/* Problem Solving — CS & DS: site stylesheet.
   Responsive: header nav collapses under 700px; sidebar reflows under 900px.
   No animations beyond instant state changes; colour is never the only signal. */

:root {
  --bg: #ffffff;
  --fg: #1a1a2e;
  --muted: #5a5a72;
  --accent: #3556d4;
  --accent-dark: #27409f;
  --accent-soft: #e8edfb;
  --border: #d8dce6;
  --code-bg: #f4f5f8;
  --code-fg: #21322e;
  --table-alt: #f7f8fa;
  --tier-b: #2e7d32;
  --tier-f: #55771f;
  --tier-i: #b26a00;
  --tier-a: #b3402e;
  --tier-e: #6a2ea0;
  --tier-bg: #f2f3f7;
}

* { box-sizing: border-box; }

html { font-size: 16px; }

body {
  margin: 0;
  font-family: "Segoe UI", system-ui, -apple-system, sans-serif;
  color: var(--fg);
  background: var(--bg);
  line-height: 1.6;
}

/* Accessibility: visible focus and skip link */
a:focus-visible, button:focus-visible, input:focus-visible {
  outline: 3px solid #f2a900;
  outline-offset: 2px;
}

.skip-link {
  position: absolute;
  left: -9999px;
  top: 0;
  background: var(--accent);
  color: #fff;
  padding: 0.5rem 1rem;
  z-index: 100;
}
.skip-link:focus { left: 0.5rem; top: 0.5rem; }

/* Header */
.site-header {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem;
  align-items: center;
  justify-content: space-between;
  padding: 0.75rem 1.25rem;
  border-bottom: 2px solid var(--accent);
  background: var(--accent-soft);
}
.brand {
  font-weight: 700;
  color: var(--accent);
  text-decoration: none;
  font-size: 1.05rem;
}
.nav-toggle {
  display: none;
  font: inherit;
  font-weight: 600;
  background: var(--bg);
  color: var(--accent);
  border: 2px solid var(--accent);
  border-radius: 6px;
  padding: 0.3rem 0.9rem;
  cursor: pointer;
}
.top-nav {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
}
.top-nav a {
  margin-left: 1rem;
  color: var(--fg);
  text-decoration: none;
  padding: 0.15rem 0.2rem;
}
.top-nav a:hover { text-decoration: underline; }
.top-nav a[aria-current="page"] {
  color: var(--accent-dark);
  font-weight: 700;
  border-bottom: 3px solid var(--accent);
}

/* Layout: sidebar + content */
.layout {
  display: flex;
  min-height: calc(100vh - 4rem);
}
.sidebar {
  width: 250px;
  flex: 0 0 250px;
  border-right: 1px solid var(--border);
  padding: 1rem;
  background: #fbfcfe;
}
.side-heading {
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--muted);
  margin: 1rem 0 0.35rem;
}
.side-heading:first-child { margin-top: 0; }
.side-link {
  display: block;
  padding: 0.2rem 0.4rem;
  color: var(--accent);
  text-decoration: none;
  border-radius: 4px;
  font-size: 0.92rem;
}
.side-link:hover { background: var(--accent-soft); }
.side-link.current {
  font-weight: 700;
  background: var(--accent-soft);
  border-left: 3px solid currentColor;
}

/* Content */
.content {
  flex: 1;
  max-width: 60rem;
  padding: 1.5rem 2rem;
  min-width: 0;
}
.content h1, .content h2, .content h3, .content h4 {
  line-height: 1.25;
  color: var(--fg);
}
.content h1 { font-size: 1.7rem; border-bottom: 2px solid var(--accent); padding-bottom: 0.3rem; }
.content h2 { font-size: 1.3rem; margin-top: 2rem; }
.content h3 { font-size: 1.1rem; margin-top: 1.5rem; }

/* Tables */
.table-wrap { overflow-x: auto; }
table {
  border-collapse: collapse;
  width: 100%;
  margin: 0.75rem 0;
  font-size: 0.95rem;
}
th, td {
  border: 1px solid var(--border);
  padding: 0.45rem 0.6rem;
  text-align: left;
}
th { background: var(--accent-soft); }
tbody tr:nth-child(even) { background: var(--table-alt); }

/* Code */
code {
  font-family: "Cascadia Code", Consolas, "Courier New", monospace;
  background: var(--code-bg);
  padding: 0.1rem 0.3rem;
  border-radius: 4px;
  font-size: 0.9em;
  color: var(--code-fg);
}
pre {
  background: var(--code-bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  overflow-x: auto;
  padding: 0.8rem 1rem;
}
pre code { background: none; padding: 0; }

/* Syntax highlighting (no colour-only meaning; purely decorative) */
.p-key { color: #0b4fa8; font-weight: 600; }
.p-str { color: #7a4a0b; }
.p-com { color: #5a6472; font-style: italic; }
.p-num { color: #8a1f5a; }

/* Blockquote */
blockquote {
  border-left: 4px solid var(--accent);
  margin: 1rem 0;
  padding: 0.5rem 1rem;
  background: var(--accent-soft);
  color: var(--fg);
}
blockquote p { margin: 0.3rem 0; }

/* Difficulty + metadata badges (always paired with text) */
.badge {
  display: inline-block;
  font-size: 0.78rem;
  font-weight: 600;
  border-radius: 999px;
  padding: 0.05rem 0.6rem;
  border: 1px solid var(--border);
  background: var(--tier-bg);
  color: var(--fg);
  white-space: nowrap;
}
.badge.t-b { color: var(--tier-b); border-color: var(--tier-b); }
.badge.t-f { color: var(--tier-f); border-color: var(--tier-f); }
.badge.t-i { color: var(--tier-i); border-color: var(--tier-i); }
.badge.t-a { color: var(--tier-a); border-color: var(--tier-a); }
.badge.t-e { color: var(--tier-e); border-color: var(--tier-e); }

/* ---------- Homepage ---------- */
.hero {
  border-bottom: 1px solid var(--border);
  padding-bottom: 1.5rem;
  margin-bottom: 1.5rem;
}
.hero h1 { font-size: 1.9rem; border-bottom: none; padding-bottom: 0; }
.hero .tagline { font-size: 1.08rem; color: var(--fg); }
.hero .audience { color: var(--muted); }
.hero-stats {
  display: flex;
  flex-wrap: wrap;
  gap: 0.6rem;
  margin: 1rem 0;
  padding: 0;
  list-style: none;
}
.hero-stats li {
  background: var(--accent-soft);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 0.25rem 0.8rem;
  font-weight: 600;
  font-size: 0.92rem;
}
.cta-row { margin: 1rem 0 0; }
.btn {
  display: inline-block;
  background: var(--accent);
  color: #fff;
  font-weight: 600;
  text-decoration: none;
  border-radius: 6px;
  padding: 0.5rem 1.1rem;
  margin: 0.25rem 0.5rem 0.25rem 0;
}
.btn:hover { background: var(--accent-dark); }
.btn.ghost {
  background: var(--bg);
  color: var(--accent);
  border: 2px solid var(--accent);
}
.btn.ghost:hover { background: var(--accent-soft); }

.block-head { margin-top: 2rem; }
.block-head h2 { margin-bottom: 0.2rem; }
.block-head p { color: var(--muted); margin-top: 0; }

.lecture-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(230px, 1fr));
  gap: 0.8rem;
  margin: 0.75rem 0 1.5rem;
  padding: 0;
  list-style: none;
}
.lecture-grid a {
  display: block;
  height: 100%;
  text-decoration: none;
  color: var(--fg);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 0.65rem 0.8rem;
  background: var(--bg);
}
.lecture-grid a:hover {
  border-color: var(--accent);
  background: var(--accent-soft);
}
.lecture-num {
  display: block;
  font-weight: 700;
  color: var(--accent);
  font-size: 0.85rem;
}
.lecture-title { display: block; font-weight: 600; margin: 0.15rem 0; }
.lecture-meta { display: block; font-size: 0.8rem; color: var(--muted); }

.materials-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(230px, 1fr));
  gap: 0.8rem;
  margin: 0.75rem 0;
  padding: 0;
  list-style: none;
}
.materials-grid a {
  display: block;
  height: 100%;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 0.65rem 0.8rem;
  text-decoration: none;
  background: var(--bg);
}
.materials-grid a:hover { border-color: var(--accent); background: var(--accent-soft); }
.materials-grid .m-title { display: block; font-weight: 700; color: var(--accent); }
.materials-grid .m-desc { display: block; font-size: 0.88rem; color: var(--muted); }

/* ---------- Search page ---------- */
.search-page .search-form { margin: 1rem 0; }
.search-box {
  font: inherit;
  width: 100%;
  max-width: 34rem;
  padding: 0.55rem 0.8rem;
  border: 2px solid var(--border);
  border-radius: 6px;
}
.search-status { color: var(--muted); margin: 0.5rem 0; }
.results { list-style: none; padding: 0; margin: 1rem 0; }
.results li {
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 0.6rem 0.9rem;
  margin-bottom: 0.6rem;
}
.results a { font-weight: 600; }
.results .r-snippet { display: block; font-size: 0.92rem; color: var(--fg); margin-top: 0.2rem; }
.results mark { background: #ffe9a8; color: var(--fg); padding: 0 0.1em; }

/* Footer */
.site-footer {
  border-top: 1px solid var(--border);
  padding: 0.8rem 1.25rem;
  font-size: 0.85rem;
  color: var(--muted);
}

/* ---------- Responsive ---------- */
@media (max-width: 900px) {
  .layout { display: block; }
  .sidebar {
    width: auto;
    border-right: none;
    border-bottom: 1px solid var(--border);
    display: flex;
    flex-wrap: wrap;
    gap: 0.25rem;
  }
  .side-heading { width: 100%; }
  .side-link { display: inline-block; border: 1px solid var(--border); margin: 0.1rem; }
  .content { padding: 1rem; }
}

@media (max-width: 700px) {
  .nav-toggle { display: inline-block; }
  .top-nav {
    flex-basis: 100%;
    display: none;
    flex-direction: column;
    align-items: stretch;
    padding: 0.5rem 0 0.25rem;
  }
  .top-nav.open { display: flex; }
  .top-nav a {
    margin-left: 0;
    padding: 0.5rem 0.4rem;
    border-bottom: 1px solid var(--border);
  }
  .top-nav a[aria-current="page"] { border-bottom: 3px solid var(--accent); }
  .hero h1 { font-size: 1.5rem; }
  .content h1 { font-size: 1.45rem; }
}