/* ═══════════════════════════════════════════════════════════════════════
   MitraSETI Tutorial Series — Shared Stylesheet
   Deep Field Labs · Space-themed, reading-optimised design
   ═══════════════════════════════════════════════════════════════════════ */

@import url('https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700&family=JetBrains+Mono:wght@400;500&family=Playfair+Display:wght@700;800&display=swap');

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

:root {
  --bg-deep:      #0b0e17;
  --bg-surface:   #111827;
  --bg-card:      #1a2035;
  --bg-code:      #0d1117;
  --text-primary: #e2e8f0;
  --text-secondary:#94a3b8;
  --text-muted:   #64748b;
  --accent:       #818cf8;
  --accent-glow:  rgba(129, 140, 248, 0.15);
  --accent-2:     #34d399;
  --accent-3:     #f59e0b;
  --accent-4:     #f472b6;
  --border:       rgba(148, 163, 184, 0.12);
  --border-glow:  rgba(129, 140, 248, 0.3);
  --radius:       12px;
  --radius-sm:    8px;
  --max-w:        780px;
  --font-body:    'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --font-display: 'Playfair Display', Georgia, serif;
  --font-mono:    'JetBrains Mono', 'Fira Code', 'Cascadia Code', monospace;
}

html {
  scroll-behavior: smooth;
  font-size: 17px;
}

body {
  font-family: var(--font-body);
  background: var(--bg-deep);
  color: var(--text-primary);
  line-height: 1.78;
  -webkit-font-smoothing: antialiased;
  min-height: 100vh;
}

/* ── Starfield background ─────────────────────────────────────────────── */
body::before {
  content: '';
  position: fixed; inset: 0;
  pointer-events: none;
  background:
    radial-gradient(1.5px 1.5px at  10%  15%, rgba(255,255,255,.35), transparent),
    radial-gradient(1px   1px   at  25%  50%, rgba(255,255,255,.2),  transparent),
    radial-gradient(1.2px 1.2px at  45%  80%, rgba(255,255,255,.25), transparent),
    radial-gradient(1px   1px   at  60%  20%, rgba(255,255,255,.15), transparent),
    radial-gradient(1.5px 1.5px at  75%  45%, rgba(255,255,255,.3),  transparent),
    radial-gradient(1px   1px   at  85%  70%, rgba(255,255,255,.2),  transparent),
    radial-gradient(1.3px 1.3px at  92%  10%, rgba(255,255,255,.25), transparent),
    radial-gradient(1px   1px   at  30%  35%, rgba(129,140,248,.25), transparent),
    radial-gradient(1px   1px   at  55%  55%, rgba(52,211,153,.2),   transparent),
    radial-gradient(1px   1px   at  80%  90%, rgba(244,114,182,.2),  transparent);
  z-index: -1;
}

/* ── Main container ───────────────────────────────────────────────────── */
.container {
  max-width: var(--max-w);
  margin: 0 auto;
  padding: 2rem 1.5rem 6rem;
}

/* ── Top navigation bar ───────────────────────────────────────────────── */
.learn-topnav {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(11, 14, 23, 0.95);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  border-bottom: 1px solid var(--border);
  padding: 0 24px;
  height: 48px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  font-size: 0.82rem;
}
.learn-topnav a {
  color: var(--text-secondary);
  text-decoration: none;
  transition: color 0.2s;
  font-weight: 500;
}
.learn-topnav a:hover {
  color: var(--accent);
  text-decoration: none;
}
.learn-nav-left {
  display: flex;
  align-items: center;
  gap: 20px;
}
.learn-nav-right {
  display: flex;
  align-items: center;
  gap: 20px;
}
.learn-topnav .brand-link {
  display: flex;
  align-items: center;
  gap: 6px;
  font-weight: 700;
  color: var(--text-primary);
  letter-spacing: -0.01em;
  font-size: 0.9rem;
}
.learn-topnav .brand-link span {
  color: var(--accent);
}
.learn-topnav .nav-sep {
  color: var(--text-muted);
  font-size: 0.75rem;
  user-select: none;
}
.learn-topnav .nav-page {
  display: flex;
  align-items: center;
  gap: 5px;
  padding: 4px 10px;
  border-radius: 6px;
  border: 1px solid transparent;
  transition: all 0.2s;
}
.learn-topnav .nav-page:hover {
  border-color: var(--border);
  background: rgba(129, 140, 248, 0.06);
}
.learn-topnav .nav-page.prev::before { content: '\2190\00a0'; font-size: 0.75rem; }
.learn-topnav .nav-page.next::after { content: '\00a0\2192'; font-size: 0.75rem; }
.learn-topnav .learn-home-link {
  padding: 4px 12px;
  border-radius: 6px;
  border: 1px solid var(--border);
  font-size: 0.78rem;
  letter-spacing: 0.03em;
  text-transform: uppercase;
  font-weight: 600;
  color: var(--accent);
}
.learn-topnav .learn-home-link:hover {
  background: rgba(129, 140, 248, 0.08);
  border-color: var(--accent);
}

/* ── Cloud CTA box ────────────────────────────────────────────────────── */
.cloud-cta-box {
  margin: 3rem 0 2rem;
  padding: 28px 32px;
  background: linear-gradient(135deg, rgba(59, 130, 246, 0.08), rgba(124, 58, 237, 0.08));
  border: 1px solid rgba(59, 130, 246, 0.2);
  border-radius: var(--radius);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
  flex-wrap: wrap;
}
.cloud-cta-box .cta-content h4 {
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--text-primary);
  margin-bottom: 6px;
}
.cloud-cta-box .cta-content p {
  font-size: 0.9rem;
  color: var(--text-secondary);
  line-height: 1.6;
  margin: 0;
}
.cloud-cta-box .cta-btn {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 10px 24px;
  background: linear-gradient(135deg, #3b82f6, #7c3aed);
  color: #fff;
  border-radius: 8px;
  font-size: 0.85rem;
  font-weight: 600;
  text-decoration: none;
  white-space: nowrap;
  transition: all 0.2s;
  box-shadow: 0 4px 16px rgba(59, 130, 246, 0.25);
}
.cloud-cta-box .cta-btn:hover {
  transform: translateY(-2px);
  box-shadow: 0 6px 24px rgba(59, 130, 246, 0.35);
  text-decoration: none;
  color: #fff;
}

@media (max-width: 600px) {
  .learn-topnav { padding: 0 12px; gap: 8px; }
  .learn-nav-left, .learn-nav-right { gap: 10px; }
  .learn-topnav .nav-page { padding: 4px 6px; font-size: 0.75rem; }
  .cloud-cta-box { flex-direction: column; text-align: center; }
}

/* ── Hero / Chapter header ────────────────────────────────────────────── */
.chapter-hero {
  text-align: center;
  padding: 3.5rem 1.5rem 3rem;
  margin-bottom: 3rem;
  position: relative;
  border-bottom: 1px solid var(--border);
}

.chapter-hero::after {
  content: '';
  position: absolute;
  bottom: -1px;
  left: 50%; transform: translateX(-50%);
  width: 120px; height: 2px;
  background: linear-gradient(90deg, transparent, var(--accent), transparent);
}

.chapter-number {
  display: inline-block;
  font-family: var(--font-mono);
  font-size: .85rem;
  font-weight: 500;
  color: var(--accent);
  letter-spacing: .15em;
  text-transform: uppercase;
  margin-bottom: .75rem;
  background: var(--accent-glow);
  padding: .25rem .85rem;
  border-radius: 999px;
  border: 1px solid var(--border-glow);
}

.chapter-hero h1 {
  font-family: var(--font-display);
  font-size: clamp(2rem, 5vw, 2.8rem);
  font-weight: 800;
  line-height: 1.2;
  color: #fff;
  margin-bottom: .75rem;
  letter-spacing: -.02em;
}

.chapter-hero .subtitle {
  font-size: 1.1rem;
  color: var(--text-secondary);
  font-weight: 300;
  max-width: 520px;
  margin: 0 auto;
  line-height: 1.6;
}

.chapter-hero .meta {
  margin-top: 1.2rem;
  font-size: .82rem;
  color: var(--text-muted);
}
.chapter-hero .meta span + span::before {
  content: '·';
  margin: 0 .5rem;
}

/* ── Headings ─────────────────────────────────────────────────────────── */
h2 {
  font-family: var(--font-display);
  font-size: 1.7rem;
  font-weight: 700;
  color: #fff;
  margin: 3rem 0 1.2rem;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
  letter-spacing: -.01em;
}

h3 {
  font-size: 1.2rem;
  font-weight: 600;
  color: var(--accent);
  margin: 2rem 0 .8rem;
}

h4 {
  font-size: 1.05rem;
  font-weight: 600;
  color: var(--text-primary);
  margin: 1.5rem 0 .6rem;
}

/* ── Paragraphs & inline ──────────────────────────────────────────────── */
p {
  margin-bottom: 1.2rem;
  color: var(--text-primary);
}

strong { color: #fff; font-weight: 600; }
em { color: var(--text-secondary); font-style: italic; }

a {
  color: var(--accent);
  text-decoration: none;
  border-bottom: 1px solid transparent;
  transition: border-color .2s;
}
a:hover { border-bottom-color: var(--accent); }

/* ── Lists ────────────────────────────────────────────────────────────── */
ul, ol {
  margin: 0 0 1.2rem 1.2rem;
  color: var(--text-primary);
}
li { margin-bottom: .4rem; }
li::marker { color: var(--accent); }

/* ── Code ─────────────────────────────────────────────────────────────── */
code {
  font-family: var(--font-mono);
  font-size: .88em;
  background: rgba(129, 140, 248, 0.1);
  color: var(--accent);
  padding: .15em .4em;
  border-radius: 4px;
}

pre {
  background: var(--bg-code);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 1.2rem 1.5rem;
  margin: 1.2rem 0 1.5rem;
  overflow-x: auto;
  line-height: 1.6;
  position: relative;
}
pre code {
  background: none;
  color: var(--text-primary);
  padding: 0;
  font-size: .85rem;
}

pre .lang-label {
  position: absolute;
  top: .5rem; right: .75rem;
  font-family: var(--font-mono);
  font-size: .7rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: .08em;
}

/* ── Tables ───────────────────────────────────────────────────────────── */
.table-wrap {
  overflow-x: auto;
  margin: 1.2rem 0 1.5rem;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
}

table {
  width: 100%;
  border-collapse: collapse;
  font-size: .92rem;
}

thead {
  background: rgba(129, 140, 248, 0.08);
}
th {
  text-align: left;
  padding: .75rem 1rem;
  font-weight: 600;
  color: var(--accent);
  font-size: .82rem;
  text-transform: uppercase;
  letter-spacing: .06em;
  border-bottom: 1px solid var(--border);
}
td {
  padding: .65rem 1rem;
  border-bottom: 1px solid var(--border);
  color: var(--text-primary);
}
tr:last-child td { border-bottom: none; }
tr:hover td { background: rgba(129, 140, 248, 0.04); }

/* ── Callout boxes ────────────────────────────────────────────────────── */
.callout {
  border-radius: var(--radius);
  padding: 1.2rem 1.5rem;
  margin: 1.5rem 0;
  border-left: 4px solid;
  position: relative;
}
.callout-icon {
  position: absolute;
  top: 1.1rem; left: -14px;
  width: 24px; height: 24px;
  border-radius: 50%;
  display: flex; align-items: center; justify-content: center;
  font-size: .8rem;
}

.callout.insight {
  background: rgba(129, 140, 248, 0.06);
  border-color: var(--accent);
}
.callout.insight .callout-icon { background: var(--accent); color: #fff; }

.callout.key-concept {
  background: rgba(52, 211, 153, 0.06);
  border-color: var(--accent-2);
}
.callout.key-concept .callout-icon { background: var(--accent-2); color: #000; }

.callout.warning {
  background: rgba(245, 158, 11, 0.06);
  border-color: var(--accent-3);
}
.callout.warning .callout-icon { background: var(--accent-3); color: #000; }

.callout.fun-fact {
  background: rgba(244, 114, 182, 0.06);
  border-color: var(--accent-4);
}
.callout.fun-fact .callout-icon { background: var(--accent-4); color: #fff; }

.callout h4 {
  margin: 0 0 .4rem;
  font-size: .9rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: .08em;
}
.callout.insight h4 { color: var(--accent); }
.callout.key-concept h4 { color: var(--accent-2); }
.callout.warning h4 { color: var(--accent-3); }
.callout.fun-fact h4 { color: var(--accent-4); }

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

/* ── Illustrations (inline SVG containers) ────────────────────────────── */
.illustration {
  margin: 2rem auto;
  padding: 1.5rem;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  text-align: center;
  overflow: hidden;
}
.illustration svg {
  max-width: 100%;
  height: auto;
}
.illustration .caption {
  display: block;
  margin-top: .8rem;
  font-size: .85rem;
  color: var(--text-muted);
  font-style: italic;
}

/* ── Formula blocks ───────────────────────────────────────────────────── */
.formula {
  text-align: center;
  padding: 1.2rem;
  margin: 1.2rem 0;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  font-family: var(--font-mono);
  font-size: 1.05rem;
  color: var(--accent);
  letter-spacing: .02em;
  overflow-x: auto;
}

/* ── ASCII art / diagrams ─────────────────────────────────────────────── */
.ascii-diagram {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 1.2rem 1.5rem;
  margin: 1.2rem 0;
  text-align: center;
  font-family: var(--font-mono);
  font-size: .82rem;
  line-height: 1.5;
  color: var(--text-secondary);
  white-space: pre;
  overflow-x: auto;
}

/* ── Navigation (prev/next) ───────────────────────────────────────────── */
.chapter-nav {
  display: flex;
  gap: 1rem;
  margin-top: 4rem;
  padding-top: 2rem;
  border-top: 1px solid var(--border);
}
.chapter-nav a {
  flex: 1;
  display: block;
  padding: 1rem 1.2rem;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  transition: all .25s;
  text-decoration: none;
}
.chapter-nav a:hover {
  border-color: var(--accent);
  background: var(--accent-glow);
}
.chapter-nav .nav-label {
  display: block;
  font-size: .75rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: .1em;
  margin-bottom: .25rem;
}
.chapter-nav .nav-title {
  font-weight: 600;
  color: var(--accent);
  font-size: .95rem;
}
.chapter-nav .next { text-align: right; }

/* ── Table of contents (sidebar on wide, inline on narrow) ────────────── */
.toc {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.5rem;
  margin: 2rem 0;
}
.toc h3 {
  color: var(--text-muted);
  font-size: .8rem;
  text-transform: uppercase;
  letter-spacing: .12em;
  margin: 0 0 .8rem;
}
.toc ol {
  list-style: none;
  margin: 0;
  padding: 0;
  counter-reset: toc;
}
.toc li {
  counter-increment: toc;
  margin: 0;
  padding: .35rem 0;
  border-bottom: 1px solid rgba(148, 163, 184, 0.06);
}
.toc li:last-child { border-bottom: none; }
.toc a {
  color: var(--text-secondary);
  font-size: .9rem;
  display: flex;
  align-items: baseline;
  gap: .5rem;
  transition: color .2s;
}
.toc a::before {
  content: counter(toc, decimal-leading-zero);
  font-family: var(--font-mono);
  font-size: .75rem;
  color: var(--accent);
  min-width: 1.5rem;
}
.toc a:hover { color: var(--accent); }

/* ── Footer ───────────────────────────────────────────────────────────── */
.footer {
  text-align: center;
  padding: 2rem 0;
  margin-top: 4rem;
  border-top: 1px solid var(--border);
  font-size: .82rem;
  color: var(--text-muted);
}
.footer a { color: var(--accent); }

/* ── Equation numbering ───────────────────────────────────────────────── */
.formula .eq-label {
  float: right;
  font-size: .8rem;
  color: var(--text-muted);
}

/* ── Keyword highlights ───────────────────────────────────────────────── */
.kw { color: var(--accent); font-weight: 600; }
.kw2 { color: var(--accent-2); font-weight: 600; }
.kw3 { color: var(--accent-3); font-weight: 600; }
.kw4 { color: var(--accent-4); font-weight: 600; }

/* ── Reading progress bar ─────────────────────────────────────────────── */
.progress-bar {
  position: fixed;
  top: 0; left: 0;
  width: 0%;
  height: 3px;
  background: linear-gradient(90deg, var(--accent), var(--accent-2));
  z-index: 9999;
  transition: width .1s linear;
}

/* ── Overview page: chapter grid ──────────────────────────────────────── */
.chapter-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 1rem;
  margin: 2rem 0;
}
@media (min-width: 540px) {
  .chapter-grid { grid-template-columns: 1fr 1fr; }
}

.chapter-card {
  display: block;
  padding: 1.2rem 1.4rem;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  transition: all .25s;
  text-decoration: none;
}
.chapter-card:hover {
  border-color: var(--accent);
  transform: translateY(-2px);
  box-shadow: 0 8px 24px rgba(129, 140, 248, 0.1);
}

.chapter-card .card-num {
  font-family: var(--font-mono);
  font-size: .75rem;
  color: var(--accent);
  letter-spacing: .1em;
}
.chapter-card .card-title {
  font-weight: 600;
  color: #fff;
  font-size: 1rem;
  margin: .3rem 0;
}
.chapter-card .card-desc {
  font-size: .85rem;
  color: var(--text-muted);
  line-height: 1.5;
}
.chapter-card .card-level {
  display: inline-block;
  margin-top: .5rem;
  font-size: .72rem;
  padding: .15rem .6rem;
  border-radius: 999px;
  border: 1px solid var(--border);
  color: var(--text-muted);
}

/* ── Responsive ───────────────────────────────────────────────────────── */
@media (max-width: 600px) {
  html { font-size: 15.5px; }
  .container { padding: 1.2rem 1rem 4rem; }
  .chapter-hero { padding: 2rem 1rem; }
  .chapter-nav { flex-direction: column; }
  pre { padding: .8rem 1rem; font-size: .8rem; }
}

/* ── Back-to-top link (added via JS if desired) ───────────────────────── */
.back-to-top {
  position: fixed;
  bottom: 2rem;
  right: 2rem;
  width: 40px;
  height: 40px;
  border-radius: 50%;
  background: var(--bg-card);
  border: 1px solid var(--border);
  color: var(--accent);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.2rem;
  cursor: pointer;
  opacity: 0;
  transition: opacity .3s, transform .2s;
  z-index: 100;
}
.back-to-top.visible { opacity: 1; }
.back-to-top:hover { transform: translateY(-2px); border-color: var(--accent); }

/* ── Smooth image transitions ─────────────────────────────────────────── */
.illustration {
  transition: transform .2s;
}
.illustration:hover {
  transform: scale(1.01);
}

/* ── Better code block styling ────────────────────────────────────────── */
pre {
  box-shadow: 0 2px 8px rgba(0,0,0,.2);
}

/* ── Callout animation ────────────────────────────────────────────────── */
.callout {
  transition: border-color .2s, box-shadow .2s;
}
.callout:hover {
  box-shadow: 0 2px 12px rgba(129, 140, 248, 0.08);
}

/* ── Chapter card improvements ────────────────────────────────────────── */
.chapter-card .card-num {
  font-size: .82rem;
  font-weight: 600;
}

/* ── Selection color ──────────────────────────────────────────────────── */
::selection {
  background: rgba(129, 140, 248, 0.3);
  color: #fff;
}

/* ── Print ────────────────────────────────────────────────────────────── */
@media print {
  body { background: #fff; color: #111; }
  body::before { display: none; }
  .progress-bar, .chapter-nav, .back-to-top { display: none; }
  a { color: #111; border: none; }
  pre { border: 1px solid #ccc; background: #f5f5f5; box-shadow: none; }
  .callout { border-left-width: 3px; }
  .illustration:hover { transform: none; }
}