:root {
  --bg: oklch(0.97 0.008 110);
  --surface: oklch(1 0 0);
  --surface-soft: oklch(0.945 0.014 110);
  --ink: oklch(0.23 0.025 135);
  --muted: oklch(0.47 0.03 135);
  --primary: oklch(0.46 0.1 112);
  --primary-hover: oklch(0.4 0.1 112);
  --tile: oklch(0.92 0.05 112);
  --tile-hover: oklch(0.87 0.075 112);
  --focus: oklch(0.55 0.14 238);
  --line: oklch(0.86 0.018 110);
  --shadow: 0 18px 42px oklch(0.22 0.035 135 / 0.12);
  --radius-lg: 1.5rem;
  --radius-md: 0.9rem;
}

* { box-sizing: border-box; }
[hidden] { display: none !important; }

body {
  min-width: 320px;
  min-height: 100vh;
  margin: 0;
  display: grid;
  place-items: center;
  padding: 2rem 1rem;
  color: var(--ink);
  background: var(--bg);
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", "Microsoft YaHei", sans-serif;
}

button { font: inherit; }
button:focus-visible { outline: 3px solid var(--focus); outline-offset: 3px; }

.game-shell { width: min(100%, 34rem); }
.hero { margin: 0 auto 1.5rem; text-align: center; }
.eyebrow { margin: 0 0 0.5rem; color: var(--primary); font-size: 0.72rem; font-weight: 800; letter-spacing: 0.12em; }
h1, h2 { margin: 0; text-wrap: balance; }
h1 { font-size: 2rem; letter-spacing: -0.04em; }
h2 { font-size: 1.5rem; letter-spacing: -0.025em; }
.intro { max-width: 30ch; margin: 0.7rem auto 0; color: var(--muted); font-size: 0.95rem; line-height: 1.6; }

.game-panel {
  padding: clamp(1.05rem, 4vw, 1.6rem);
  border: 1px solid var(--line);
  border-radius: var(--radius-lg);
  background: var(--surface);
  box-shadow: var(--shadow);
}

.difficulty, .actions { display: flex; gap: 0.65rem; }
.difficulty { margin-bottom: 1.25rem; }
.difficulty-button, .button {
  min-height: 2.75rem;
  border: 1px solid transparent;
  border-radius: 0.75rem;
  cursor: pointer;
  font-weight: 700;
  transition: background-color 180ms ease, color 180ms ease, border-color 180ms ease, box-shadow 180ms ease, transform 180ms ease;
}
.difficulty-button { flex: 1; color: var(--muted); background: var(--surface-soft); }
.difficulty-button:hover { color: var(--ink); background: oklch(0.91 0.025 110); }
.difficulty-button.is-active { color: var(--surface); background: var(--primary); box-shadow: 0 6px 14px oklch(0.46 0.1 112 / 0.25); }

.board {
  --gap: 0.55rem;
  position: relative;
  gap: var(--gap);
  width: min(100%, 28rem);
  aspect-ratio: 1;
  margin: 0 auto;
  border-radius: 1.15rem;
  background: oklch(0.84 0.025 110);
  overflow: hidden;
}
.board[data-size="3"] { --size: 3; }
.board[data-size="4"] { --size: 4; }

/* 棋盘是直接操作区域：禁止文本选中、拖拽和移动端长按菜单。 */
.board, .tile, .empty-cell {
  -webkit-touch-callout: none;
  -webkit-user-select: none;
  user-select: none;
}
.tile { touch-action: manipulation; }

.tile, .empty-cell {
  position: absolute;
  min-width: 0;
  min-height: 0;
  border-radius: 0.8rem;
}
.tile {
  border: 0;
  color: var(--ink);
  background: var(--tile);
  box-shadow: 0 4px 0 oklch(0.71 0.055 112), 0 8px 16px oklch(0.24 0.03 135 / 0.12);
  cursor: pointer;
  font-size: clamp(1.15rem, 7vw, 2rem);
  font-weight: 800;
  will-change: transform;
  transition: transform 55ms cubic-bezier(.22, 1, .36, 1), background-color 55ms ease, box-shadow 55ms ease;
}
.tile:hover:not(:disabled) { background: var(--tile-hover); box-shadow: 0 5px 0 oklch(0.67 0.075 112), 0 10px 18px oklch(0.24 0.03 135 / 0.15); }
.tile:active:not(:disabled) { box-shadow: 0 1px 0 oklch(0.67 0.075 112); }
.tile:disabled { cursor: default; }
.tile.is-movable { outline: 2px solid oklch(0.61 0.09 112 / 0.42); outline-offset: -2px; }
.empty-cell { background: oklch(0.76 0.02 110 / 0.52); }
.board.is-positioning .tile,
.board.is-positioning .empty-cell { transition: none; }

.game-stats { display: grid; grid-template-columns: repeat(4, 1fr); gap: 0.5rem; margin: 1.25rem 0; }
.stat { min-width: 0; padding: 0.75rem 0.45rem; border-radius: var(--radius-md); text-align: center; background: var(--surface-soft); }
.stat dt { margin-bottom: 0.28rem; color: var(--muted); font-size: 0.72rem; }
.stat dd { margin: 0; font-size: clamp(0.9rem, 3.8vw, 1.08rem); font-weight: 800; font-variant-numeric: tabular-nums; }
.stat-best { background: oklch(0.92 0.045 112); }

.actions { flex-wrap: wrap; }
.button { flex: 1 1 8rem; padding: 0.65rem 0.85rem; }
.button:hover:not(:disabled) { transform: translateY(-2px); }
.button:active:not(:disabled) { transform: translateY(0); }
.button-primary { color: var(--surface); background: var(--primary); box-shadow: 0 6px 14px oklch(0.46 0.1 112 / 0.22); }
.button-primary:hover:not(:disabled) { background: var(--primary-hover); }
.button-secondary { color: var(--ink); border-color: var(--line); background: var(--surface); }
.button-secondary:hover:not(:disabled) { background: var(--surface-soft); }
.button:disabled { color: oklch(0.62 0.02 110); background: oklch(0.93 0.008 110); box-shadow: none; cursor: not-allowed; }
.status { min-height: 1.4rem; margin: 1rem 0 0; color: var(--muted); font-size: 0.88rem; text-align: center; }

.dialog-backdrop { position: fixed; z-index: 10; inset: 0; display: grid; place-items: center; padding: 1rem; background: oklch(0.15 0.02 135 / 0.42); }
.win-dialog { width: min(100%, 24rem); padding: 2rem; border-radius: var(--radius-lg); text-align: center; background: var(--surface); box-shadow: 0 24px 60px oklch(0.15 0.02 135 / 0.25); }
.win-mark { display: grid; place-items: center; width: 3rem; height: 3rem; margin: 0 auto 1rem; border-radius: 50%; color: var(--surface); background: var(--primary); font-size: 1.5rem; font-weight: 800; }
.win-dialog > .eyebrow { margin-bottom: 0.45rem; }
.win-dialog > p:not(.eyebrow) { margin: 0.7rem 0 1.25rem; color: var(--muted); line-height: 1.6; }
.win-dialog .button { width: 100%; }

@media (max-width: 390px) {
  body { padding: 1rem 0.7rem; }
  .game-panel { border-radius: 1.2rem; }
  .board { --gap: 0.42rem; }
  .game-stats { gap: 0.35rem; }
  .stat { padding-inline: 0.25rem; }
  .stat dt { font-size: 0.68rem; }
}

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