/* ============================================================
   Data Structures Learning Portal — shared design system
   ============================================================ */

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

:root {
  --bg:        #0B1B2B;
  --panel:     #13263B;
  --panel-2:   #1B344F;
  --line:      #25425F;
  --teal:      #2DD4BF;
  --teal-dim:  #1C8A7C;
  --amber:     #F2A93B;
  --text:      #EAF2F5;
  --text-dim:  #7C93A8;
  --good:      #4ADE80;
  --bad:       #F26B6B;

  --display: 'Space Grotesk', sans-serif;
  --body: 'Source Serif 4', Georgia, serif;
  --mono: 'JetBrains Mono', monospace;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: var(--body);
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
}

/* Ambient technical backdrop: faint circuit/graph grid, fixed, behind everything */
.bg-grid {
  position: fixed;
  inset: 0;
  z-index: -2;
  background-image:
    radial-gradient(circle at 18% 22%, rgba(45, 212, 191, 0.10), transparent 40%),
    radial-gradient(circle at 82% 78%, rgba(242, 169, 59, 0.08), transparent 42%),
    linear-gradient(rgba(124, 147, 168, 0.07) 1px, transparent 1px),
    linear-gradient(90deg, rgba(124, 147, 168, 0.07) 1px, transparent 1px);
  background-size: auto, auto, 42px 42px, 42px 42px;
  pointer-events: none;
}

a { color: inherit; }

.wrap {
  max-width: 1040px;
  margin: 0 auto;
  padding: 0 24px;
}

/* ---------- Top nav ---------- */
.site-nav {
  position: sticky;
  top: 0;
  z-index: 10;
  background: rgba(11, 27, 43, 0.88);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--line);
}
.site-nav .wrap {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding-top: 14px;
  padding-bottom: 14px;
}
.site-nav .brand {
  font-family: var(--display);
  font-weight: 700;
  font-size: 1.05rem;
  letter-spacing: 0.01em;
  text-decoration: none;
  color: var(--text);
}
.site-nav .brand span { color: var(--teal); }
.site-nav .links { display: flex; gap: 22px; }
.site-nav .links a {
  font-family: var(--display);
  font-size: 0.92rem;
  text-decoration: none;
  color: var(--text-dim);
  transition: color 0.15s ease;
}
.site-nav .links a:hover,
.site-nav .links a:focus-visible { color: var(--teal); }

/* ---------- Headings & basic type ---------- */
h1, h2, h3 {
  font-family: var(--display);
  font-weight: 700;
  letter-spacing: -0.01em;
  margin: 0 0 0.5em;
}
h1 { font-size: clamp(2rem, 4vw, 2.8rem); }
h2 { font-size: clamp(1.4rem, 2.6vw, 1.8rem); color: var(--teal); }
h3 { font-size: 1.15rem; color: var(--amber); }

p { color: var(--text); }
.muted { color: var(--text-dim); }

code, .mono { font-family: var(--mono); font-size: 0.9em; }

/* ---------- Cards / panels ---------- */
.panel {
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 14px;
  padding: 28px;
}

footer {
  text-align: center;
  color: var(--text-dim);
  font-family: var(--display);
  font-size: 0.85rem;
  padding: 36px 0 48px;
}

/* ---------- Focus visibility ---------- */
a:focus-visible, button:focus-visible, input:focus-visible {
  outline: 2px solid var(--teal);
  outline-offset: 3px;
}

@media (prefers-reduced-motion: reduce) {
  * { animation-duration: 0.001ms !important; transition-duration: 0.001ms !important; }
}

@media (max-width: 640px) {
  .site-nav .links { gap: 12px; font-size: 0.8rem; }
  .panel { padding: 20px; }
}

/* ============================================================
   Unit pages
   ============================================================ */

.breadcrumb {
  font-family: var(--mono);
  font-size: 0.82rem;
  color: var(--text-dim);
  padding: 22px 0 0;
}
.breadcrumb a { color: var(--teal); text-decoration: none; }
.breadcrumb a:hover { text-decoration: underline; }

.unit-hero { padding: 18px 0 6px; }
.unit-hero .unit-tag {
  font-family: var(--mono);
  color: var(--teal);
  font-size: 0.82rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
}
.unit-hero h1 { margin-top: 6px; }
.unit-hero .lede {
  font-size: 1.08rem;
  color: var(--text-dim);
  max-width: 680px;
}

.diagram-box {
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 14px;
  padding: 22px;
  margin: 28px 0;
  overflow-x: auto;
}
.diagram-box svg { display: block; margin: 0 auto; max-width: 100%; height: auto; }
.diagram-caption {
  text-align: center;
  font-family: var(--mono);
  font-size: 0.8rem;
  color: var(--text-dim);
  margin-top: 12px;
}

.notes-section { margin: 30px 0; }
.notes-section h2 { margin-top: 0; }
.notes-section ul { padding-left: 22px; color: var(--text); }
.notes-section li { margin-bottom: 8px; }

.complexity-table {
  width: 100%;
  border-collapse: collapse;
  margin: 18px 0;
  font-family: var(--mono);
  font-size: 0.88rem;
}
.complexity-table th, .complexity-table td {
  border: 1px solid var(--line);
  padding: 9px 12px;
  text-align: left;
}
.complexity-table th {
  background: var(--panel-2);
  color: var(--amber);
  font-family: var(--display);
  font-weight: 600;
}
.complexity-table td { color: var(--text); }

/* ---- Quiz ---- */
.quiz-section { margin: 40px 0 20px; }
.quiz-q {
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 12px;
  padding: 18px 20px;
  margin-bottom: 16px;
}
.quiz-question {
  font-family: var(--display);
  font-weight: 600;
  color: var(--text);
  margin: 0 0 12px;
}
.quiz-choices {
  display: flex;
  flex-direction: column;
  gap: 8px;
}
.quiz-choice {
  font-family: var(--body);
  font-size: 0.96rem;
  text-align: left;
  background: var(--bg);
  color: var(--text);
  border: 1px solid var(--line);
  border-radius: 8px;
  padding: 10px 14px;
  cursor: pointer;
  transition: border-color 0.12s ease, background 0.12s ease;
}
.quiz-choice:hover:not(:disabled) { border-color: var(--teal); }
.quiz-choice:disabled { cursor: default; }
.quiz-choice.correct {
  border-color: var(--good);
  background: rgba(74, 222, 128, 0.1);
}
.quiz-choice.incorrect {
  border-color: var(--bad);
  background: rgba(242, 107, 107, 0.1);
}
.quiz-feedback {
  margin: 12px 0 0;
  font-size: 0.92rem;
  color: var(--text-dim);
}
.quiz-summary {
  font-family: var(--display);
  color: var(--text);
  margin-top: 10px;
  display: flex;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
}
.quiz-retry {
  font-family: var(--display);
  font-size: 0.85rem;
  background: transparent;
  border: 1px solid var(--teal-dim);
  color: var(--teal);
  border-radius: 7px;
  padding: 7px 14px;
  cursor: pointer;
}
.quiz-retry:hover { background: rgba(45, 212, 191, 0.08); }

.unit-nav {
  display: flex;
  justify-content: space-between;
  gap: 16px;
  margin: 44px 0 12px;
  font-family: var(--display);
  font-size: 0.9rem;
}
.unit-nav a {
  text-decoration: none;
  color: var(--text-dim);
  border: 1px solid var(--line);
  border-radius: 9px;
  padding: 10px 16px;
  transition: border-color 0.12s ease, color 0.12s ease;
}
.unit-nav a:hover { border-color: var(--teal); color: var(--teal); }