@import url('https://fonts.googleapis.com/css2?family=Source+Serif+4:ital,opsz,wght@0,8..60,300;0,8..60,400;0,8..60,600;0,8..60,700;1,8..60,400&family=Inter:wght@400;500;600&family=JetBrains+Mono:ital,wght@0,400;0,500;1,400&display=swap');

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

:root {
  --bg:         #ffffff;
  --bg-warm:    #f7f5f2;
  --bg-code:    #1d1f21;
  --border:     #ddd9d3;
  --border-dk:  #c8c3bb;
  --text:       #1a1917;
  --muted:      #5c574f;
  --dim:        #9c968d;
  --accent:     #d4621a;
  --accent-bg:  #fef4ed;
  --link:       #c0571a;

  /* syntax — dark bg */
  --syn-kw:    #f0883e;
  --syn-fn:    #79b8ff;
  --syn-str:   #85e89d;
  --syn-cmt:   #6a737d;
  --syn-ty:    #b392f0;
  --syn-num:   #79c0ff;

  --sidebar-w: 248px;
  --serif:     'Source Serif 4', Georgia, serif;
  --sans:      'Inter', system-ui, sans-serif;
  --mono:      'JetBrains Mono', 'Fira Code', monospace;
}

html { scroll-behavior: smooth; }
body {
  background: var(--bg);
  color: var(--text);
  font-family: var(--serif);
  font-size: 17px;
  line-height: 1.75;
  -webkit-font-smoothing: antialiased;
}

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

/* ── Top bar ── */
.topbar {
  height: 44px;
  background: var(--bg-warm);
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  padding: 0 20px;
  position: sticky;
  top: 0;
  z-index: 20;
  gap: 0;
}
.site-name {
  font-family: var(--sans);
  font-size: 14px;
  font-weight: 600;
  color: var(--text);
  text-decoration: none;
  letter-spacing: -0.01em;
  margin-right: 24px;
  flex-shrink: 0;
}
.site-name span { color: var(--accent); }
.site-name:hover { text-decoration: none; color: var(--text); }

.topbar-nav {
  display: flex;
  align-items: center;
  gap: 0;
  flex: 1;
}
.topbar-nav a {
  font-family: var(--sans);
  font-size: 13px;
  color: var(--muted);
  text-decoration: none;
  padding: 0 14px;
  height: 44px;
  display: flex;
  align-items: center;
  border-right: 1px solid var(--border);
  transition: background 0.1s, color 0.1s;
}
.topbar-nav a:first-child { border-left: 1px solid var(--border); }
.topbar-nav a:hover { background: var(--bg); color: var(--text); }
.topbar-nav a.active { background: var(--bg); color: var(--accent); font-weight: 500; }

.menu-btn {
  display: none;
  margin-left: auto;
  background: none;
  border: 1px solid var(--border);
  color: var(--muted);
  width: 30px; height: 30px;
  border-radius: 3px;
  cursor: pointer;
  font-size: 14px;
  align-items: center;
  justify-content: center;
  font-family: var(--sans);
}

/* ── Layout ── */
.layout {
  display: flex;
  min-height: calc(100vh - 44px);
}

/* ── Sidebar ── */
.sidebar {
  width: var(--sidebar-w);
  flex-shrink: 0;
  background: var(--bg-warm);
  border-right: 1px solid var(--border);
  padding: 20px 0 40px;
  position: sticky;
  top: 44px;
  height: calc(100vh - 44px);
  overflow-y: auto;
  scrollbar-width: thin;
  scrollbar-color: var(--border) transparent;
}

.nav-group { margin-bottom: 4px; }
.nav-label {
  font-family: var(--sans);
  font-size: 10px;
  font-weight: 600;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--dim);
  padding: 12px 18px 5px;
  display: block;
}
.nav-link {
  display: flex;
  align-items: baseline;
  gap: 7px;
  padding: 5px 18px;
  font-family: var(--sans);
  font-size: 13px;
  color: var(--muted);
  text-decoration: none;
  line-height: 1.4;
  transition: color 0.1s, background 0.1s;
  border-left: 2px solid transparent;
}
.nav-link:hover { color: var(--text); text-decoration: none; background: rgba(0,0,0,0.03); }
.nav-link.active {
  color: var(--accent);
  border-left-color: var(--accent);
  background: var(--accent-bg);
  font-weight: 500;
}
.nav-num {
  font-family: var(--mono);
  font-size: 10px;
  color: var(--dim);
  flex-shrink: 0;
  min-width: 18px;
}
.nav-link.active .nav-num { color: var(--accent); }

/* ── Overlay ── */
.overlay {
  display: none;
  position: fixed; inset: 0;
  background: rgba(0,0,0,0.4);
  z-index: 40;
}
.overlay.open { display: block; }

/* ── Main content ── */
.main { flex: 1; min-width: 0; }

/* ── Article ── */
.article {
  max-width: 1080px;
  margin: 0 auto;
  padding: 44px 56px 100px;
}

/* ── Page header ── */
.page-header {
  margin-bottom: 36px;
  padding-bottom: 20px;
  border-bottom: 1px solid var(--border);
}
.page-label {
  font-family: var(--sans);
  font-size: 11px;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--dim);
  margin-bottom: 8px;
}
.page-title {
  font-family: var(--serif);
  font-size: 30px;
  font-weight: 700;
  letter-spacing: -0.02em;
  line-height: 1.15;
  color: var(--text);
  margin-bottom: 10px;
}
.page-intro {
  font-size: 17px;
  color: var(--muted);
  line-height: 1.7;
  max-width: 620px;
}

/* ── Section headings ── */
h2 {
  font-family: var(--serif);
  font-size: 22px;
  font-weight: 700;
  letter-spacing: -0.015em;
  color: var(--text);
  margin: 48px 0 14px;
  line-height: 1.25;
}
h2:first-child { margin-top: 0; }
h3 {
  font-family: var(--sans);
  font-size: 12px;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--accent);
  margin: 28px 0 8px;
}

/* ── Body text ── */
p {
  margin-bottom: 16px;
  color: var(--text);
  font-size: 17px;
  line-height: 1.75;
}
p.muted { color: var(--muted); }
p:last-child { margin-bottom: 0; }
ul, ol { padding-left: 22px; margin-bottom: 16px; }
li { margin-bottom: 5px; font-size: 17px; line-height: 1.65; }
strong { font-weight: 600; }
em { font-style: italic; }

p code, li code {
  font-family: var(--mono);
  font-size: 13.5px;
  background: #f0ede8;
  border: 1px solid var(--border);
  border-radius: 3px;
  padding: 1px 5px;
  color: var(--text);
}

hr { border: none; border-top: 1px solid var(--border); margin: 36px 0; }

/* ── Callout boxes ── */
.callout {
  border-left: 3px solid;
  padding: 14px 18px;
  margin: 24px 0;
  border-radius: 0 3px 3px 0;
}
.callout-title {
  font-family: var(--sans);
  font-size: 11px;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  margin-bottom: 5px;
}
.callout p { font-size: 15px; line-height: 1.65; margin: 0; }
.callout-key   { border-color: var(--accent);  background: var(--accent-bg); }
.callout-key   .callout-title { color: var(--accent); }
.callout-tip   { border-color: #2e7d32; background: #f1f8f1; }
.callout-tip   .callout-title { color: #2e7d32; }
.callout-warn  { border-color: #c0960a; background: #fdf9ed; }
.callout-warn  .callout-title { color: #9a7308; }

/* ── Two column ── */
.two-col {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 48px;
  align-items: start;
  margin: 24px 0 36px;
}
.two-col.lean { grid-template-columns: 5fr 6fr; }
.sticky { position: sticky; top: 64px; }

/* ── Code block ── */
.code-block {
  background: var(--bg-code);
  border-radius: 4px;
  overflow: hidden;
  font-size: 13px;
  margin: 0 0 16px;
  border: 1px solid #333;
}
.code-top {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 7px 14px;
  background: #161819;
  border-bottom: 1px solid #333;
}
.code-fname {
  font-family: var(--mono);
  font-size: 11px;
  color: #8a8a8a;
}
.copy-btn {
  font-family: var(--mono);
  font-size: 11px;
  color: #666;
  background: none;
  border: none;
  cursor: pointer;
  padding: 2px 0;
  transition: color 0.1s;
}
.copy-btn:hover { color: #aaa; }
pre {
  padding: 18px;
  overflow-x: auto;
  font-family: var(--mono);
  font-size: 13px;
  line-height: 1.8;
  color: #e1e4e8;
}

/* Syntax */
.kw  { color: var(--syn-kw); }
.fn  { color: var(--syn-fn); }
.str { color: var(--syn-str); }
.cmt { color: var(--syn-cmt); font-style: italic; }
.ty  { color: var(--syn-ty); }
.num { color: var(--syn-num); }

/* ── Inline code block (single-column) ── */
.code-block.inline { max-width: 680px; }

/* ── Cheat sheet grid ── */
.cheat-section { margin-bottom: 44px; }
.cheat-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
  gap: 0;
  border: 1px solid var(--border);
  border-radius: 4px;
  overflow: hidden;
}
.cheat-cell {
  border-right: 1px solid var(--border);
  border-bottom: 1px solid var(--border);
  display: flex;
  flex-direction: column;
}
.cheat-cell:nth-child(2n) { border-right: none; }
.cheat-cell-head {
  font-family: var(--sans);
  font-size: 11px;
  font-weight: 600;
  color: var(--muted);
  background: var(--bg-warm);
  padding: 7px 14px;
  border-bottom: 1px solid var(--border);
  text-transform: uppercase;
  letter-spacing: 0.07em;
}
.cheat-cell pre {
  flex: 1;
  padding: 14px;
  font-size: 12.5px;
  line-height: 1.75;
  background: var(--bg-code);
  color: #e1e4e8;
}

/* ── Exercise ── */
.exercise-box {
  border: 1px solid var(--border);
  border-radius: 4px;
  overflow: hidden;
  margin-bottom: 36px;
}
.exercise-head {
  background: var(--bg-warm);
  border-bottom: 1px solid var(--border);
  padding: 18px 24px;
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 16px;
}
.exercise-meta {}
.exercise-label {
  font-family: var(--sans);
  font-size: 11px;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--dim);
  margin-bottom: 4px;
}
.exercise-title {
  font-family: var(--serif);
  font-size: 20px;
  font-weight: 700;
  color: var(--text);
  letter-spacing: -0.01em;
}
.diff {
  font-family: var(--sans);
  font-size: 11px;
  font-weight: 600;
  padding: 3px 10px;
  border-radius: 2px;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  flex-shrink: 0;
}
.diff-easy   { background: #e8f5e9; color: #2e7d32; }
.diff-medium { background: var(--accent-bg); color: var(--accent); }
.diff-hard   { background: #f3e5f5; color: #7b1fa2; }

.exercise-body { padding: 24px; }
.exercise-desc {
  font-size: 16px;
  color: var(--muted);
  margin-bottom: 20px;
  line-height: 1.65;
}
.req-list { list-style: none; padding: 0; border-top: 1px solid var(--border); margin: 0; }
.req-list li {
  display: flex;
  gap: 14px;
  align-items: baseline;
  padding: 10px 0;
  border-bottom: 1px solid var(--border);
  font-size: 15px;
  color: var(--text);
}
.req-n {
  font-family: var(--mono);
  font-size: 11px;
  color: var(--dim);
  flex-shrink: 0;
  min-width: 22px;
}

/* ── Solution toggle ── */
.sol-toggle {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 11px 16px;
  background: var(--bg-warm);
  border: 1px solid var(--border);
  border-radius: 3px;
  cursor: pointer;
  user-select: none;
  font-family: var(--sans);
  font-size: 13.5px;
  font-weight: 500;
  color: var(--muted);
  transition: background 0.1s, color 0.1s;
  margin-top: 28px;
}
.sol-toggle:hover { background: var(--border); color: var(--text); }
.sol-toggle.open { color: var(--accent); border-color: var(--accent); background: var(--accent-bg); }
.sol-icon { font-family: var(--mono); font-size: 16px; color: var(--accent); width: 14px; }
.sol-hint { margin-left: auto; font-family: var(--mono); font-size: 11px; color: var(--dim); }
.sol-body { display: none; margin-top: 28px; }
.sol-body.open { display: block; }

/* ── Page nav footer ── */
.page-footer {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
  margin-top: 56px;
  padding-top: 20px;
  border-top: 1px solid var(--border);
  font-family: var(--sans);
  font-size: 14px;
}
.page-footer a {
  color: var(--link);
  font-weight: 500;
  text-decoration: none;
}
.page-footer a:hover { text-decoration: underline; }

/* ── Homepage ── */
.home-hero {
  padding: 52px 56px 40px;
  max-width: 1080px;
  margin: 0 auto;
  border-bottom: 1px solid var(--border);
  margin-bottom: 0;
}
.home-hero h1 {
  font-family: var(--serif);
  font-size: 34px;
  font-weight: 700;
  letter-spacing: -0.025em;
  line-height: 1.15;
  color: var(--text);
  margin-bottom: 14px;
}
.home-hero p {
  font-size: 17px;
  color: var(--muted);
  max-width: 540px;
  margin-bottom: 0;
}

.home-toc {
  padding: 0 56px 80px;
  max-width: 1080px;
  margin: 0 auto;
}
.toc-section { padding: 32px 0; border-bottom: 1px solid var(--border); }
.toc-section:last-child { border-bottom: none; }
.toc-section-head {
  display: flex;
  align-items: baseline;
  gap: 12px;
  margin-bottom: 12px;
}
.toc-section-title {
  font-family: var(--sans);
  font-size: 13px;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.09em;
  color: var(--text);
}
.toc-section-count {
  font-family: var(--mono);
  font-size: 11px;
  color: var(--dim);
}
.toc-items { display: flex; flex-direction: column; gap: 1px; }
.toc-item {
  display: flex;
  align-items: baseline;
  gap: 10px;
  padding: 8px 12px;
  text-decoration: none;
  color: var(--text);
  font-size: 15px;
  font-family: var(--serif);
  transition: background 0.1s;
  border-radius: 2px;
  margin: 0 -12px;
}
.toc-item:hover { background: var(--bg-warm); text-decoration: none; color: var(--link); }
.toc-num { font-family: var(--mono); font-size: 11px; color: var(--dim); min-width: 20px; flex-shrink: 0; }
.toc-desc { font-size: 13px; color: var(--dim); margin-left: auto; font-family: var(--sans); }
.toc-diff { font-family: var(--sans); font-size: 10.5px; font-weight: 600; text-transform: uppercase; letter-spacing: 0.06em; color: var(--dim); margin-left: 6px; }

/* ── Mobile ── */
@media (max-width: 860px) {
  .sidebar {
    position: fixed;
    left: -100%; top: 44px; bottom: 0;
    z-index: 50;
    transition: left 0.2s;
    width: min(var(--sidebar-w), 82vw);
    height: calc(100vh - 44px);
  }
  .sidebar.open { left: 0; }
  .menu-btn { display: flex; }
  .topbar-nav { display: none; }
  .article { padding: 28px 22px 72px; }
  .home-hero { padding: 32px 22px 28px; }
  .home-toc { padding: 0 22px 60px; }
  .two-col { grid-template-columns: 1fr; }
  .two-col.lean { grid-template-columns: 1fr; }
  .sticky { position: static; }
  .cheat-grid { grid-template-columns: 1fr; }
  .cheat-cell:nth-child(2n) { border-right: 1px solid var(--border); }
}