:root {
  --bg: #ffffff;
  --bg2: #f7f7f5;
  --bg3: #efefec;
  --text: #1a1a1a;
  --muted: #5c5c5c;
  --accent: #1a1a1a;
  --link: #0b57d0;
  --border: #e2e2de;
  --warn: #8a5a00;
  --font: "Source Serif 4", "Iowan Old Style", Georgia, serif;
  --sans: "DM Sans", "Segoe UI", system-ui, sans-serif;
  --mono: "IBM Plex Mono", ui-monospace, monospace;
  --max: 720px;
  --wide: 960px;
}

*, *::before, *::after { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: var(--sans);
  background: var(--bg);
  color: var(--text);
  line-height: 1.65;
  font-size: 17px;
  -webkit-font-smoothing: antialiased;
}
a { color: var(--link); text-decoration: underline; text-underline-offset: 2px; }
a:hover { color: #083a91; }
img, iframe, embed, object { max-width: 100%; }

.site-header {
  position: sticky; top: 0; z-index: 50;
  background: rgba(255,255,255,0.94);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--border);
}
.nav {
  max-width: var(--wide); margin: 0 auto;
  display: flex; flex-wrap: wrap; gap: 0.35rem 1.1rem;
  align-items: baseline; padding: 0.85rem 1.25rem;
}
.nav .brand {
  font-family: var(--sans);
  font-weight: 650; color: var(--text);
  text-decoration: none; margin-right: 0.5rem;
  letter-spacing: -0.02em;
}
.nav .brand:hover { color: var(--link); }
.nav a { color: var(--muted); font-size: 0.9rem; text-decoration: none; }
.nav a:hover, .nav a[aria-current="page"] { color: var(--text); text-decoration: underline; }

main { max-width: var(--max); margin: 0 auto; padding: 2rem 1.25rem 4.5rem; }
main.wide { max-width: var(--wide); }

h1, h2, h3, h4 { font-family: var(--sans); letter-spacing: -0.02em; line-height: 1.25; font-weight: 650; }
h1 { font-size: clamp(1.75rem, 4vw, 2.25rem); margin: 0 0 0.65rem; }
h2 {
  font-size: 1.2rem; margin: 2.4rem 0 0.85rem;
  padding-bottom: 0.4rem; border-bottom: 1px solid var(--border);
}
h3 { font-size: 1.05rem; margin: 1.75rem 0 0.55rem; color: var(--text); }
h4 { font-size: 0.95rem; margin: 1.2rem 0 0.4rem; }
p { margin: 0.65rem 0; }
.lead { color: var(--muted); font-size: 1.05rem; max-width: 38rem; }
.muted { color: var(--muted); }
.meta { font-size: 0.85rem; color: var(--muted); font-family: var(--sans); }

.hero { padding: 0.5rem 0 0.25rem; }
.hero h1 { font-size: clamp(2rem, 5vw, 2.6rem); }
.tagline { font-family: var(--font); font-size: 1.15rem; color: var(--muted); max-width: 34rem; }

.pill-row { display: flex; flex-wrap: wrap; gap: 0.4rem; margin: 1.1rem 0; }
.pill {
  display: inline-block; padding: 0.18rem 0.55rem;
  border: 1px solid var(--border); border-radius: 999px;
  font-size: 0.75rem; color: var(--muted); background: var(--bg2);
  font-family: var(--sans);
}

.btn-row { display: flex; flex-wrap: wrap; gap: 0.55rem; margin: 1.35rem 0; }
.btn {
  display: inline-flex; align-items: center; gap: 0.35rem;
  padding: 0.5rem 0.9rem; border-radius: 4px;
  border: 1px solid var(--border); background: var(--bg);
  color: var(--text); font-size: 0.88rem; cursor: pointer;
  font-family: var(--sans); text-decoration: none;
}
.btn:hover { border-color: #bbb; background: var(--bg2); color: var(--text); text-decoration: none; }
.btn.primary { background: var(--text); border-color: var(--text); color: #fff; font-weight: 600; }
.btn.primary:hover { background: #333; border-color: #333; color: #fff; }

.grid2 {
  display: grid; gap: 0.85rem;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  margin: 1rem 0;
}
.card {
  background: var(--bg2); border: 1px solid var(--border);
  border-radius: 6px; padding: 1rem 1.05rem;
}
.card h3 { margin-top: 0; }
.card .metric {
  font-family: var(--sans); font-size: 1.45rem; font-weight: 700;
  letter-spacing: -0.03em;
}
.card ul { margin: 0.4rem 0 0; padding-left: 1.1rem; }
.card li { margin: 0.25rem 0; font-size: 0.92rem; }

table {
  width: 100%; border-collapse: collapse; font-size: 0.88rem;
  margin: 0.75rem 0 1.25rem; overflow-x: auto; display: block;
  font-family: var(--sans);
}
@media (min-width: 700px) { table { display: table; } }
th, td {
  border: 1px solid var(--border); padding: 0.45rem 0.55rem;
  text-align: left; vertical-align: top;
}
th { background: var(--bg3); color: var(--muted); font-weight: 600; }
td { background: var(--bg); }
tr:nth-child(even) td { background: var(--bg2); }
.grade-S, .grade-A { color: #0a7a3e; font-weight: 650; }
.grade-B { color: #1a6b3c; }
.grade-C { color: var(--warn); }
.grade-D, .grade-E { color: #a33; }

.toc {
  background: var(--bg2); border: 1px solid var(--border);
  border-radius: 6px; padding: 0.85rem 1.1rem; margin: 1rem 0 1.5rem;
  font-family: var(--sans); font-size: 0.92rem;
}
.toc ul { margin: 0.4rem 0 0; padding-left: 1.15rem; columns: 1; }
@media (min-width: 640px) { .toc ul { columns: 2; column-gap: 1.5rem; } }
.toc li { margin: 0.2rem 0; break-inside: avoid; }

.cheat {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-left: 3px solid var(--text);
  border-radius: 0 6px 6px 0;
  padding: 1rem 1.15rem 1.25rem;
  margin: 1rem 0 2rem;
}
.cheat h2 { margin-top: 0; border: 0; padding: 0; }
.cheat dl { margin: 0.5rem 0; }
.cheat dt {
  font-weight: 650; margin-top: 0.7rem;
  font-family: var(--mono); font-size: 0.86rem;
}
.cheat dd { margin: 0.15rem 0 0; color: var(--muted); font-size: 0.95rem; font-family: var(--font); }
.formula {
  font-family: var(--mono); font-size: 0.84rem;
  background: #fff; border: 1px solid var(--border);
  border-radius: 4px; padding: 0.5rem 0.65rem; margin: 0.4rem 0;
  overflow-x: auto; white-space: pre-wrap; color: #222;
}
.cheat-topic { margin: 0.85rem 0 1rem; }
.cheat-topic h4 {
  margin: 0 0 0.35rem;
  font-family: var(--sans);
  font-size: 0.95rem;
  font-weight: 650;
  letter-spacing: -0.01em;
}
.cheat .note-list li { color: #333; }

.qa { margin: 1.4rem 0; padding: 1rem 0; border-bottom: 1px solid var(--border); }
.qa:last-child { border-bottom: 0; }
.qa .q {
  font-family: var(--sans); font-weight: 650; font-size: 1.02rem;
  margin: 0 0 0.55rem; letter-spacing: -0.01em;
}
.qa .a { font-family: var(--font); color: #222; margin: 0; }
.qa .a p { margin: 0.5rem 0; }
.qa .tip {
  margin-top: 0.65rem; font-size: 0.86rem; color: var(--muted);
  font-family: var(--sans); padding: 0.45rem 0.65rem;
  background: var(--bg2); border-radius: 4px;
}

.topic { margin: 2rem 0; padding-top: 0.25rem; }
.topic .links { margin-top: 0.85rem; font-size: 0.88rem; font-family: var(--sans); }
.topic .links a { display: inline-block; margin-right: 0.85rem; margin-bottom: 0.35rem; }
.course-list { font-size: 0.86rem; color: var(--muted); margin: 0.5rem 0 1rem; font-family: var(--sans); }
.course-list code { color: #333; font-family: var(--mono); font-size: 0.8rem; }

.pdf-wrap {
  background: var(--bg2); border: 1px solid var(--border);
  border-radius: 6px; overflow: hidden; min-height: 70vh;
}
.pdf-wrap embed, .pdf-wrap iframe { width: 100%; height: 75vh; border: 0; }

.site-footer {
  max-width: var(--wide); margin: 0 auto;
  padding: 1.25rem 1.25rem 2.5rem;
  border-top: 1px solid var(--border);
  color: var(--muted); font-size: 0.8rem; font-family: var(--sans);
}

/* —— Roadmap —— */
.roadmap-progress { margin: 1rem 0 1.5rem; }
.progress-bar {
  height: 8px; background: var(--bg3); border-radius: 999px; overflow: hidden;
  margin: 0.55rem 0 0.75rem;
}
.progress-bar > div {
  height: 100%; width: 0%; background: var(--text); border-radius: 999px;
  transition: width 0.25s ease;
}
.roadmap-grid {
  display: grid; gap: 1rem;
  grid-template-columns: 1fr;
}
@media (min-width: 720px) {
  .roadmap-grid { grid-template-columns: 1fr 1fr; }
}
.roadmap-card-head {
  display: flex; flex-wrap: wrap; gap: 0.35rem 0.5rem;
  align-items: baseline; justify-content: space-between;
  margin-bottom: 0.35rem;
}
.roadmap-card-head h3 { margin: 0; }
.track-check {
  display: flex; align-items: flex-start; gap: 0.45rem;
  font-size: 0.9rem; color: var(--muted); cursor: pointer;
  font-family: var(--sans);
}
.track-check input { margin-top: 0.25rem; accent-color: var(--text); }
.track-list { list-style: none; padding: 0; margin: 0.75rem 0; }
.track-list li { margin: 0.4rem 0; }
.track-check.is-done span { color: var(--text); text-decoration: line-through; opacity: 0.75; }

/* —— Math knowledge graph —— */
.graph-toolbar { margin: 1rem 0 1.25rem; }
.graph-stats {
  display: flex; flex-wrap: wrap; gap: 0.75rem 1.25rem;
  align-items: baseline; margin-bottom: 0.75rem;
}
.graph-filter {
  display: inline-flex; flex-direction: column; gap: 0.25rem;
  font-size: 0.8rem; color: var(--muted); margin: 0.35rem 0.85rem 0.35rem 0;
  font-family: var(--sans);
}
.graph-filter select, .graph-filter input {
  font: inherit; color: var(--text); padding: 0.35rem 0.5rem;
  border: 1px solid var(--border); border-radius: 4px; background: #fff;
  min-width: 11rem;
}
.graph-legend {
  display: flex; flex-wrap: wrap; gap: 0.45rem 0.85rem;
  margin-top: 0.75rem; font-size: 0.78rem; color: var(--muted);
  font-family: var(--sans);
}
.graph-legend-item { display: inline-flex; align-items: center; gap: 0.35rem; }
.graph-legend-item i {
  width: 0.65rem; height: 0.65rem; border-radius: 50%; display: inline-block;
}
.graph-layout {
  display: grid; gap: 1rem;
  grid-template-columns: 1fr;
}
@media (min-width: 900px) {
  .graph-layout { grid-template-columns: 1fr min(280px, 32%); align-items: start; }
}
.graph-panel {
  border: 1px solid var(--border); border-radius: 8px;
  background: var(--bg2); overflow: hidden; min-height: 420px;
}
.math-graph {
  width: 100%; height: min(70vh, 640px); touch-action: none; cursor: grab;
  position: relative;
}
.math-graph:active { cursor: grabbing; }
.math-graph canvas { display: block; width: 100%; height: 100%; }
.graph-detail h2 { margin-top: 0.35rem; border: 0; padding: 0; }
.graph-detail .detail-track { margin: 0.85rem 0; }
.graph-detail {
  max-height: min(70vh, 640px);
  overflow-y: auto;
  position: sticky; top: 4.5rem;
}
.graph-detail h3 { font-size: 0.95rem; margin: 1.1rem 0 0.45rem; }
.detail-topics { margin: 0.35rem 0 0.75rem; }
.detail-topics .track-check { font-size: 0.86rem; line-height: 1.4; }
.graph-report { margin-top: 1.5rem; }
.graph-report summary {
  cursor: pointer; font-family: var(--sans); list-style: none;
}
.graph-report summary::-webkit-details-marker { display: none; }
.graph-report[open] summary { margin-bottom: 0.75rem; }

@media print {
  .site-header, .site-footer, .btn-row, .no-print,
  .graph-panel, .graph-toolbar { display: none !important; }
  body { background: #fff; color: #000; font-size: 11pt; }
  .cheat, .card, td, th, .qa .tip { background: #fff !important; color: #000; border-color: #ccc; }
  a { color: #000; text-decoration: none; }
  main { max-width: none; padding: 0; }
}