);
  margin-top: var(--spacing-sm);
}

.tag {
  display: inline-block;
  padding: 0.25rem 0.75rem;
  background-color: var(--bg-light);
  color: var(--primary-blue);
  border-radius: 20px;
  font-size: 0.85rem;
  font-weight: 500;
  border: 1px solid var(--border-gray);
}

/* ===================================
   Experience Timeline
   =================================== */

.experience-item {
  margin-bottom: var(--spacing-xl);
  padding-bottom: var(--spacing-xl);
  border-bottom: 1px solid var(--border-gray);
}

.experience-item:last-child {
  border-bottom: none;
}

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

.experience-title {
  font-size: 1.5rem;
  color: var(--primary-blue);
  margin-bottom: var(--spacing-xs);
}

.experience-company {
  font-size: 1.125rem;
  font-weight: 600;
  color: var(--text-primary);
  margin-bottom: var(--spacing-xs);
}

.experience-date {
  color: var(--text-secondary);
  font-size: 0.95rem;
  font-style: italic;
}

.experience-item ul {
  margin-left: var(--spacing-lg);
  margin-top: var(--spacing-sm);
}

.experience-item li {
  margin-bottom: var(--spacing-sm);
}

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

footer {
  background-color: var(--text-primary);
  color: white;
  padding: var(--spacing-xl) 0 var(--spacing-lg) 0;
  margin-top: var(--spacing-2xl);
}

.footer-content {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 var(--spacing-md);
  text-align: center;
}

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

.footer-links a {
  color: white;
  text-decoration: none;
  transition: color var(--transition-fast);
}

.footer-links a:hover {
  color: var(--secondary-cyan);
  text-decoration: none;
}

.footer-text {
  color: #9ca3af;
  font-size: 0.9rem;
}

/* ===================================
   Contact Form
   =================================== */

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

.contact-item {
  display: flex;
  align-items: center;
  gap: var(--spacing-sm);
  margin-bottom: var(--spacing-md);
  font-size: 1.125rem;
}

.contact-item strong {
  color: var(--primary-blue);
  min-width: 80px;
}

.contact-form {
  max-width: 600px;
  margin: 0 auto;
}

.form-group {
  margin-bottom: var(--spacing-md);
}

.form-group label {
  display: block;
  margin-bottom: var(--spacing-xs);
  font-weight: 600;
  color: var(--text-primary);
}

.form-group input,
.form-group textarea {
  width: 100%;
  padding: var(--spacing-sm);
  border: 1px solid var(--border-gray);
  border-radius: 6px;
  font-family: var(--font-base);
  font-size: 1rem;
  transition: border-color var(--transition-fast);
}

.form-group input:focus,
.form-group textarea:focus {
  outline: none;
  border-color: var(--primary-blue);
  box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.1);
}

.form-group textarea {
  resize: vertical;
  min-height: 150px;
}

/* ===================================
   Research Paper Specific
   =================================== */

.paper-abstract {
  background-color: var(--bg-light);
  border-left: 4px solid var(--primary-blue);
  padding: var(--spacing-lg);
  margin-bottom: var(--spacing-xl);
  border-radius: 4px;
}

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

.paper-section h2 {
  color: var(--primary-blue);
  border-bottom: 2px solid var(--border-gray);
  padding-bottom: var(--spacing-sm);
}

/* ===================================
   Print Styles (for Resume)
   =================================== */

@media print {
  .navbar, footer, .btn {
    display: none;
  }

  body {
    font-size: 12pt;
    color: black;
  }

  .container {
    max-width: 100%;
    padding: 0;
  }

  .section {
    padding: 1rem 0;
  }

  .experience-item {
    page-break-inside: avoid;
  }

  a {
    color: black;
    text-decoration: none;
  }

  .card {
    border: 1px solid #ddd;
    box-shadow: none;
    page-break-inside: avoid;
  }
}

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

@media (max-width: 768px) {
  .container {
    padding: 0 var(--spacing-sm);
  }

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

  .btn-group {
    flex-direction: column;
    width: 100%;
  }

  .btn {
    width: 100%;
  }

  .skills-grid {
    grid-template-columns: 1fr;
  }
}

/* ===================================
   Accessibility
   =================================== */

:focus {
  outline: 2px solid var(--primary-blue);
  outline-offset: 2px;
}

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