/*
 * gridshift
 *
 * No web fonts: upstream shipped three weights of Clear Sans in four formats,
 * roughly 400 KB, to draw about a dozen numbers. A system stack with tabular
 * figures renders the same board with nothing to download.
 */

:root {
  color-scheme: light dark;

  --bg: #eef1f5;
  --panel: #ffffff;
  --board: #cfd6e0;
  --cell-bg: #dee3ea;
  --ink: #161c25;
  --ink-2: #4d5766;
  --ink-3: #7b8697;
  --rule: #d3d9e2;
  --accent: #1f6f63;
  --accent-ink: #ffffff;
  --focus: #1f6f63;
  --notice-bg: #f6eedd;
  --notice-ink: #7a5c16;

  --gap: 10px;
  --radius: 6px;
  --n: 4;
  --cell: 0px;

  --tile-2:    #e8edf3; --tile-2-ink:    #3a4453;
  --tile-4:    #dbe5ee; --tile-4-ink:    #33404f;
  --tile-8:    #a8d8cf; --tile-8-ink:    #123a34;
  --tile-16:   #79c5b7; --tile-16-ink:   #0d332d;
  --tile-32:   #46ab9b; --tile-32-ink:   #04241f;
  --tile-64:   #2f9c8a; --tile-64-ink:   #ffffff;
  --tile-128:  #e8c477; --tile-128-ink:  #3b2c0b;
  --tile-256:  #e0a34a; --tile-256-ink:  #2f2207;
  --tile-512:  #d4823a; --tile-512-ink:  #ffffff;
  --tile-1024: #c76242; --tile-1024-ink: #ffffff;
  --tile-2048: #b34a5a; --tile-2048-ink: #ffffff;
  --tile-high: #6f4b8e; --tile-high-ink: #ffffff;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #0d1219;
    --panel: #151c26;
    --board: #1c2532;
    --cell-bg: #222c3a;
    --ink: #e6ebf2;
    --ink-2: #a8b3c2;
    --ink-3: #788494;
    --rule: #27313f;
    --accent: #2f9c8a;
    --accent-ink: #04241f;
    --focus: #63c7b5;
    --notice-bg: #2a2316;
    --notice-ink: #dcb96e;

    --tile-2:    #2b3543; --tile-2-ink:    #c3ccd9;
    --tile-4:    #35414f; --tile-4-ink:    #d6dee8;
    --tile-8:    #26564e; --tile-8-ink:    #b9e6dc;
    --tile-16:   #2c7466; --tile-16-ink:   #d6f2ea;
    --tile-32:   #2f9c8a; --tile-32-ink:   #04241f;
    --tile-64:   #46b8a3; --tile-64-ink:   #04241f;
    --tile-128:  #9a7526; --tile-128-ink:  #f7e8c4;
    --tile-256:  #c08b30; --tile-256-ink:  #241905;
    --tile-512:  #d4823a; --tile-512-ink:  #241505;
    --tile-1024: #c76242; --tile-1024-ink: #ffffff;
    --tile-2048: #b34a5a; --tile-2048-ink: #ffffff;
    --tile-high: #8a63a8; --tile-high-ink: #ffffff;
  }
}

* { box-sizing: border-box; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font-family: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  font-size: 16px;
  line-height: 1.5;
  -webkit-font-smoothing: antialiased;
}

.shell {
  max-width: 560px;
  margin: 0 auto;
  padding-inline: 16px;
  padding-block: 24px 48px;
  display: flex;
  flex-direction: column;
  gap: 20px;
}

.skip {
  position: absolute;
  left: -9999px;
  top: 0;
  background: var(--accent);
  color: var(--accent-ink);
  padding: 10px 16px;
  z-index: 10;
}
.skip:focus { left: 8px; top: 8px; }

.sr-only {
  position: absolute;
  width: 1px; height: 1px;
  margin: -1px; padding: 0;
  overflow: hidden;
  clip: rect(0 0 0 0);
  clip-path: inset(50%);
  white-space: nowrap;
  border: 0;
}

/* masthead ---------------------------------------------------------------- */

.masthead {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 16px;
  flex-wrap: wrap;
}

h1 {
  margin: 0;
  font-size: 32px;
  letter-spacing: -0.02em;
  font-weight: 650;
}

.tagline { margin: 2px 0 0; color: var(--ink-3); font-size: 14px; }

.readouts { display: flex; gap: 8px; }

.readout {
  background: var(--panel);
  border: 1px solid var(--rule);
  border-radius: var(--radius);
  padding: 6px 12px;
  min-width: 72px;
  text-align: right;
}

.readout-key {
  display: block;
  font-size: 10.5px;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-3);
}

.readout-value {
  display: block;
  font-size: 20px;
  font-weight: 600;
  font-variant-numeric: tabular-nums;
  line-height: 1.2;
}

/* toolbar ----------------------------------------------------------------- */

.toolbar {
  display: flex;
  align-items: flex-end;
  gap: 10px;
  flex-wrap: wrap;
}

.field { display: flex; flex-direction: column; gap: 4px; }

.field label {
  font-size: 10.5px;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-3);
}

select, .btn {
  font: inherit;
  font-size: 14px;
  padding: 8px 12px;
  border-radius: var(--radius);
  border: 1px solid var(--rule);
  background: var(--panel);
  color: var(--ink);
  cursor: pointer;
}

.btn-primary {
  background: var(--accent);
  border-color: var(--accent);
  color: var(--accent-ink);
  font-weight: 600;
}

.btn:disabled { opacity: 0.45; cursor: not-allowed; }

:focus-visible { outline: 2px solid var(--focus); outline-offset: 2px; }

.seed {
  margin: 0 0 8px auto;
  font-size: 12.5px;
  color: var(--ink-3);
  font-variant-numeric: tabular-nums;
}

.notice {
  margin: 0;
  padding: 10px 14px;
  background: var(--notice-bg);
  color: var(--notice-ink);
  border-radius: var(--radius);
  font-size: 13.5px;
}

/* board ------------------------------------------------------------------- */

.board-wrap { position: relative; }

.board {
  position: relative;
  aspect-ratio: 1;
  width: 100%;
  max-width: 100%;
  padding: var(--gap);
  background: var(--board);
  border-radius: calc(var(--radius) + 4px);
  touch-action: none;
}

.cells, .tiles {
  position: absolute;
  inset: var(--gap);
}

.cells {
  display: grid;
  grid-template-columns: repeat(var(--n), 1fr);
  grid-template-rows: repeat(var(--n), 1fr);
  gap: var(--gap);
}

.cell { background: var(--cell-bg); border-radius: var(--radius); }

.tile {
  position: absolute;
  top: 0;
  left: 0;
  width: var(--cell);
  height: var(--cell);
  display: grid;
  place-items: center;
  border-radius: var(--radius);
  background: var(--tile-2);
  color: var(--tile-2-ink);
  font-weight: 650;
  font-variant-numeric: tabular-nums;
  transform: translate(
    calc(var(--x) * (var(--cell) + var(--gap))),
    calc(var(--y) * (var(--cell) + var(--gap)))
  );
  transition: transform 120ms ease-out;
  will-change: transform;
}

.tile-value {
  /* Shrink the label as the board grows and the numbers get longer. */
  font-size: clamp(13px, calc(var(--cell) * 0.42), 40px);
}

.tile[data-value="2"]    { background: var(--tile-2);    color: var(--tile-2-ink); }
.tile[data-value="4"]    { background: var(--tile-4);    color: var(--tile-4-ink); }
.tile[data-value="8"]    { background: var(--tile-8);    color: var(--tile-8-ink); }
.tile[data-value="16"]   { background: var(--tile-16);   color: var(--tile-16-ink); }
.tile[data-value="32"]   { background: var(--tile-32);   color: var(--tile-32-ink); }
.tile[data-value="64"]   { background: var(--tile-64);   color: var(--tile-64-ink); }
.tile[data-value="128"]  { background: var(--tile-128);  color: var(--tile-128-ink); }
.tile[data-value="256"]  { background: var(--tile-256);  color: var(--tile-256-ink); }
.tile[data-value="512"]  { background: var(--tile-512);  color: var(--tile-512-ink); }
.tile[data-value="1024"] { background: var(--tile-1024); color: var(--tile-1024-ink); font-size: 0.88em; }
.tile[data-value="2048"] { background: var(--tile-2048); color: var(--tile-2048-ink); font-size: 0.88em; }
.tile[data-value="4096"],
.tile[data-value="8192"],
.tile[data-value="huge"] { background: var(--tile-high); color: var(--tile-high-ink); font-size: 0.8em; }

.tile.is-new { animation: appear 140ms ease-out; }
.tile.is-merged { animation: pop 140ms ease-out; }
.tile.is-retiring { z-index: -1; }

@keyframes appear {
  from { transform: translate(calc(var(--x) * (var(--cell) + var(--gap))), calc(var(--y) * (var(--cell) + var(--gap)))) scale(0.4); opacity: 0; }
}

@keyframes pop {
  50% { transform: translate(calc(var(--x) * (var(--cell) + var(--gap))), calc(var(--y) * (var(--cell) + var(--gap)))) scale(1.12); }
}

/* overlay ----------------------------------------------------------------- */

.overlay {
  position: absolute;
  inset: 0;
  display: grid;
  place-items: center;
  padding: 16px;
  background: color-mix(in srgb, var(--board) 88%, transparent);
  border-radius: calc(var(--radius) + 4px);
  backdrop-filter: blur(2px);
}

.overlay[hidden] { display: none; }

.overlay-card {
  background: var(--panel);
  border: 1px solid var(--rule);
  border-radius: var(--radius);
  padding: 20px 22px;
  text-align: center;
  max-width: 320px;
}

.overlay-card h2 { margin: 0 0 6px; font-size: 22px; }
.overlay-card p { margin: 0 0 16px; color: var(--ink-2); font-size: 14.5px; }
.overlay-actions { display: flex; gap: 8px; justify-content: center; flex-wrap: wrap; }

/* panels ------------------------------------------------------------------ */

.panel {
  background: var(--panel);
  border: 1px solid var(--rule);
  border-radius: var(--radius);
  padding: 16px 18px;
}

.panel h2 {
  margin: 0 0 12px;
  font-size: 12px;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-3);
  font-weight: 600;
}

.panel p { margin: 0; font-size: 14px; color: var(--ink-2); }
.panel a { color: var(--accent); }

.stats, .keys { margin: 0; display: grid; gap: 10px; }
.stats { grid-template-columns: repeat(auto-fit, minmax(120px, 1fr)); }
.stats dt, .keys dt { font-size: 12px; color: var(--ink-3); margin: 0; }
.stats dd { margin: 2px 0 0; font-size: 20px; font-weight: 600; font-variant-numeric: tabular-nums; }
.keys div { display: flex; align-items: baseline; gap: 12px; justify-content: space-between; }
.keys dd { margin: 0; font-size: 14px; color: var(--ink-2); }

kbd {
  display: inline-block;
  font: inherit;
  font-size: 12px;
  padding: 1px 6px;
  border: 1px solid var(--rule);
  border-bottom-width: 2px;
  border-radius: 4px;
  background: var(--bg);
  margin-right: 2px;
}

@media (max-width: 420px) {
  h1 { font-size: 26px; }
  .readout { min-width: 62px; padding: 5px 10px; }
  .readout-value { font-size: 17px; }
  .seed { margin-left: 0; width: 100%; }
}

@media (prefers-reduced-motion: reduce) {
  .tile { transition: none; }
  .tile.is-new, .tile.is-merged { animation: none; }
}