/* ======================================================================
   Apple-Style Research Blog — Complete Stylesheet
   ====================================================================== */

/* ----------------------------- CSS Variables ----------------------------- */
:root {
  /* Light theme */
  --bg-primary: #ffffff;
  --bg-secondary: #f5f5f7;
  --bg-tertiary: #fbfbfd;
  --bg-card: #ffffff;
  --bg-code: #f5f5f7;
  --bg-nav: rgba(255, 255, 255, 0.72);

  --text-primary: #1d1d1f;
  --text-secondary: #6e6e73;
  --text-tertiary: #86868b;
  --text-inverse: #f5f5f7;

  --accent: #0071e3;
  --accent-hover: #0077ED;
  --accent-gradient: linear-gradient(135deg, #0071e3 0%, #2997ff 100%);

  --border-color: rgba(0, 0, 0, 0.08);
  --border-nav: rgba(0, 0, 0, 0.12);
  --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.04);
  --shadow-md: 0 4px 18px rgba(0, 0, 0, 0.06);
  --shadow-lg: 0 12px 40px rgba(0, 0, 0, 0.08);
  --shadow-card-hover: 0 16px 48px rgba(0, 0, 0, 0.12);

  --radius-sm: 8px;
  --radius-md: 12px;
  --radius-lg: 18px;
  --radius-xl: 24px;

  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'SF Pro Text', 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
  --font-mono: 'SF Mono', 'Fira Code', 'Fira Mono', Menlo, Consolas, monospace;

  --max-width: 980px;
  --max-width-narrow: 720px;
  --nav-height: 52px;

  --transition-fast: 0.2s cubic-bezier(0.25, 0.1, 0.25, 1);
  --transition-medium: 0.4s cubic-bezier(0.25, 0.1, 0.25, 1);
  --transition-slow: 0.6s cubic-bezier(0.25, 0.1, 0.25, 1);
}

/* Dark mode */
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg-primary: #000000;
    --bg-secondary: #1c1c1e;
    --bg-tertiary: #0d0d0d;
    --bg-card: #1c1c1e;
    --bg-code: #1c1c1e;
    --bg-nav: rgba(0, 0, 0, 0.72);

    --text-primary: #f5f5f7;
    --text-secondary: #a1a1a6;
    --text-tertiary: #6e6e73;
    --text-inverse: #1d1d1f;

    --accent: #2997ff;
    --accent-hover: #64b5f6;

    --border-color: rgba(255, 255, 255, 0.08);
    --border-nav: rgba(255, 255, 255, 0.12);
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.2);
    --shadow-md: 0 4px 18px rgba(0, 0, 0, 0.3);
    --shadow-lg: 0 12px 40px rgba(0, 0, 0, 0.4);
    --shadow-card-hover: 0 16px 48px rgba(0, 0, 0, 0.5);
  }
}

[data-theme="dark"] {
  --bg-primary: #000000;
  --bg-secondary: #1c1c1e;
  --bg-tertiary: #0d0d0d;
  --bg-card: #1c1c1e;
  --bg-code: #1c1c1e;
  --bg-nav: rgba(0, 0, 0, 0.72);

  --text-primary: #f5f5f7;
  --text-secondary: #a1a1a6;
  --text-tertiary: #6e6e73;
  --text-inverse: #1d1d1f;

  --accent: #2997ff;
  --accent-hover: #64b5f6;

  --border-color: rgba(255, 255, 255, 0.08);
  --border-nav: rgba(255, 255, 255, 0.12);
  --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.2);
  --shadow-md: 0 4px 18px rgba(0, 0, 0, 0.3);
  --shadow-lg: 0 12px 40px rgba(0, 0, 0, 0.4);
  --shadow-card-hover: 0 16px 48px rgba(0, 0, 0, 0.5);
}


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

html {
  font-size: 17px;
  scroll-behavior: smooth;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-rendering: optimizeLegibility;
}

body {
  font-family: var(--font-sans);
  color: var(--text-primary);
  background-color: var(--bg-primary);
  line-height: 1.47059;
  letter-spacing: -0.022em;
  overflow-x: hidden;
  transition: background-color var(--transition-medium), color var(--transition-medium);
}

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

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

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

::selection {
  background-color: var(--accent);
  color: #fff;
}


/* ----------------------------- Navigation ----------------------------- */
.site-nav {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--nav-height);
  background: var(--bg-nav);
  backdrop-filter: saturate(180%) blur(20px);
  -webkit-backdrop-filter: saturate(180%) blur(20px);
  border-bottom: 1px solid var(--border-nav);
  z-index: 9999;
  transition: background-color var(--transition-medium);
}

.nav-inner {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 22px;
  height: 100%;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.nav-brand {
  font-size: 1.1rem;
  font-weight: 600;
  color: var(--text-primary);
  letter-spacing: -0.03em;
  text-decoration: none;
  transition: opacity var(--transition-fast);
}

.nav-brand:hover {
  opacity: 0.72;
  color: var(--text-primary);
}

.nav-links {
  display: flex;
  align-items: center;
  gap: 28px;
  list-style: none;
}

.nav-links a {
  font-size: 0.82rem;
  font-weight: 400;
  color: var(--text-secondary);
  letter-spacing: -0.008em;
  transition: color var(--transition-fast);
  text-decoration: none;
}

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

.nav-links a.active {
  color: var(--text-primary);
  font-weight: 500;
}

/* Theme toggle */
.theme-toggle {
  background: none;
  border: none;
  cursor: pointer;
  color: var(--text-secondary);
  font-size: 1rem;
  padding: 4px;
  border-radius: 50%;
  transition: color var(--transition-fast), background var(--transition-fast);
  display: flex;
  align-items: center;
  justify-content: center;
  width: 32px;
  height: 32px;
}

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

/* Mobile nav */
.nav-mobile-toggle {
  display: none;
  background: none;
  border: none;
  cursor: pointer;
  color: var(--text-primary);
  font-size: 1.3rem;
  padding: 4px;
}

@media (max-width: 640px) {
  .nav-links {
    display: none;
    position: absolute;
    top: var(--nav-height);
    left: 0;
    right: 0;
    background: var(--bg-nav);
    backdrop-filter: saturate(180%) blur(20px);
    -webkit-backdrop-filter: saturate(180%) blur(20px);
    flex-direction: column;
    padding: 20px 22px;
    gap: 16px;
    border-bottom: 1px solid var(--border-nav);
  }

  .nav-links.open {
    display: flex;
  }

  .nav-mobile-toggle {
    display: block;
  }
}


/* ----------------------------- Hero Section ----------------------------- */
.hero {
  padding: 160px 22px 80px;
  text-align: center;
  max-width: var(--max-width);
  margin: 0 auto;
}

.hero-overline {
  display: inline-block;
  font-size: 0.82rem;
  font-weight: 500;
  color: var(--accent);
  letter-spacing: 0.02em;
  text-transform: uppercase;
  margin-bottom: 16px;
}

.hero h1 {
  font-size: clamp(2.4rem, 6vw, 4rem);
  font-weight: 700;
  letter-spacing: -0.04em;
  line-height: 1.08;
  color: var(--text-primary);
  margin-bottom: 20px;
}

.hero-subtitle {
  font-size: clamp(1.1rem, 2.5vw, 1.35rem);
  color: var(--text-secondary);
  max-width: 600px;
  margin: 0 auto 36px;
  line-height: 1.5;
  font-weight: 400;
  letter-spacing: -0.01em;
}

.hero-links {
  display: flex;
  gap: 14px;
  justify-content: center;
  flex-wrap: wrap;
}

.hero-links a {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  padding: 10px 24px;
  font-size: 0.88rem;
  font-weight: 500;
  border-radius: 980px;
  transition: all var(--transition-fast);
  letter-spacing: -0.01em;
}

.btn-primary {
  background: var(--accent);
  color: #fff !important;
}

.btn-primary:hover {
  background: var(--accent-hover);
  color: #fff !important;
  transform: scale(1.04);
}

.btn-secondary {
  background: transparent;
  color: var(--accent) !important;
  border: 1px solid var(--accent);
}

.btn-secondary:hover {
  background: var(--accent);
  color: #fff !important;
  transform: scale(1.04);
}


/* ----------------------------- Section Titles ----------------------------- */
.section-title {
  font-size: clamp(1.6rem, 3.5vw, 2.4rem);
  font-weight: 700;
  letter-spacing: -0.03em;
  color: var(--text-primary);
  text-align: center;
  margin-bottom: 12px;
}

.section-subtitle {
  font-size: 1.06rem;
  color: var(--text-secondary);
  text-align: center;
  margin-bottom: 48px;
  font-weight: 400;
}


/* ----------------------------- Post Cards Grid ----------------------------- */
.posts-section {
  padding: 60px 22px 80px;
  max-width: var(--max-width);
  margin: 0 auto;
}

.posts-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: 24px;
}

.post-card {
  background: var(--bg-card);
  border-radius: var(--radius-lg);
  border: 1px solid var(--border-color);
  overflow: hidden;
  transition: transform var(--transition-medium), box-shadow var(--transition-medium), border-color var(--transition-medium);
  text-decoration: none;
  display: flex;
  flex-direction: column;
}

.post-card:hover {
  transform: translateY(-4px);
  box-shadow: var(--shadow-card-hover);
  border-color: transparent;
}

.post-card-body {
  padding: 28px 24px 24px;
  flex: 1;
  display: flex;
  flex-direction: column;
}

.post-card-categories {
  display: flex;
  gap: 8px;
  flex-wrap: wrap;
  margin-bottom: 12px;
}

.post-card-category {
  font-size: 0.7rem;
  font-weight: 500;
  color: var(--accent);
  background: rgba(0, 113, 227, 0.08);
  padding: 3px 10px;
  border-radius: 980px;
  letter-spacing: 0.01em;
  text-transform: uppercase;
}

[data-theme="dark"] .post-card-category {
  background: rgba(41, 151, 255, 0.15);
}

.post-card-title {
  font-size: 1.18rem;
  font-weight: 600;
  color: var(--text-primary);
  letter-spacing: -0.02em;
  line-height: 1.3;
  margin-bottom: 10px;
  flex: 1;
}

.post-card-excerpt {
  font-size: 0.88rem;
  color: var(--text-secondary);
  line-height: 1.55;
  margin-bottom: 16px;
  display: -webkit-box;
  -webkit-line-clamp: 3;
  -webkit-box-orient: vertical;
  overflow: hidden;
}

.post-card-meta {
  display: flex;
  align-items: center;
  gap: 12px;
  font-size: 0.76rem;
  color: var(--text-tertiary);
  font-weight: 400;
}

.post-card-meta span {
  display: flex;
  align-items: center;
  gap: 4px;
}


/* ----------------------------- Pagination ----------------------------- */
.pagination {
  display: flex;
  justify-content: center;
  align-items: center;
  gap: 8px;
  padding: 40px 22px 80px;
}

.pagination a,
.pagination span {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-width: 40px;
  height: 40px;
  padding: 0 12px;
  border-radius: var(--radius-sm);
  font-size: 0.88rem;
  font-weight: 500;
  transition: all var(--transition-fast);
  text-decoration: none;
}

.pagination a {
  color: var(--text-secondary);
  border: 1px solid var(--border-color);
}

.pagination a:hover {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}

.pagination .current {
  background: var(--accent);
  color: #fff;
  border: 1px solid var(--accent);
}

.pagination .disabled {
  color: var(--text-tertiary);
  border: 1px solid var(--border-color);
  opacity: 0.4;
  pointer-events: none;
}


/* ----------------------------- Article / Post ----------------------------- */
.article-header {
  padding: 120px 22px 40px;
  max-width: var(--max-width-narrow);
  margin: 0 auto;
  text-align: center;
}

.article-categories {
  display: flex;
  gap: 8px;
  justify-content: center;
  flex-wrap: wrap;
  margin-bottom: 16px;
}

.article-category {
  font-size: 0.7rem;
  font-weight: 600;
  color: var(--accent);
  background: rgba(0, 113, 227, 0.08);
  padding: 4px 12px;
  border-radius: 980px;
  letter-spacing: 0.02em;
  text-transform: uppercase;
}

[data-theme="dark"] .article-category {
  background: rgba(41, 151, 255, 0.15);
}

.article-header h1 {
  font-size: clamp(1.8rem, 4.5vw, 2.8rem);
  font-weight: 700;
  letter-spacing: -0.035em;
  line-height: 1.12;
  color: var(--text-primary);
  margin-bottom: 16px;
}

.article-meta {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 16px;
  font-size: 0.82rem;
  color: var(--text-tertiary);
  font-weight: 400;
}

.article-meta span {
  display: flex;
  align-items: center;
  gap: 4px;
}

.article-divider {
  width: 48px;
  height: 2px;
  background: var(--accent-gradient);
  margin: 40px auto;
  border-radius: 2px;
}

.article-content {
  max-width: var(--max-width-narrow);
  margin: 0 auto;
  padding: 0 22px 80px;
  font-size: 1.06rem;
  line-height: 1.72;
  color: var(--text-primary);
}

.article-content h1,
.article-content h2,
.article-content h3,
.article-content h4 {
  font-weight: 600;
  letter-spacing: -0.025em;
  margin-top: 2.2em;
  margin-bottom: 0.7em;
  color: var(--text-primary);
}

.article-content h1 { font-size: 1.8rem; }
.article-content h2 { font-size: 1.5rem; }
.article-content h3 { font-size: 1.24rem; }
.article-content h4 { font-size: 1.06rem; }

.article-content p {
  margin-bottom: 1.35em;
}

.article-content ul,
.article-content ol {
  margin-bottom: 1.35em;
  padding-left: 1.5em;
}

.article-content li {
  margin-bottom: 0.4em;
}

.article-content blockquote {
  border-left: 3px solid var(--accent);
  margin: 1.6em 0;
  padding: 12px 20px;
  background: var(--bg-secondary);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  color: var(--text-secondary);
  font-style: italic;
}

.article-content img {
  border-radius: var(--radius-md);
  margin: 2em auto;
  box-shadow: var(--shadow-md);
}

.article-content em {
  font-style: italic;
}

.article-content strong {
  font-weight: 600;
  color: var(--text-primary);
}

.article-content a {
  color: var(--accent);
  text-decoration: underline;
  text-decoration-color: rgba(0, 113, 227, 0.3);
  text-underline-offset: 2px;
  transition: text-decoration-color var(--transition-fast);
}

.article-content a:hover {
  text-decoration-color: var(--accent);
}

.article-content table {
  width: 100%;
  border-collapse: collapse;
  margin: 1.6em 0;
  font-size: 0.94rem;
}

.article-content th,
.article-content td {
  padding: 10px 14px;
  border: 1px solid var(--border-color);
  text-align: left;
}

.article-content th {
  background: var(--bg-secondary);
  font-weight: 600;
  font-size: 0.82rem;
  text-transform: uppercase;
  letter-spacing: 0.02em;
  color: var(--text-secondary);
}

/* Code blocks */
.article-content code {
  font-family: var(--font-mono);
  font-size: 0.88em;
  background: var(--bg-code);
  padding: 2px 6px;
  border-radius: 4px;
  color: var(--text-primary);
}

.article-content pre {
  background: var(--bg-secondary);
  border-radius: var(--radius-md);
  padding: 20px 24px;
  overflow-x: auto;
  margin: 1.6em 0;
  border: 1px solid var(--border-color);
}

.article-content pre code {
  background: none;
  padding: 0;
  font-size: 0.85rem;
  line-height: 1.6;
}

.highlight {
  background: var(--bg-secondary);
  border-radius: var(--radius-md);
  padding: 20px 24px;
  overflow-x: auto;
  margin: 1.6em 0;
  border: 1px solid var(--border-color);
}

.highlight pre {
  background: none;
  padding: 0;
  margin: 0;
  border: none;
}

.highlight code {
  background: none;
  padding: 0;
}


/* ----------------------------- Page Layout ----------------------------- */
.page-header {
  padding: 120px 22px 32px;
  max-width: var(--max-width-narrow);
  margin: 0 auto;
  text-align: center;
}

.page-header h1 {
  font-size: clamp(2rem, 5vw, 3rem);
  font-weight: 700;
  letter-spacing: -0.04em;
  color: var(--text-primary);
}

.page-content {
  max-width: var(--max-width-narrow);
  margin: 0 auto;
  padding: 0 22px 80px;
  font-size: 1.06rem;
  line-height: 1.72;
  color: var(--text-primary);
}

.page-content h2 {
  font-size: 1.5rem;
  font-weight: 600;
  letter-spacing: -0.025em;
  margin-top: 2em;
  margin-bottom: 0.7em;
}

.page-content p {
  margin-bottom: 1.35em;
}

.page-content a {
  color: var(--accent);
}


/* ----------------------------- Footer ----------------------------- */
.site-footer {
  border-top: 1px solid var(--border-color);
  padding: 40px 22px;
  text-align: center;
  background: var(--bg-tertiary);
}

.footer-inner {
  max-width: var(--max-width);
  margin: 0 auto;
}

.footer-links {
  display: flex;
  justify-content: center;
  gap: 24px;
  margin-bottom: 20px;
  list-style: none;
}

.footer-links a {
  font-size: 0.82rem;
  color: var(--text-secondary);
  transition: color var(--transition-fast);
}

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

.footer-copy {
  font-size: 0.76rem;
  color: var(--text-tertiary);
  letter-spacing: -0.01em;
}


/* ----------------------------- Scroll Animations ----------------------------- */
.fade-in {
  opacity: 0;
  transform: translateY(20px);
  transition: opacity var(--transition-slow), transform var(--transition-slow);
}

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

.stagger-1 { transition-delay: 0.05s; }
.stagger-2 { transition-delay: 0.1s; }
.stagger-3 { transition-delay: 0.15s; }
.stagger-4 { transition-delay: 0.2s; }
.stagger-5 { transition-delay: 0.25s; }
.stagger-6 { transition-delay: 0.3s; }


/* ----------------------------- Responsive ----------------------------- */
@media (max-width: 768px) {
  .hero {
    padding: 120px 20px 60px;
  }

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

  .post-card-body {
    padding: 22px 20px 20px;
  }

  .article-header {
    padding: 100px 20px 32px;
  }

  .article-content,
  .page-content {
    padding: 0 20px 60px;
  }

  .hero-links {
    flex-direction: column;
    align-items: center;
  }

  .hero-links a {
    width: 100%;
    max-width: 280px;
    justify-content: center;
  }
}

@media (max-width: 480px) {
  html {
    font-size: 16px;
  }

  .hero h1 {
    font-size: 2rem;
  }

  .nav-inner {
    padding: 0 16px;
  }

  .posts-section {
    padding: 40px 16px 60px;
  }
}


/* ----------------------------- MathJax ----------------------------- */
.MathJax {
  font-size: 1em !important;
}


/* ----------------------------- 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;
}