/* solve/ : warm paper, black ink, one red accent. The interactive model sits in a dark "screen" panel (.lab). */
:root {
  --bg: #f5f2ea; --panel: #fffdf8; --panel2: #efeadf; --line: #ddd6c6; --ink: #18181b; --dim: #67635a;
  --acc: #d2382a; --on-acc: #fff; --cyan: #0b7285; --red: #c92a2a; --green: #2b8a3e; --amber: #b7791f;
  --mono: "JetBrains Mono", ui-monospace, "SF Mono", Menlo, Consolas, monospace;
  --serif: "Newsreader", "Iowan Old Style", Georgia, serif;
  --sans: -apple-system, BlinkMacSystemFont, "SF Pro Text", "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
}
* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; scroll-behavior: smooth; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 17px/1.6 var(--sans); }
a { color: inherit; }
img, video { max-width: 100%; display: block; }
.wrap { max-width: 1120px; margin: 0 auto; padding: 0 16px; }
.wrap.narrow { max-width: 720px; }
main.wrap { padding-bottom: 72px; }

/* header and footer */
header.site { border-bottom: 1px solid var(--line); background: var(--bg); position: sticky; top: 0; z-index: 20; }
.bar { display: flex; align-items: center; gap: 22px; height: 60px; }
.logo { display: flex; align-items: center; gap: 8px; text-decoration: none; font: 700 19px/1 var(--mono); letter-spacing: -.02em; }
.logo i, .kicker i { font-style: normal; color: var(--acc); }
header nav { display: flex; gap: 4px; flex: 1; }
header nav a { text-decoration: none; font-size: 15px; color: var(--dim); padding: 8px 10px; border-radius: 8px; }
header nav a:hover, header nav a.on { color: var(--ink); background: var(--panel2); }
.ig { font: 500 13px/1 var(--mono); text-decoration: none; padding: 10px 14px; border: 1px solid var(--line); border-radius: 999px; background: var(--panel); white-space: nowrap; }
.ig:hover { border-color: var(--ink); }
footer.site { border-top: 1px solid var(--line); padding: 28px 0 40px; color: var(--dim); font-size: 14px; }
footer.site b { font-family: var(--mono); color: var(--ink); }
.fine { font-size: 13px; color: var(--dim); margin: 8px 0 0; }

/* type */
h1 { font: 500 clamp(34px, 6.4vw, 60px)/1.05 var(--serif); letter-spacing: -.02em; margin: 10px 0 16px; text-wrap: balance; }
h1 em, h2 em { font-style: italic; color: var(--acc); }
h2 { font: 500 26px/1.2 var(--serif); margin: 44px 0 12px; letter-spacing: -.01em; }
h2 small { display: block; font: 500 12px/1 var(--mono); letter-spacing: .1em; text-transform: uppercase; color: var(--dim); margin-bottom: 8px; }
h3 { font-size: 17px; margin: 0 0 4px; }
p { margin: 0 0 14px; }
.lede { color: var(--dim); font-size: 19px; max-width: 62ch; }
.kicker { font: 700 15px/1 var(--mono); margin: 0; color: var(--dim); }
.strip { display: inline-block; font: 500 13px/1.4 var(--mono); letter-spacing: .08em; text-transform: uppercase; color: var(--cyan); background: #e2eef0; padding: 8px 12px; margin: 0 0 16px; }
.hero { padding: 56px 0 28px; }
.hero.small { padding: 40px 0 12px; }
.hero.small h1 { font-size: clamp(32px, 5vw, 46px); }
.cta { display: flex; gap: 10px; flex-wrap: wrap; margin-top: 22px; }
.rule { display: flex; align-items: center; gap: 16px; margin: 48px 0 22px; font: 500 12px/1 var(--mono); letter-spacing: .14em; text-transform: uppercase; color: var(--dim); }
.rule::before, .rule::after { content: ""; flex: 1; border-top: 1px dashed var(--line); }

/* buttons, tags */
.btn { display: inline-flex; align-items: center; gap: 6px; font: 600 14px/1 var(--sans); padding: 12px 16px; border-radius: 10px; border: 1px solid var(--line); background: var(--panel); color: var(--ink); cursor: pointer; text-decoration: none; }
.btn:hover { border-color: var(--ink); }
.btn.go { background: var(--ink); color: var(--bg); border-color: var(--ink); }
.btn.go:hover { background: var(--acc); border-color: var(--acc); color: var(--on-acc); }
.btn:disabled { opacity: .55; cursor: default; }
.tag { font: 500 12px/1 var(--mono); padding: 7px 10px; border: 1px solid var(--line); border-radius: 999px; background: var(--panel); white-space: nowrap; }
.tag.done { background: var(--green); border-color: var(--green); color: #fff; }
.lvl { font: 700 12px/1 var(--mono); padding: 7px 10px; border-radius: 999px; white-space: nowrap; }
.lvl.easy { color: var(--green); background: #e3f1e4; } .lvl.medium { color: var(--amber); background: #f6ebd3; } .lvl.hard { color: var(--red); background: #f8e1dd; }

/* cards */
.cards { display: grid; grid-template-columns: repeat(3, 1fr); gap: 14px; }
.pcard { display: flex; flex-direction: column; background: var(--panel); border: 1px solid var(--line); border-radius: 14px; overflow: hidden; text-decoration: none; transition: transform .15s, border-color .15s; }
.pcard:hover { border-color: var(--ink); transform: translateY(-2px); }
.thumb { display: block; aspect-ratio: 16 / 11; overflow: hidden; background: #0c101a; }
.thumb img { width: 100%; height: 100%; object-fit: cover; object-position: 50% 38%; }
.ptxt { display: flex; flex-direction: column; gap: 8px; padding: 16px; align-items: flex-start; flex: 1; }
.ptxt small, .post small { font: 500 11px/1.3 var(--mono); letter-spacing: .08em; text-transform: uppercase; color: var(--dim); }
.ptxt strong { font: 500 21px/1.2 var(--serif); flex: 1; }
.posts { display: grid; grid-template-columns: repeat(auto-fit, minmax(260px, 1fr)); gap: 14px; }
.post { display: flex; flex-direction: column; gap: 8px; padding: 18px; background: var(--panel); border: 1px solid var(--line); border-radius: 14px; text-decoration: none; }
.post:hover { border-color: var(--ink); }
.post strong { font: 500 23px/1.2 var(--serif); }
.post span { color: var(--dim); font-size: 15px; }

.rcard .sum { color: var(--dim); font-size: 15px; }
.rcard:not(:has(.thumb)) { border-top: 3px solid var(--ink); }
.rcard[hidden] { display: none; }
.cta.center { justify-content: center; }
.reeltags { margin: 8px 0 20px; }
.igframe { width: 100%; height: 640px; border: 1px solid var(--line); border-radius: 16px; background: var(--panel); }
.reel .prose { margin-top: 0; }
.reel h1 { max-width: 20ch; }

/* problem list */
.filters { display: grid; gap: 12px; margin-bottom: 12px; }
.chips { display: flex; gap: 8px; flex-wrap: wrap; }
.chip { font: 500 14px/1 var(--sans); padding: 10px 14px; border-radius: 999px; border: 1px solid var(--line); background: var(--panel); color: var(--ink); cursor: pointer; }
.chip i { font: 500 12px/1 var(--mono); font-style: normal; color: var(--dim); margin-left: 4px; }
.chip.on { background: var(--ink); color: var(--bg); border-color: var(--ink); }
.chip.on i { color: #b9b4a8; }
.tools { display: flex; gap: 10px; align-items: center; flex-wrap: wrap; }
.search { display: flex; align-items: center; gap: 8px; padding: 0 12px; height: 40px; border: 1px solid var(--line); border-radius: 10px; background: var(--panel); color: var(--dim); flex: 1; min-width: 180px; max-width: 320px; }
.search input { border: 0; outline: 0; background: none; font: inherit; font-size: 15px; color: var(--ink); width: 100%; min-width: 0; }
.search:focus-within { border-color: var(--ink); }
select { height: 40px; padding: 0 10px; border: 1px solid var(--line); border-radius: 10px; background: var(--panel); font: inherit; font-size: 15px; color: var(--ink); }
.solved { margin-left: auto; font: 500 13px/1 var(--mono); color: var(--dim); }
.rows { border: 1px solid var(--line); border-radius: 14px; overflow: hidden; background: var(--panel); }
.row { display: grid; grid-template-columns: 22px minmax(0, 1fr) 110px 150px 76px 92px; gap: 12px; align-items: center; padding: 13px 16px; text-decoration: none; border-bottom: 1px solid var(--line); }
.row:last-of-type { border-bottom: 0; }
.row:nth-of-type(even) { background: #faf7f0; }
.row:hover { background: var(--panel2); }
.row[hidden] { display: none; }
.st { width: 16px; height: 16px; border-radius: 50%; border: 1.5px solid #bdb6a6; }
.row.done .st { border-color: var(--green); background: var(--green) url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16'%3E%3Cpath d='M4.3 8.4l2.4 2.4 5-5.2' fill='none' stroke='%23fff' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'/%3E%3C/svg%3E") center / 14px no-repeat; }
.name { min-width: 0; }
.name b { display: block; font-weight: 600; font-size: 16px; }
.name small { display: block; color: var(--dim); font-size: 13.5px; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.meta { font-size: 14px; color: var(--dim); white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.row .lvl { justify-self: start; background: none; padding: 0; font-size: 13px; }
.extras { display: flex; gap: 6px; justify-content: flex-end; }
.extras i { font: 500 10.5px/1 var(--mono); font-style: normal; text-transform: uppercase; letter-spacing: .06em; padding: 5px 6px; border: 1px solid var(--line); border-radius: 6px; color: var(--dim); }
.none { padding: 22px 16px; margin: 0; color: var(--dim); }

/* problem page */
.crumbs { display: flex; gap: 8px; align-items: center; padding: 22px 0 14px; font: 500 13px/1.3 var(--mono); color: var(--dim); flex-wrap: wrap; }
.crumbs a { color: var(--dim); } .crumbs b { color: var(--ink); font-weight: 500; }
.ptags { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; }
.problem h1 { font-size: clamp(30px, 5vw, 50px); max-width: 22ch; }
.split { display: grid; grid-template-columns: 320px minmax(0, 1fr); gap: 36px; margin-top: 26px; align-items: start; }
.watch { position: sticky; top: 78px; display: grid; gap: 10px; }
.watch video, .inlinevid video { width: 100%; aspect-ratio: 9 / 16; background: #0c101a; border-radius: 16px; border: 1px solid var(--line); }
.btn.reel { justify-content: center; }
.solution > :first-child { margin-top: 0; }

.card { background: var(--panel); border: 1px solid var(--line); border-radius: 16px; padding: 18px; }
.question .given { font: 500 13px/1.5 var(--mono); color: var(--dim); margin-bottom: 6px; }
.question .ask { font: 500 26px/1.2 var(--serif); margin-bottom: 14px; text-wrap: balance; }
.opts { display: grid; grid-template-columns: 1fr 1fr; gap: 10px; }
.opts button { display: flex; align-items: center; gap: 12px; padding: 12px; border-radius: 12px; border: 1.5px solid var(--line); background: var(--bg); color: var(--ink); font: 600 18px/1.2 var(--sans); cursor: pointer; text-align: left; transition: border-color .15s, transform .1s; }
.opts button:hover { border-color: var(--ink); }
.opts button:active { transform: scale(.98); }
.opts button i { flex: none; width: 34px; height: 34px; border-radius: 50%; background: var(--ink); color: var(--bg); display: grid; place-items: center; font: 700 14px/1 var(--mono); font-style: normal; }
.opts button.right { border-color: var(--green); background: #e3f1e4; } .opts button.right i { background: var(--green); color: #fff; }
.opts button.wrong { border-color: var(--red); background: #f8e1dd; } .opts button.wrong i { background: var(--red); color: #fff; }
.feedback { margin-top: 12px; padding: 12px 14px; border-radius: 10px; background: var(--panel2); border-left: 4px solid var(--acc); display: none; }
.feedback.show { display: block; } .feedback.ok { border-left-color: var(--green); } .feedback.no { border-left-color: var(--red); }
.hint { font-size: 14px; color: var(--dim); margin-top: 10px; }

/* the model: a dark screen inside the page (lab.js draws in dark-theme colours) */
.lab { --panel2: #1a2030; --line: #2a3145; --ink: #f4f0e6; --dim: #aab2c8; --acc: #ffb238; --on-acc: #0c101a; --cyan: #58d6e8; --red: #ff6048; --green: #78e28c;
  background: #0c101a; border-color: #0c101a; color: var(--ink); }
.lab canvas { display: block; width: 100%; height: 340px; border-radius: 10px; background: #080b13; border: 1px solid var(--line); touch-action: pan-y; }
.controls { display: grid; gap: 12px; margin-top: 14px; }
.ctl { display: grid; grid-template-columns: 1fr auto; gap: 2px 12px; align-items: center; }
.ctl label { font-size: 14px; color: var(--dim); }
.ctl output { font: 700 15px/1 var(--mono); color: var(--cyan); }
.ctl input[type=range] { grid-column: 1 / -1; width: 100%; accent-color: var(--acc); height: 28px; margin: 0; }
.btns { display: flex; gap: 8px; flex-wrap: wrap; margin-top: 12px; }
.lab .btn { background: var(--panel2); color: var(--ink); border-color: var(--line); }
.lab .btn:hover { border-color: var(--acc); }
.lab .btn.go { background: var(--acc); color: var(--on-acc); border-color: var(--acc); }
.readout { display: grid; grid-template-columns: repeat(auto-fit, minmax(130px, 1fr)); gap: 10px; margin-top: 14px; }
.readout div { background: var(--panel2); border-radius: 10px; padding: 10px 12px; min-width: 0; }
.readout span { display: block; font-size: 12px; color: var(--dim); margin-bottom: 4px; }
.readout b { font: 700 19px/1.1 var(--mono); color: var(--ink); font-variant-numeric: tabular-nums; }
.readout b.acc { color: var(--acc); } .readout b.cyan { color: var(--cyan); } .readout b.red { color: var(--red); } .readout b.green { color: var(--green); }

.steps { counter-reset: s; display: grid; gap: 10px; }
.step { display: none; position: relative; padding: 14px 16px 14px 58px; background: var(--panel); border: 1px solid var(--line); border-radius: 14px; }
.step.show { display: block; animation: rise .25s ease-out; }
.step::before { counter-increment: s; content: counter(s); position: absolute; left: 14px; top: 14px; width: 30px; height: 30px; border-radius: 50%; background: var(--ink); color: var(--bg); font: 700 14px/30px var(--mono); text-align: center; }
.step p:last-child { margin-bottom: 0; }
.step.answer { border-color: var(--green); background: #f0f7ee; }
.step.answer::before { content: "✓"; background: var(--green); color: #fff; }
@keyframes rise { from { opacity: 0; transform: translateY(6px); } to { opacity: 1; transform: none; } }
@media (prefers-reduced-motion: reduce) { .step.show { animation: none; } html { scroll-behavior: auto; } .pcard { transition: none; } }
.eq { font: 500 16px/1.5 var(--mono); color: var(--acc); background: var(--panel2); border-radius: 8px; padding: 10px 12px; margin: 8px 0; overflow-x: auto; white-space: nowrap; }
.eq b { color: var(--ink); }
.simbox { border-left: 4px solid var(--cyan); }
.simbox ul, .card ul { margin: 0; padding-left: 20px; }
.simbox li, .card li { margin-bottom: 6px; }
.tablewrap { overflow-x: auto; }
table { width: 100%; border-collapse: collapse; font-size: 15px; }
th, td { text-align: left; padding: 8px 6px; border-bottom: 1px solid var(--line); vertical-align: top; }
th { font: 500 12px/1 var(--mono); letter-spacing: .08em; text-transform: uppercase; color: var(--dim); }
.solution td:first-child { font-weight: 700; white-space: nowrap; }
details { background: var(--panel); border: 1px solid var(--line); border-radius: 14px; padding: 0 16px; margin-bottom: 10px; }
summary { cursor: pointer; padding: 14px 0; font-weight: 600; }
details[open] summary { color: var(--acc); }
details p:last-child { margin-bottom: 16px; }
.pager { display: flex; justify-content: space-between; gap: 10px; margin-top: 44px; }
.pager a { flex: 1; padding: 14px 16px; border-radius: 14px; border: 1px solid var(--line); background: var(--panel); text-decoration: none; font-weight: 600; }
.pager a:hover { border-color: var(--ink); }
.pager span { flex: 1; }
.pager a small { display: block; font: 500 11px/1 var(--mono); letter-spacing: .1em; text-transform: uppercase; color: var(--dim); margin-bottom: 6px; }
.pager a:last-child { text-align: right; }

/* articles */
.article main { padding-top: 36px; }
.byline { font: 500 13px/1 var(--mono); color: var(--dim); margin: 0 0 6px; }
.article h1 { font-size: clamp(32px, 5.4vw, 50px); }
.prose { font: 400 20px/1.6 var(--serif); margin-top: 28px; }
.prose h2 { font-size: 28px; margin-top: 40px; }
.prose h3 { font: 600 17px/1.3 var(--sans); margin-top: 28px; }
.prose a, .lede a, .fine a, footer a, figcaption a { color: var(--acc); }
.prose table { font-family: var(--sans); margin: 8px 0 20px; }
.prose code { font: 500 .82em var(--mono); background: var(--panel2); padding: 2px 6px; border-radius: 5px; }
.prose pre { background: #0c101a; color: #f4f0e6; padding: 14px 16px; border-radius: 12px; overflow-x: auto; }
.prose pre code { background: none; padding: 0; font-size: 14px; }
.prose blockquote { margin: 20px 0; padding: 4px 0 4px 18px; border-left: 3px solid var(--acc); font-style: italic; }
.prose li { margin-bottom: 6px; }
.inlinevid { margin: 26px auto 0; max-width: 300px; }
.inlinevid figcaption { font-size: 13px; color: var(--dim); margin-top: 8px; text-align: center; }

@media (max-width: 900px) {
  .split { grid-template-columns: 1fr; gap: 22px; }
  .watch { position: static; max-width: 300px; margin: 0 auto; width: 100%; }
  .reel .watch { max-width: 340px; }
  .cards { grid-template-columns: 1fr 1fr; }
  .row { grid-template-columns: 22px minmax(0, 1fr) 70px; }
  .row .topic, .row .robot, .row .extras { display: none; }
}
@media (max-width: 600px) {
  .ig { display: none; }
  .bar { gap: 8px; }
  header nav { gap: 0; }
  header nav a { font-size: 14px; padding: 8px 7px; }
  .logo { font-size: 17px; }
  .igframe { height: 600px; }
  .cards { grid-template-columns: 1fr; }
  .thumb { aspect-ratio: 16 / 9; }
  .hero { padding-top: 36px; }
  .solved { margin-left: 0; width: 100%; }
  .opts button { font-size: 16px; padding: 10px; }
  .lab canvas { height: 300px; }
  .name small { white-space: normal; }
  .prose { font-size: 18px; }
}

/* freebies: keyword chips, banner on the post, callout on the reel page */
.kwchip { display: inline-block; font: 700 11px/1 var(--mono); letter-spacing: .08em; color: var(--on-acc); background: var(--acc); padding: 4px 7px; border-radius: 5px; vertical-align: 1px; }
.kwban { font-size: 15px; color: var(--dim); background: var(--panel); border: 1px dashed var(--acc); border-radius: 10px; padding: 12px 14px; }
.kwban .kwchip { margin-right: 6px; }
.kwbox { display: block; margin: 28px 0 0; padding: 16px 18px; border: 1px solid var(--ink); border-left: 5px solid var(--acc); border-radius: 10px; background: var(--panel); text-decoration: none; color: var(--ink); }
.kwbox:hover { background: var(--panel2); }
.kwbox small { display: block; font: 500 12px/1.3 var(--mono); letter-spacing: .06em; text-transform: uppercase; color: var(--acc); margin-bottom: 6px; }
.kwbox strong { display: block; font: 500 21px/1.25 var(--serif); }
.kwbox span { display: block; color: var(--dim); font-size: 15px; margin-top: 4px; }
.prose .kwbox, .prose .kwbox:hover { text-decoration: none; }
.kwbox strong { color: var(--ink); }
@media (max-width: 600px) { header nav a[href="/about/"] { display: none; } }