:root {
  --bg: #0f1216;
  --panel: #171b21;
  --panel-2: #1e242c;
  --line: #2a313b;
  --text: #e7ebf0;
  --muted: #8b95a3;
  --accent: #4d9fff;
  --good: #35c46a;
  --bad: #ff5a5a;
  --radius: 12px;
}
@media (prefers-color-scheme: light) {
  :root {
    --bg: #f4f6f9; --panel: #ffffff; --panel-2: #eef1f6; --line: #dde2ea;
    --text: #161a20; --muted: #616b7a; --accent: #1668d6;
  }
}
* { box-sizing: border-box; }
html, body { height: 100%; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font: 15px/1.5 -apple-system, BlinkMacSystemFont, "Segoe UI", Inter, system-ui, sans-serif;
  -webkit-font-smoothing: antialiased;
}

/* ---------------------------------------------------------- header */
.top {
  display: flex; align-items: center; gap: 16px;
  padding: 14px 20px; border-bottom: 1px solid var(--line);
  background: var(--panel);
}
.top h1 {
  margin: 0; font-size: 15px; font-weight: 700; letter-spacing: .08em;
  text-transform: uppercase; color: var(--muted); white-space: nowrap;
}
.tabs { display: flex; gap: 4px; margin-left: auto; }
.tab {
  border: 0; background: transparent; color: var(--muted);
  font: inherit; font-weight: 600; padding: 7px 14px; border-radius: 99px; cursor: pointer;
}
.tab:hover { color: var(--text); }
.tab.is-on { background: var(--panel-2); color: var(--text); }
.icon-btn {
  border: 0; background: transparent; color: var(--muted); cursor: pointer;
  font-size: 20px; line-height: 1; padding: 6px 8px; border-radius: 8px;
}
.icon-btn:hover { color: var(--text); background: var(--panel-2); }

main { max-width: 1080px; margin: 0 auto; padding: 22px 20px 60px; }
.view { display: none; }
.view.is-on { display: block; }

/* ---------------------------------------------------------- train */
.scoreboard {
  display: grid; grid-template-columns: repeat(4, 1fr); gap: 10px;
  max-width: 520px; margin: 0 auto 22px;
}
.score {
  background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 10px 6px; text-align: center;
}
.score-v {
  display: block; font-size: 22px; font-weight: 700;
  font-variant-numeric: tabular-nums; line-height: 1.2;
}
.score-k { font-size: 11px; text-transform: uppercase; letter-spacing: .07em; color: var(--muted); }

.stage { text-align: center; }
.cube { width: min(300px, 62vw); margin: 0 auto; }
.cube svg { width: 100%; height: auto; display: block; }
.cube.is-good { animation: pop .28s ease; }
.cube.is-bad { animation: shake .32s ease; }
@keyframes pop { 50% { transform: scale(1.045); } }
@keyframes shake {
  20% { transform: translateX(-9px); } 45% { transform: translateX(8px); }
  70% { transform: translateX(-5px); } 100% { transform: translateX(0); }
}

.feedback {
  /* Fixed, not min-height: the result must never push the input or the case
     buttons down the page. */
  height: 50px; margin: 14px 0 10px; overflow: hidden;
  font-weight: 600; font-size: 16px;
}
.feedback.good { color: var(--good); }
.feedback.bad { color: var(--bad); }
.feedback .alg {
  display: block; margin-top: 3px; font-weight: 500; font-size: 13px; color: var(--muted);
  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
}

#answer {
  width: min(320px, 80vw); padding: 12px 16px; text-align: center;
  font: 600 20px/1 ui-monospace, SFMono-Regular, Menlo, monospace;
  color: var(--text); background: var(--panel); border: 2px solid var(--line);
  border-radius: var(--radius); outline: none;
}
#answer::placeholder { font-weight: 400; color: var(--muted); font-size: 15px; }
#answer:focus { border-color: var(--accent); }
#answer.good { border-color: var(--good); }
#answer.bad { border-color: var(--bad); }

.hint { color: var(--muted); font-size: 12.5px; margin-top: 16px; }
kbd {
  background: var(--panel-2); border: 1px solid var(--line); border-bottom-width: 2px;
  border-radius: 5px; padding: 1px 5px; font: 600 11px/1.6 ui-monospace, Menlo, monospace;
}

/* ---------------------------------------------------------- why panel */
.why {
  max-width: 460px; margin: 20px auto 0; padding: 12px 14px;
  border: 1px solid var(--line); border-radius: var(--radius);
  background: var(--panel); color: var(--muted);
  font-size: 13.5px; line-height: 1.55; text-align: left;
}
.why[hidden] { display: none; }
.why-branch { font-size: 12.5px; color: var(--muted); margin-bottom: 4px; }
.why-rule { color: var(--text); font-weight: 600; font-size: 15px; line-height: 1.45; }
.why-credit { margin-top: 8px; font-size: 11.5px; color: var(--muted); }
.why-credit a { color: var(--muted); text-decoration: underline; }
.why-credit a:hover { color: var(--accent); }

/* ---------------------------------------------------- answer pad (touch) */
/* On a phone there is no keyboard worth using, so the 21 cases become
   three rows of seven buttons instead of a text field. */
.answer-pad { display: none; }
.pad-btn {
  border: 1px solid var(--line); background: var(--panel); color: var(--text);
  font: 600 14px/1 inherit; padding: 0; height: 42px; border-radius: 9px;
  cursor: pointer; -webkit-tap-highlight-color: transparent;
}
.pad-btn:active { background: var(--panel-2); }
.pad-btn:focus { outline: none; }
.pad-btn:focus-visible { outline: 2px solid var(--accent); outline-offset: 1px; }
.pad-btn.is-right { background: var(--good); border-color: var(--good); color: #08130c; }
.pad-btn.is-wrong { background: var(--bad); border-color: var(--bad); color: #fff; }

@media (max-width: 640px) {
  #answer-form, .hint { display: none; }
  .answer-pad {
    display: grid; grid-template-columns: repeat(7, 1fr); gap: 6px;
    max-width: 460px; margin: 0 auto;
  }
  .cube { width: min(230px, 54vw); }
  .scoreboard { gap: 8px; margin-bottom: 14px; }
  .score { padding: 7px 4px; }
  .feedback { margin: 10px 0; height: 62px; font-size: 15px; }
  /* Long algorithms wrap to two lines on a narrow screen; keep them inside
     the reserved box. */
  .feedback .alg { font-size: 12px; line-height: 1.45; }
  .stage-btn { margin-top: 12px; }
  .why { margin-top: 14px; font-size: 13px; padding: 10px 12px; }
}
@media (max-width: 380px) {
  .pad-btn { font-size: 13px; height: 38px; }
  .answer-pad { gap: 4px; }
}

/* ---------------------------------------------------------- stats */
.panel-head { display: flex; align-items: center; gap: 12px; margin-bottom: 6px; }
.panel-head h2 { font-size: 17px; margin: 0; }
.ghost-btn {
  margin-left: auto; border: 1px solid var(--line); background: transparent;
  color: var(--muted); font: inherit; font-size: 13px; padding: 6px 12px;
  border-radius: 8px; cursor: pointer;
}
.ghost-btn:hover { color: var(--bad); border-color: var(--bad); }
.session-line { color: var(--muted); font-size: 13.5px; margin-bottom: 16px; }
.stats-table { width: 100%; border-collapse: collapse; font-variant-numeric: tabular-nums; }
.stats-table th {
  text-align: right; font-size: 11px; text-transform: uppercase; letter-spacing: .06em;
  color: var(--muted); padding: 8px 10px; border-bottom: 1px solid var(--line); font-weight: 600;
}
.stats-table th:first-child, .stats-table td:first-child { text-align: left; }
.stats-table td { text-align: right; padding: 9px 10px; border-bottom: 1px solid var(--line); }
.stats-table tbody tr:hover { background: var(--panel); }
.stats-table .name { font-weight: 700; }
.stats-table .none { color: var(--muted); }
.bar { display: inline-block; height: 6px; border-radius: 3px; background: var(--good); vertical-align: middle; margin-right: 8px; }

/* ---------------------------------------------------------- reference */
.blurb { color: var(--muted); max-width: 62ch; margin: 0 0 20px; font-size: 14px; }
.blurb a { color: var(--accent); }
.ref-group { margin-bottom: 26px; }
.ref-head { display: flex; align-items: baseline; gap: 10px; margin-bottom: 8px; }
.ref-head h3 { margin: 0; font-size: 16px; }
.ref-head code {
  font-family: ui-monospace, SFMono-Regular, Menlo, monospace; font-size: 12px; color: var(--muted);
}
.ref-row { display: flex; flex-wrap: wrap; gap: 10px; }
.ref-cube {
  width: 104px; background: var(--panel); border: 1px solid var(--line);
  border-radius: 10px; padding: 8px;
}
.ref-cube svg { width: 100%; height: auto; display: block; }

/* ---------------------------------------------------------- drawer */
.drawer-backdrop {
  position: fixed; inset: 0; background: rgba(0,0,0,.5);
  opacity: 0; pointer-events: none; transition: opacity .18s;
}
.drawer-backdrop.is-on { opacity: 1; pointer-events: auto; }
.drawer {
  position: fixed; top: 0; right: 0; bottom: 0; width: min(400px, 92vw);
  background: var(--panel); border-left: 1px solid var(--line);
  padding: 16px 20px 40px; overflow-y: auto;
  transform: translateX(101%); transition: transform .22s ease;
}
.drawer.is-on { transform: none; }
.drawer-head { display: flex; align-items: center; margin-bottom: 6px; }
.drawer-head h2 { font-size: 17px; margin: 0; }
.drawer-head .icon-btn { margin-left: auto; font-size: 24px; }
.drawer h3 {
  font-size: 11px; text-transform: uppercase; letter-spacing: .07em;
  color: var(--muted); margin: 22px 0 10px;
}
.presets { display: flex; flex-wrap: wrap; gap: 6px; margin-bottom: 12px; }
.chip-btn {
  border: 1px solid var(--line); background: transparent; color: var(--muted);
  font: inherit; font-size: 12px; padding: 5px 10px; border-radius: 99px; cursor: pointer;
}
.chip-btn:hover { color: var(--text); border-color: var(--accent); }
.case-picker { display: grid; grid-template-columns: repeat(4, 1fr); gap: 6px; }
.case-chip {
  border: 1px solid var(--line); background: transparent; color: var(--muted);
  font: 600 14px/1 inherit; padding: 9px 0; border-radius: 8px; cursor: pointer;
}
.case-chip.is-on { background: var(--accent); border-color: var(--accent); color: #fff; }
.opt { display: flex; gap: 10px; align-items: flex-start; padding: 9px 0; cursor: pointer; }
.opt input { margin-top: 3px; accent-color: var(--accent); width: 16px; height: 16px; }
.opt b { font-weight: 600; font-size: 14px; display: block; }
.opt em { font-style: normal; color: var(--muted); font-size: 12.5px; }

@media (max-width: 560px) {
  .top { gap: 8px; padding: 12px 14px; }
  .top h1 { font-size: 13px; }
  .tab { padding: 6px 10px; font-size: 14px; }
  .score-v { font-size: 18px; }
  .stats-table th, .stats-table td { padding: 8px 5px; font-size: 13px; }
}

.stage-btn { margin: 4px auto 0; display: block; }
.stage-btn:hover { color: var(--accent); border-color: var(--accent); }