/* ── BASE THEME ── */
:root {
  --bg: #070b14;
  --surface2: #0d1220;
  --card: #111827;
  --card2: #141d2e;
  --border: rgba(255,255,255,0.07);
  --t1: #f1f5ff;
  --t2: #94a3b8;
  --t3: #475569;
  --blue: #3b82f6;
  --cyan: #06b6d4;
  --green: #10b981;
  --purple: #8b5cf6;
  --yellow: #f59e0b;
  --red: #ef4444;
  --accent: #3b82f6;
  --accent2: #06b6d4;
  --accent-rgb: 59, 130, 246;
}

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

body {
  background: var(--bg);
  font-family: 'Inter', sans-serif;
  color: var(--t1);
  min-height: 100vh;
}

/* ── NAV ── */
nav, .nav {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(7,11,20,0.9);
  backdrop-filter: blur(16px);
  border-bottom: 1px solid var(--border);
  padding: 14px 40px;
  display: flex;
  align-items: center;
  justify-content: space-between;
}
.nav-brand {
  font-size: 15px;
  font-weight: 700;
  color: var(--t2);
  text-decoration: none;
}
.nav-links { display:flex; gap:8px; }
.nav-links a {
  font-size: 13px;
  color: var(--t3);
  text-decoration: none;
  padding: 5px 12px;
  border-radius: 7px;
}
.nav-links a:hover { color: var(--t1); background: rgba(255,255,255,0.06); }

/* ── PAGE HEADER ── */
.page-header {
  padding: 50px 40px 36px;
  max-width: 1100px;
  margin: 0 auto;
}
.breadcrumb {
  font-size: 13px;
  color: var(--t3);
  margin-bottom: 18px;
}
.breadcrumb a { color: var(--t2); text-decoration: none; }
.topic-badge {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 5px 14px;
  border-radius: 20px;
  font-size: 11px;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.8px;
  background: rgba(var(--accent-rgb), 0.12);
  border: 1px solid rgba(var(--accent-rgb), 0.2);
  color: #93c5fd;
  margin-bottom: 16px;
}
.page-header h1 {
  font-size: clamp(26px, 4vw, 46px);
  font-weight: 900;
  letter-spacing: -1.5px;
  background: linear-gradient(135deg, #60a5fa, #a78bfa);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  margin-bottom: 12px;
}
.page-header p {
  font-size: 16px;
  color: var(--t2);
  max-width: 640px;
  line-height: 1.65;
}
.page-meta { display:flex; gap:14px; margin-top:20px; flex-wrap:wrap; }
.meta-pill {
  font-size: 12px;
  color: var(--t3);
  background: rgba(255,255,255,0.05);
  padding: 4px 12px;
  border-radius: 6px;
}

/* ── CONTENT ── */
.content {
  max-width: 1100px;
  margin: 0 auto;
  padding: 0 40px 80px;
}

/* ── SECTIONS ── */
.section { margin-bottom: 36px; }
.section-title {
  font-size: 20px;
  font-weight: 800;
  letter-spacing: -0.3px;
  margin-bottom: 18px;
  display: flex;
  align-items: center;
  gap: 10px;
}
.section-title::before {
  content: '';
  width: 3px;
  height: 20px;
  background: linear-gradient(180deg, var(--blue), var(--purple));
  border-radius: 2px;
}

/* ── CARDS ── */
.card {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 14px;
  padding: 24px;
}
.card + .card { margin-top: 14px; }
.card-title {
  font-size: 15px;
  font-weight: 700;
  margin-bottom: 12px;
  color: var(--t1);
}

/* ── TEXT ── */
p { font-size: 14px; color: var(--t2); line-height: 1.7; }
strong { color: var(--t1); }

/* ── CODE BLOCKS ── */
.code-block {
  white-space: pre;
  background: #080c18;
  border: 1px solid rgba(255,255,255,0.05);
  border-radius: 12px;
  padding: 20px 22px;
  font-family: 'JetBrains Mono', monospace;
  font-size: 13px;
  line-height: 1.75;
  overflow-x: auto;
  margin: 14px 0;
  position: relative;
}
.lb {
  position: absolute;
  top: 10px;
  right: 14px;
  font-size: 10px;
  font-weight: 700;
  color: var(--t3);
  text-transform: uppercase;
}

/* Code wrap (arrays style) */
.code-wrap {
  background: #080c18;
  border: 1px solid rgba(255,255,255,0.05);
  border-radius: 12px;
  overflow: hidden;
  margin: 14px 0;
}
.code-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 10px 18px;
  background: rgba(255,255,255,0.03);
  border-bottom: 1px solid rgba(255,255,255,0.05);
}
.code-lang {
  font-size: 10px;
  font-weight: 700;
  color: var(--t3);
  text-transform: uppercase;
  letter-spacing: 0.5px;
}
.copy-btn {
  font-size: 11px;
  font-weight: 600;
  padding: 3px 10px;
  border-radius: 5px;
  background: rgba(255,255,255,0.06);
  border: 1px solid var(--border);
  color: var(--t3);
  cursor: pointer;
  transition: all 0.2s;
}
.copy-btn:hover { color: var(--t1); background: rgba(255,255,255,0.1); }
.code-body {
  padding: 18px 22px;
  font-family: 'JetBrains Mono', monospace;
  font-size: 13px;
  line-height: 1.75;
  overflow-x: auto;
  white-space: pre;
}

/* Syntax highlight classes */
.ck { color: #a78bfa; }
.cf { color: #60a5fa; }
.cs { color: #6ee7b7; }
.cn { color: #fcd34d; }
.cc { color: #475569; }

/* ── OPERATIONS TABLE ── */
.ops-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 13px;
}
.ops-table th {
  text-align: left;
  font-size: 11px;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.8px;
  color: var(--t3);
  padding: 10px 14px;
  border-bottom: 1px solid var(--border);
}
.ops-table td {
  padding: 10px 14px;
  color: var(--t2);
  border-bottom: 1px solid rgba(255,255,255,0.03);
  font-family: 'JetBrains Mono', monospace;
  font-size: 12px;
}
.ops-table td:first-child {
  color: var(--t1);
  font-weight: 600;
  font-family: 'Inter', sans-serif;
  font-size: 13px;
}
.ops-table td:last-child {
  font-family: 'Inter', sans-serif;
  font-size: 12px;
  color: var(--t3);
}
.og { color: #6ee7b7 !important; }
.oy { color: #fcd34d !important; }

/* ── CALLOUTS ── */
.callout {
  border-radius: 10px;
  padding: 14px 18px;
  margin: 14px 0;
  font-size: 14px;
  line-height: 1.6;
}
.c-tip { background: rgba(16,185,129,0.08); border-left: 3px solid var(--green); color: var(--t2); }
.c-info { background: rgba(59,130,246,0.08); border-left: 3px solid var(--blue); color: var(--t2); }

/* ── PROBLEM CARDS ── */
.problem-card {
  background: var(--card2);
  border: 1px solid var(--border);
  border-radius: 12px;
  margin-bottom: 14px;
  overflow: hidden;
}
.prob-header {
  padding: 16px 20px;
  display: flex;
  align-items: center;
  gap: 12px;
  cursor: pointer;
  transition: background 0.15s;
}
.prob-header:hover { background: rgba(255,255,255,0.03); }
.prob-num {
  font-size: 13px;
  font-weight: 700;
  font-family: 'JetBrains Mono', monospace;
  color: var(--t3);
  min-width: 40px;
}
.prob-title { font-size: 15px; font-weight: 700; flex: 1; }
.chevron {
  font-size: 12px;
  color: var(--t3);
  transition: transform 0.2s;
}
.diff-e {
  font-size: 11px; font-weight: 700;
  padding: 2px 9px; border-radius: 5px;
  background: rgba(16,185,129,0.12); color: #6ee7b7;
}
.diff-m {
  font-size: 11px; font-weight: 700;
  padding: 2px 9px; border-radius: 5px;
  background: rgba(245,158,11,0.12); color: #fcd34d;
}
.diff-h {
  font-size: 11px; font-weight: 700;
  padding: 2px 9px; border-radius: 5px;
  background: rgba(239,68,68,0.12); color: #fca5a5;
}
.prob-body {
  border-top: 1px solid var(--border);
  padding: 18px 20px;
  display: none;
}
.prob-body.open { display: block; }
.prob-desc {
  font-size: 14px;
  color: var(--t2);
  line-height: 1.6;
  margin-bottom: 14px;
}
.prob-insight {
  background: rgba(59,130,246,0.08);
  border-left: 3px solid var(--blue);
  border-radius: 6px;
  padding: 12px 16px;
  font-size: 13px;
  color: var(--t2);
  margin-bottom: 14px;
}
.prob-insight strong { color: #93c5fd; }

/* ── TWO COL ── */
.two-col { display: grid; grid-template-columns: 1fr 1fr; gap: 16px; }

/* ── PAGE NAV ── */
.page-nav {
  display: flex;
  justify-content: space-between;
  padding: 24px 40px;
  max-width: 1100px;
  margin: 0 auto;
  border-top: 1px solid var(--border);
}
.btn-nav {
  padding: 11px 22px;
  border-radius: 10px;
  font-size: 13px;
  font-weight: 700;
  text-decoration: none;
}
.btn-prev { background: rgba(255,255,255,0.06); color: var(--t2); }
.btn-prev:hover { background: rgba(255,255,255,0.1); }
.btn-next {
  background: linear-gradient(135deg, var(--blue), var(--purple));
  color: #fff;
  box-shadow: 0 4px 20px rgba(59,130,246,0.3);
}
.btn-next:hover { box-shadow: 0 6px 28px rgba(59,130,246,0.45); }

/* ── SCROLL PROGRESS BAR ── */
.scroll-progress {
  position: fixed; top: 0; left: 0; height: 3px; width: 0%;
  background: linear-gradient(90deg, var(--purple), var(--blue), var(--cyan));
  z-index: 200; transition: width 0.1s linear;
  box-shadow: 0 0 10px rgba(139,92,246,0.5);
}

/* ── BACK TO TOP ── */
.to-top {
  position: fixed; bottom: 28px; right: 28px; z-index: 200;
  width: 46px; height: 46px; border-radius: 14px;
  display: flex; align-items: center; justify-content: center;
  background: rgba(15,23,42,0.7); backdrop-filter: blur(16px);
  border: 1px solid var(--border); color: var(--t1);
  font-size: 18px; cursor: pointer;
  opacity: 0; transform: translateY(16px) scale(0.9); pointer-events: none;
  transition: all 0.3s cubic-bezier(0.16,1,0.3,1);
  box-shadow: 0 8px 32px rgba(0,0,0,0.35);
}
.to-top.show { opacity: 1; transform: translateY(0) scale(1); pointer-events: auto; }
.to-top:hover {
  border-color: rgba(139,92,246,0.4);
  background: rgba(139,92,246,0.15);
  transform: translateY(-3px) scale(1.05);
}

/* ── SCROLLBAR ── */
::-webkit-scrollbar { width: 6px; }
::-webkit-scrollbar-thumb { background: rgba(255,255,255,0.1); border-radius: 3px; }