2048 · mimo-v2.6-pro

Let me build a solid 2048 game. I'll use absolutely positioned tiles with CSS transforms for smooth animation.

Design:
- 4x4 grid, board container with background cells.
- Tiles absolutely positioned, animated with transform translate.
- State: tiles array with id, value, row, col.
- Move logic: process lines, merge once per tile per move.

Implementation plan:
- Grid coordinate system: row 0 top, col 0 left.
- For each move direction, iterate lines in order toward the target edge.

Merge algorithm on a 1D array of tile refs (ordered from the edge they slide toward):
```
function moveLine(tiles) { // tiles = array of tile objects in order from edge
  const result = [];
  const merged = [];
  let score = 0;
  for (const t of tiles) {
    const last = result[result.length-1];
    if (last && last.value === t.value && !last.merged) {
      last.merged = true;
      last.value *= 2;
      score += last.value;
      merged.push({from: t, into: last});
      // tile t is removed (animated to last's position then removed)
    } else {
      result.push({...t, merged:false});
    }
  }
}
```

But careful: "merged once per tile per move" — the flag approach handles [2,2,2,2] -> [4,4]. Actually the standard: iterate from edge, if last result tile equals current and last wasn't already produced by merge this move, merge. That gives [4,4] for [2,2,2,2]. Yes.

Wait, but "merged once per tile" — with the flag on the merged result, [2,2,2,2] -> results: push 2; next 2 merges into it -> 4 (flagged); next 2: last is 4, flagged anyway, value differs; push 2; next 2 merges -> 4. Result [4,4]. Good.

Animation approach: keep DOM elements per tile. On move:
1. Compute new positions for each existing tile (which tile ids survive, which merge).
2. Set transform to new positions (CSS transition).
3. For merged tiles: both tiles animate to same cell; then after transition, remove the merged-away tiles and update the surviving tile's value with a pop animation; spawn new tile with appear animation.
4. Lock input during animation (e.g., 120ms), queue or ignore.

Simplest robust approach: perform state update immediately (so rapid input can't corrupt), but lock input for animation duration (~130ms). Actually "without allowing rapid input to corrupt board state" — locking is fine. I'll implement a lock that ignores keys during animation, plus optionally buffer one move. Simpler: ignore during lock.

Let me write the tile DOM approach:
- Each tile: div.tile with class tile-2 etc, inner text, style transform translate(x,y).
- Position via CSS variables: cell size computed in JS? Better: use percentages. Board padding + cell size in px via CSS custom props. Use `--cell: 105px` etc. But viewport resize... Use fixed-ish layout centered; fine for 1200x800 viewport. Let's make board 460x460 with 15px gap? Let's do: board size 480px, padding 12px, gap 12px, cell = (480 - 2*12 - 3*12)/4 = (480-24-36)/4 = 420/4 = 105px.

Tile positioning: transform: translate(calc(col * (cell+gap) + padding), ...) — I'll compute px in JS from constants. Simpler to just compute in JS: CELL=105, GAP=12, PAD=12. left/top computed = PAD + col*(CELL+GAP).

Use `transform: translate(Xpx, Ypx)` with transition on transform.

Merging DOM: for a merge, the two tiles both move to target cell. Then after transition end (timeout), remove the absorbed tile element, update the survivor's value and add `.merged` pop class.

Spawn: new tile appended with `.spawn` class (scale 0 -> 1 animation).

Game over check: no empty cells and no adjacent equal.

Win: reaching 2048 — show win message overlay with "Keep going" button to continue. Only trigger once.

Score display with +N float animation maybe — nice but optional. I'll add a small score bump.

Also support arrow keys plus maybe WASD/touch? Keep arrows required; add WASD and swipe optionally. Keep it simple but maybe add WASD—no harm. I'll add on-screen buttons? Not required. Keep arrows + WASD maybe. I'll include touch swipe lightly? Skip to save time; maybe include simple touch handlers quickly — it's cheap. I'll skip to reduce risk, actually include swipe since it's like 15 lines. Fine, include.

Testing: I'll use browser tools to run and inspect. Also I can write a quick node test of the merge logic extracted? The logic is inline in HTML. I could test via browser console eval... The browser tool has inspect/console. Let me just test with keyboard moves in the browser and inspect DOM.

Let me write the file.

Key structure:

```js
const SIZE = 4;
let tiles = []; // {id, value, r, c, el}
let score = 0;
let won = false, over = false, keepPlaying = false;
let locked = false;
```

Move:
```js
function move(dir) {
  if (locked || over || (won && !keepPlaying)) return;
  // build lines
  const vectors = {left:{dr:0,dc:-1}, right:..., up:..., down:...};
  // For each line, get tiles ordered from the edge
  ...
}
```

Line construction: For 'left': for each row r, tiles in that row sorted by col ascending (edge is col 0). For 'right': col descending. 'up': each col, row ascending. 'down': row descending.

Then apply merge algorithm producing newCol/newRow positions.

Implementation:

```js
function move(dir) {
  if (locked || gameOver || (won && !keepPlaying)) return;
  const order = [];
  if (dir === 'left') for (let r=0;r<4;r++) order.push(tiles.filter(t=>t.r===r).sort((a,b)=>a.c-b.c));
  ...
```

Then for each line:
```js
  let moved = false, gained = 0;
  const removals = [];
  for (const line of lines) {
    const placed = []; // {tile, merged}
    for (const t of line) {
      const last = placed[placed.length-1];
      if (last && last.tile.value === t.value && !last.mergedThis) {
        last.mergedThis = true;
        last.tile.value *= 2;  // mutate survivor value immediately
        gained += last.tile.value;
        // absorbed tile t: mark for removal, but keep its DOM to animate
        t.absorbedInto = last.tile;
        removals.push(t);
        moved = true;
      } else {
        placed.push({tile: t, mergedThis: false});
      }
    }
    // assign positions
    placed.forEach((p, i) => {
      const newR = ..., newC = ...;
      if (p.tile.r !== newR || p.tile.c !== newC) moved = true;
      p.tile.r = newR; p.tile.c = newC;
    });
  }
```

Careful: position assignment depends on dir and line index. I'll build lines with a function that also returns the target coordinates given index i.

For left: line = row r, target (r, i). right: (r, 3-i). up: (i, c). down: (3-i, c).

For absorbed tiles, set their r/c to the survivor's target so they animate there.

Note the survivor value mutation happens before rendering — but we render after computing all; for merge animation, we want the survivor DOM to keep showing old value until the merge completes, then flip to new value. So store `survivor.pendingValue = new value` and don't update DOM text until after animation.

Then render:
- For each surviving tile: set transform to new pos (transition animates).
- For each absorbed tile: set transform to survivor's pos; after ANIM ms remove element.
- After ANIM ms: update survivor text/value class, add 'pop' class, spawn new tile, unlock, check game state.

But state (tiles array, score) is updated immediately — good for correctness.

Actually the tile value in the model is already doubled; DOM text lags during animation only. Fine.

Edge: if a merge happens but tile didn't move (e.g., [2,2] left at col 0,1 -> [4,0]: survivor stays at col0, absorbed moves from col1 to col0). moved = true due to absorbed moving? I set moved=true on merge always. Good — a merge is a change.

Wait: is a merge always a board change? Yes.

Now spawn: choose random empty cell.

Game over check after spawn: if no empty and no adjacent equals -> gameOver.

Win check: after merge, if any tile value >= 2048 and !won -> won = true, show overlay.

Rendering tiles: I keep DOM elements persistent per tile id, updated in render.

Let me write the code now.

CSS colors: classic 2048 palette.

Overlay for win: absolute over board with message + "Keep going" button. Game over overlay with "Try again".

Also "New Game" button.

Score bump animation: add class then remove.

Prevent scrolling: `e.preventDefault()` on arrow keys, and CSS `overflow: hidden`? Page should fit. I'll set body overflow hidden maybe not needed but ok to keep page short. Use `e.preventDefault()` for arrows always.

Input lock: locked=true during animation; ignore further keys. Optionally queue last key. I'll just ignore (safe).

Hmm, "Animate movement and merging without allowing rapid input to corrupt board state" — ignoring is acceptable. But maybe buffering one input makes it feel better. I'll buffer one move and apply after animation finishes — but careful with correctness: buffered move applies to updated state. That's fine and can't corrupt. I'll implement `pendingDir` — if locked, store latest key (overwrite). After animation completes, if pendingDir and game active, execute it.

Let me write it.

For tests in the browser, I want deterministic control. Maybe expose `window.__game` with state for inspection. I'll expose a small debug API: `window.game2048 = { move, newGame, getState, setBoard(...) }` — helpful for verification of merge logic via console. Yes, add `setBoard(rows)` to set up test boards. That makes verification easy.

Now write the file.

Rendering details:

```js
function renderAll() {
  // tiles elements created on demand
}
```

I'll do: `ensureEl(t)` creates the div and appends to board layer; `positionEl(t)` sets transform.

Full render function `syncTiles()` iterates tiles, ensures elements exist, positions and sets value classes.

For move animation, I want old values displayed during slide. So syncTiles would set the doubled value text... Hmm. Let's separate: during a move I only call `positionEl` for movement and handle text update after timeout. So don't call full sync during move; call it after (spawn etc.).

Plan for move():
1. compute changes
2. if !moved: return (maybe shake? no)
3. score += gained; update score display
4. for each tile in tiles (survivors + absorbed): positionEl(t)
5. locked = true
6. setTimeout(ANIM_TIME):
   - remove absorbed elements from DOM and from tiles array
   - for merged survivors: update text/class, add pop class
   - spawn tile (add to tiles, create element with spawn class)
   - update score display with bump
   - check win / game over, update status
   - locked = false; if pendingDir, execute

Wait — but absorbed tiles must be removed from `tiles` array for the state to be right... but the state is used for game-over checks etc. If I remove them after the timeout, the intermediate state has duplicates (two tiles same cell with same value... actually absorbed has old value equal to survivor's old value; survivor model value already doubled). During the animation the model has both. If a buffered move executes after cleanup, fine. Game over check happens after cleanup. OK.

Actually simpler: remove absorbed from `tiles` array immediately (state correct), keep their elements in DOM with a class `.dying`, remove elements after timeout. That's cleaner. Do that.

Let's write.

Value colors:
2: #eee4da, 4: #ede0c8, 8: #f2b179, 16: #f59563, 32: #f67c5f, 64: #f65e3b, 128: #edcf72, 256: #edcc61, 512: #edc850, 1024: #edc53f, 2048: #edc22e, 4096: #3c3a32...

Text colors: dark for 2/4, light otherwise.

Tile font sizes: shrink for 4-5 digits.

Now the code. Let me write it out carefully.

```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>2048</title>
<style>...</style>
</head>
<body>
<main class="wrap">
  <header>
    <h1>2048</h1>
    <div class="scores">
      <div class="score-box" id="score-box"><span class="label">Score</span><span id="score">0</span></div>
      <div class="score-box"><span class="label">Best</span><span id="best">0</span></div>
    </div>
  </header>
  <div class="sub">
    <p>Join the tiles, get to <strong>2048!</strong> Use <strong>arrow keys</strong>.</p>
    <button id="new-game">New Game</button>
  </div>
  <div class="board" id="board">
    <div class="grid-bg" id="grid-bg"></div>
    <div class="tile-layer" id="tile-layer"></div>
    <div class="overlay" id="overlay" hidden>
      <div class="overlay-inner">
        <p id="overlay-msg"></p>
        <div class="overlay-btns">
          <button id="overlay-primary"></button>
          <button id="overlay-secondary" hidden></button>
        </div>
      </div>
    </div>
  </div>
  <p class="hint">Arrow keys / WASD to move. Tiles merge once per move.</p>
</main>
<script>...</script>
</body>
</html>
```

Overlay: win → "You win!" with Keep going + New Game; game over → "Game Over" with Try again.

I'll implement overlay via function showOverlay(kind).

Now JS details for grid bg: 16 cell divs created in JS or just static via JS loop.

Time constants: ANIM = 110ms.

Write the file now.

One more consideration: `won` overlay shows only first time reaching 2048; continue sets keepPlaying = true.

Also best score in localStorage — sandboxed may block localStorage? Usually works but could throw in some sandboxes. Wrap in try/catch.

Now writing.

For the debug/testing API:

```js
window.game2048 = {
  move: (d)=>move(d),
  newGame,
  getState: () => ({score, tiles: tiles.map(t=>({v:t.value,r:t.r,c:t.c})), over: gameOver, won}),
  setBoard(rows, opts) {...}
};
```

setBoard: rows 4x4 array of numbers; clears DOM, creates tiles; sets score to opts.score||0.

This helps me test merges precisely.

Now write.

JS careful coding:

```js
const SIZE = 4;
const CELL = 105, GAP = 12, PAD = 12;
const ANIM = 110;
```

Board CSS width = PAD*2 + SIZE*CELL + (SIZE-1)*GAP = 24 + 420 + 36 = 480.

position: x = PAD + c*(CELL+GAP), y = PAD + r*(CELL+GAP).

Tile element style: width/height = CELL px, transform translate(x,y).

OK.

Handle resize? Board fixed at 480px, fine for 1200x800. Maybe scale down with transform for small screens — skip.

Write it.

Move function in detail:

```js
function buildLines(dir) {
  const lines = [];
  for (let i = 0; i < SIZE; i++) {
    const line = [];
    for (let j = 0; j < SIZE; j++) {
      let r, c;
      if (dir === 'left') { r = i; c = j; }
      else if (dir === 'right') { r = i; c = SIZE - 1 - j; }
      else if (dir === 'up') { r = j; c = i; }
      else { r = SIZE - 1 - j; c = i; }
      const t = tiles.find(t => t.r === r && t.c === c);
      if (t) line.push({ tile: t, r, c, index: j });
    }
    lines.push(line);
  }
  return lines;
}
```

Wait for left, the target position for the i-th placed tile is (i_row, j_index). In general target = position of slot j in the line as constructed: for left slot j has (r=i, c=j) — the slot coordinates. So I can just use the slot's (r,c) as target. Good: store slot r,c and use it as target.

Then:

```js
function move(dir) {
  if (locked || gameOver || (won && !keepPlaying)) return false;
  const lines = buildLines(dir);
  let moved = false, gained = 0;
  const absorbed = [];
  const mergedPairs = []; // {survivor, newValue}
  for (const line of lines) {
    const placed = [];
    for (const item of line) {
      const t = item.tile;
      const last = placed[placed.length - 1];
      if (last && !last.merged && last.tile.value === t.value) {
        last.merged = true;
        last.tile.value *= 2;
        gained += last.tile.value;
        t.dying = true;
        t.absorbTarget = last.tile;
        absorbed.push(t);
        moved = true;
      } else {
        placed.push({ tile: t, merged: false, slot: item });
      }
    }
    placed.forEach((p, i) => {
      const target = ... // slot i of line? 
    });
  }
}
```

Hmm careful: placed index i corresponds to slot index i? placed items are a subsequence of line items; since we place them left to right, placed[i] occupies slot i. Yes because line items are ordered from edge and each placed item takes the next free slot.

But I need slot coordinates for slot index i = line[i]'s (r,c). Since line[i] exists for i < placed.length (placed.length <= line.length and placed[i] came from line item index >= i). Actually placed[i] is from line item at index >= i, so line[i] exists. Use line[i]'s r,c as target. Good — simpler: target = line[i] coordinates.

Hmm, careful: placed[i] always comes from line item index >= i, so line[i] exists. Use line[i]'s r,c as target. Good.

Then:
```js
    placed.forEach((p, i) => {
      const tr = line[i].r, tc = line[i].c;
      if (p.tile.r !== tr || p.tile.c !== tc) moved = true;
      p.tile.r = tr; p.tile.c = tc;
    });
    // absorbed tiles move to survivor's position
    for (const a of absorbed) { ... } // handled globally after
```

Set absorbed tiles' r/c to their target's r/c after loop, globally:
```js
  for (const a of absorbed) { a.r = a.absorbTarget.r; a.c = a.absorbTarget.c; }
```

Then if (!moved) return false;

Then remove absorbed from `tiles` array: `tiles = tiles.filter(t => !t.dying);` but keep their DOM elements (mark class 'dying').

Then render positions for all tiles still in `tiles` plus dying elements.

Then animation completion.

Score update: score += gained.

Let me write `updateScore()`.

Ok writing now. Also handle: keydown handler maps keys; store pendingDir when locked.

Game over check:

```js
function hasMoves() {
  if (tiles.length < SIZE*SIZE) return true;
  const grid = gridValues();
  for r,c: check right and down neighbors equal.
}
```

Spawn: random empty cell.

Now write out.

I'll now write the file.

Let me write the file.

Wait: there's a bug in setBoard: `if (opts.random) randFn = opts.random;` persists. Fine.

Another bug: in `spawnTile`, randFn is used. In newGame after setBoard with custom random, that persists — fine.

Bug: in `finishMove`, `result.merged` may include duplicates? Each survivor merges at most once per move (last.merged flag), so no duplicates.

Bug: dying tile elements — `t.el.classList.add("dying")` and later `tileLayerEl.querySelectorAll(".tile.dying")` removes all. Fine.

Potential issue: `pop-in` keyframes use `var(--pos)` which is set on the element — the `--pos` variable is used in animation transform. Good.

But merged pop animation: after setting className to "... merged", the animation runs with --pos scale — good.

Issue: `transition: transform 110ms` combined with animation transform on spawn — during spawn animation the transform is overridden by the animation; fine.

Another issue: the `.tile.merged` class stays on the element forever after the animation. Next move's className rewrite (`refreshTileLook`? not used) — actually in move() we only call setTransform, not className rewrite. So the "merged" class persists on surviving tiles, and if it merges again later, setting className again to the same string won't restart the animation (no reflow trick). Let me handle: in finishMove, set className without "merged", force reflow, then add "merged". Also for spawn: spawnTile adds "spawn" class to a fresh element — fine.

Also in move(), I should clear stale "spawn"/"merged" classes? Not necessary since animations are one-shot. But a persistent `merged` class on an element that then gets moved — the pop animation already finished, so no visual issue. But for repeat merges the animation needs restarting. Let me fix that in finishMove.

Also the win check: "allow continuing after a win" — done via Keep going.

Also note: `updateScore(0)` at setBoard doesn't remove the bump class... it only adds when gained>0. Fine.

Let me also double check the `move()` early return when `won && !keepPlaying`: after winning, the overlay appears and moves are blocked until "Keep going". That's standard 2048 behavior.

Now fix the merged animation restart, then test in browser.

Now test in browser. First run it, inspect console and screenshot.

Board renders with two tiles. But tiles appear tiny — the tiles are ~22px instead of 105px! Why? Because `.tile` width/height use `var(--cell)` = 105px... but the screenshot shows tiny tiles. Hmm, look: the tile at (421,217) is small. The grid cells are ~105px. So tiles are rendered small.

Wait — CSS `--cell: 105px` defined on `:root`. `.tile { width: var(--cell) }` should be 105px. Unless... the font-size 46px text "2" appears tiny too, suggesting the whole tile is scaled down? The text "2" appears small in the tile. Hmm, maybe the screenshot is scaled? No, the board is 480px wide shown at 480px.

Ah — I see: `animation: pop-in` sets `transform: var(--pos) scale(0) -> scale(1)`. The animation may still be running... no, it's done.

Wait, maybe `--pos` is not defined at the time the animation... no.

Actually maybe the issue: the `pop-in` keyframe ends at scale(1) but the animation is fill... after animation ends, transform returns to the element's style `transform: translate(...)` which is set. Fine.

Hmm, but the tiles look ~22x22 px with a "2" in it. That's scale(0.2)? Maybe `--cell` is not resolved in `.tile` because of something? Let me inspect computed styles via browser inspect.

Actually! I bet the issue: `transform: var(--pos) scale(0)` — during the animation the transform property is set. And the `transition: transform 110ms`... after animation completes, the element's transform goes back to inline style. That's fine.

Wait, maybe the tile IS 105px but with a huge `transform: scale`? No.

Let me just inspect the DOM to see computed width.

The second frame shows tiles at full size — the first screenshot just caught the spawn animation mid-way (scale(0)->scale(1) with 140ms). Fine.

Now test keyboard moves and merge logic. Use setBoard via browser? The browser tool doesn't allow console eval directly... Actions: reload, inspect, screenshot, motion, click, drag, key, type, scroll. No eval. Hmm. So I can't call game2048.setBoard from the tool. I can use key presses and check the DOM output.

To verify merges deterministically, I could temporarily create a test harness page... but the deliverable must end as the game. Alternative: run the game logic in node by extracting the script? I can write a small node test that replicates the move logic by loading index.html's script with a DOM stub... that's heavy.

Better: use the browser with key presses and observe. To get deterministic merges, I can use `setBoard` triggered by... hmm, `type` action types text into a focused element. Not helpful for eval.

Alternative approach: add a URL-hash-driven test? I could temporarily add a script that reads `location.hash` and sets up a board. But the sandbox iframe may not allow hash control.

Simplest: write a temporary node test file that pulls the move-logic out of index.html via regex and runs it with a minimal DOM stub. Actually simpler: implement a tiny DOM stub in node (document.createElement returns objects with style/classList/appendChild etc.) and evaluate the whole inline script. That's maybe 60 lines of stub. It gives me deterministic verification of [2,2,2,2] -> [4,4], [2,2,4,0] -> [4,4], spawn counts, score, game over.

Let me do that — quick and reliable. Then also do live keypress checks in the browser.

DOM stub needs:
- document.getElementById -> element stubs for the ids used
- document.createElement
- element: className (string, but classList used too), textContent, style (object with setProperty, transform), appendChild, removeChild, firstChild, querySelectorAll, addEventListener, setAttribute, classList {add, remove}, offsetWidth
- document.addEventListener
- window.setTimeout, window.localStorage, window.game2048 assignment.

`tileLayerEl.querySelectorAll(".tile.dying")` — I need a working querySelectorAll: I can implement it by filtering children by class. Let me write it.

Also `gridBgEl` buildGrid appends 16 cells.

Let me write test.js (temporary, delete after) in the cwd... The instructions say end with index.html present; extra temp files should be removed. I'll create test harness in /tmp outside the cwd to keep cwd clean.

Implementation of stub:

```js
class El {
  constructor(tag){ this.tagName=tag; this.children=[]; this.style=new Proxy? }
}
```

style: needs setProperty and arbitrary props. Use plain object with setProperty function.

classList: add/remove toggling a Set, but className is also set directly. Simplify: store classes in `el._classes` Set; define className getter/setter that syncs; classList add/remove operate on it.

querySelectorAll(sel): parse ".tile.dying" -> classes; return children (recursive) matching all classes.

firstChild: children[0] || null.

Let me write it.

Also the script uses `document.addEventListener`, `boardEl.addEventListener`, `document.getElementById("new-game").addEventListener`.

Run script with `vm` or just eval in a context where document/window defined. Use `new Function('document','window', code)`? The code is an IIFE referencing document and window globals. I'll wrap: `const fn = new Function('document','window','setTimeout', code)` — but code uses `window.setTimeout` and bare `document`. Both provided as params. Also `Math`, `localStorage` via window.localStorage. Good.

Extraction: read index.html, regex the <script>...</script> content.

Let me write it.

Run the test quickly.

Let me analyze the failures — several are likely test harness artifacts:

1. "exactly one tile spawned -> 3": board started with 4 tiles in row 0, after move left merges into 2 tiles in row 0 + spawn = 3 tiles total. My test expected 5 — test bug (I assumed 5 tiles from 4... no: 4 tiles merge to 2, +1 spawn = 3). Test bug. ✓ game correct.

2. "[2,0,2,2] left -> [4,2]": result grid shows row0 = "4 0 2 0" and a spawn at (3,3) = 4. So row 0 has 4 at c0 and 2 at c2. Expected [4,2] at cols 0,1. Hmm! That's a real bug? Let's think: [2,0,2,2] left. Line order left: tiles at c0=2, c2=2, c3=2. placed: push t(2@0). next t(2@c2): last value 2 == 2, not merged → merge into 4 at slot0, absorbed. next t(2@c3): last.merged true → push. placed = [t0(4), t3(2)], assign slots 0 and 1 → (0,0) and (0,1). So expected 4 at c0, 2 at c1. But output shows 2 at c2!

Hmm, that means my position assignment is wrong. `line[p]` for slot p... For 'left', line[j] has (r=i, c=j) so line[0]=(0,0), line[1]=(0,1), line[2]=(0,2). placed[0] → line[0] = (0,0); placed[1] → line[1] = (0,1). That looks right.

But wait — the test's check uses `r.state.tiles` after flush. After the merge animation, `finishMove` sets merged survivors... the tile at c2 = 2: that would be the spawned tile? The spawned tile is at (3,3) with value 4. Row 0 shows "4 0 2 0" — so there is a 2 at (0,2). Hmm.

Oh! I see the bug: in `move()`, I filter `tiles` to alive BEFORE `finishMove`, but `setTransform(tiles[i])` is called... fine. But the merge position assignment: the absorbed tile t at c2 gets `t.r/t.c` set to survivor's position... fine.

Hmm wait, maybe the issue is in the test harness flush order? `flush()` runs timers; move() schedules finishMove via window.setTimeout which pushes to timers. flush runs it. Then spawnTile happens in finishMove.

Let me actually debug: maybe the problem is `buildLines` finds tiles by scanning `tiles` array — fine.

Hmm, but the check in the test: `r.state.tiles.filter(t => t.r === 0)` — includes ALL row-0 tiles, sorted by c. Output "4 0 2 0" from grid() means a tile with value 2 at (0,2).

Wait... maybe the move is "left" but the grid shows 4 at (0,0) and 2 at (0,2)? Let me recheck test 3's board: `[[2, 0, 2, 2], ...]`. Hmm what if `setBoard` created tiles in order and `buildLines` matched... 

Actually hold on. Test 5 check "unchanged move: no spawn" passed and shows tiles preserved. Test 1 worked ([2,2,2,2] → [4,4]). Test 3 differs by a gap.

OH WAIT. I see it. In `move()`:

```js
var alive = [];
for (var i = 0; i < tiles.length; i++) {
  if (tiles[i].absorbInto) { ... } else { alive.push(tiles[i]); setTransform(tiles[i]); }
}
```

That's fine.

Hmm, but `applyMove`'s slot assignment loop:
```js
for (var p = 0; p < placed.length; p++) {
  var slot = line[p];
```
placed[p].tile gets line[p] coordinates. Correct.

Unless... `placed` merges mutate `last.tile.value` — that's the survivor.

Let me just debug directly with a tiny node run printing state after each step.

3. Test "down" failure: [2,2,4] in col 0 rows 0-2, move down → expected 4 at row2? Let's compute: line order for 'down': j=0 → r=3, j=1 → r=2, j=2 → r=1, j=3 → r=0. Tiles at r=0(2), r=1(2), r=2(4) → in line order: [t(4@r2), t(2@r1), t(2@r0)]. placed: push 4; next 2 != 4 → push 2; next 2 == 2 and last not merged → merge → 4 at slot1 = (r=2). So result: 4 at r3 (slot0), 4 at r2 (slot1). Test output: "4 at row1, 4 at row2, spawn 4 at (3,3)". So the tile that should be at r3 is at r1?? Output grid: "0 0 0 0 | 4 0 0 0 | 4 0 0 0 | 0 0 0 4" → 4 at (1,0) and (2,0), spawn at (3,3).

So slot assignment is off: it looks like the tiles got slots in the original order rather than compressed... Actually it looks like positions were assigned as: t(4) → r2? and merge result at ... hmm.

Result tiles at r1 and r2 in col 0. Original tiles: 4@r2, 2@r1, 2@r0. If NO position update happened at all, they'd be at r0,r1,r2 (three tiles). But we have two tiles at r1,r2: merged pair (2@r1 + 2@r0) → survivor at r1 (its original position, unchanged), and 4 at r2 (unchanged). So NO position movement happened at all for this case! And in test 3: 2@c0, 2@c2, 2@c3 → merge c2+c3? No... result 4@c0 and 2@c2: so t@c0 and t@c2 merged? No — values: 4@c0 (merged 2+2) and 2@c2.

Hmm in test 3: tiles at c0, c2, c3. If position assignment didn't happen: placed = [t@c0 (merged with t@c2 → 4)], and t@c3 pushed with... no wait placed[1] = t@c3 would stay at c3, but result shows 2 at c2.

Hmm confusing. Let me think about `move()` again... Actually! The bug might be in `move()`'s alive loop: `setTransform` — but also `finishMove` runs later and uses `result.merged`. But the state (r/c) is already set in applyMove. So state should be right regardless of rendering.

Unless... `applyMove` never assigns because `moved` was already true and... no.

Hmm, test 1 passed with correct positions ([4,4] at c0,c1). Test 2 passed ([4,4] at c0,c1). Test 4 right passed. Up passed. So position assignment works in some cases.

Test 3 difference: there's a GAP in the line. Let me re-look at the loop:

```js
for (var i = 0; i < line.length; i++) {
  var t = line[i].tile;
  ...
}
for (var p = 0; p < placed.length; p++) {
  var slot = line[p];
  if (placed[p].tile.r !== slot.r || placed[p].tile.c !== slot.c) moved = true;
  placed[p].tile.r = slot.r;
  placed[p].tile.c = slot.c;
}
```

Hmm wait — is `placed[p].tile` maybe the same object as `line[p].tile`? For test 3: line = [t0(c0), t1(c2), t2(c3)]. placed after loop: 
- i=0: placed=[{t0}]
- i=1: t1.value 2 == t0.value 2, last.merged false → merge: t0.value=4, t1.absorbInto=t0, absorbed=[t1], merged=[t0], placed=[{t0, merged:true}]
- i=2: t2.value 2, last = {t0, merged:true} → last.merged is true → push {t2}. placed=[{t0},{t2}].
- slots: placed[0] → line[0] = (0,0): t0 already there. placed[1] → line[1] = (0,2): t2 at c3 → moves to c2. 

So expected state: t0=4 at (0,0), t2=2 at (0,2). And that's EXACTLY what the test output shows: "4 0 2 0"! 

So my test expectation "[4,2]" at cols 0,1 was wrong! Wait no — the spec says moving [2, 2, 4, 0] left produces [4, 4, 0, 0]. For [2,0,2,2] left the correct 2048 result is [4,2,0,0] — tiles slide to the edge! The 2 at c3 should end at c1, not c2. So the bug IS real: gaps are not closed.

Why? Because `line` only contains tiles that exist (I skip empty slots), so `line[p]` isn't the p-th board slot — it's the p-th existing tile's slot. I need slot coordinates from the *position index in the line* j, not from `line[p]`.

Right! In buildLines I push only found tiles, so line entries' (r,c) are the tiles' original positions, not slot coordinates. I need to store the slot coordinate (r,c) computed from j at construction time for each of the SIZE slots.

Fix: build lines as arrays of SIZE slots each with slot r,c and tile (or null). Then in applyMove, iterate slots with tile != null.

Similarly the "down" test failure: line slots (from edge r3 upward): slot0=(3,0), slot1=(2,0), slot2=(1,0), slot3=(0,0). Tiles: 4@r2 → found at j=1; 2@r1 at j=2; 2@r0 at j=3. So line (existing only) = [{tile:4@(2,0)}, {tile:2@(1,0)}, {tile:2@(0,0)}]. placed: [4@r2, merged 2+2 at slot1 = line[1] = (2,0)... wait placed[0]=4 gets line[0]=(2,0) — wrong, should be (3,0). That explains the output (4 stayed at r2, merged 2 at r1... hmm merged tile is 2@r1 with value 4 staying at (2,0)? placed[1]=t2? no.

Anyway: fix buildLines to include slot coordinates.

Test failure 5: "full board with equal neighbours still has moves" FAIL: board [[2,4,8,16],[16,8,4,2],[2,4,8,16],[16,8,4,2]] — equal neighbours: (0,0)=2 and (1,0)=16? no. Adjacent equal pairs: row0: 2,4,8,16 none equal horizontally; rows similar. Column: (0,0)=2,(1,0)=16,(2,0)=2,(3,0)=16 — no equals. So actually there are NO equal adjacent tiles — my test board is wrong (it's a valid "stuck" board). Test bug. Let me use a board with an actual equal pair.

Failure 6: "stuck board -> game over overlay -> You win!": the stuck board [[2,4,8,16],[32,64,128,256],[512,1024,2,4],[8,16,32,64]] contains 1024... no 2048. But overlay msg says "You win!" — because previous test (test 7) set `won`... no, setBoard resets won. Hmm, `showOverlay("win")` triggered because maxTile >= 2048? No. Actually in test 8's stuck board, the "move left" on a full stuck board returns false (no change), so no finishMove... then `s.over` false and overlay msg is leftover "You win!" from test 7. Wait but the check "stuck board has no moves" passed. The move('left') returns false → no game over detection! That's the actual gap: game over should be detected even when the player presses a key that doesn't change the board. In real play, game over is detected after a move + spawn; but on a stuck board reached without any further change... Actually in real gameplay the board becomes stuck exactly after a move+spawn, and finishMove's hasMoves check catches it. But if somehow it isn't caught (e.g., setBoard), it stays. Safer: in `move()`, when the move doesn't change the board, also check hasMoves() and show game over. Let me add that: after `if (!result.moved)`, check `if (!hasMoves()) { gameOver = true; showOverlay('over'); }`. That's more robust and matches "Game Over only when no valid move remains".

Failure 7: spawn distribution NaN: counts 0 — because in that test I used a full board with one empty at (3,3) and move "left" on [[2,4,8,16],[32,64,128,256],[512,1024,2,4],[8,16,32,0]] → moving left changes the board (row3: [8,16,32,0] → unchanged! left already packed). Row 0-2 unchanged too. So no move → no spawn. Test bug: I should move right so row3's 32 moves... Actually to force a spawn at a known cell, simpler: count spawned tiles in general. Let me just fix the test: use board where moving left changes something and count the tile values that are new. Simpler: after each iteration, compare before/after tiles; count the new tile's value. Let me restructure: setBoard with board [[2,0,0,0],...] and move left → spawn. Then find the tile that wasn't there before (by position). Simplest: record tiles before spawn... Actually `getState` before move and after flush; the new tile is the one at a cell not occupied before. Let me just find tiles whose (r,c) was empty before and value is 2 or 4.

Let me fix:
1. Real bug: buildLines slot coordinates (gap closing).
2. Real improvement: detect game over on an unchanged move.
3. Fix test bugs.

Then re-run tests and do a quick browser check. Time is short — be efficient.

Implementation fix for buildLines: return lines where each entry = {r, c, tile} for every slot (tile may be undefined). Then applyMove iterates slots, skips null tiles, and assigns placed[p] to line[p]... careful: placed[p] should map to the p-th *occupied* slot? No — placed[i] occupies slot index i (from the edge), and slot i is line[i] regardless of whether it had a tile. Since placed.length <= number of occupied slots <= SIZE, line[p] exists for p < placed.length. Good — that's what I have, but line must include ALL slots.

Let me rewrite buildLines to include empty slots with tile: null, and in applyMove skip null entries when building placed.

Now fix the test harness bugs: test 1 expectation (3 tiles total), test 3 now valid, test 5 board needs a real equal pair, test 6 expects game over after move attempt, test 7 spawn counting.

Test 1: "exactly one tile spawned" — 4 tiles → 2 merged + spawn = 3. Change check to `r.state.tiles.length === 3` and name "exactly one tile spawned after the move".

Test 5 board: [[2,4,8,16],[16,8,4,2],[2,4,8,16],[16,8,4,2]] — replace with one having equal neighbours: [[2,4,8,16],[16,8,4,2],[2,4,8,16],[16,8,2,2]] → (3,2)=2,(3,3)=2 equal → has moves.

Test 6: after g.move("left") on stuck board, expect over true with "Game Over".

Test 7: rewrite spawn counting: board with one empty cell and a change... simpler: setBoard rows [[2,4,8,16],[32,64,128,256],[512,1024,2,4],[8,16,0,0]] and move "right" → row3 becomes [0,0,8,16] → changes, spawns at one of (3,0)/(3,1). Count new tile: find tiles whose cell was empty before.

Let me just edit the test file.

Also test 3's expectation now: [2,0,2,2] left → tiles at c0=4, c1=2 — with the fix. And test "stuck board -> game over overlay" now passes with the unchanged-move game-over detection. Run.

All 33 pass. Now a quick live browser check: keyboard move adds a tile, New Game resets. Time is short.

Let me do: reload, press ArrowLeft a few times, screenshot/inspect DOM to see tile count, score; click New Game and check 2 tiles + score 0.

Note the DOM output shows tile text values which is enough.

Initial: two tiles (2 and 4), score 0. Now press arrow keys and check a new tile spawns and merges update score.

Hmm — the DOM still shows "2 4" and score 0, but the screenshot shows a big 4 at (0,1) and a 2 at (2,3)... The 4 is mid-animation? Actually the 4 appears large at position (0,1) — it was at (0,1) already. Wait initial screenshot showed 4 at (0,1) small (mid spawn animation) and 2 at (2,3). After ArrowLeft: 4 should move to (0,0) and 2 to (2,0), and a new tile spawn. But the screenshot shows 4 at (0,1) and 2 at (2,3) unchanged, and DOM unchanged.

Maybe the key event didn't reach the document? The key action sends a key to the page — maybe focus is on the iframe body. Hmm, the dom output is identical. Possibly the move didn't happen because... `move("left")`: 4 at (0,1) → should move to (0,0). moved=true. Unless keys aren't delivered.

Wait — the screenshot shows the tiles at full size now (post animation), still at original positions. So no move happened. Key press likely wasn't dispatched to the document listener, or e.key differs.

Let me check: browser key action with text "ArrowLeft" — maybe it needs the key name differently. Let me try clicking on the board first to ensure focus, then press. Or check console for errors.

Alternatively the page might be inside a sandboxed iframe and key events go to the iframe — should be fine.

Let me try pressing again and inspecting; maybe the first frame captured was before processing. Let me press ArrowLeft and then inspect in a separate call.

Interesting — the ArrowDown frame caught mid-animation (tiles sliding, 2 tiles merging shown as overlapping2s and a "+4" score bump). Score is now 4, best 4. So the previous ArrowLeft DID work (it moved 4 and 2 left and spawned a 2), and this ArrowDown merged... wait score 4 means a merge of 2+2=4 happened. The screenshot shows animation in progress.

DOM now: "2 4 2" — three tiles with score 4. Hmm, if a merge happened we'd expect fewer tiles. Actually the DOM listing may be caught mid-animation (dying tiles still in DOM).

Wait — DOM text lists "2 4 2" and score 4. Let me inspect again now that animation should be complete.

Now inspect — but note the dom snapshot from a new call. Also the ArrowLeft first press: the DOM showed unchanged... maybe the frame was captured before the state updated (0ms). Actually the screenshot after ArrowLeft showed tiles at same positions — but then ArrowDown produced score 4 with a merge, meaning ArrowLeft had spawned a 2 (a merge of 2+2 = 4 happened on ArrowDown). Wait, initial board had a 2 and a 4. After ArrowLeft: 2 and 4 both to col 0, spawn one tile (2 or 4). Then ArrowDown merges if two2s... The dom showed "2 4 2" — three tiles: 4, 2, 2. ArrowDown would merge the two 2s → 4. Score 4 ✓.

So the ArrowLeft did work; the first screenshot just rendered pre-move state (frame captured at 0ms before animation). Fine.

Now inspect the current state.

Hmm — the DOM says "4 4 2" but the screenshot shows 2 at (0,0), 4 at (2,0), 4 at (3,0). Mismatch in ordering, but consistent values (2,4,4). Score 4, best 4.

Wait, DOM lists "4 4 2" but visually 2 at top... DOM order is the order of tile elements in the layer (creation order), not board order. Fine.

But wait: the screenshot shows 2 at (0,0) and 4 at (2,0),(3,0). But earlier state was tiles at col 0: 4 at (0,0)? and 2 at (1,0), 2 at (2,0) after left... then ArrowDown: 2s at rows 1,2 merge → 4 at row 3? Hmm result shows 4 at rows 2 and 3 and 2 at row 0. That means the 4 was at row... Let's reconstruct: after ArrowLeft: 4@(0,0), 2@(0,?)... whatever, the merge produced score +4 correctly and tiles are packed at the bottom of col 0 minus spawn at (0,0). Spawn at (0,0) = 2. So col 0: 2 at row0 (spawn), 4 at row2, 4 at row3. But wait, if two 2s merged into 4 at row3 and the original 4 slid to row2 — yes that's consistent with ArrowDown merging2+2=4 and one spawn. 

Now test New Game button click.

New Game: score 0, best 4 preserved, exactly two tiles. 

Now verify page doesn't scroll on arrow keys — the page fits in 800px height? Total ~660px, so no scrolling possible anyway. preventDefault is in the handler.

Let me also quickly verify rapid key input doesn't corrupt (press a few keys fast) and check a final screenshot. And verify the file is valid and self-contained (no external URLs).

Quick: grep for http/cdn in index.html.

One more live check: rapid input (spam keys) then inspect for consistency. Let me do3 quick key presses in a row and inspect.

ArrowUp after New Game: the two tiles moved up? They were at (0,2) and (1,1) — already top rows? The 4 was at (1,1), stayed. The 2 at (0,2) stayed. Unchanged move → no spawn. Score still 0, tiles 2. Correct (no spawn after unchanged move).

Hmm, but wait — DOM shows "4 2" and tiles unchanged: consistent.

Now spam several keys quickly for corruption check.