/* ============================================================
   ALE · Minimalist Equation Visualizer — Styles
   ============================================================ */

:root {
  --bg: #fafaf9;
  --fg: #1a1a1a;
  --fg-muted: #8a8a8a;
  --fg-faint: #c5c5c5;
  --accent: #2563eb;
  --accent-light: #dbeafe;
  --accent-glow: rgba(37, 99, 235, 0.12);
  --highlight: #fbbf24;
  --highlight-bg: rgba(251, 191, 36, 0.15);
  --surface: #ffffff;
  --border: #e5e5e5;
  --radius: 12px;
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
  --font-serif: 'EB Garamond', 'Georgia', serif;
  --transition: 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

/* Dark mode (optional, auto-detects) */
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #0f0f0f;
    --fg: #e5e5e5;
    --fg-muted: #737373;
    --fg-faint: #404040;
    --surface: #1a1a1a;
    --border: #2a2a2a;
    --accent-light: #1e3a5f;
    --accent-glow: rgba(37, 99, 235, 0.2);
    --highlight-bg: rgba(251, 191, 36, 0.1);
  }
}

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

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

body {
  background: var(--bg);
  color: var(--fg);
  font-family: var(--font-sans);
  line-height: 1.6;
  overflow-x: hidden;
  padding-bottom: 100px;
}

/* ---- Progress bar ---- */
#progress-bar {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 2px;
  z-index: 1000;
  background: var(--border);
}
#progress-fill {
  height: 100%;
  width: 0%;
  background: var(--accent);
  transition: width 0.4s ease;
}

/* ---- Header ---- */
#header {
  position: fixed;
  top: 2px;
  left: 0;
  right: 0;
  z-index: 100;
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 14px 32px;
  background: var(--bg);
  opacity: 0.95;
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border);
}
.header-left { display: flex; align-items: baseline; gap: 12px; }
.logo {
  font-weight: 600;
  font-size: 1.1rem;
  letter-spacing: 0.08em;
  color: var(--accent);
}
.subtitle {
  font-size: 0.78rem;
  color: var(--fg-muted);
  font-weight: 400;
  letter-spacing: 0.02em;
}
.header-right { display: flex; align-items: center; gap: 16px; }
#eq-counter {
  font-size: 0.78rem;
  color: var(--fg-muted);
  font-variant-numeric: tabular-nums;
}
#btn-toc {
  background: none;
  border: none;
  color: var(--fg-muted);
  cursor: pointer;
  padding: 4px;
  border-radius: 6px;
  transition: var(--transition);
}
#btn-toc:hover { color: var(--fg); background: var(--accent-glow); }

/* ---- TOC Panel ---- */
#toc-panel {
  position: fixed;
  top: 0;
  right: 0;
  width: 320px;
  height: 100vh;
  background: var(--surface);
  border-left: 1px solid var(--border);
  z-index: 200;
  padding: 24px;
  overflow-y: auto;
  transform: translateX(100%);
  transition: transform 0.35s cubic-bezier(0.4, 0, 0.2, 1);
}
#toc-panel.visible { transform: translateX(0); }
#toc-panel.hidden { pointer-events: none; }
#toc-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.3);
  z-index: 199;
  opacity: 0;
  transition: opacity 0.3s;
  pointer-events: none;
}
#toc-overlay.visible { opacity: 1; pointer-events: auto; }
.toc-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 20px;
}
.toc-header h3 {
  font-size: 0.9rem;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--fg-muted);
}
#btn-toc-close {
  background: none;
  border: none;
  font-size: 1.4rem;
  color: var(--fg-muted);
  cursor: pointer;
}
.toc-item {
  display: block;
  padding: 10px 12px;
  margin-bottom: 2px;
  border-radius: 8px;
  text-decoration: none;
  color: var(--fg);
  font-size: 0.85rem;
  cursor: pointer;
  transition: var(--transition);
}
.toc-item:hover { background: var(--accent-glow); }
.toc-item.active { background: var(--accent-light); color: var(--accent); font-weight: 500; }
.toc-item .toc-number {
  color: var(--fg-muted);
  font-size: 0.75rem;
  margin-right: 8px;
  font-variant-numeric: tabular-nums;
}
.toc-section-label {
  font-size: 0.7rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--fg-faint);
  padding: 16px 12px 6px;
  font-weight: 500;
}

/* ---- Main equation flow ---- */
#equation-flow {
  max-width: 720px;
  margin: 0 auto;
  padding: 100px 24px 60px;
}

/* ---- Equation card ---- */
.eq-card {
  position: relative;
  margin-bottom: 12px;
  padding: 28px 32px;
  border-radius: var(--radius);
  background: var(--surface);
  border: 1px solid var(--border);
  transition: opacity 0.5s ease, transform 0.5s ease, border-color 0.3s ease;
  opacity: 0;
  transform: translateY(20px);
}
.eq-card.visible {
  opacity: 1;
  transform: translateY(0);
}
.eq-card.faded {
  opacity: 0.35;
}
.eq-card.faded:hover {
  opacity: 0.7;
}
.eq-card.current {
  opacity: 1;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px var(--accent-glow);
}
.eq-card.referenced {
  opacity: 0.85;
  border-color: var(--highlight);
  box-shadow: 0 0 0 3px var(--highlight-bg);
}

/* Equation number badge */
.eq-number {
  position: absolute;
  top: -10px;
  left: 20px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 20px;
  padding: 2px 12px;
  font-size: 0.7rem;
  color: var(--fg-muted);
  font-weight: 500;
  letter-spacing: 0.04em;
}
.eq-card.current .eq-number {
  background: var(--accent);
  color: white;
  border-color: var(--accent);
}

/* Equation label / title */
.eq-label {
  font-family: var(--font-serif);
  font-size: 0.92rem;
  color: var(--fg-muted);
  margin-bottom: 12px;
  font-style: italic;
}

/* Math container */
.eq-math {
  font-size: 1.15rem;
  overflow-x: auto;
  padding: 8px 0;
  text-align: center;
}
.eq-math .katex-display {
  margin: 0;
}

/* Description / annotation */
.eq-desc {
  font-size: 0.82rem;
  color: var(--fg-muted);
  margin-top: 14px;
  line-height: 1.65;
  font-family: var(--font-serif);
}

/* Term highlights within equations */
.eq-term {
  display: inline-block;
  position: relative;
  cursor: pointer;
  transition: var(--transition);
  border-radius: 4px;
  padding: 2px 4px;
  margin: -2px -4px;
}
.eq-term:hover {
  background: var(--accent-glow);
}
.eq-term.highlighted {
  background: var(--highlight-bg);
  box-shadow: 0 0 0 2px var(--highlight);
}

/* Tooltip for term info */
.term-tooltip {
  position: absolute;
  bottom: calc(100% + 8px);
  left: 50%;
  transform: translateX(-50%);
  background: var(--fg);
  color: var(--bg);
  padding: 8px 14px;
  border-radius: 8px;
  font-size: 0.78rem;
  white-space: nowrap;
  pointer-events: none;
  opacity: 0;
  transition: opacity 0.2s;
  z-index: 50;
}
.eq-term:hover .term-tooltip {
  opacity: 1;
}

/* Transformation annotation */
.eq-transform {
  display: flex;
  align-items: center;
  gap: 8px;
  margin: -4px 0 8px 0;
  padding: 0 32px;
  font-size: 0.75rem;
  color: var(--fg-faint);
}
.eq-transform svg {
  flex-shrink: 0;
}
.eq-transform-label {
  font-style: italic;
  font-family: var(--font-serif);
}

/* Reference links */
.eq-ref-link {
  color: var(--accent);
  cursor: pointer;
  text-decoration: none;
  border-bottom: 1px dashed var(--accent);
  font-weight: 500;
  transition: var(--transition);
}
.eq-ref-link:hover {
  background: var(--accent-glow);
  border-bottom-style: solid;
}

/* ---- Bottom navigation ---- */
#bottom-nav {
  position: fixed;
  bottom: 0;
  left: 0;
  right: 0;
  display: flex;
  align-items: center;
  gap: 16px;
  padding: 16px 32px;
  background: var(--bg);
  opacity: 0.97;
  backdrop-filter: blur(12px);
  border-top: 1px solid var(--border);
  z-index: 100;
}
#btn-prev, #btn-next {
  background: none;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 8px 10px;
  color: var(--fg);
  cursor: pointer;
  transition: var(--transition);
  display: flex;
  align-items: center;
}
#btn-prev:hover, #btn-next:hover {
  background: var(--accent-glow);
  border-color: var(--accent);
}
#btn-prev:disabled, #btn-next:disabled {
  opacity: 0.3;
  cursor: not-allowed;
}

/* Timeline */
#timeline {
  flex: 1;
  position: relative;
  height: 20px;
  display: flex;
  align-items: center;
}
#timeline-track {
  width: 100%;
  height: 2px;
  background: var(--border);
  border-radius: 1px;
  position: relative;
}
#timeline-progress {
  height: 100%;
  background: var(--accent);
  border-radius: 1px;
  width: 0%;
  transition: width 0.4s ease;
}
#timeline-dots {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 100%;
  display: flex;
  align-items: center;
}
.timeline-dot {
  position: absolute;
  width: 8px;
  height: 8px;
  border-radius: 50%;
  background: var(--border);
  border: 2px solid var(--bg);
  cursor: pointer;
  transition: var(--transition);
  transform: translateX(-50%);
}
.timeline-dot.active {
  background: var(--accent);
  width: 10px;
  height: 10px;
}
.timeline-dot.visited {
  background: var(--accent);
  opacity: 0.5;
}
.timeline-dot:hover {
  transform: translateX(-50%) scale(1.4);
}
.timeline-dot.section-start {
  width: 10px;
  height: 10px;
}

/* ---- Reference connection lines ---- */
.ref-lines-svg {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  pointer-events: none;
  z-index: 50;
}
.ref-line {
  fill: none;
  stroke: var(--highlight);
  stroke-width: 1.5;
  stroke-dasharray: 6 4;
  opacity: 0;
  transition: opacity 0.4s ease;
}
.ref-line.visible {
  opacity: 0.6;
}

/* ---- Section divider ---- */
.section-divider {
  text-align: center;
  padding: 40px 0 24px;
  position: relative;
}
.section-divider::before {
  content: '';
  position: absolute;
  top: 50%;
  left: 10%;
  right: 10%;
  height: 1px;
  background: var(--border);
}
.section-title {
  position: relative;
  display: inline-block;
  background: var(--bg);
  padding: 0 20px;
  font-family: var(--font-serif);
  font-size: 1.1rem;
  color: var(--fg);
  font-weight: 400;
}
.section-subtitle {
  display: block;
  font-size: 0.75rem;
  color: var(--fg-faint);
  margin-top: 4px;
  font-family: var(--font-sans);
  letter-spacing: 0.05em;
  text-transform: uppercase;
}

/* ---- Animations ---- */
@keyframes fadeInUp {
  from { opacity: 0; transform: translateY(24px); }
  to   { opacity: 1; transform: translateY(0); }
}
@keyframes pulse {
  0%, 100% { box-shadow: 0 0 0 3px var(--accent-glow); }
  50% { box-shadow: 0 0 0 6px var(--accent-glow); }
}
.eq-card.entering {
  animation: fadeInUp 0.5s cubic-bezier(0.4, 0, 0.2, 1) forwards;
}
.eq-card.current {
  animation: pulse 2.5s ease-in-out infinite;
}

/* ---- Responsive ---- */
@media (max-width: 640px) {
  #header { padding: 12px 16px; }
  #bottom-nav { padding: 12px 16px; }
  #equation-flow { padding: 80px 16px 60px; }
  .eq-card { padding: 20px 18px; }
  .eq-math { font-size: 1rem; }
  #toc-panel { width: 280px; }
}

/* ---- Scrollbar ---- */
::-webkit-scrollbar { width: 6px; }
::-webkit-scrollbar-track { background: transparent; }
::-webkit-scrollbar-thumb { background: var(--fg-faint); border-radius: 3px; }
::-webkit-scrollbar-thumb:hover { background: var(--fg-muted); }

/* ---- Comparison panel ---- */
#compare-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.55);
  z-index: 299;
  opacity: 0;
  transition: opacity 0.35s ease;
  pointer-events: none;
  backdrop-filter: blur(4px);
}
#compare-overlay.visible {
  opacity: 1;
  pointer-events: auto;
}

#compare-panel {
  position: fixed;
  bottom: 0;
  left: 50%;
  transform: translateX(-50%) translateY(100%);
  width: min(96vw, 1100px);
  max-height: 85vh;
  z-index: 300;
  background: var(--surface);
  border: 1px solid var(--border);
  border-bottom: none;
  border-radius: var(--radius) var(--radius) 0 0;
  box-shadow: 0 -8px 40px rgba(0, 0, 0, 0.15);
  transition: transform 0.4s cubic-bezier(0.4, 0, 0.2, 1);
  display: flex;
  flex-direction: column;
  overflow: hidden;
}
#compare-panel.visible {
  transform: translateX(-50%) translateY(0);
}

.compare-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 16px 24px;
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
}
.compare-title {
  font-size: 0.8rem;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--fg-muted);
}
#btn-compare-close {
  background: none;
  border: none;
  font-size: 1.5rem;
  color: var(--fg-muted);
  cursor: pointer;
  padding: 0 4px;
  line-height: 1;
  transition: var(--transition);
  border-radius: 6px;
}
#btn-compare-close:hover {
  color: var(--fg);
  background: var(--accent-glow);
}

.compare-body {
  display: flex;
  align-items: stretch;
  gap: 0;
  padding: 24px;
  overflow-y: auto;
  flex: 1;
}

.compare-card {
  flex: 1;
  min-width: 0;
  padding: 24px 20px;
  border-radius: var(--radius);
  border: 1px solid var(--border);
  background: var(--bg);
  position: relative;
  display: flex;
  flex-direction: column;
  gap: 8px;
}
.compare-card .compare-badge {
  position: absolute;
  top: -10px;
  left: 16px;
  padding: 2px 12px;
  background: var(--accent);
  color: white;
  font-size: 0.65rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  border-radius: 20px;
}
.compare-card .compare-badge.ref {
  background: var(--highlight);
  color: #1a1a1a;
}
.compare-eq-number {
  font-size: 0.78rem;
  color: var(--fg-muted);
  font-weight: 500;
  font-variant-numeric: tabular-nums;
  margin-top: 4px;
}
.compare-eq-label {
  font-family: var(--font-serif);
  font-size: 0.9rem;
  color: var(--fg);
  font-style: italic;
}
.compare-eq-math {
  font-size: 1.05rem;
  overflow-x: auto;
  padding: 12px 0;
  text-align: center;
}
.compare-eq-math .katex-display {
  margin: 0;
}
.compare-eq-desc {
  font-size: 0.78rem;
  color: var(--fg-muted);
  line-height: 1.55;
  font-family: var(--font-serif);
}

.compare-arrow {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 6px;
  padding: 0 16px;
  color: var(--fg-faint);
  flex-shrink: 0;
}
.compare-arrow svg {
  transform: rotate(180deg);
}
.compare-arrow-label {
  font-size: 0.65rem;
  font-family: var(--font-sans);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--fg-faint);
  writing-mode: vertical-rl;
  text-orientation: mixed;
}

.compare-footer {
  padding: 12px 24px 16px;
  border-top: 1px solid var(--border);
  display: flex;
  justify-content: center;
  flex-shrink: 0;
}
.compare-goto-btn {
  background: none;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 8px 20px;
  color: var(--accent);
  font-size: 0.8rem;
  font-weight: 500;
  cursor: pointer;
  transition: var(--transition);
  font-family: var(--font-sans);
}
.compare-goto-btn:hover {
  background: var(--accent-glow);
  border-color: var(--accent);
}

/* Responsive: stack vertically on narrow screens */
@media (max-width: 700px) {
  .compare-body {
    flex-direction: column;
    gap: 12px;
  }
  .compare-arrow {
    flex-direction: row;
    padding: 8px 0;
    writing-mode: horizontal-tb;
  }
  .compare-arrow svg {
    transform: rotate(-90deg);
  }
  .compare-arrow-label {
    writing-mode: horizontal-tb;
    text-orientation: initial;
  }
  #compare-panel {
    max-height: 92vh;
  }
}