@import url('https://fonts.googleapis.com/css2?family=Source+Serif+4:ital,opsz,wght@0,8..60,400;0,8..60,600;1,8..60,400&family=DM+Sans:wght@400;500;700&display=swap');

/* ─── Design tokens ───────────────────────────────────────────────────────── */
:root {
  --bg:          #0d1117;
  --bg-surface:  #161b22;
  --bg-card:     #1c2333;
  --border:      #30363d;
  --text:        #e6edf3;
  --text-muted:  #8b949e;
  --accent:      #d4a017;       /* amber */
  --accent-dim:  #8b6914;
  --link:        #58a6ff;
  --link-hover:  #79c0ff;
  --tag-bg:      #1f3a2a;
  --tag-text:    #3fb950;
  --code-bg:     #161b22;

  --font-serif:  'Source Serif 4', Georgia, serif;
  --font-sans:   'DM Sans', system-ui, sans-serif;
  --font-mono:   'SFMono-Regular', Consolas, 'Liberation Mono', Menlo, monospace;

  --max-w:       900px;
  --chapter-w:   720px;
}

/* ─── Reset ───────────────────────────────────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; }
body {
  background: var(--bg);
  color: var(--text);
  font-family: var(--font-serif);
  font-size: 18px;
  line-height: 1.75;
}
a { color: var(--link); text-decoration: none; }
a:hover { color: var(--link-hover); text-decoration: underline; }
img { max-width: 100%; }

/* ─── Site header ─────────────────────────────────────────────────────────── */
.site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: var(--bg-surface);
  border-bottom: 1px solid var(--border);
}
.header-inner {
  max-width: var(--max-w);
  margin: 0 auto;
  padding: 0 1.5rem;
  height: 56px;
  display: flex;
  align-items: center;
  justify-content: space-between;
}
.site-title {
  font-family: var(--font-sans);
  font-weight: 700;
  font-size: 1rem;
  color: var(--accent);
  letter-spacing: 0.03em;
}
.site-title:hover { text-decoration: none; color: var(--accent); }
.top-nav { display: flex; gap: 1.5rem; font-family: var(--font-sans); font-size: 0.875rem; }

/* Chapter header breadcrumb */
.chapter-header {
  background: var(--bg-surface);
  border-bottom: 1px solid var(--border);
  padding: 0.5rem 1.5rem;
  font-family: var(--font-sans);
  font-size: 0.8rem;
  color: var(--text-muted);
  max-width: 100%;
}
.chapter-header a { color: var(--text-muted); }
.chapter-header a:hover { color: var(--link); }
.chapter-header .sep { margin: 0 0.5rem; }

/* ─── Hero ────────────────────────────────────────────────────────────────── */
.hero {
  background: linear-gradient(135deg, #0d1117 0%, #1a2038 100%);
  border-bottom: 1px solid var(--border);
  padding: 5rem 1.5rem;
  text-align: center;
}
.hero-inner { max-width: var(--max-w); margin: 0 auto; }
.hero h1 {
  font-size: clamp(2rem, 5vw, 3.5rem);
  font-weight: 700;
  color: var(--accent);
  margin-bottom: 1rem;
  letter-spacing: -0.02em;
}
.hero-sub {
  font-size: 1.125rem;
  color: var(--text-muted);
  max-width: 600px;
  margin: 0 auto 2rem;
  font-family: var(--font-sans);
}
.btn-primary {
  display: inline-block;
  background: var(--accent);
  color: #000;
  font-family: var(--font-sans);
  font-weight: 600;
  font-size: 0.95rem;
  padding: 0.65rem 1.75rem;
  border-radius: 6px;
  transition: background 0.15s;
}
.btn-primary:hover { background: var(--accent-dim); color: #fff; text-decoration: none; }

/* ─── Author section ─────────────────────────────────────────────────────── */
.author-section {
  border-top: 1px solid var(--border);
  border-bottom: 1px solid var(--border);
  padding: 2.5rem 1.5rem;
  background: var(--bg-surface);
}
.author-inner {
  max-width: var(--max-w);
  margin: 0 auto;
  display: flex;
  align-items: flex-start;
  gap: 2rem;
}
.author-bio { flex: 1; }
.author-bio h2 {
  font-family: var(--font-sans);
  font-size: 1.25rem;
  font-weight: 700;
  color: var(--text);
  margin-bottom: 0.4rem;
}
.author-badge {
  display: inline-block;
  font-family: var(--font-mono);
  font-size: 0.72rem;
  background: #2a1f00;
  color: var(--accent);
  border: 1px solid var(--accent-dim);
  border-radius: 4px;
  padding: 0.15rem 0.55rem;
  margin-bottom: 0.85rem;
  letter-spacing: 0.03em;
}
.author-bio p {
  font-family: var(--font-sans);
  font-size: 0.9rem;
  color: var(--text-muted);
  line-height: 1.65;
  margin-bottom: 1rem;
  max-width: 560px;
}
.author-links {
  display: flex;
  flex-wrap: wrap;
  gap: 0.6rem;
}
.author-links a {
  font-family: var(--font-mono);
  font-size: 0.78rem;
  color: var(--text-muted);
  border: 1px solid var(--border);
  border-radius: 4px;
  padding: 0.25rem 0.7rem;
  transition: border-color 0.15s, color 0.15s;
}
.author-links a:hover {
  border-color: var(--accent);
  color: var(--accent);
  text-decoration: none;
}
@media (max-width: 500px) {
  .author-inner { flex-direction: column; gap: 1rem; }
}

/* ─── Chapter grid ────────────────────────────────────────────────────────── */
.chapter-grid {
  max-width: var(--max-w);
  margin: 4rem auto;
  padding: 0 1.5rem;
}
.chapter-grid h2 {
  font-family: var(--font-sans);
  font-size: 0.875rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--text-muted);
  margin-bottom: 1.5rem;
}
.grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 1rem;
}
.card {
  display: block;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1.25rem 1.5rem;
  transition: border-color 0.15s, transform 0.15s;
  position: relative;
}
.card:hover { border-color: var(--accent-dim); transform: translateY(-2px); text-decoration: none; }
.card-num {
  font-family: var(--font-mono);
  font-size: 0.75rem;
  color: var(--text-muted);
  display: block;
  margin-bottom: 0.5rem;
}
.card h3 { font-size: 1rem; font-family: var(--font-sans); color: var(--text); margin-bottom: 0.4rem; }
.card p  { font-size: 0.875rem; color: var(--text-muted); font-family: var(--font-sans); line-height: 1.5; }
.card-tag {
  display: inline-block;
  margin-top: 0.75rem;
  font-family: var(--font-mono);
  font-size: 0.7rem;
  background: var(--tag-bg);
  color: var(--tag-text);
  border-radius: 4px;
  padding: 0.1rem 0.4rem;
}
.card-tag.soon {
  background: #1c2333;
  color: var(--text-muted);
}
.card.card-soon {
  opacity: 0.45;
  cursor: default;
}
.card.card-soon:hover {
  border-color: var(--border);
  transform: none;
}

/* ─── Chapter page layout ─────────────────────────────────────────────────── */
.chapter-body {
  max-width: var(--chapter-w);
  margin: 0 auto;
  padding: 3rem 1.5rem 5rem;
}
.chapter-body h1 {
  font-size: clamp(1.75rem, 4vw, 2.5rem);
  color: var(--accent);
  margin-bottom: 0.5rem;
  letter-spacing: -0.02em;
}
.chapter-body .subtitle {
  font-family: var(--font-sans);
  font-size: 1rem;
  color: var(--text-muted);
  margin-bottom: 2.5rem;
}
.chapter-body h2 {
  font-size: 1.4rem;
  margin-top: 2.5rem;
  margin-bottom: 0.75rem;
  color: var(--text);
  border-bottom: 1px solid var(--border);
  padding-bottom: 0.3rem;
}
.chapter-body h3 {
  font-size: 1.1rem;
  margin-top: 1.75rem;
  margin-bottom: 0.5rem;
  color: var(--text);
}
.chapter-body p { margin-bottom: 1.25rem; }
.chapter-body ul, .chapter-body ol {
  margin: 0 0 1.25rem 1.5rem;
}
.chapter-body li { margin-bottom: 0.4rem; }
.chapter-body blockquote {
  border-left: 3px solid var(--accent);
  padding: 0.5rem 1.25rem;
  margin: 1.5rem 0;
  color: var(--text-muted);
  font-style: italic;
}
.chapter-body code {
  font-family: var(--font-mono);
  font-size: 0.85em;
  background: var(--code-bg);
  border: 1px solid var(--border);
  border-radius: 4px;
  padding: 0.1em 0.35em;
}
.chapter-body pre {
  background: var(--code-bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 1rem 1.25rem;
  overflow-x: auto;
  margin-bottom: 1.5rem;
}
.chapter-body pre code { background: none; border: none; padding: 0; font-size: 0.875rem; }

/* ─── Simulation panel ────────────────────────────────────────────────────── */
.sim-section {
  margin-top: 3rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  overflow: hidden;
  background: var(--bg-surface);
}
.sim-section .sim-title {
  background: var(--bg-card);
  border-bottom: 1px solid var(--border);
  padding: 0.75rem 1.25rem;
  font-family: var(--font-sans);
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--text-muted);
  letter-spacing: 0.05em;
  text-transform: uppercase;
}
.sim-controls {
  padding: 1rem 1.25rem;
  border-bottom: 1px solid var(--border);
  display: flex;
  flex-wrap: wrap;
  gap: 1.5rem;
  background: var(--bg-card);
}
.slider-group {
  display: flex;
  flex-direction: column;
  gap: 0.3rem;
  min-width: 200px;
}
.slider-group label {
  font-family: var(--font-sans);
  font-size: 0.8rem;
  color: var(--text-muted);
}
.slider-group .slider-row {
  display: flex;
  align-items: center;
  gap: 0.75rem;
}
.slider-group input[type=range] {
  flex: 1;
  accent-color: var(--accent);
}
.slider-group .val-display {
  font-family: var(--font-mono);
  font-size: 0.8rem;
  color: var(--accent);
  min-width: 3.5rem;
  text-align: right;
}
.sim-plot {
  width: 100%;
  min-height: 360px;
  padding: 0.5rem;
}
.sim-status {
  padding: 0.5rem 1.25rem;
  font-family: var(--font-sans);
  font-size: 0.75rem;
  color: var(--text-muted);
  border-top: 1px solid var(--border);
  min-height: 2rem;
}

/* ─── Chapter navigation footer ──────────────────────────────────────────── */
.chapter-nav-footer {
  display: flex;
  justify-content: space-between;
  margin-top: 4rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border);
  font-family: var(--font-sans);
  font-size: 0.875rem;
}
.chapter-nav-footer a {
  display: flex;
  flex-direction: column;
  gap: 0.2rem;
  max-width: 40%;
}
.chapter-nav-footer .nav-label { color: var(--text-muted); font-size: 0.75rem; }
.chapter-nav-footer .nav-title { color: var(--link); }
.chapter-nav-footer a:hover .nav-title { color: var(--link-hover); text-decoration: underline; }
.chapter-nav-footer .next { text-align: right; margin-left: auto; }

/* ─── Site footer ─────────────────────────────────────────────────────────── */
.site-footer {
  text-align: center;
  padding: 2rem 1.5rem;
  border-top: 1px solid var(--border);
  font-family: var(--font-sans);
  font-size: 0.8rem;
  color: var(--text-muted);
}

/* ─── Placeholder / coming soon ──────────────────────────────────────────── */
.placeholder-notice {
  background: var(--bg-card);
  border: 1px dashed var(--border);
  border-radius: 6px;
  padding: 2rem;
  text-align: center;
  font-family: var(--font-sans);
  font-size: 0.9rem;
  color: var(--text-muted);
  margin: 2rem 0;
}

/* ─── Responsive ─────────────────────────────────────────────────────────── */
@media (max-width: 600px) {
  .grid { grid-template-columns: 1fr; }
  .sim-controls { flex-direction: column; }
  .chapter-nav-footer { flex-direction: column; gap: 1rem; }
  .chapter-nav-footer .next { text-align: left; margin-left: 0; }
}