/* ============================================================
   SolveLog — bold, vibrant, white-forward (Brilliant-style).
   Electric blue + lime accents. Display: Bricolage Grotesque,
   Body: Hanken Grotesk, Mono: JetBrains Mono.
   One stylesheet for the whole site.
   ============================================================ */

/* ---------------- tokens ---------------- */
:root {
  --bg: #FFFFFF;
  --bg-tint: #F4F7FF;
  --card: #FFFFFF;
  --surface: #F3F6FC;
  --line: #E4E9F2;
  --ink: #0E1726;
  --ink-2: #45506A;
  --ink-3: #6B7790;

  --accent: #2563FF;
  --accent-deep: #1748D4;
  --accent-soft: #EAF0FF;
  --lime: #65C30A;
  --lime-soft: #EEF8E0;

  --easy: #16A34A;
  --medium: #D98A00;
  --hard: #E23B54;

  --radius: 18px;
  --radius-sm: 11px;
  --radius-xs: 8px;
  --shadow: 0 1px 2px rgba(14,23,38,.04), 0 6px 20px rgba(14,23,38,.06);
  --shadow-lift: 0 2px 6px rgba(14,23,38,.06), 0 16px 40px rgba(37,99,255,.12);

  --font-display: "Bricolage Grotesque", system-ui, sans-serif;
  --font-body: "Hanken Grotesk", system-ui, sans-serif;
  --font-mono: "JetBrains Mono", ui-monospace, monospace;

  --code-bg: #0C1426;
  --code-ink: #E7ECF6;
  --nav-h: 64px;
}

html[data-theme="dark"] {
  --bg: #0B1120;
  --bg-tint: #0F1729;
  --card: #131C30;
  --surface: #161F35;
  --line: #25324B;
  --ink: #EEF2FB;
  --ink-2: #AEB8CF;
  --ink-3: #7E8AA6;

  --accent: #5B8CFF;
  --accent-deep: #7AA2FF;
  --accent-soft: #18233E;
  --lime: #8FE04A;
  --lime-soft: #1A2A18;

  --easy: #34D27E;
  --medium: #E6B14E;
  --hard: #FF6B80;

  --shadow: 0 1px 2px rgba(0,0,0,.3), 0 6px 20px rgba(0,0,0,.35);
  --shadow-lift: 0 2px 6px rgba(0,0,0,.4), 0 16px 40px rgba(0,0,0,.5);
  --code-bg: #060B16;
  --code-ink: #DCE3F1;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
@media (prefers-reduced-motion: reduce) { html { scroll-behavior: auto; } }

body {
  margin: 0;
  font-family: var(--font-body);
  background: var(--bg);
  color: var(--ink);
  font-size: 16px;
  line-height: 1.6;
  -webkit-font-smoothing: antialiased;
}

a { color: var(--accent); text-decoration: none; }
a:hover { color: var(--accent-deep); }

:focus-visible {
  outline: 3px solid var(--accent);
  outline-offset: 2px;
  border-radius: 4px;
}

h1, h2, h3, h4 { font-family: var(--font-display); letter-spacing: -0.02em; line-height: 1.08; margin: 0; }

.wrap { max-width: 1120px; margin: 0 auto; padding: 0 24px; }
.mono { font-family: var(--font-mono); }

/* ---------------- skip link ----------------
   Hidden until focused, so keyboard and screen-reader users can jump past the
   nav, search field and theme toggle straight to <main>.

   Clipped rather than moved off-screen: `clip-path` hides it without taking it
   out of the tab order, and there is no transition, so the reveal is immediate
   and cannot be missed. `position: fixed` with a z-index above the sticky nav
   (which is z-index 60) keeps it on top once shown. */
.skip-link {
  position: fixed; left: 12px; top: 12px; z-index: 200;
  padding: 10px 16px; border-radius: 10px;
  background: var(--accent); color: #fff; font-weight: 700;
  text-decoration: none;
  clip-path: inset(50%);
}
.skip-link:focus,
.skip-link:focus-visible {
  clip-path: none;
  outline: 3px solid var(--ink);
  outline-offset: 2px;
}

/* ---------------- nav ---------------- */
.nav {
  position: sticky; top: 0; z-index: 60;
  height: var(--nav-h);
  background: color-mix(in srgb, var(--bg) 82%, transparent);
  backdrop-filter: blur(14px) saturate(140%);
  -webkit-backdrop-filter: blur(14px) saturate(140%);
  border-bottom: 1px solid var(--line);
}
.nav-inner { max-width: 1120px; margin: 0 auto; padding: 0 24px; height: 100%; display: flex; align-items: center; gap: 18px; }
.brand { display: flex; align-items: center; gap: 10px; font-family: var(--font-display); font-weight: 800; font-size: 21px; color: var(--ink); letter-spacing: -0.03em; }
.brand:hover { color: var(--ink); }
.brand-mark {
  display: inline-flex; align-items: center; justify-content: center;
  width: 30px; height: 30px; border-radius: 9px;
  background: var(--accent); color: #fff;
  font-family: var(--font-mono); font-size: 14px; font-weight: 700;
}
.nav-links { display: flex; gap: 4px; margin-left: 8px; }
.nav-links a {
  font-weight: 600; font-size: 14.5px; color: var(--ink-2);
  padding: 8px 14px; border-radius: 999px; transition: .16s;
}
.nav-links a:hover { color: var(--ink); background: var(--surface); }
.nav-links a.active { color: var(--accent); background: var(--accent-soft); }
.nav-search { margin-left: auto; position: relative; }
.nav-search input {
  font-family: var(--font-body); font-size: 14px;
  width: 210px; max-width: 38vw;
  padding: 9px 14px 9px 34px;
  border: 1px solid var(--line); border-radius: 999px;
  background: var(--surface); color: var(--ink);
  transition: .16s;
}
.nav-search input:focus { width: 250px; border-color: var(--accent); background: var(--bg); outline: none; box-shadow: 0 0 0 3px var(--accent-soft); }
.nav-search .ns-icon { position: absolute; left: 12px; top: 50%; transform: translateY(-50%); color: var(--ink-3); font-size: 14px; pointer-events: none; }
/* Phones: brand + links + search + toggle overflow 375px, which scrolls the
   whole page sideways. Tighten the bar and let the search shrink instead. */
@media (max-width: 560px) {
  .nav-inner { padding: 0 12px; gap: 8px; }
  .nav-links { margin-left: 0; gap: 0; }
  .nav-links a { padding-left: 8px; padding-right: 8px; }
  .nav-search { min-width: 0; flex: 1 1 auto; }
  .nav-search input, .nav-search input:focus { width: 100%; max-width: none; }
}
.theme-toggle {
  display: inline-flex; align-items: center; justify-content: center;
  width: 40px; height: 40px; flex-shrink: 0;
  border: 1px solid var(--line); border-radius: 11px;
  background: var(--card); color: var(--ink); cursor: pointer; font-size: 17px;
  transition: .16s;
}
.theme-toggle:hover { border-color: var(--accent); color: var(--accent); }
.tt-moon { display: none; }
html[data-theme="dark"] .tt-sun { display: none; }
html[data-theme="dark"] .tt-moon { display: inline; }

/* ---------------- buttons / pills / chips ---------------- */
.btn {
  display: inline-flex; align-items: center; gap: 8px;
  font-family: var(--font-body); font-weight: 700; font-size: 15px;
  padding: 12px 22px; border-radius: 12px;
  border: 1px solid var(--line); background: var(--card); color: var(--ink);
  cursor: pointer; transition: transform .14s, box-shadow .14s, background .14s, border-color .14s;
}
.btn:hover { transform: translateY(-2px); box-shadow: var(--shadow); color: var(--ink); }
.btn-primary { background: var(--accent); border-color: var(--accent); color: #fff; }
.btn-primary:hover { background: var(--accent-deep); border-color: var(--accent-deep); color: #fff; box-shadow: var(--shadow-lift); }
@media (prefers-reduced-motion: reduce) { .btn:hover { transform: none; } }

.pill {
  display: inline-block; font-family: var(--font-mono); font-size: 11.5px; font-weight: 700;
  padding: 3px 10px; border-radius: 999px; letter-spacing: .01em;
}
.pill-easy { color: var(--easy); background: color-mix(in srgb, var(--easy) 14%, transparent); }
.pill-medium { color: var(--medium); background: color-mix(in srgb, var(--medium) 16%, transparent); }
.pill-hard { color: var(--hard); background: color-mix(in srgb, var(--hard) 14%, transparent); }

.chip {
  display: inline-block; font-family: var(--font-mono); font-size: 11.5px; font-weight: 600;
  padding: 3px 9px; border-radius: 7px; color: var(--ink-2);
  background: var(--surface); border: 1px solid var(--line);
}

/* ---------------- card ---------------- */
.card { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); box-shadow: var(--shadow); }
.card-hover { transition: transform .16s, box-shadow .16s, border-color .16s; }
.card-hover:hover { transform: translateY(-3px); box-shadow: var(--shadow-lift); border-color: color-mix(in srgb, var(--accent) 40%, var(--line)); }
@media (prefers-reduced-motion: reduce) { .card-hover:hover { transform: none; } }

/* ---------------- reveal animation ---------------- */
[data-reveal] { opacity: 0; transform: translateY(16px); }
[data-reveal].in { opacity: 1; transform: none; transition: opacity .5s ease, transform .5s cubic-bezier(.2,.7,.2,1); }
@media (prefers-reduced-motion: reduce) { [data-reveal] { opacity: 1 !important; transform: none !important; } }

/* ============================================================
   HOME
   ============================================================ */
.hero { padding: 72px 0 36px; text-align: center; }
.hero h1 { font-size: clamp(40px, 6.4vw, 80px); font-weight: 800; max-width: 17ch; margin: 0 auto 20px; }
.hero h1 .accent { color: var(--accent); position: relative; white-space: nowrap; }
.hero h1 .ul { position: relative; }
.hero h1 .ul::after {
  content: ""; position: absolute; left: -2%; right: -2%; bottom: .04em; height: .28em;
  background: var(--lime); border-radius: 999px; z-index: -1; opacity: .85;
}
.hero p.lede { font-size: clamp(16px, 1.7vw, 20px); color: var(--ink-2); max-width: 56ch; margin: 0 auto 30px; }
.hero-cta { display: flex; gap: 12px; justify-content: center; flex-wrap: wrap; }

.stats { display: grid; grid-template-columns: repeat(5, 1fr); gap: 14px; margin: 44px 0 8px; }
.stat-tile { padding: 22px 18px; text-align: center; border-radius: var(--radius); }
.stat-num { font-family: var(--font-mono); font-weight: 700; font-size: 34px; color: var(--accent); line-height: 1; }
.stat-tile.t-easy .stat-num { color: var(--easy); }
.stat-tile.t-medium .stat-num { color: var(--medium); }
.stat-tile.t-hard .stat-num { color: var(--hard); }
.stat-tile.t-pat .stat-num { color: var(--lime); }
.stat-label { font-size: 13px; color: var(--ink-3); margin-top: 6px; font-weight: 600; }

.section { margin: 64px 0; }
.section-head { display: flex; align-items: baseline; justify-content: space-between; gap: 14px; margin-bottom: 22px; flex-wrap: wrap; }
.section-head h2 { font-size: clamp(24px, 3vw, 34px); font-weight: 800; }
.section-head a { font-weight: 700; font-size: 14px; }

.feat-grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: 16px; }
.pcard { display: flex; flex-direction: column; padding: 20px; text-decoration: none; color: var(--ink); height: 100%; }
.pcard:hover { color: var(--ink); }
.pcard-top { display: flex; align-items: center; justify-content: space-between; margin-bottom: 12px; }
.pcard-num { font-family: var(--font-mono); font-size: 12px; font-weight: 700; color: var(--ink-3); }
.pcard h3 { font-size: 19px; font-weight: 700; margin-bottom: 8px; letter-spacing: -0.02em; }
.pcard:hover h3 { color: var(--accent); }
.pcard p { font-size: 14px; color: var(--ink-2); margin: 0 0 14px; flex: 1; display: -webkit-box; -webkit-line-clamp: 3; -webkit-box-orient: vertical; overflow: hidden; }
.pcard-tags { display: flex; gap: 6px; flex-wrap: wrap; }

.recent-list { border: 1px solid var(--line); border-radius: var(--radius); overflow: hidden; background: var(--card); box-shadow: var(--shadow); }
.recent-row { display: grid; grid-template-columns: 56px 1fr auto auto; gap: 14px; align-items: center; padding: 14px 18px; border-bottom: 1px solid var(--line); text-decoration: none; color: var(--ink); transition: background .14s; }
.recent-row:last-child { border-bottom: 0; }
.recent-row:hover { background: var(--bg-tint); color: var(--ink); }
.recent-row .rr-num { font-family: var(--font-mono); font-size: 13px; color: var(--ink-3); font-weight: 700; }
.recent-row .rr-title { font-weight: 600; }
.recent-row .rr-cat { font-size: 12.5px; color: var(--ink-3); font-family: var(--font-mono); }
@media (max-width: 620px) { .recent-row { grid-template-columns: 44px 1fr auto; } .recent-row .rr-cat { display: none; } }

.pat-grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: 14px; }
.pat-card { display: flex; flex-direction: column; gap: 4px; padding: 18px 20px; text-decoration: none; color: var(--ink); }
.pat-card:hover { color: var(--ink); }
.pat-card .pat-top { display: flex; align-items: center; justify-content: space-between; }
.pat-card .pat-name { font-family: var(--font-display); font-weight: 700; font-size: 18px; letter-spacing: -0.02em; }
.pat-card:hover .pat-name { color: var(--accent); }
.pat-card .pat-count { font-family: var(--font-mono); font-weight: 700; font-size: 12px; color: var(--accent); background: var(--accent-soft); padding: 3px 10px; border-radius: 999px; }
.pat-card .pat-blurb { font-size: 13.5px; color: var(--ink-3); }

/* ---------------- footer ---------------- */
/* Reserve the height up front: an unfilled <ins> is 0px tall, so without this
   the whole page jumps down the moment the ad arrives. */
.ad-slot { max-width: 1120px; margin: 56px auto 0; padding: 0 24px; min-height: 100px; }
.footer { border-top: 1px solid var(--line); margin-top: 70px; }
.footer-inner { max-width: 1120px; margin: 0 auto; padding: 28px 24px 40px; display: flex; align-items: center; justify-content: space-between; gap: 16px; flex-wrap: wrap; }
.footer .f-brand { font-family: var(--font-display); font-weight: 800; font-size: 18px; }
.footer .f-count { font-family: var(--font-mono); font-size: 12.5px; color: var(--ink-3); }

/* ============================================================
   BROWSE
   ============================================================ */
.browse-head { padding: 44px 0 20px; }
.browse-head h1 { font-size: clamp(32px, 4.6vw, 56px); font-weight: 800; margin-bottom: 8px; }
.browse-head p { color: var(--ink-2); font-size: 16px; }

.toolbar { display: grid; grid-template-columns: 1fr auto; gap: 12px; align-items: center; padding: 14px; margin-bottom: 18px; }
.toolbar .tb-search { position: relative; }
.toolbar .tb-search input { width: 100%; font-family: var(--font-body); font-size: 15px; padding: 11px 14px 11px 38px; border: 1px solid var(--line); border-radius: 12px; background: var(--surface); color: var(--ink); }
.toolbar .tb-search input:focus { outline: none; border-color: var(--accent); background: var(--bg); box-shadow: 0 0 0 3px var(--accent-soft); }
.toolbar .tb-search .ns-icon { position: absolute; left: 14px; top: 50%; transform: translateY(-50%); color: var(--ink-3); }
.toolbar .tb-controls { display: flex; gap: 8px; flex-wrap: wrap; }
.toolbar select { font-family: var(--font-body); font-weight: 600; font-size: 14px; padding: 10px 12px; border: 1px solid var(--line); border-radius: 10px; background: var(--card); color: var(--ink); cursor: pointer; }

/* custom dropdown (progressive enhancement over native select) */
.dd { position: relative; }
.dd .dd-native { position: absolute; width: 1px; height: 1px; opacity: 0; pointer-events: none; }
.dd-btn { display: inline-flex; align-items: center; gap: 10px; font-family: var(--font-body); font-weight: 600; font-size: 14px; padding: 10px 12px; border: 1px solid var(--line); border-radius: 10px; background: var(--card); color: var(--ink); cursor: pointer; transition: border-color .14s, box-shadow .14s; }
.dd-btn:hover { border-color: var(--ink-3); }
.dd.open .dd-btn { border-color: var(--accent); box-shadow: 0 0 0 3px var(--accent-soft); }
.dd-arr { font-size: 11px; color: var(--ink-3); transition: transform .16s; }
.dd.open .dd-arr { transform: rotate(180deg); }
.dd-menu { position: absolute; top: calc(100% + 6px); left: 0; min-width: max(100%, 190px); z-index: 60; background: var(--card); border: 1px solid var(--line); border-radius: 12px; box-shadow: 0 12px 32px rgba(15, 23, 42, .14); padding: 6px; opacity: 0; transform: translateY(-4px) scale(.98); pointer-events: none; transition: opacity .14s, transform .14s; max-height: 320px; overflow: auto; }
.dd.open .dd-menu { opacity: 1; transform: none; pointer-events: auto; }
.dd-item { display: flex; align-items: center; gap: 8px; width: 100%; text-align: left; font-family: var(--font-body); font-weight: 500; font-size: 14px; padding: 9px 10px; border: 0; border-radius: 8px; background: transparent; color: var(--ink); cursor: pointer; }
.dd-item:hover { background: var(--surface); }
.dd-item.on { font-weight: 700; color: var(--accent); background: var(--accent-soft); }
.dd-item .dd-check { width: 14px; font-size: 12px; color: var(--accent); visibility: hidden; }
.dd-item.on .dd-check { visibility: visible; }
.dd-menu { scrollbar-width: thin; scrollbar-color: var(--line) transparent; }
.dd-menu::-webkit-scrollbar { width: 8px; }
.dd-menu::-webkit-scrollbar-track { background: transparent; }
.dd-menu::-webkit-scrollbar-thumb { background: var(--line); border-radius: 8px; }
html[data-theme="dark"] .dd-menu { box-shadow: 0 12px 32px rgba(0, 0, 0, .5); }
.seg { display: inline-flex; border: 1px solid var(--line); border-radius: 10px; overflow: hidden; }
.seg button { font-family: var(--font-body); font-weight: 600; font-size: 13.5px; padding: 10px 14px; border: 0; background: var(--card); color: var(--ink-2); cursor: pointer; transition: .14s; }
.seg button:hover { background: var(--surface); }
.seg button.active { background: var(--accent); color: #fff; }

.result-meta { font-family: var(--font-mono); font-size: 12.5px; color: var(--ink-3); margin: 6px 2px 14px; }
.browse-list { border: 1px solid var(--line); border-radius: var(--radius); overflow: hidden; background: var(--card); box-shadow: var(--shadow); }
.brow-row { display: grid; grid-template-columns: 60px 1fr 150px 96px; gap: 14px; align-items: center; padding: 15px 18px; border-bottom: 1px solid var(--line); text-decoration: none; color: var(--ink); transition: background .14s; }
.brow-row:last-child { border-bottom: 0; }
.brow-row:hover { background: var(--bg-tint); color: var(--ink); }
.brow-row .br-num { font-family: var(--font-mono); font-size: 13px; font-weight: 700; color: var(--ink-3); }
.brow-row .br-main .br-title { font-weight: 600; }
.brow-row .br-main .br-tags { display: flex; gap: 5px; flex-wrap: wrap; margin-top: 4px; }
.brow-row .br-cat { font-family: var(--font-mono); font-size: 12px; color: var(--ink-3); }
@media (max-width: 700px) { .toolbar { grid-template-columns: 1fr; } .brow-row { grid-template-columns: 44px 1fr auto; } .brow-row .br-cat { display: none; } }

.pager { display: flex; align-items: center; justify-content: center; gap: 8px; margin: 24px 0 8px; }
.pager button { font-family: var(--font-mono); font-weight: 600; font-size: 13px; padding: 8px 13px; border: 1px solid var(--line); border-radius: 9px; background: var(--card); color: var(--ink); cursor: pointer; }
.pager button:hover:not(:disabled) { border-color: var(--accent); color: var(--accent); }
.pager button:disabled { opacity: .4; cursor: default; }
.pager .pg-info { font-family: var(--font-mono); font-size: 12.5px; color: var(--ink-3); padding: 0 6px; }

.empty { text-align: center; padding: 60px 20px; color: var(--ink-3); }
.empty .e-emoji { font-size: 34px; }
.empty h3 { font-size: 20px; color: var(--ink); margin: 12px 0 6px; }

/* ============================================================
   PROBLEM PAGE
   ============================================================ */
.wrap.wrap-wide { max-width: 1320px; }
.prob-layout { display: grid; grid-template-columns: 200px 1fr; gap: 32px; padding: 30px 0 60px; align-items: start; }
.toc { position: sticky; top: calc(var(--nav-h) + 22px); }
.toc .toc-label { font-family: var(--font-mono); font-size: 11px; text-transform: uppercase; letter-spacing: .08em; color: var(--ink-3); margin-bottom: 10px; }
.toc a { display: block; font-size: 13.5px; color: var(--ink-2); padding: 7px 12px; border-radius: 8px; border-left: 2px solid transparent; transition: .14s; }
.toc a:hover { background: var(--surface); color: var(--ink); }
.toc a.active { color: var(--accent); border-left-color: var(--accent); background: var(--accent-soft); font-weight: 600; }
@media (max-width: 900px) { .prob-layout { grid-template-columns: 1fr; gap: 0; } .toc { display: none; } }

.breadcrumb { font-family: var(--font-mono); font-size: 12.5px; color: var(--ink-3); margin-bottom: 8px; }
.breadcrumb a { color: var(--ink-3); }
.breadcrumb a:hover { color: var(--accent); }

.psec { margin-bottom: 40px; scroll-margin-top: calc(var(--nav-h) + 18px); }
.psec-num { font-family: var(--font-mono); font-size: 12px; font-weight: 700; color: var(--accent); letter-spacing: .04em; }
.psec h2 { font-size: 26px; font-weight: 800; margin: 4px 0 16px; }

/* 01 intro */
.intro-card { padding: 26px; }
.intro-meta { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; margin-bottom: 14px; }
.intro-meta .lc-tag { font-family: var(--font-mono); font-size: 11px; font-weight: 700; color: #fff; background: #FFA116; padding: 3px 9px; border-radius: 6px; }
.intro-meta .lc-num { font-family: var(--font-mono); font-size: 13px; color: var(--ink-3); font-weight: 700; }
.intro-card h1 { font-size: clamp(26px, 3.4vw, 38px); font-weight: 800; margin-bottom: 14px; }
.intro-ask { font-size: 16.5px; color: var(--ink-2); margin-bottom: 16px; }
.intro-ask code { font-family: var(--font-mono); font-size: .9em; background: var(--surface); padding: 2px 6px; border-radius: 6px; color: var(--accent-deep); }
.intro-tags { display: flex; gap: 6px; flex-wrap: wrap; margin-bottom: 16px; }
.intro-link { font-weight: 700; font-size: 14px; }

/* 02 intuition callout */
.callout { padding: 24px 26px; border-radius: var(--radius); background: var(--accent-soft); border: 1px solid color-mix(in srgb, var(--accent) 22%, var(--line)); position: relative; overflow: hidden; }
.callout::before { content: ""; position: absolute; left: 0; top: 0; bottom: 0; width: 5px; background: var(--lime); }
.callout .co-icon { font-size: 22px; }
.callout p { font-size: 16.5px; color: var(--ink); margin: 8px 0 0; }
.callout code { font-family: var(--font-mono); font-size: .88em; background: color-mix(in srgb, var(--accent) 12%, transparent); padding: 2px 6px; border-radius: 6px; }

/* 03 approach steps */
.steps { display: flex; flex-direction: column; gap: 14px; }
.step { display: grid; grid-template-columns: 40px 1fr; gap: 16px; padding: 20px; }
.step .step-n { font-family: var(--font-mono); font-weight: 700; font-size: 15px; color: #fff; background: var(--accent); width: 36px; height: 36px; border-radius: 10px; display: flex; align-items: center; justify-content: center; }
.step h3 { font-size: 18px; font-weight: 700; margin-bottom: 5px; }
.step p { font-size: 15px; color: var(--ink-2); margin: 0; }
.step p code, .edge code { font-family: var(--font-mono); font-size: .88em; background: var(--surface); padding: 1px 5px; border-radius: 5px; color: var(--accent-deep); }

/* 04 solution + demo */
.solve-grid { display: grid; grid-template-columns: minmax(0, 1.55fr) minmax(320px, 1fr); gap: 20px; align-items: stretch; }
.solve-grid > * { min-width: 0; }   /* let columns contain their scrollable children instead of clipping */
/* both panels fill the row's shared height so their tops AND bottoms line up */
.solve-grid .code-panel, .solve-grid .demo-panel { height: 100%; display: flex; flex-direction: column; }
@media (max-width: 1180px) { .solve-grid { grid-template-columns: 1fr; align-items: start; } .solve-grid .code-panel, .solve-grid .demo-panel { height: auto; } }

.code-panel { background: var(--code-bg); border-radius: var(--radius); overflow: hidden; border: 1px solid color-mix(in srgb, #fff 8%, var(--code-bg)); }
.code-head { display: flex; align-items: center; justify-content: space-between; padding: 11px 16px; border-bottom: 1px solid rgba(255,255,255,.08); }
.code-head .ch-lang { font-family: var(--font-mono); font-size: 12px; color: #9fb0cc; font-weight: 600; }
.code-head .ch-dots { display: flex; gap: 6px; }
.code-head .ch-dots i { width: 11px; height: 11px; border-radius: 50%; display: inline-block; }
.ch-dots i:nth-child(1) { background: #ff5f57; } .ch-dots i:nth-child(2) { background: #febc2e; } .ch-dots i:nth-child(3) { background: #28c840; }
.copy-btn { font-family: var(--font-mono); font-size: 12px; color: #9fb0cc; background: rgba(255,255,255,.06); border: 1px solid rgba(255,255,255,.12); border-radius: 7px; padding: 5px 10px; cursor: pointer; transition: .14s; }
.copy-btn:hover { color: #fff; background: rgba(255,255,255,.14); }
.code-body { margin: 0; padding: 14px 0; overflow: auto; flex: 1 1 auto; min-height: 0; font-family: var(--font-mono); font-size: 13px; line-height: 1.75; }
.code-line { display: grid; grid-template-columns: 18px 38px 1fr; padding: 0 16px 0 0; min-width: max-content; }
.code-line .cl-arrow { color: var(--lime); font-weight: 700; text-align: center; opacity: 0; position: sticky; left: 0; background: var(--code-bg); }
.code-line .cl-no { color: #54637f; text-align: right; padding-right: 14px; user-select: none; position: sticky; left: 18px; background: var(--code-bg); }
.code-line .cl-src { color: var(--code-ink); white-space: pre; padding-right: 8px; }
.code-line.cl-active { background: rgba(91,140,255,.18); box-shadow: inset 3px 0 0 var(--lime); }
.code-line.cl-active .cl-arrow { opacity: 1; background: #11203c; }
.code-line.cl-active .cl-no { color: var(--lime); background: #11203c; }
@media (prefers-reduced-motion: no-preference) { .code-line.cl-active { transition: background .15s; } }
/* token colors */
.tk-kw { color: #ff9ed2; } .tk-fn { color: #82d0ff; } .tk-str { color: #9be38b; } .tk-num { color: #ffcb6b; } .tk-com { color: #5f7194; font-style: italic; } .tk-self { color: #c792ea; }

/* demo */
.demo-panel { padding: 16px; }
.sim-ex { display: flex; gap: 7px; flex-wrap: wrap; margin-bottom: 12px; }
.sim-ex-btn { font-family: var(--font-mono); font-size: 11.5px; font-weight: 600; padding: 6px 11px; border: 1px solid var(--line); border-radius: 999px; background: var(--card); color: var(--ink-2); cursor: pointer; transition: .14s; }
.sim-ex-btn:hover { border-color: var(--accent); color: var(--accent); }
.sim-ex-btn.active { background: var(--accent); border-color: var(--accent); color: #fff; }
.sim-stage { min-height: 168px; display: flex; align-items: center; justify-content: center; padding: 18px 6px; background: var(--bg-tint); border-radius: var(--radius-sm); border: 1px solid var(--line); overflow: hidden; }
.sim-viz { width: 100%; min-width: 0; overflow-x: auto; overflow-y: hidden; padding: 8px 0 20px; }
.sim-vars { display: flex; flex-wrap: wrap; gap: 8px; margin: 14px 0 10px; }
.var { display: flex; flex-direction: column; gap: 1px; padding: 7px 11px; border: 1px solid var(--line); border-radius: 9px; background: var(--card); min-width: 60px; }
.var-k { font-family: var(--font-mono); font-size: 10px; color: var(--ink-3); text-transform: uppercase; letter-spacing: .03em; }
.var-v { font-family: var(--font-mono); font-size: 14px; font-weight: 700; color: var(--ink); }
.var-sub { font-size: 10px; color: var(--ink-3); }
.var.v-flash { border-color: var(--accent); box-shadow: 0 0 0 2px var(--accent-soft); }
.var.v-hit { border-color: var(--lime); background: var(--lime-soft); }
.var.v-hit .var-v { color: color-mix(in srgb, var(--lime) 70%, var(--ink)); }
.var.v-over { border-color: var(--hard); }
.sim-note { font-size: 14px; color: var(--ink-2); min-height: 40px; padding: 10px 12px; background: var(--surface); border-radius: 9px; }
.sim-note code { font-family: var(--font-mono); font-size: .9em; background: var(--card); padding: 1px 5px; border-radius: 5px; color: var(--accent-deep); }
.sim-bar { height: 5px; background: var(--surface); border-radius: 999px; margin: 12px 0; overflow: hidden; }
.sim-progress { height: 100%; width: 0; background: var(--accent); border-radius: 999px; transition: width .3s ease; }
.sim-controls { display: flex; align-items: center; gap: 7px; flex-wrap: wrap; }
.sim-btn { font-family: var(--font-body); font-weight: 600; font-size: 13px; padding: 8px 13px; border: 1px solid var(--line); border-radius: 9px; background: var(--card); color: var(--ink); cursor: pointer; transition: .14s; }
.sim-btn:hover { border-color: var(--accent); color: var(--accent); }
.sim-btn-primary { background: var(--accent); border-color: var(--accent); color: #fff; }
.sim-btn-primary:hover { background: var(--accent-deep); color: #fff; }
.sim-btn-primary.playing { background: var(--hard); border-color: var(--hard); }
.sim-count { font-family: var(--font-mono); font-size: 12px; color: var(--ink-3); margin-left: auto; }
.sim-out { margin-top: 12px; max-height: 0; opacity: 0; overflow: hidden; transition: .3s; }
.sim-out.out-show { max-height: 90px; opacity: 1; padding: 12px 14px; background: var(--lime-soft); border: 1px solid color-mix(in srgb, var(--lime) 40%, var(--line)); border-radius: 10px; }
.sim-out .out-tag { font-family: var(--font-mono); font-size: 10px; text-transform: uppercase; letter-spacing: .05em; color: color-mix(in srgb, var(--lime) 60%, var(--ink)); display: block; margin-bottom: 4px; }
.sim-out code { font-family: var(--font-mono); font-size: 14px; font-weight: 700; color: var(--ink); }

/* viz primitives */
.viz-array, .viz-bars { display: flex; gap: 6px; align-items: flex-end; flex-wrap: nowrap; width: max-content; max-width: none; margin: 0 auto; }
.viz-array.is-text { gap: 3px; }
.cell { position: relative; min-width: 38px; padding: 0 8px; height: 44px; display: flex; align-items: center; justify-content: center; font-family: var(--font-mono); font-weight: 700; font-size: 15px; background: var(--card); border: 1.5px solid var(--line); border-radius: 9px; color: var(--ink); transition: .18s; }
.is-text .cell { min-width: 26px; padding: 0 2px; height: 38px; font-size: 14px; }
.cell-idx { position: absolute; bottom: -16px; font-style: normal; font-size: 9px; color: var(--ink-3); font-weight: 500; }
.cell.c-i { border-color: var(--accent); background: var(--accent-soft); color: var(--accent-deep); transform: translateY(-3px); }
.cell.c-j { border-color: var(--medium); }
.cell.c-win { border-color: var(--accent); background: color-mix(in srgb, var(--accent) 8%, var(--card)); }
.cell.c-done { border-color: var(--lime); background: var(--lime-soft); }
.cell.c-hit { border-color: var(--lime); background: var(--lime); color: #08240a; }
.ptr-row { display: flex; gap: 6px; width: max-content; margin: 22px auto 0; }
.is-text + .ptr-row { gap: 3px; }
.ptr-cell { min-width: 38px; display: flex; justify-content: center; }
.is-text ~ .ptr-row .ptr-cell { min-width: 26px; }
.ptr-tag { font-family: var(--font-mono); font-size: 10px; font-weight: 700; color: var(--accent); background: var(--accent-soft); padding: 1px 6px; border-radius: 5px; }
.viz-array.n-lg, .ptr-row.n-lg { gap: 4px; }
.viz-array.n-lg .cell { min-width: 28px; height: 38px; font-size: 13px; padding: 0 4px; }
.ptr-row.n-lg .ptr-cell { min-width: 28px; }
.viz-array.n-xl, .ptr-row.n-xl { gap: 3px; }
.viz-array.n-xl .cell { min-width: 21px; height: 34px; font-size: 11.5px; padding: 0 3px; }
.ptr-row.n-xl .ptr-cell { min-width: 21px; }
.viz-array.is-text.n-lg .cell { min-width: 22px; height: 34px; font-size: 12.5px; }
.viz-array.is-text.n-xl .cell { min-width: 18px; height: 32px; font-size: 11.5px; }
.is-text.n-lg ~ .ptr-row .ptr-cell { min-width: 22px; }
.is-text.n-xl ~ .ptr-row .ptr-cell { min-width: 18px; }

.viz-bars { height: 150px; align-items: flex-end; }
.bar { position: relative; width: 34px; min-height: 8px; background: var(--accent); border-radius: 6px 6px 0 0; display: flex; align-items: flex-start; justify-content: center; transition: .18s; }
.bar.bar-neg { background: var(--ink-3); }
.bar.c-i { background: var(--accent-deep); transform: scaleY(1.02); }
.bar.c-done { background: var(--lime); }
.bar.c-hit { background: var(--lime); }
.bar.c-win { background: color-mix(in srgb, var(--accent) 70%, var(--lime)); }
.bar-val { font-family: var(--font-mono); font-size: 11px; font-weight: 700; color: #fff; margin-top: -17px; position: absolute; top: 0; }
.bar .cell-idx { bottom: -16px; }

.viz-seq { text-align: center; margin-bottom: 14px; font-family: var(--font-mono); font-size: 20px; letter-spacing: 4px; }
.seq-ch { color: var(--ink-3); }
.seq-ch.seq-past { color: var(--ink-2); }
.seq-ch.seq-now { color: var(--accent); background: var(--accent-soft); border-radius: 5px; padding: 0 3px; }
.viz-stack { position: relative; display: flex; flex-direction: column-reverse; align-items: center; gap: 5px; min-height: 60px; justify-content: flex-end; }
.stk-item { font-family: var(--font-mono); font-weight: 700; min-width: 44px; text-align: center; padding: 7px 0; background: var(--accent-soft); border: 1.5px solid var(--accent); border-radius: 8px; color: var(--accent-deep); }
.stk-empty { color: var(--ink-3); font-family: var(--font-mono); font-size: 13px; }
.stk-label { font-style: normal; font-family: var(--font-mono); font-size: 10px; color: var(--ink-3); margin-top: 4px; }

.viz-list { display: flex; gap: 0; align-items: center; flex-wrap: nowrap; width: max-content; margin: 0 auto; }
.viz-list.n-lg .node { min-width: 32px; height: 34px; font-size: 13px; }
.viz-list.n-lg .n-arrow { padding: 0 3px; font-size: 13px; }
.node-wrap { display: inline-flex; align-items: center; }
.node { font-family: var(--font-mono); font-weight: 700; min-width: 40px; height: 40px; display: flex; align-items: center; justify-content: center; border: 1.5px solid var(--line); border-radius: 9px; background: var(--card); color: var(--ink); transition: .18s; }
.node.n-cur, .node.n-slow { border-color: var(--accent); background: var(--accent-soft); color: var(--accent-deep); }
.node.n-fast { border-color: var(--medium); box-shadow: 0 0 0 2px color-mix(in srgb, var(--medium) 24%, transparent); }
.node.n-head { border-color: var(--lime); background: var(--lime-soft); }
.node.n-hit { border-color: var(--lime); background: var(--lime); color: #08240a; }
.node.n-flip { border-color: var(--lime); }
.n-arrow { color: var(--ink-3); padding: 0 6px; font-weight: 700; }
.n-arrow.n-loop { color: var(--hard); font-family: var(--font-mono); font-size: 11px; }

.viz-rows { display: flex; flex-direction: column; align-items: center; gap: 5px; width: max-content; margin: 0 auto; }
.tri-row { display: flex; gap: 5px; }
.tri-row.tri-now .tri-cell { border-color: var(--accent); background: var(--accent-soft); color: var(--accent-deep); }
.tri-cell { font-family: var(--font-mono); font-weight: 700; font-size: 12px; min-width: 30px; padding: 5px 0; text-align: center; border: 1.5px solid var(--line); border-radius: 7px; background: var(--card); }
.tri-cell.tri-cur { border-color: var(--accent); background: var(--accent-soft); color: var(--accent-deep); }
.tri-cell.tri-hit { border-color: var(--lime); background: var(--lime-soft); }
.tri-cell.tri-bad { border-color: var(--hard); background: transparent; color: var(--hard); }

.viz-grid { display: flex; flex-direction: column; gap: 5px; width: max-content; margin: 0 auto; }
.grid-row { display: flex; gap: 5px; }
.grid-cell { font-family: var(--font-mono); font-weight: 700; min-width: 38px; height: 38px; display: flex; align-items: center; justify-content: center; border: 1.5px solid var(--line); border-radius: 8px; background: var(--card); transition: .18s; }
.grid-cell.g-zero { color: var(--hard); border-color: var(--hard); }
.grid-cell.g-mark { background: color-mix(in srgb, var(--hard) 12%, var(--card)); }
.grid-cell.g-active { border-color: var(--accent); background: var(--accent-soft); color: var(--accent-deep); transform: scale(1.06); }
.grid-cell.g-done { background: color-mix(in srgb, var(--lime) 14%, var(--card)); border-color: color-mix(in srgb, var(--lime) 45%, var(--line)); }
.grid-cell.g-src { border-color: var(--medium); background: color-mix(in srgb, var(--medium) 16%, var(--card)); color: var(--ink); }
.grid-cell.g-head { border-color: transparent; background: transparent; color: var(--ink-3); font-size: 12px; font-weight: 600; }

/* recursion tree — backtracking demos */
.viz-tree { display: flex; width: max-content; margin: 0 auto; padding: 2px 4px; }
.tk { position: relative; display: flex; flex-direction: column; align-items: center; padding: 12px 3px 0; }
.tk::before { content: ""; position: absolute; top: 0; left: 50%; width: 1.5px; height: 12px; background: var(--line); }
.tk::after { content: ""; position: absolute; top: 0; left: 0; right: 0; height: 1.5px; background: var(--line); }
.tk:first-child::after { left: 50%; }
.tk:last-child::after { right: 50%; }
.tk:only-child::after { display: none; }
.viz-tree > .tk { padding-top: 0; }
.viz-tree > .tk::before, .viz-tree > .tk::after { display: none; }
.tstem { width: 1.5px; height: 9px; background: var(--line); margin-top: 2px; }
.tkids { display: flex; align-items: flex-start; }
.tnode { font-family: var(--font-mono); font-weight: 700; font-size: 13px; min-width: 30px; padding: 4px 8px; text-align: center; border: 1.5px solid var(--line); border-radius: 8px; background: var(--card); color: var(--ink); transition: .18s; }
.tnode.t-path { border-color: var(--accent); background: var(--accent-soft); color: var(--accent-deep); }
.tnode.t-active { box-shadow: 0 0 0 3px var(--accent-soft); border-color: var(--accent-deep); }
.tnode.t-hit { border-color: var(--lime); background: var(--lime); color: #08240a; }
.tnode.t-done { opacity: .45; }
.tnode.t-dead { opacity: .4; border-style: dashed; color: var(--ink-3); }
.viz-tree.t-md .tnode { font-size: 11.5px; min-width: 24px; padding: 3px 6px; }
.viz-tree.t-md .tk { padding: 10px 2px 0; }
.viz-tree.t-sm .tnode { font-size: 10.5px; min-width: 20px; padding: 2px 4px; border-radius: 6px; }
.viz-tree.t-sm .tk { padding: 9px 1.5px 0; }
.viz-tree.t-sm .tk::before { height: 9px; }

.viz-intervals { width: 100%; max-width: 460px; margin: 0 auto; }
.iv-label { font-family: var(--font-mono); font-size: 10px; text-transform: uppercase; color: var(--ink-3); margin: 8px 0 4px; letter-spacing: .05em; }
.iv-track { position: relative; height: 26px; margin-bottom: 4px; background: var(--surface); border-radius: 6px; }
.iv-bar { position: absolute; top: 0; height: 26px; display: flex; align-items: center; justify-content: center; font-family: var(--font-mono); font-size: 11px; font-weight: 700; border-radius: 6px; color: #fff; }
.iv-in { background: var(--accent); opacity: .8; }
.iv-out { background: var(--lime); color: #08240a; }
.iv-active { outline: 2px solid var(--ink); opacity: 1; }

/* 05 edge cases */
.edges { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; }
@media (max-width: 760px) { .edges { grid-template-columns: 1fr; } }
@media (max-width: 640px) {
  .cell { min-width: 30px; height: 38px; font-size: 13px; padding: 0 5px; }
  .is-text .cell { min-width: 22px; height: 34px; font-size: 12px; }
  .ptr-cell { min-width: 30px; }
  .is-text ~ .ptr-row .ptr-cell { min-width: 22px; }
  .viz-array.n-lg .cell, .ptr-row.n-lg .ptr-cell { min-width: 24px; }
  .viz-array.n-xl .cell, .ptr-row.n-xl .ptr-cell { min-width: 18px; }
  .viz-array.is-text.n-xl .cell, .is-text.n-xl ~ .ptr-row .ptr-cell { min-width: 16px; }
  .bar { width: 26px; }
  .node { min-width: 32px; height: 34px; font-size: 13px; }
  .viz-list.n-lg .node { min-width: 26px; height: 30px; font-size: 12px; }
  .grid-cell { min-width: 32px; height: 32px; font-size: 13px; }
  .tnode { font-size: 11px; min-width: 24px; padding: 3px 5px; }
  .viz-tree.t-sm .tnode { font-size: 9.5px; min-width: 17px; padding: 2px 3px; }
}
.edge { padding: 18px 20px; }
.edge .edge-case { font-weight: 700; font-size: 15px; margin-bottom: 6px; display: flex; gap: 8px; align-items: flex-start; }
.edge .edge-case::before { content: "⚠"; color: var(--medium); }
.edge .edge-handle { font-size: 14px; color: var(--ink-2); margin: 0; }

/* 06 complexity */
.cplx { display: grid; grid-template-columns: 1fr 1fr; gap: 16px; }
@media (max-width: 620px) { .cplx { grid-template-columns: 1fr; } }
.cplx-card { padding: 24px; text-align: center; }
.cplx-card .cc-label { font-family: var(--font-mono); font-size: 11px; text-transform: uppercase; letter-spacing: .08em; color: var(--ink-3); }
.cplx-card .cc-val { font-family: var(--font-mono); font-weight: 700; font-size: 38px; margin: 6px 0; }
.cplx-card.c-time .cc-val { color: var(--accent); }
.cplx-card.c-space .cc-val { color: var(--lime); }
.cplx-note { grid-column: 1 / -1; font-size: 14px; color: var(--ink-2); padding: 14px 18px; background: var(--surface); border-radius: var(--radius-sm); }

/* prev/next */
.prevnext { display: flex; justify-content: space-between; gap: 14px; margin-top: 30px; }
.prevnext a { flex: 1; padding: 16px 18px; border: 1px solid var(--line); border-radius: var(--radius); background: var(--card); transition: .14s; }
.prevnext a:hover { border-color: var(--accent); box-shadow: var(--shadow); }
.prevnext .pn-dir { font-family: var(--font-mono); font-size: 11px; color: var(--ink-3); }
.prevnext .pn-title { font-weight: 700; color: var(--ink); margin-top: 2px; }
.prevnext .pn-next { text-align: right; }

@media (max-width: 560px) { .stats { grid-template-columns: repeat(2, 1fr); } .feat-grid, .pat-grid { grid-template-columns: 1fr; } .hero { padding: 48px 0 24px; } }
@media (max-width: 820px) and (min-width: 561px) { .feat-grid, .pat-grid { grid-template-columns: repeat(2, 1fr); } }