/* ============================================
   Design System - Warren Buffett Letters
   Following Vercel Web Interface Guidelines
   ============================================ */

/* CSS Custom Properties */
:root {
  /* Colors - Dark Theme (Default) */
  --bg-primary: #0a0a0a;
  --bg-secondary: #111111;
  --bg-card: rgba(255, 255, 255, 0.03);
  --bg-card-hover: rgba(255, 255, 255, 0.08);
  --border-subtle: rgba(255, 255, 255, 0.08);
  --border-hover: rgba(255, 255, 255, 0.15);

  --text-primary: #fafafa;
  --text-secondary: rgba(255, 255, 255, 0.65);
  --text-tertiary: rgba(255, 255, 255, 0.45);

  --accent: #3b82f6;
  --accent-glow: rgba(59, 130, 246, 0.25);
  --accent-hover: #60a5fa;

  --gradient-hero: linear-gradient(135deg, rgba(59, 130, 246, 0.15) 0%, rgba(147, 51, 234, 0.15) 100%);
  --gradient-text: linear-gradient(135deg, #60a5fa 0%, #a78bfa 100%);

  /* Typography */
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --font-chinese: 'Noto Sans SC', 'PingFang SC', 'Microsoft YaHei', sans-serif;

  /* Spacing */
  --space-xs: 0.25rem;
  --space-sm: 0.5rem;
  --space-md: 1rem;
  --space-lg: 1.5rem;
  --space-xl: 2rem;
  --space-2xl: 3rem;
  --space-3xl: 4rem;

  /* Border Radius */
  --radius-sm: 0.375rem;
  --radius-md: 0.75rem;
  --radius-lg: 1rem;
  --radius-xl: 1.5rem;

  /* Transitions */
  --transition-fast: 150ms cubic-bezier(0.4, 0, 0.2, 1);
  --transition-base: 200ms cubic-bezier(0.4, 0, 0.2, 1);
  --transition-slow: 300ms cubic-bezier(0.4, 0, 0.2, 1);

  /* Shadows */
  --shadow-card: 0 4px 24px rgba(0, 0, 0, 0.3);
  --shadow-glow: 0 0 60px var(--accent-glow);

  /* Focus Ring */
  --focus-ring: 0 0 0 2px var(--bg-primary), 0 0 0 4px var(--accent);

  color-scheme: dark;
}

/* Light Theme */
[data-theme="light"] {
  --bg-primary: #fafafa;
  --bg-secondary: #ffffff;
  --bg-card: rgba(0, 0, 0, 0.02);
  --bg-card-hover: rgba(0, 0, 0, 0.05);
  --border-subtle: rgba(0, 0, 0, 0.08);
  --border-hover: rgba(0, 0, 0, 0.15);

  --text-primary: #171717;
  --text-secondary: rgba(0, 0, 0, 0.65);
  --text-tertiary: rgba(0, 0, 0, 0.45);

  --gradient-hero: linear-gradient(135deg, rgba(59, 130, 246, 0.08) 0%, rgba(147, 51, 234, 0.08) 100%);
  --shadow-card: 0 4px 24px rgba(0, 0, 0, 0.08);

  color-scheme: light;
}

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

html {
  font-size: 16px;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

body {
  font-family: var(--font-sans);
  background: var(--bg-primary);
  color: var(--text-primary);
  line-height: 1.6;
  min-height: 100vh;
}

/* Focus Visible - Vercel Guidelines Compliant */
:focus {
  outline: none;
}

:focus-visible {
  outline: none;
  box-shadow: var(--focus-ring);
}

/* Respect reduced motion */
@media (prefers-reduced-motion: reduce) {

  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}

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

.app {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

/* Header */
.header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(10, 10, 10, 0.8);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border-subtle);
  padding: var(--space-md) var(--space-lg);
}

[data-theme="light"] .header {
  background: rgba(250, 250, 250, 0.8);
}

.header-content {
  max-width: 1200px;
  margin: 0 auto;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.logo {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
  font-weight: 600;
  font-size: 1.125rem;
}

.logo-icon {
  font-size: 1.5rem;
}

.theme-toggle {
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-md);
  padding: var(--space-sm) var(--space-md);
  cursor: pointer;
  color: var(--text-primary);
  transition: background var(--transition-fast), border-color var(--transition-fast);
  touch-action: manipulation;
  font-size: 1rem;
}

.theme-toggle:hover {
  background: var(--bg-card-hover);
  border-color: var(--border-hover);
}

.theme-icon-light {
  display: none;
}

[data-theme="light"] .theme-icon-dark {
  display: none;
}

[data-theme="light"] .theme-icon-light {
  display: inline;
}

/* Hero Section */
.hero {
  position: relative;
  padding: var(--space-3xl) var(--space-lg);
  text-align: center;
  overflow: hidden;
}

.hero-content {
  max-width: 800px;
  margin: 0 auto;
  position: relative;
  z-index: 1;
}

.hero-title {
  margin-bottom: var(--space-lg);
  text-wrap: balance;
}

.title-chinese {
  display: block;
  font-family: var(--font-chinese);
  font-size: clamp(2.5rem, 8vw, 4.5rem);
  font-weight: 700;
  background: var(--gradient-text);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
  margin-bottom: var(--space-sm);
  letter-spacing: 0.05em;
}

.title-english {
  display: block;
  font-size: clamp(1rem, 3vw, 1.375rem);
  font-weight: 500;
  color: var(--text-secondary);
}

.hero-subtitle {
  font-size: 1.125rem;
  color: var(--text-secondary);
  max-width: 600px;
  margin: 0 auto var(--space-xl);
  line-height: 1.7;
}

.hero-stats {
  display: flex;
  justify-content: center;
  gap: var(--space-2xl);
  flex-wrap: wrap;
}

.stat {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: var(--space-xs);
}

.stat-number {
  font-size: 2rem;
  font-weight: 700;
  font-variant-numeric: tabular-nums;
  background: var(--gradient-text);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.stat-label {
  font-size: 0.875rem;
  color: var(--text-tertiary);
  text-transform: uppercase;
  letter-spacing: 0.1em;
}

.hero-glow {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 600px;
  height: 400px;
  background: var(--gradient-hero);
  filter: blur(100px);
  opacity: 0.6;
  pointer-events: none;
}

/* Filters Section */
.filters {
  padding: var(--space-lg) var(--space-lg);
  position: sticky;
  top: 60px;
  z-index: 50;
  background: var(--bg-primary);
}

.filters-content {
  max-width: 1200px;
  margin: 0 auto;
  display: flex;
  flex-direction: column;
  gap: var(--space-md);
}

.search-box {
  position: relative;
  max-width: 400px;
}

.search-icon {
  position: absolute;
  left: var(--space-md);
  top: 50%;
  transform: translateY(-50%);
  font-size: 1rem;
  opacity: 0.5;
}

.search-input {
  width: 100%;
  padding: var(--space-sm) var(--space-md) var(--space-sm) 2.75rem;
  font-size: 1rem;
  font-family: inherit;
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-md);
  color: var(--text-primary);
  transition: border-color var(--transition-fast), background var(--transition-fast);
}

.search-input::placeholder {
  color: var(--text-tertiary);
}

.search-input:hover {
  border-color: var(--border-hover);
}

.search-input:focus {
  border-color: var(--accent);
  background: var(--bg-secondary);
}

.decade-filters {
  display: flex;
  gap: var(--space-sm);
  flex-wrap: wrap;
}

.decade-btn {
  padding: var(--space-sm) var(--space-md);
  font-size: 0.875rem;
  font-weight: 500;
  font-family: inherit;
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-sm);
  color: var(--text-secondary);
  cursor: pointer;
  transition: background var(--transition-fast), border-color var(--transition-fast), color var(--transition-fast);
  touch-action: manipulation;
}

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

.decade-btn.active {
  background: var(--accent);
  border-color: var(--accent);
  color: white;
}

/* Letters Grid */
.letters-section {
  flex: 1;
  padding: var(--space-lg);
}

.letters-grid {
  max-width: 1200px;
  margin: 0 auto;
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(140px, 1fr));
  gap: var(--space-md);
}

/* Letter Card */
.letter-card {
  position: relative;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: var(--space-xl) var(--space-md);
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-lg);
  text-decoration: none;
  color: var(--text-primary);
  transition: transform var(--transition-base), background var(--transition-fast), border-color var(--transition-fast), box-shadow var(--transition-base);
  cursor: pointer;
  overflow: hidden;
}

.letter-card::before {
  content: '';
  position: absolute;
  inset: 0;
  background: linear-gradient(135deg, transparent 50%, var(--accent-glow) 150%);
  opacity: 0;
  transition: opacity var(--transition-base);
}

.letter-card:hover {
  transform: translateY(-4px);
  background: var(--bg-card-hover);
  border-color: var(--border-hover);
  box-shadow: var(--shadow-card);
}

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

.letter-year {
  font-size: 1.75rem;
  font-weight: 700;
  font-variant-numeric: tabular-nums;
  position: relative;
  z-index: 1;
}

.letter-label {
  font-size: 0.75rem;
  color: var(--text-tertiary);
  text-transform: uppercase;
  letter-spacing: 0.1em;
  margin-top: var(--space-xs);
  position: relative;
  z-index: 1;
  opacity: 0;
  transform: translateY(4px);
  transition: opacity var(--transition-fast), transform var(--transition-fast);
}

.letter-card:hover .letter-label {
  opacity: 1;
  transform: translateY(0);
  color: var(--accent);
}

.letter-topic {
  font-size: 0.75rem;
  color: var(--text-secondary);
  text-align: center;
  margin-top: var(--space-xs);
  position: relative;
  z-index: 1;
  line-height: 1.3;
  transition: color var(--transition-fast);
}

.letter-card:hover .letter-topic {
  color: var(--text-primary);
}

/* Special styling for notable years */
.letter-card[data-notable="true"] .letter-year {
  background: var(--gradient-text);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

/* No Results */
.no-results {
  text-align: center;
  padding: var(--space-3xl);
  color: var(--text-secondary);
}

.no-results-icon {
  font-size: 3rem;
  display: block;
  margin-bottom: var(--space-md);
}

/* Footer */
.footer {
  padding: var(--space-xl) var(--space-lg);
  text-align: center;
  border-top: 1px solid var(--border-subtle);
  margin-top: auto;
}

.footer-content {
  max-width: 1200px;
  margin: 0 auto;
}

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

.footer a:hover {
  color: var(--accent-hover);
  text-decoration: underline;
}

.footer-note {
  margin-top: var(--space-sm);
  font-size: 0.875rem;
  color: var(--text-tertiary);
}

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

@media (max-width: 768px) {
  .hero {
    padding: var(--space-2xl) var(--space-md);
  }

  .hero-stats {
    gap: var(--space-lg);
  }

  .stat-number {
    font-size: 1.5rem;
  }

  .filters {
    padding: var(--space-md);
  }

  .letters-grid {
    grid-template-columns: repeat(auto-fill, minmax(100px, 1fr));
    gap: var(--space-sm);
  }

  .letter-card {
    padding: var(--space-lg) var(--space-sm);
  }

  .letter-year {
    font-size: 1.25rem;
  }
}

@media (max-width: 480px) {
  .decade-filters {
    overflow-x: auto;
    flex-wrap: nowrap;
    padding-bottom: var(--space-sm);
    -webkit-overflow-scrolling: touch;
  }

  .decade-btn {
    flex-shrink: 0;
  }
}

/* Safe area insets for notched devices */
@supports (padding: env(safe-area-inset-bottom)) {
  .footer {
    padding-bottom: calc(var(--space-xl) + env(safe-area-inset-bottom));
  }
}

/* ============================================
   Reader View
   ============================================ */

.reader-view {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: 100;
  display: flex;
  justify-content: center;
  align-items: center;
  padding: var(--space-md);
  opacity: 0;
  pointer-events: none;
  transition: opacity var(--transition-normal);
}

.reader-view:not([hidden]) {
  opacity: 1;
  pointer-events: auto;
}

.reader-view[hidden] {
  display: none;
}

.reader-backdrop {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: rgba(0, 0, 0, 0.4);
  backdrop-filter: blur(4px);
  -webkit-backdrop-filter: blur(4px);
  z-index: 1;
}

[data-theme="dark"] .reader-backdrop {
  background: rgba(0, 0, 0, 0.6);
}

.reader-content-container {
  position: relative;
  width: 100%;
  max-width: 800px;
  height: 90vh;
  max-height: 90vh;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  box-shadow: 0 20px 50px rgba(0, 0, 0, 0.2);
  z-index: 2;
  display: flex;
  flex-direction: column;
  transform: translateY(20px);
  transition: transform var(--transition-normal);
  overflow: hidden;
}

.reader-view:not([hidden]) .reader-content-container {
  transform: translateY(0);
}

.reader-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: var(--space-lg);
  border-bottom: 1px solid var(--border);
  background: var(--bg-card);
}

.reader-header h2 {
  font-family: var(--font-primary);
  font-size: 1.25rem;
  font-weight: 600;
  color: var(--text-primary);
  margin: 0;
}

.reader-actions {
  display: flex;
  align-items: center;
  gap: var(--space-md);
}

.external-link-btn {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: 0.875rem;
  color: var(--text-secondary);
  text-decoration: none;
  padding: 6px 12px;
  border-radius: var(--radius-md);
  border: 1px solid var(--border);
  transition: all var(--transition-fast);
}

.external-link-btn:hover {
  background: var(--bg-secondary);
  color: var(--text-primary);
}

.close-reader-btn {
  background: none;
  border: none;
  cursor: pointer;
  color: var(--text-secondary);
  padding: 4px;
  border-radius: var(--radius-md);
  transition: color var(--transition-fast), background var(--transition-fast);
  display: flex;
  align-items: center;
  justify-content: center;
}

.close-reader-btn:hover {
  color: var(--text-primary);
  background: var(--bg-secondary);
}

.reader-body {
  flex: 1;
  overflow-y: auto;
  padding: var(--space-xl) var(--space-2xl);
  font-family: var(--font-secondary);
  /* Serif for reading? Or just clean sans */
  font-family: 'Georgia', serif;
  /* Serif for reading */
  font-size: 1.125rem;
  line-height: 1.7;
  color: var(--text-primary);
}

[data-theme="dark"] .reader-body {
  color: #e0e0e0;
}

/* Markdown Styles inside reader */
.reader-body p {
  margin-bottom: 1.5em;
}

.reader-body h3 {
  font-family: var(--font-primary);
  font-size: 1.5rem;
  margin-top: 2em;
  margin-bottom: 1em;
  font-weight: 600;
}

/* Scrollbar for reader */
.reader-body::-webkit-scrollbar {
  width: 8px;
}

.reader-body::-webkit-scrollbar-track {
  background: transparent;
}

.reader-body::-webkit-scrollbar-thumb {
  background-color: var(--border);
  border-radius: 4px;
}

.reader-body::-webkit-scrollbar-thumb:hover {
  background-color: var(--text-tertiary);
}

@media (max-width: 768px) {
  .reader-content-container {
    height: 100vh;
    max-height: 100vh;
    width: 100%;
    border-radius: 0;
    max-width: none;
  }

  .reader-body {
    padding: var(--space-lg);
    font-size: 1rem;
  }
}