/* Toolkit -- worked problems for the facts TCS papers take for granted. */

:root {
  --bg: #f5f3ee;
  --grid: rgba(28, 36, 64, 0.045);
  --card: #ffffff;
  --card-2: #f8f7f3;
  --ink: #171b26;
  --ink-2: #525869;
  --ink-3: #868b9b;
  --line: #e3dfd5;
  --track: #e9e5db;
  --shadow: 0 1px 2px rgba(23, 27, 38, 0.05), 0 10px 28px -10px rgba(23, 27, 38, 0.14);
  --shadow-lift: 0 2px 4px rgba(23, 27, 38, 0.06), 0 18px 36px -12px rgba(23, 27, 38, 0.22);

  --accent: #2a78d6;
  --on-accent: #ffffff;
  --good: #13a355;
  --good-ink: #0b7a3e;
  --good-soft: #e3f6eb;
  --bad: #e5484d;
  --bad-soft: #fde8e8;
  --warn: #c98500;
  --warn-soft: #fbf1d8;

  /* chart series: the validated categorical order (blue, orange, aqua, yellow, magenta, green, violet, red) */
  --s1: #2a78d6; --s2: #eb6834; --s3: #1baf7a; --s4: #eda100;
  --s5: #e87ba4; --s6: #008300; --s7: #6250d6; --s8: #e34948;
  --chart-grid: #e8e6df;
  --chart-axis: #c3c2b7;
  --chart-text: #77756f;
  --neg: #e34948;
  --pos: #2a78d6;

  /* one hue per area */
  --a-prob: #eb6834;
  --a-dist: #1baf7a;
  --a-stat: #d55181;
  --a-asym: #2a78d6;
  --a-graph: #6250d6;
  --a-four: #c98500;
  --a-alg: #008300;

  --display: "Space Grotesk", ui-sans-serif, system-ui, sans-serif;
  --body: "Inter", ui-sans-serif, system-ui, -apple-system, "Segoe UI", sans-serif;
  --radius: 18px;
  color-scheme: light;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0f1218;
    --grid: rgba(255, 255, 255, 0.035);
    --card: #171b24;
    --card-2: #1c212c;
    --ink: #eceef4;
    --ink-2: #a7acbb;
    --ink-3: #6f7587;
    --line: #29303f;
    --track: #29303f;
    --shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 12px 30px -12px rgba(0, 0, 0, 0.6);
    --shadow-lift: 0 2px 6px rgba(0, 0, 0, 0.35), 0 20px 40px -14px rgba(0, 0, 0, 0.7);
    --accent: #3987e5;
    --good: #34c77b; --good-ink: #4bd88d; --good-soft: rgba(52, 199, 123, 0.13);
    --bad: #ff6b6f; --bad-soft: rgba(255, 107, 111, 0.13);
    --warn: #f2b33d; --warn-soft: rgba(242, 179, 61, 0.13);
    --s1: #3987e5; --s2: #d95926; --s3: #199e70; --s4: #c98500;
    --s5: #d55181; --s6: #008300; --s7: #9085e9; --s8: #e66767;
    --chart-grid: #262a33; --chart-axis: #3a3f4b; --chart-text: #8d909a;
    --neg: #e66767; --pos: #3987e5;
    --a-prob: #e2662f; --a-dist: #23b07f; --a-stat: #e0679a; --a-asym: #4b91e8;
    --a-graph: #9085e9; --a-four: #d9a21c; --a-alg: #2ea043;
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #0f1218;
  --grid: rgba(255, 255, 255, 0.035);
  --card: #171b24;
  --card-2: #1c212c;
  --ink: #eceef4;
  --ink-2: #a7acbb;
  --ink-3: #6f7587;
  --line: #29303f;
  --track: #29303f;
  --shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 12px 30px -12px rgba(0, 0, 0, 0.6);
  --shadow-lift: 0 2px 6px rgba(0, 0, 0, 0.35), 0 20px 40px -14px rgba(0, 0, 0, 0.7);
  --accent: #3987e5;
  --good: #34c77b; --good-ink: #4bd88d; --good-soft: rgba(52, 199, 123, 0.13);
  --bad: #ff6b6f; --bad-soft: rgba(255, 107, 111, 0.13);
  --warn: #f2b33d; --warn-soft: rgba(242, 179, 61, 0.13);
  --s1: #3987e5; --s2: #d95926; --s3: #199e70; --s4: #c98500;
  --s5: #d55181; --s6: #008300; --s7: #9085e9; --s8: #e66767;
  --chart-grid: #262a33; --chart-axis: #3a3f4b; --chart-text: #8d909a;
  --neg: #e66767; --pos: #3987e5;
  --a-prob: #e2662f; --a-dist: #23b07f; --a-stat: #e0679a; --a-asym: #4b91e8;
  --a-graph: #9085e9; --a-four: #d9a21c; --a-alg: #2ea043;
  color-scheme: dark;
}

*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }

body {
  margin: 0;
  min-height: 100vh;
  font-family: var(--body);
  font-size: 16px;
  line-height: 1.55;
  color: var(--ink);
  background-color: var(--bg);
  background-image:
    linear-gradient(var(--grid) 1px, transparent 1px),
    linear-gradient(90deg, var(--grid) 1px, transparent 1px);
  background-size: 24px 24px;
  -webkit-font-smoothing: antialiased;
}

button, input, select { font: inherit; color: inherit; }
a { color: var(--accent); text-underline-offset: 3px; }
code { font-size: 0.88em; background: var(--card-2); border: 1px solid var(--line); padding: 0 5px; border-radius: 5px; }
:focus-visible { outline: 3px solid color-mix(in srgb, var(--accent) 55%, transparent); outline-offset: 2px; }

#app { max-width: 920px; margin: 0 auto; padding: 0 18px 40px; }

/* ------------------------------------------------------------ top bar */

.topbar { display: flex; align-items: center; justify-content: space-between; gap: 12px; padding: 18px 0 12px; }
.brand {
  display: flex; align-items: center; gap: 10px;
  font-family: var(--display); font-weight: 700; font-size: 1.35rem; letter-spacing: -0.02em;
  color: var(--ink); text-decoration: none;
}
.logo { width: 30px; height: 30px; flex: none; }
.logo rect { fill: var(--accent); }
.logo .base { stroke: #fff; stroke-opacity: 0.55; stroke-width: 1.6; stroke-linecap: round; }
.logo .curve { fill: none; stroke: #fff; stroke-width: 2.4; stroke-linecap: round; transition: d 0.4s; }
.brand:hover .logo .curve { d: path("M5 23c5 0 6.5-1 8-4.6C14.5 14.5 14.8 5 16 5s1.5 9.5 3 13.4c1.5 3.6 3 4.6 8 4.6"); }

.top-nav { display: flex; align-items: center; gap: 4px; }
.top-nav a {
  padding: 7px 12px; border-radius: 10px;
  color: var(--ink-2); font-weight: 600; font-size: 0.93rem; text-decoration: none;
}
.top-nav a:hover { color: var(--ink); background: var(--card); }
.top-nav a.on { color: var(--ink); background: var(--card); box-shadow: inset 0 0 0 1px var(--line); }
.icon-btn {
  width: 38px; height: 38px; margin-left: 6px;
  display: grid; place-items: center; padding: 0;
  border: 1px solid var(--line); border-radius: 999px; background: var(--card); cursor: pointer;
}
.icon-btn:hover { border-color: var(--ink-3); }
.icon-btn svg { width: 18px; height: 18px; fill: none; stroke: currentColor; stroke-width: 1.8; stroke-linecap: round; stroke-linejoin: round; }

.foot { margin-top: 40px; color: var(--ink-3); font-size: 0.82rem; text-align: center; }
.foot code { font-size: 0.85em; }

/* ------------------------------------------------------------ shared bits */

.eyebrow {
  font-family: var(--display); font-size: 0.74rem; font-weight: 600;
  letter-spacing: 0.1em; text-transform: uppercase; color: var(--ink-3); margin: 0;
}
h1, h2, h3 { font-family: var(--display); letter-spacing: -0.02em; line-height: 1.2; }
.card {
  background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  box-shadow: var(--shadow);
}
.muted { color: var(--ink-2); }
.small { font-size: 0.88rem; }

.btn {
  display: inline-flex; align-items: center; justify-content: center; gap: 8px;
  border-radius: 12px; border: 0; padding: 10px 18px;
  font-family: var(--display); font-weight: 600; font-size: 0.98rem;
  cursor: pointer; text-decoration: none;
  transition: transform 0.12s, box-shadow 0.15s, background 0.2s, opacity 0.2s, border-color 0.15s;
}
.btn svg { width: 17px; height: 17px; fill: none; stroke: currentColor; stroke-width: 2; stroke-linecap: round; stroke-linejoin: round; }
.btn.primary {
  background: var(--accent); color: var(--on-accent);
  box-shadow: 0 6px 16px -6px color-mix(in srgb, var(--accent) 70%, transparent), inset 0 -3px 0 rgba(0, 0, 0, 0.14);
}
.btn.primary:hover { transform: translateY(-1px); }
.btn.primary:active { transform: translateY(1px); box-shadow: inset 0 -1px 0 rgba(0, 0, 0, 0.14); }
.btn.primary:disabled { background: var(--track); color: var(--ink-3); box-shadow: none; cursor: default; transform: none; }
.btn.ghost { background: var(--card); border: 1px solid var(--line); color: var(--ink); }
.btn.ghost:hover { border-color: var(--ink-3); }
.btn.ghost:disabled { opacity: 0.5; cursor: default; }
.btn.small { padding: 6px 12px; font-size: 0.86rem; border-radius: 10px; }
.btn.big { padding: 13px 26px; font-size: 1.05rem; }
.link-btn {
  background: none; border: 0; padding: 2px 4px; cursor: pointer;
  color: var(--ink-2); font-size: 0.85rem; text-decoration: underline; text-underline-offset: 3px;
}
.link-btn:hover { color: var(--ink); }

.chips { display: flex; flex-wrap: wrap; gap: 7px; }
.chip {
  border: 1.5px solid var(--line); background: transparent; color: var(--ink-2);
  border-radius: 999px; padding: 4px 12px; font-size: 0.86rem; font-weight: 500; cursor: pointer;
  transition: background 0.15s, color 0.15s, border-color 0.15s, transform 0.12s;
}
.chip:hover { border-color: var(--ink-3); }
.chip:active { transform: scale(0.96); }
.chip.on { background: var(--ink); border-color: var(--ink); color: var(--bg); }

.area-dot { display: inline-block; width: 9px; height: 9px; border-radius: 3px; background: var(--c, var(--accent)); flex: none; }

/* ------------------------------------------------------------ home */

.hero { padding: 14px 0 6px; }
.hero h1 { font-size: clamp(1.7rem, 4.6vw, 2.4rem); margin: 0 0 10px; max-width: 20ch; text-wrap: balance; }
.hero p { margin: 0; max-width: 64ch; color: var(--ink-2); font-size: 1.02rem; }
.hero-actions { display: flex; gap: 10px; flex-wrap: wrap; margin-top: 18px; }

.area-block { margin-top: 30px; }
.area-head { display: flex; align-items: center; gap: 10px; margin: 0 0 12px; }
.area-head h2 { font-size: 1.08rem; margin: 0; }
.area-head .area-dot { width: 11px; height: 11px; }
.area-head .count { color: var(--ink-3); font-size: 0.82rem; }

.topic-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(255px, 1fr)); gap: 12px; }
.topic-card {
  --c: var(--accent);
  position: relative; display: flex; flex-direction: column; gap: 6px;
  padding: 16px 16px 14px 18px; text-decoration: none; color: var(--ink);
  background: var(--card); border: 1px solid var(--line); border-radius: 16px; box-shadow: var(--shadow);
  overflow: hidden; transition: transform 0.18s, box-shadow 0.18s, border-color 0.18s;
}
.topic-card::before { content: ""; position: absolute; left: 0; top: 0; bottom: 0; width: 4px; background: var(--c); }
.topic-card:hover { transform: translateY(-2px); box-shadow: var(--shadow-lift); border-color: color-mix(in srgb, var(--c) 45%, var(--line)); }
.topic-name { font-family: var(--display); font-weight: 700; font-size: 1.06rem; letter-spacing: -0.015em; line-height: 1.25; }
.topic-blurb { color: var(--ink-2); font-size: 0.88rem; line-height: 1.45; flex: 1; }
.topic-foot { display: flex; align-items: center; gap: 10px; margin-top: 6px; font-size: 0.78rem; color: var(--ink-3); }
.meter { flex: 1; height: 6px; border-radius: 999px; background: var(--track); overflow: hidden; display: flex; gap: 0; }
.meter i { display: block; height: 100%; background: var(--good); }
.meter i.h { background: var(--warn); }
.meter i.r { background: var(--ink-3); opacity: 0.5; }

/* ------------------------------------------------------------ practice setup */

.setup { padding: 22px 24px; margin-top: 8px; }
.setup h1 { font-size: 1.6rem; margin: 0 0 6px; }
.setup > p { margin: 0 0 6px; color: var(--ink-2); }
.setup-row { margin-top: 18px; }
.setup-row-head { display: flex; align-items: baseline; gap: 10px; margin-bottom: 8px; }
.setup-row-head h3 { font-size: 0.98rem; margin: 0; }
.area-chips { display: grid; grid-template-columns: 190px 1fr; gap: 8px 14px; align-items: start; margin-top: 4px; }
.area-chips .area-name { display: flex; align-items: center; gap: 8px; font-size: 0.86rem; font-weight: 600; color: var(--ink-2); padding-top: 5px; cursor: pointer; background: none; border: 0; text-align: left; }
.area-chips .area-name:hover { color: var(--ink); }
.setup-foot { display: flex; align-items: center; justify-content: space-between; gap: 16px; margin-top: 22px; flex-wrap: wrap; }
.setup-count { color: var(--ink-3); font-size: 0.85rem; }

.segs { display: inline-flex; padding: 3px; border-radius: 12px; background: var(--card-2); border: 1px solid var(--line); gap: 2px; }
.segs button {
  border: 0; background: transparent; padding: 5px 12px; border-radius: 9px; cursor: pointer;
  font-size: 0.86rem; font-weight: 600; color: var(--ink-2);
}
.segs button:hover { color: var(--ink); }
.segs button.on { background: var(--ink); color: var(--bg); }

/* ------------------------------------------------------------ topic page */

.crumbs { display: flex; align-items: center; gap: 8px; font-size: 0.86rem; color: var(--ink-3); margin: 4px 0 6px; }
.crumbs a { color: var(--ink-2); text-decoration: none; }
.crumbs a:hover { color: var(--ink); text-decoration: underline; }
.topic-head { --c: var(--accent); margin: 2px 0 14px; }
.topic-head h1 { font-size: clamp(1.7rem, 4.4vw, 2.3rem); margin: 6px 0 6px; }
.topic-head p { margin: 0; color: var(--ink-2); max-width: 66ch; }
.area-tag { display: inline-flex; align-items: center; gap: 7px; font-size: 0.78rem; font-weight: 600; color: var(--ink-2); letter-spacing: 0.02em; }

.tabs { display: flex; gap: 4px; border-bottom: 1px solid var(--line); margin: 18px 0 20px; }
.tabs a {
  padding: 9px 14px; margin-bottom: -1px; border-bottom: 2px solid transparent;
  color: var(--ink-2); font-weight: 600; text-decoration: none; font-size: 0.95rem;
}
.tabs a:hover { color: var(--ink); }
.tabs a.on { color: var(--ink); border-bottom-color: var(--c, var(--accent)); }
.tabs .badge { display: inline-block; margin-left: 6px; padding: 0 7px; border-radius: 999px; background: var(--track); font-size: 0.75rem; color: var(--ink-2); }

.learn { display: flex; flex-direction: column; gap: 22px; }
.learn-intro { font-size: 1.04rem; line-height: 1.65; max-width: 70ch; }
.learn-intro p { margin: 0 0 10px; }
.learn h2 { font-size: 1.2rem; margin: 0 0 10px; }

.facts { display: grid; gap: 10px; }
.fact { padding: 14px 18px; border-left: 4px solid var(--c, var(--accent)); }
.fact-name { font-family: var(--display); font-weight: 700; font-size: 0.95rem; margin-bottom: 2px; }
.fact-tex { overflow-x: auto; overflow-y: hidden; padding: 2px 0; }
.fact-tex .katex-display { margin: 6px 0; }
.fact-when { color: var(--ink-2); font-size: 0.9rem; }

details.sec { padding: 0; overflow: hidden; }
details.sec > summary {
  list-style: none; cursor: pointer; padding: 14px 18px;
  font-family: var(--display); font-weight: 700; font-size: 1.02rem;
  display: flex; align-items: center; justify-content: space-between; gap: 10px;
}
details.sec > summary::-webkit-details-marker { display: none; }
details.sec > summary::after { content: "+"; font-size: 1.3rem; color: var(--ink-3); font-weight: 500; transition: transform 0.2s; }
details.sec[open] > summary::after { transform: rotate(45deg); }
details.sec > .sec-body { padding: 0 18px 16px; line-height: 1.65; }
.sec-body p { margin: 0 0 10px; }
.sec-body ul, .sec-body ol { margin: 0 0 10px; padding-left: 22px; }

.sec-body table { border-collapse: collapse; width: 100%; font-size: 0.92rem; margin: 4px 0 10px; }
.sec-body th, .sec-body td { text-align: left; padding: 7px 10px; border-bottom: 1px solid var(--line); vertical-align: top; }
.sec-body th { font-family: var(--display); font-weight: 600; color: var(--ink-2); font-size: 0.82rem; }
.sec-body .table-wrap { overflow-x: auto; }

.playground { padding: 18px 20px 20px; }
.playground h2 { margin-bottom: 4px; }
.playground .pg-caption { margin: 0 0 14px; color: var(--ink-2); font-size: 0.93rem; }
.thumbs { margin: 0; padding-left: 20px; }
.thumbs li { margin: 0 0 8px; line-height: 1.55; }
.uses { margin: 0; padding: 0; list-style: none; display: grid; gap: 8px; }
.uses li { padding: 10px 14px; border-radius: 12px; background: var(--card-2); border: 1px solid var(--line); font-size: 0.93rem; }
.learn-cta { display: flex; align-items: center; gap: 14px; flex-wrap: wrap; padding: 16px 20px; }
.learn-cta p { margin: 0; flex: 1; min-width: 200px; color: var(--ink-2); }
.see-also { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; font-size: 0.88rem; color: var(--ink-3); }
.see-also a { color: var(--ink-2); }

.prob-list { display: flex; flex-direction: column; gap: 8px; }
.prob-row {
  display: grid; grid-template-columns: 22px 1fr auto; align-items: center; gap: 12px;
  padding: 12px 16px; border-radius: 14px; background: var(--card); border: 1px solid var(--line);
  text-decoration: none; color: var(--ink); transition: border-color 0.15s, transform 0.15s;
}
.prob-row:hover { border-color: var(--ink-3); transform: translateX(2px); }
.prob-row .pr-title { font-weight: 600; }
.prob-row .pr-sub { color: var(--ink-3); font-size: 0.82rem; }
.prob-list-head { display: flex; justify-content: space-between; align-items: center; gap: 12px; margin-bottom: 12px; flex-wrap: wrap; }
.prob-list-head p { margin: 0; color: var(--ink-2); font-size: 0.92rem; }

.status { width: 18px; height: 18px; border-radius: 50%; border: 2px solid var(--line); display: grid; place-items: center; }
.status svg { width: 11px; height: 11px; fill: none; stroke: #fff; stroke-width: 3; stroke-linecap: round; stroke-linejoin: round; }
.status.solved { background: var(--good); border-color: var(--good); }
.status.hinted { background: var(--warn); border-color: var(--warn); }
.status.revealed { background: var(--ink-3); border-color: var(--ink-3); }

.level { display: inline-flex; gap: 3px; align-items: center; }
.level i { width: 7px; height: 7px; border-radius: 50%; background: var(--track); }
.level i.on { background: var(--ink-2); }
.level-name { font-size: 0.78rem; color: var(--ink-3); margin-left: 6px; }

/* ------------------------------------------------------------ problem card */

.play-head { display: flex; align-items: center; justify-content: space-between; gap: 12px; margin: 6px 0 14px; flex-wrap: wrap; }
.pdots { display: flex; gap: 6px; }
.pdot { width: 11px; height: 11px; border-radius: 50%; background: var(--track); }
.pdot.current { background: transparent; box-shadow: inset 0 0 0 2.5px var(--accent); }
.pdot.solved { background: var(--good); }
.pdot.hinted { background: var(--warn); }
.pdot.revealed { background: var(--ink-3); }

.pcard { --c: var(--accent); padding: 22px 26px 24px; position: relative; }
.pcard.enter { animation: enter 0.35s cubic-bezier(.2, .8, .2, 1); }
@keyframes enter { from { opacity: 0; transform: translateY(12px); } to { opacity: 1; transform: none; } }
.pcard.shake { animation: shake 0.42s; }
@keyframes shake {
  0%, 100% { transform: none; } 20% { transform: translateX(-7px); } 40% { transform: translateX(6px); }
  60% { transform: translateX(-4px); } 80% { transform: translateX(2px); }
}
.pmeta { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; margin-bottom: 8px; font-size: 0.8rem; color: var(--ink-3); }
.pill {
  display: inline-flex; align-items: center; gap: 6px;
  font-size: 0.72rem; font-weight: 700; letter-spacing: 0.06em; text-transform: uppercase;
  color: color-mix(in srgb, var(--c) 75%, var(--ink)); background: color-mix(in srgb, var(--c) 13%, transparent);
  padding: 3px 9px; border-radius: 999px; text-decoration: none;
}
.pmeta .spacer { flex: 1; }
.ptitle { font-size: clamp(1.25rem, 3.4vw, 1.55rem); margin: 0 0 10px; }
.prompt { font-size: 1.04rem; line-height: 1.65; }
.prompt p { margin: 0 0 10px; }
.prompt .katex-display { margin: 10px 0; overflow-x: auto; overflow-y: hidden; padding: 2px 0; }
.figure { margin: 12px 0 4px; }

.answer { margin-top: 18px; }
.field {
  display: flex; align-items: center; gap: 10px;
  border: 2px solid var(--line); border-radius: 14px; background: var(--card-2);
  padding: 0 14px; transition: border-color 0.15s, box-shadow 0.15s;
}
.field:focus-within { border-color: var(--accent); box-shadow: 0 0 0 4px color-mix(in srgb, var(--accent) 16%, transparent); }
.field.right { border-color: var(--good); }
.field.wrong { border-color: var(--bad); }
.field input {
  flex: 1; min-width: 0; border: 0; outline: 0; background: transparent;
  font-family: var(--display); font-size: 1.25rem; font-weight: 600; padding: 11px 0;
}
.field input::placeholder { color: var(--ink-3); font-weight: 500; font-size: 0.98rem; font-family: var(--body); }
.field .eq { color: var(--ink-2); font-size: 0.95rem; white-space: nowrap; font-variant-numeric: tabular-nums; }
.field .eq.err { color: var(--ink-3); font-style: italic; }
.field .unit { color: var(--ink-3); font-size: 0.9rem; white-space: nowrap; }
.answer-note { margin: 6px 2px 0; font-size: 0.8rem; color: var(--ink-3); }

.choices { display: grid; gap: 8px; }
.choices.cols-2 { grid-template-columns: 1fr 1fr; }
.choice {
  display: flex; align-items: center; gap: 12px; text-align: left;
  padding: 11px 14px; border-radius: 13px; border: 2px solid var(--line); background: var(--card-2);
  cursor: pointer; font-size: 0.98rem; line-height: 1.4;
  transition: border-color 0.15s, background 0.2s, transform 0.12s;
}
.choice:hover:not(:disabled) { border-color: var(--accent); }
.choice:active:not(:disabled) { transform: scale(0.99); }
.choice .key {
  flex: none; width: 24px; height: 24px; border-radius: 7px; display: grid; place-items: center;
  font-family: var(--display); font-weight: 700; font-size: 0.8rem; background: var(--track); color: var(--ink-2);
}
.choice.right { border-color: var(--good); background: var(--good-soft); }
.choice.right .key { background: var(--good); color: #fff; }
.choice.wrong { border-color: var(--bad); background: var(--bad-soft); }
.choice.wrong .key { background: var(--bad); color: #fff; }
.choice:disabled { cursor: default; }
.choice:disabled:not(.right):not(.wrong) { opacity: 0.55; }

.pactions { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; margin-top: 14px; }
.pactions .grow { flex: 1; }
.feedback { margin-top: 12px; padding: 10px 14px; border-radius: 12px; font-size: 0.95rem; display: none; }
.feedback.show { display: block; animation: enter 0.25s ease; }
.feedback.right { background: var(--good-soft); color: var(--good-ink); }
.feedback.wrong { background: var(--bad-soft); color: color-mix(in srgb, var(--bad) 80%, var(--ink)); }
.feedback.close { background: var(--warn-soft); color: color-mix(in srgb, var(--warn) 70%, var(--ink)); }
.feedback b { font-family: var(--display); }

.hints { display: flex; flex-direction: column; gap: 8px; margin-top: 12px; }
.hint {
  padding: 10px 14px 10px 38px; border-radius: 12px; background: var(--card-2); border: 1px dashed var(--line);
  position: relative; font-size: 0.95rem; animation: enter 0.25s ease;
}
.hint::before {
  content: "Hint " attr(data-n); position: absolute; left: 0; top: 0; bottom: 0; width: 28px;
  writing-mode: vertical-rl; transform: rotate(180deg); text-align: center;
  font-size: 0.62rem; font-weight: 700; letter-spacing: 0.08em; text-transform: uppercase; color: var(--ink-3);
}

.solution { margin-top: 20px; padding-top: 18px; border-top: 1px dashed var(--line); line-height: 1.65; }
.solution.in { animation: enter 0.35s cubic-bezier(.2, .8, .2, 1); }
.solution h3 { font-size: 1rem; margin: 0 0 8px; display: flex; align-items: center; gap: 10px; }
.solution h3 .ans { margin-left: auto; font-family: var(--display); font-weight: 700; font-size: 0.95rem; color: var(--ink-2); }
.solution p { margin: 0 0 10px; }
.solution .katex-display { overflow-x: auto; overflow-y: hidden; padding: 2px 0; }
.solution .visual { margin: 14px 0 6px; padding: 14px 16px; border-radius: 14px; background: var(--card-2); border: 1px solid var(--line); }
.takeaway {
  margin-top: 14px; padding: 12px 16px; border-radius: 12px;
  background: color-mix(in srgb, var(--accent) 8%, var(--card)); border-left: 3px solid var(--accent);
  font-size: 0.97rem;
}
.takeaway b { font-family: var(--display); }

.fu-cta { margin-top: 18px; display: flex; align-items: center; gap: 12px; flex-wrap: wrap; }
.fu-cta .teaser { color: var(--ink-2); font-size: 0.93rem; flex: 1; min-width: 200px; }
.fu-wrap { margin-top: 16px; padding-left: 18px; border-left: 3px solid color-mix(in srgb, var(--accent) 35%, var(--line)); }
.fu-wrap .pcard { box-shadow: none; }
.fu-label { font-family: var(--display); font-size: 0.74rem; font-weight: 700; letter-spacing: 0.1em; text-transform: uppercase; color: var(--accent); }

.next-row { display: flex; justify-content: flex-end; gap: 10px; margin-top: 18px; flex-wrap: wrap; }

/* ------------------------------------------------------------ summary */

.summary { text-align: center; padding: 26px 24px; }
.summary h1 { font-size: 1.6rem; margin: 6px 0; }
.summary .big { font-family: var(--display); font-weight: 700; font-size: clamp(3rem, 12vw, 4.6rem); line-height: 1; color: var(--accent); }
.summary .of { font-family: var(--display); color: var(--ink-3); font-size: 1.2rem; font-weight: 600; }
.recap { list-style: none; margin: 20px 0 0; padding: 0; display: flex; flex-direction: column; gap: 8px; text-align: left; }
.recap li { display: grid; grid-template-columns: 22px 1fr auto; gap: 12px; align-items: center; padding: 10px 14px; border-radius: 12px; background: var(--card-2); border: 1px solid var(--line); }
.recap a { color: var(--ink); text-decoration: none; font-weight: 600; }
.recap a:hover { text-decoration: underline; }
.recap .rt { font-size: 0.8rem; color: var(--ink-3); }
.summary .actions { display: flex; justify-content: center; gap: 10px; margin-top: 22px; flex-wrap: wrap; }

/* ------------------------------------------------------------ controls & stats */

.controls { display: flex; flex-wrap: wrap; gap: 10px 18px; align-items: center; margin: 0 0 12px; }
.ctl { display: flex; align-items: center; gap: 8px; font-size: 0.88rem; }
.ctl-name { color: var(--ink-2); font-weight: 600; white-space: nowrap; }
.ctl input[type="range"] { width: 150px; accent-color: var(--accent); }
.ctl output { min-width: 3.4em; font-variant-numeric: tabular-nums; font-family: var(--display); font-weight: 600; }
.ctl select { padding: 5px 8px; border-radius: 9px; border: 1px solid var(--line); background: var(--card); }
.seg-ctl .segs button { padding: 4px 10px; font-size: 0.82rem; }
.toggle-ctl { cursor: pointer; }
.toggle-ctl input { position: absolute; opacity: 0; pointer-events: none; }
.switch { flex: none; width: 34px; height: 20px; border-radius: 999px; background: var(--track); position: relative; transition: background 0.2s; }
.switch::after { content: ""; position: absolute; top: 3px; left: 3px; width: 14px; height: 14px; border-radius: 50%; background: var(--card); box-shadow: 0 1px 3px rgba(0, 0, 0, 0.25); transition: transform 0.2s; }
.toggle-ctl input:checked + .switch { background: var(--accent); }
.toggle-ctl input:checked + .switch::after { transform: translateX(14px); }
.toggle-ctl input:focus-visible + .switch { outline: 3px solid color-mix(in srgb, var(--accent) 55%, transparent); outline-offset: 2px; }

.stats { display: grid; grid-template-columns: repeat(auto-fit, minmax(120px, 1fr)); gap: 8px; margin: 12px 0 0; }
.stat { padding: 9px 12px; border-radius: 12px; background: var(--card); border: 1px solid var(--line); display: flex; flex-direction: column; gap: 1px; min-width: 0; }
.solution .stat, .playground .stat { background: var(--card-2); }
.solution .visual .stat { background: var(--card); }
.stat-label { font-size: 0.74rem; color: var(--ink-3); font-weight: 600; }
.stat-value { font-family: var(--display); font-weight: 700; font-size: 1.1rem; overflow-wrap: anywhere; }
.stat-sub { font-size: 0.75rem; color: var(--ink-3); }

/* ------------------------------------------------------------ charts */

.chart { position: relative; }
.chart-legend { display: flex; flex-wrap: wrap; gap: 4px 16px; margin: 0 0 6px; font-size: 0.82rem; color: var(--ink-2); }
.chart-legend span { display: inline-flex; align-items: center; gap: 6px; }
.sw { display: inline-block; flex: none; }
.sw.sq { width: 10px; height: 10px; border-radius: 3px; }
.sw.dot { width: 9px; height: 9px; border-radius: 50%; }
.sw.ln { width: 18px; height: 0; border-top: 2.5px solid var(--c); border-radius: 2px; }
.sw.ln.dash { border-top-style: dashed; }
.chart-box { position: relative; width: 100%; }
.chart-svg { display: block; overflow: visible; touch-action: pan-y; }
.chart-svg .grid { stroke: var(--chart-grid); stroke-width: 1; shape-rendering: crispEdges; }
.chart-svg .baseline { stroke: var(--chart-axis); stroke-width: 1; shape-rendering: crispEdges; }
.chart-svg .tick { fill: var(--chart-text); font-size: 11px; font-variant-numeric: tabular-nums; }
.chart-svg .axis-label { fill: var(--ink-2); font-size: 11.5px; font-weight: 600; }
.chart-svg .mark-label { fill: var(--ink-2); font-size: 11px; font-weight: 600; paint-order: stroke; stroke: var(--card); stroke-width: 3px; stroke-linejoin: round; }
.solution .visual .chart-svg .mark-label { stroke: var(--card-2); }
.chart-svg .crosshair { stroke: var(--ink-3); stroke-width: 1; }
.chart-svg .hit { cursor: crosshair; }
.chart-tip {
  position: absolute; z-index: 5; pointer-events: none; min-width: 140px;
  padding: 8px 10px; border-radius: 10px; background: var(--card); border: 1px solid var(--line);
  box-shadow: var(--shadow-lift); font-size: 0.8rem; line-height: 1.5;
}
.chart-tip .tip-head { font-weight: 700; color: var(--ink); margin-bottom: 2px; }
.chart-tip div { display: flex; align-items: center; gap: 6px; color: var(--ink-2); white-space: nowrap; }
.chart-tip b { margin-left: auto; padding-left: 12px; color: var(--ink); font-variant-numeric: tabular-nums; }
.chart-note { margin: 6px 0 0; font-size: 0.82rem; color: var(--ink-3); }
.viz-row { display: grid; grid-template-columns: 1fr 1fr; gap: 16px; }
.viz-caption { font-size: 0.85rem; color: var(--ink-2); margin: 0 0 6px; font-weight: 600; }

/* custom SVG pictures (graphs, urns, bins, timelines) */
.pic { width: 100%; height: auto; display: block; }
.pic text { fill: var(--ink-2); font-size: 12px; font-family: var(--body); }
.pic .edge { stroke: var(--chart-axis); stroke-width: 1.6; }
.pic .edge.hot { stroke: var(--accent); stroke-width: 2.6; }
.pic .node { fill: var(--card); stroke: var(--ink-3); stroke-width: 1.6; }
.pic .node.hot { fill: var(--accent); stroke: var(--accent); }
.pic .walker { fill: var(--s2); stroke: var(--card); stroke-width: 2.5; }
.pic .muted-text { fill: var(--ink-3); }
.pic .ink { fill: var(--ink); }

.truth-table { display: grid; gap: 4px; }
.tt-cell {
  display: flex; align-items: center; justify-content: space-between; gap: 8px;
  padding: 5px 10px; border-radius: 9px; border: 1px solid var(--line); background: var(--card);
  font-family: var(--display); font-size: 0.86rem; cursor: pointer; font-variant-numeric: tabular-nums;
}
.tt-cell:hover { border-color: var(--ink-3); }
.tt-cell .out { width: 30px; text-align: center; border-radius: 6px; font-weight: 700; color: #fff; }
.tt-cell .out.p { background: var(--pos); }
.tt-cell .out.m { background: var(--neg); }

/* ------------------------------------------------------------ misc */

.loading { text-align: center; padding: 80px 20px; color: var(--ink-2); }
.empty { padding: 40px 20px; text-align: center; color: var(--ink-2); }
.kbd { font-family: var(--display); font-size: 0.8em; padding: 1px 6px; border-radius: 6px; border: 1px solid var(--line); background: var(--card-2); }

@media (max-width: 640px) {
  #app { padding: 0 14px 32px; }
  .topbar { padding-top: 12px; }
  .top-nav a { padding: 6px 9px; }
  .pcard { padding: 18px 16px 20px; }
  .setup { padding: 18px 16px; }
  .area-chips { grid-template-columns: 1fr; gap: 4px; }
  .area-chips .area-name { padding-top: 8px; }
  .choices.cols-2 { grid-template-columns: 1fr; }
  .viz-row { grid-template-columns: 1fr; }
  .ctl input[type="range"] { width: 120px; }
  .fu-wrap { padding-left: 10px; }
  .playground { padding: 16px 14px; }
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { animation-duration: 0.01ms !important; transition-duration: 0.01ms !important; }
}