2048 · qwen3.8-27b

Build the game. Let's write index.html with a tile-2048 implementation that has animations using absolutely positioned tile elements.

Key design: the board state is a 4x4 grid of tile objects {id, value, row, col, merged}. Tiles are DOM elements positioned by CSS transform. Animation about 120ms, input locked during animation.

Write the code carefully.

Implementation plan:
- `tiles` array, each {id, value, row, col, el}.
- move(dir): compute the new position of each row/column; standard merge logic: traverse in the direction order, and merge adjacent tiles of the same value only once.
- Mark merged tiles for deletion (after animation), and spawn.
- Lock input during animation (about 130ms), using a `locked` flag.
- Win detection: value 2048 → show "You win!" overlay with "Keep going" and "New Game".
- Game over: no empty cells and no adjacent equal cells → "Game over" overlay with "New Game".
- Score display, New Game button.
- Arrow keys preventDefault.

Render: container #board is position relative, with grid cell backgrounds; tiles are absolute, transition transform.

Size: use CSS custom properties for size; simpler: board is 400px (or responsive), tile size computed in JS? Use percentages: each cell 25%; tile width is calc(25% - gap). Use transform: translate in percentages of its own size... transform translate percentages are relative to the element's own size. Easier: position in px from a fixed size computed in JS, or use left/top in px. Simplest and most robust: fixed size 420px board, compute tile positions in px via JS and set transform.

Let's go with this: cell size 90px, gap 10px, padding 10px. Board = 10*2 + 4*90 + 3*10 = 410px. Position: x = 10 + col*100, y = 10 + row*100.

Animation: transition transform 120ms; merged tile: move to target, then after transition show a "pop" scale animation, then remove the underlying tile and update the value.

Merge implementation details: two tiles a, b (b is behind a in movement direction). Target = a's final position. b moves to the same position (below in z-index), then gets removed; a.value *= 2, score += a.value, pop class.

Implementation:
```js
function move(dr, dc) {
  if (locked || over) return;
  // build grid
  const grid = Array.from({length:4},()=>Array(4).fill(null));
  tiles.forEach(t => grid[t.row][t.col] = t);
  let moved = false, merges = [];
  const lines = [];
  // traversal order: for each row/col depending on direction
  for (let i=0;i<4;i++){
    const seq = [];
    for (let j=0;j<4;j++){
      const r = dr===0? i : (dc===0 ? (3-j) : j);
      const c = dc===0? j : (dr===0 ? (3-j) : i);
      const t = grid[r][c];
      if (t) seq.push(t);
    }
    // seq ordered from the edge being moved toward
    let pos = 0;
    for (let k=0;k<seq.length;k++){
      const t = seq[k];
      if (k+1<seq.length && seq[k+1].value===t.value && pos===targetOf(k)?? ) 
```
Simpler standard: iterate k; if next tile has the same value → merge: t goes to index pos, next tile also to index pos, mark merge. Otherwise t goes to pos, pos++. Wait, careful: when merging, t takes slot pos and next tile also goes to slot pos, pos++ only once.

```js
let pos = 0;
for (let k=0;k<seq.length;k++){
  const t = seq[k];
  if (k+1 < seq.length && seq[k+1].value === t.value) {
    // merge t and seq[k+1] at pos
    assignPos(t, i, pos, dr, dc);
    assignPos(seq[k+1], i, pos, dr, dc);
    merges.push([t, seq[k+1]]);
    k++; // skip merged tile
    moved = true;
  } else {
    assignPos(t, i, pos, dr, dc);
  }
  pos++;
}
```
Hmm, wait — need to be careful about "moved" detection: even if a tile merges, its position may not have changed (e.g., [2,2,0,0] moved right → both go to the right, position changes; but [2,2,0,0] moved left → no position change, but merge occurs). Actually [2,2] moved left: positions unchanged but merge happens — the board changes so it counts as a move. So moved = any tile's position changed, or a merge occurred. In the code above, merges are always marked moved=true. But [2,2,0,0] moved left: tile 0 stays at pos 0, tile 1 stays at pos 1 → merge! Is this correct in 2048? Moving [2,2,0,0] left — since they're already at the left edge, the tiles don't move, but do they merge? In real 2048... no. In real 2048, a move only merges if tiles slide. If tiles don't move, no merge. Hmm, is this right? Real 2048: moving left on [2,2,0,0]: the tiles are already flush left; do they merge into [4,0,0,0]? Let me think about the standard implementation: in the standard algorithm (Gabriele Cirulli's), for each row, traverse from the left; tile at index 0 is 2, already placed; next tile 2: "same as previous tile and previous tile hasn't merged" → merge! Yes, the standard 2048 actually does merge [2,2,0,0] left into [4,0,0,0]. Hmm wait, really? In Cirulli's implementation: traverse cells left to right; skip nulls; compare to previous placed tile; if equal and !merged → merge. In [2,2,0,0], the first 2 is placed at 0, the second 2 is adjacent and equal → merge into 4. And position: doesn't move. So yes, in standard 2048 it merges and the board changed (a 4 is created) so it counts as a valid move.

But wait — the task says "after a move that changes the board, spawn". A merge changes the board, so spawn. Good.

Hmm but actually, would real 2048 do this? I think so — in the game, tiles only merge when they collide during a move... actually in the real game, moving left with [2,2,0,0] on the left edge — I think it does merge. Hmm, let me reconsider. In Cirulli's code, moves are computed from the actual grid; the merge check is about adjacency in the traversal order, not about movement. In [2,2,0,0], the tiles are adjacent and equal, so moving left → merge. Yes, I'm fairly confident real 2048 merges in this case.

Either way, my implementation handles it consistently: if a merge occurs, the board changed.

assignPos: for each line index i and slot pos, compute row/col based on dr, dc:
- left (dc=-1): row=i, col=pos
- right (dc=1): row=i, col=3-pos
- up (dr=-1): row=pos, col=i
- down (dr=1): row=3-pos, col=i

For tracking moves, compare to old row/col before setting.

After computing all positions and merges: if not moved → return (no spawn). Otherwise set locked, update transforms, after 120ms: apply merges (remove the "source" tile's element, double the target's value, pop animation), spawn a new tile, check win/lose, unlock.

Score: += sum of new merged values.

Win: track a `won` flag; on merge if target value === 2048 and !won → won=true, show "You win!" overlay with Keep going / New Game. If keepGoing, no more win overlay.

Game over: after spawn, check if any move is possible (empty cells, or adjacent equal cells). If not, over=true, show overlay.

Spawn: random empty cell, value 2 with 90%, 4 with 10%. New tile appears with "new" pop-in animation (scale from 0).

Rendering: tile element div.tile.t{value} with position absolute, width/height 90px, set transform via translate(x,y). Use style.transform.

Color map for 2, 4, ..., 2048, and super.

Also prevent arrow scrolling: keydown on window, if arrow, e.preventDefault().

Also prevent holding a key triggering rapid moves? Locking during 120ms animation is sufficient; "prevents board state corruption from rapid input" — the locked flag handles this.

Let's write the file.

CSS: dark-ish 2048 style. Board container position relative, size 410px. Background grid cells: 16 divs positioned in a 4x4 grid (use CSS grid with gap inside an inset-10px container). Easier: #grid {position:absolute; inset:10px; display:grid; grid-template: repeat(4, 90px)/repeat(4,90px); gap:10px;} with 16 .cell.

Tile layer on top: #tiles absolute inset:0. Tile transform: translate(10 + col*100 px, 10 + row*100 px).

Score header: "2048" title, score box, New Game button.

Font size: small for large values.

Alright, let's write it.

Win overlay: div.overlay with message and buttons.

After writing, run in browser, use browser tool to type arrow keys, screenshot, check console.

The browser "key" action: can input keys like "ArrowLeft". Click "New Game" to test reset.

Let's write the code.

```js
const SIZE = 4, CELL = 90, GAP = 10, PAD = 10;
let tiles = [], nextId = 1, score = 0, locked = false, over = false, won = false, keepGoing = false;

function posToXY(r, c){ return [PAD + c*(CELL+GAP), PAD + r*(CELL+GAP)]; }

function makeTile(r, c, v, isNew){
  const el = document.createElement('div');
  el.className = 'tile t' + v;
  el.textContent = v;
  const [x,y] = posToXY(r,c);
  el.style.transform = `translate(${x}px, ${y}px)`;
  if (isNew) el.classList.add('new');
  tileLayer.appendChild(el);
  const t = {id: nextId++, value: v, row: r, col: c, el};
  return t;
}
```

Tile value class for colors: t2, t4, t8, t16, t32, t64, t128, t256, t512, t1024, t2048, tbig.

newGame: clear tileLayer, score=0, flags, tiles=[spawn, spawn].

move(dr,dc):
```js
if (locked || over) return;
const grid = ...;
let moved = false;
const merges = [];
for (let i=0;i<4;i++){
  const seq = [];
  for (let j=0;j<4;j++){
    let r,c;
    if (dc!==0){ r=i; c = dc===1 ? 3-j : j; }
    else { c=i; r = dr===1 ? 3-j : j; }
    if (grid[r][c]) seq.push(grid[r][c]);
  }
  let pos=0;
  for (let k=0;k<seq.length;k++){
    const t = seq[k];
    if (k+1<seq.length && seq[k+1].value===t.value){
      const n = seq[k+1];
      place(t,i,pos,dr,dc); place(n,i,pos,dr,dc);
      merges.push({survivor:t, removed:n, newValue:t.value*2});
      k++; moved=true;
    } else {
      place(t,i,pos,dr,dc);
    }
    pos++;
  }
}
if (!moved) return;
locked = true;
// update transforms
requestAnimationFrame / force reflow then set transform
```
Since tiles already have transform set at creation, changing t.row/col and then setting el.style.transform triggers CSS transition. But need to ensure the browser registered the old transform — since we're setting a new value it should be fine. To be safe, force reflow before setting? Tiles were rendered at the old position on a previous frame, so setting a new transform will animate. Should be fine.

Actually place() computes r,c and if different from t.row/col, sets moved=true, then updates.

Then:
```js
setTimeout(()=>{
  for (const m of merges){
    m.removed.el.remove();
    const idx = tiles.indexOf(m.removed); if (idx>=0) tiles.splice(idx,1);
    m.survivor.value = m.newValue;
    m.survivor.el.textContent = m.newValue;
    m.survivor.el.className = 'tile t'+m.newValue;
    m.survivor.el.classList.add('pop');
    score += m.newValue;
    if (m.newValue===2048) won = true;
  }
  updateScore();
  spawnTile();
  if (won && !keepGoing){ showOverlay('win'); }
  else if (!movesAvailable()){ over = true; showOverlay('lose'); }
  locked = false;
}, 130);
```
Wait: the pop class on survivor — set className to 'tile tN' then add 'pop' — but that removes the 'new' class which is fine. pop animation is a scale keyframe; add it after the transform transition has ended (130ms), which is fine.

spawnTile: randomly pick from empty cells.

movesAvailable: any grid cell null → true; any adjacent equal → true.

showOverlay('win'|'lose'): overlay div with title and buttons; win: "You win!" + Keep going (sets keepGoing=true, hides overlay) + New Game; lose: "Game Over" + New Game.

Score element.

keydown: arrow map; preventDefault on arrows. Also space? Not needed.

Now write the file and test in browser.

Wait, there's a bug: `place` sets `moved = true`, but `moved` is a local variable of `move`, and `place` is a top-level function, so does it end up creating a global? In JS, `moved = true` inside `place` (which is not in strict scope) — since the IIFE has 'use strict', this assignment creates a reference to an undeclared variable → in strict mode it throws a ReferenceError! Fix needed: either pass moved around, or have place return a bool value.

Also, in the pop animation's keyframes I'm using a var for --x/--y, but the element's transform is set inline to translate(...), and since the animation doesn't specify a from/to for transform, the base transform is used. The 50% frame has scale. But the element's transform transition also applies — no problem. However, the keyframe at 50% uses translate(var(--x),var(--y)) scale(1.18), which matches. OK.

For the appearance animation on new tiles: the transform scale(0.4) translate(...) in `from` — the order of scale and translate: scale is applied after translate (since the transform functions are applied right-to-left... actually, in the list `scale(0.4) translate(x)`, translate is applied first, then scale is applied to the coordinates → the position gets shrunk. Hmm, translate(x,y) is inside scale(0.4): the point is scaled after being moved → the final offset becomes 0.4x. That would make new tiles appear at the wrong position during the animation, then jump. Fix: translate(var(--x), var(--y)) scale(0.4).

Also, the appearance animation has a 120ms delay with backwards fill — it appears after the movement finishes, good.

Fixing the place function.

Wait: in the merge branch, place(t,...) can also change the position, and moved is set to true unconditionally — that's fine.

However, there's a subtle issue: in the merge branch, the tile that doesn't move but merges (e.g., moving [2,2,0,0] to the left) is still counted as a change to moved → it becomes a spawn. This matches the behavior of the standard 2048.

Alright, let's test in the browser.

Two tiles are shown in the DOM, but they're not visible in the screenshot? The tiles are probably behind... In the DOM the tiles show "2 2", but nothing is visible in the screenshot. Maybe the animation delay: the 'new' animation has a 120ms delay with backwards fill (opacity 0) — but the screenshot was taken at +1ms, so the tiles are still hidden. That's the cause. Let me wait and take another screenshot. Also, I'll use JS evaluation for testing... no, there's no eval tool. I'll verify using the key action and the DOM text from inspect.

Let's press ArrowLeft and then inspect.

(Thinking) The screenshot was taken during the transition (tiles in the middle of moving, and the DOM text at the time of capture is stale). Both 2s are in row 2, and moving left should merge them into a 4. Wait — but in the screenshot two "2"s are displayed next to each other in the center. The key press was executed, and the DOM shows "2\n2" — perhaps the DOM read happened before the 130ms timeout that applies the merge. Let me check again now.

merge is working: 2+2 → 4, score = 4, and one new tile (2) was spawned.

Next, test: press ArrowDown, ArrowUp, etc. After that, verify that the score resets and the number of tiles becomes 2 with New Game. Also test rapid input: pressing keys in quick succession shouldn't break anything (since it's locked during animation).

Press ArrowDown.

The DOM still shows "4 2" — the move animation is probably still in progress (the screenshot was taken at +0ms). Let me press another key and take a screenshot later. Actually, the tiles in the screenshot haven't changed from before (the 4 in row 3, column 1, and the 2 in row 3, column 3) — the DOM was captured at the same moment. Wait, but ArrowDown should move both down to row 4. The screenshot looks identical... maybe the key press happened but the frame was captured before the animation? The transform transition is 120ms, so the screenshot at +0ms should show the starting position. Let me check now.

ArrowDown moved 4 and 2 to the bottom row, and the new 2 was spawned in the top right. Sliding is working, and the score stays at 4 (no merge).

Next up is a rapid input test: press multiple keys in quick succession, and after that, click New Game to verify the reset.

The 4 and 2 in the bottom row slid right (in the middle of the animation). Let's send consecutive keys and verify that the board is consistent, then click "New Game".

Board state: 2 in the top row (col 2), 2 in the 3rd row (col 2), 4 and 2 in the bottom row (sliding left, 2 is behind the 4). No corruption, 4 tiles. Next, click New Game.

New Game works: score resets to 0, DOM shows exactly two tiles "2, 2" (in the +0ms frame, in the hidden state before the 120ms spawn animation kicks in). Take another screenshot to confirm the tiles are being displayed.

Confirmed: After the new game, two 2s are displayed, score is 0, and the board is clean. No errors in the console (only sandbox warnings from the harness itself).

Also a quick check: that arrow keys don't scroll the page — the page fits without scrolling, and I've confirmed that preventDefault is implemented; in any case it's not visually verifiable. The win/game-over overlays are unverified (can't reach 2048 in a reasonable time), and the rapid input lock is only partially verified (no breakage was observed through consecutive key presses; the lock is a code-level guard).

Done — reporting.