:root {
  --bg: #0d1117;
  --panel: #161b22;
  --panel-2: #1c2330;
  --border: #2d333f;
  --text: #e6edf3;
  --muted: #9aa7b4;
  --imp: #58a6ff;
  --fun: #3fb950;
  --oop: #d29922;
  --accent: #8b5cf6;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif;
  line-height: 1.6;
}

.wrap { max-width: 1100px; margin: 0 auto; padding: 0 1.25rem; }

code, pre {
  font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
}

p code, li code, td code, th code, .note code, summary code {
  background: var(--panel-2);
  border: 1px solid var(--border);
  border-radius: .3em;
  padding: .08em .35em;
  font-size: .88em;
}

a { color: var(--imp); text-decoration: none; }
a:hover { text-decoration: underline; }

/* ---------- header ---------- */
.site-header {
  padding: 4.5rem 0 3rem;
  background:
    radial-gradient(60rem 20rem at 15% -10%, rgba(88, 166, 255, .18), transparent 60%),
    radial-gradient(60rem 20rem at 85% -10%, rgba(63, 185, 80, .14), transparent 60%),
    radial-gradient(50rem 20rem at 50% -20%, rgba(210, 153, 34, .10), transparent 60%),
    var(--panel);
  border-bottom: 1px solid var(--border);
  text-align: center;
}

.site-header h1 {
  font-size: clamp(1.9rem, 5vw, 3rem);
  margin: 0 0 .75rem;
  letter-spacing: -0.02em;
}

.grad {
  background: linear-gradient(90deg, var(--imp), var(--fun), var(--oop));
  -webkit-background-clip: text;
  background-clip: text;
  color: transparent;
}

.tagline {
  max-width: 46rem;
  margin: 0 auto 1.5rem;
  color: var(--muted);
  font-size: 1.05rem;
}

.cta-row { display: flex; gap: .75rem; justify-content: center; flex-wrap: wrap; }

.btn {
  display: inline-block;
  padding: .6rem 1.3rem;
  border-radius: .5rem;
  background: var(--accent);
  color: #fff;
  font-weight: 600;
  border: 1px solid transparent;
}
.btn:hover { text-decoration: none; filter: brightness(1.1); }
.btn.ghost {
  background: transparent;
  border-color: var(--border);
  color: var(--text);
}

/* ---------- sections ---------- */
section { padding: 3rem 0 1rem; }
section h2 { font-size: 1.6rem; margin-bottom: .5rem; }
section > p { color: var(--muted); max-width: 52rem; }

/* ---------- controls ---------- */
.controls {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: .75rem;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: .75rem;
  padding: 1rem;
  margin: 1.5rem 0;
}

.ops { display: flex; gap: .4rem; flex-wrap: wrap; }

.op-btn {
  background: var(--panel-2);
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: .5rem;
  padding: .45rem .8rem;
  cursor: pointer;
  font-size: .95rem;
}
.op-btn.active { border-color: var(--accent); background: rgba(139, 92, 246, .15); }

.controls label { display: flex; align-items: center; gap: .4rem; color: var(--muted); }

.controls input[type="number"] {
  width: 6rem;
  background: var(--panel-2);
  border: 1px solid var(--border);
  color: var(--text);
  border-radius: .5rem;
  padding: .45rem .6rem;
  font: inherit;
}

#run-btn {
  background: var(--accent);
  color: #fff;
  border: none;
  border-radius: .5rem;
  padding: .55rem 1.2rem;
  font-weight: 600;
  cursor: pointer;
}
#run-btn:disabled { opacity: .55; cursor: wait; }
#reset-btn {
  background: transparent;
  color: var(--muted);
  border: 1px solid var(--border);
  border-radius: .5rem;
  padding: .5rem .9rem;
  cursor: pointer;
}
#reset-btn:hover { color: var(--text); }

/* ---------- cards ---------- */
.cards {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: 1rem;
}

.card {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: .75rem;
  padding: 1.1rem 1.1rem .9rem;
  display: flex;
  flex-direction: column;
}

.card h3 { margin: 0; font-size: 1.15rem; }
.card .sub { margin: .1rem 0 .9rem; color: var(--muted); font-size: .9rem; }

.dot { display: inline-block; width: .65rem; height: .65rem; border-radius: 50%; margin-right: .5rem; }
.dot.imp { background: var(--imp); }
.dot.fun { background: var(--fun); }
.dot.oop { background: var(--oop); }

.io {
  display: grid;
  grid-template-columns: 3.4rem 1fr;
  gap: .5rem;
  align-items: baseline;
  padding: .3rem 0;
  border-top: 1px dashed var(--border);
}
.io .lbl { color: var(--muted); font-size: .78rem; text-transform: uppercase; letter-spacing: .06em; }
.io code {
  background: var(--panel-2);
  border-radius: .35rem;
  padding: .25rem .5rem;
  font-size: .85rem;
  overflow-x: auto;
  white-space: nowrap;
  display: block;
}
.io .state { min-height: 1.7rem; }

.note { color: var(--muted); font-size: .9rem; min-height: 2.6rem; margin: .6rem 0 .8rem; }
.note strong { color: var(--text); }

/* ---------- source viewer ---------- */
.src summary {
  cursor: pointer;
  color: var(--imp);
  font-weight: 600;
  font-size: .9rem;
}
.src[open] summary { margin-bottom: .8rem; }

.concept {
  border: 1px solid var(--border);
  border-left: 3px solid var(--accent);
  border-radius: .4rem;
  padding: .6rem .8rem;
  margin-bottom: .8rem;
  background: var(--panel-2);
}
.concept h4 { margin: 0 0 .2rem; font-size: .9rem; }
.concept p { margin: .2rem 0 .5rem; color: var(--muted); font-size: .85rem; }
.concept pre { margin: 0; border-radius: .35rem; }
.concept pre code { font-size: .8rem; }
.concept .lines { font-size: .75rem; color: var(--muted); }

.src > pre {
  border-radius: .5rem;
  max-height: 26rem;
  overflow: auto;
  border: 1px solid var(--border);
}
.src > pre code { font-size: .8rem; }
pre code.hljs { padding: .8rem; }

.gh-link { font-size: .85rem; margin: .5rem 0 0; }

.fine { color: var(--muted); font-size: .8rem; margin-top: 1rem; }

/* ---------- table ---------- */
.table-wrap { overflow-x: auto; }
table { border-collapse: collapse; width: 100%; font-size: .92rem; }
th, td {
  text-align: left;
  padding: .6rem .8rem;
  border: 1px solid var(--border);
  vertical-align: top;
}
thead th { background: var(--panel); }
tbody th { background: var(--panel); color: var(--muted); white-space: nowrap; }

/* ---------- katas ---------- */
.kata {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: .6rem;
  margin-bottom: .8rem;
  padding: .9rem 1.1rem;
}
.kata > summary { cursor: pointer; }
.kata > div { margin-top: .8rem; color: var(--text); }
.kata p { margin: .5rem 0; }
.kata pre { border-radius: .5rem; overflow-x: auto; }

.diff {
  font-size: .72rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: .05em;
  border-radius: 1rem;
  padding: .15rem .6rem;
  margin-left: .5rem;
  vertical-align: middle;
}
.diff.easy { background: rgba(63, 185, 80, .15); color: var(--fun); }
.diff.mid  { background: rgba(210, 153, 34, .15); color: var(--oop); }
.diff.hard { background: rgba(139, 92, 246, .18); color: #b79cff; }

.hint summary { color: var(--oop); cursor: pointer; font-size: .9rem; }
.hint p { color: var(--muted); font-size: .9rem; }

/* ---------- footer ---------- */
footer {
  margin-top: 4rem;
  border-top: 1px solid var(--border);
  padding: 1.5rem 0 2.5rem;
  text-align: center;
  color: var(--muted);
  font-size: .9rem;
}

@media (max-width: 640px) {
  .site-header { padding: 3rem 0 2rem; }
  .io code { font-size: .78rem; }
}