2048 · gemma-4-26b-a4b-it

The objective is to build a playable 2048 game in a single `index.html` file.

    *   4x4 grid.
    *   Tiles merge adjacent equal tiles (one merge per tile per move).
    *   Score increases by the value of the new merged tile.
    *   Example: `[2, 2, 2, 2]` left -> `[4, 4, 0, 0]`.
    *   Spawning: Start with 2 tiles. After a move that changes the board, spawn one tile in an empty cell (90% chance for 2, 10% chance for 4).
    *   Input: Arrow keys (prevent scrolling).
    *   Animations: Smooth movement and merging.
    *   Win/Loss: Win at 2048, Game Over when no moves left.
    *   UI: Score, Win message, New Game button.
    *   No external assets/CDNs.
    *   Self-contained `index.html`.

    *   HTML: Container for score, board, win message, and new game button.
    *   CSS: Grid layout for the 4x4 board, styling for tiles (different colors for different values), and animations (transitions for movement).
    *   JavaScript:
        *   State management: Board (2D array or flat array), current score, game status (playing, won, game over).
        *   Input handling: `keydown` event listener for arrow keys.
        *   Game logic:
            *   `move(direction)` function.
            *   `slideAndMerge(row)` function.
            *   `spawnTile()` function.
            *   `isGameOver()` function.
            *   `checkWin()` function.
        *   Rendering: Update the DOM based on the game state. Use CSS transitions or requestAnimationFrame for smooth movement. Actually, using absolute positioning with `translate` might be easier for smooth animations of tiles moving between cells.

    *   Represent tiles as objects with `value` and `position` (x, y).
    *   Grid: 4x4.
    *   Score: Integer.
    *   Moves: Arrow keys (Up, Down, Left, Right).

    *   Use a `transform: translate(x, y)` approach for smooth movement.
    *   Each tile will have a DOM element.
    *   The board container will be the reference point.
    *   When tiles move, update their `transform` property.

    *   `board`: 4x4 array.
    *   `score`: Total score.
    *   `cells`: Grid cells (visual only).
    *   `tiles`: Array of active tiles with their properties.

    1.  Create an empty grid (4x4).
    2.  Initialize with two tiles (2s).
    3.  Listen for arrow keys.
    4.  On key down:
        a.  Capture current state for potential comparison (to see if move changed anything).
        b.  Calculate new state by sliding and merging in the chosen direction.
        c.  If state changed:
            i.  Update board.
            ii. Update score.
            iii. Spawn new tile (2 or 4).
            iv. Update UI/animations.
            v.  Check for Win/Loss.
    5.  `isGameOver`: Check if any adjacent tiles are equal or if there is an empty cell.

    *   `[2, 2, 2, 2]` -> `[4, 4, 0, 0]` is standard 2048.
    *   The rule "Adjacent equal tiles merge once per tile per move" means in a row `[2, 2, 4, 4]`, a left move should result in `[4, 8, 0, 0]`? Wait, let's re-read.
    *   "Adjacent equal tiles merge once per tile per move".
    *   Example: `[2, 2, 2, 2]` -> `[4, 4, 0, 0]`.
    *   Example: `[2, 2, 4, 0]` -> `[4, 4, 0, 0]`.
    *   Standard 2048 logic:
        *   For each row/column:
        *   Filter out zeros.
        *   Iterate through the non-zero elements and merge adjacent equals.
        *   Fill remaining with zeros.

    *   Board container: `position: relative`.
    *   Cells: `position: absolute` (or grid/flex).
    *   Tiles: `position: absolute`, with `transition: transform 0.1s, opacity 0.1s`.

    *   HTML Structure.
    *   CSS (Grid, Tiles, Animations).
    *   JS (Core logic, Rendering).

    *   `tiles`: array of objects `{ value, row, col, id }`.
    *   `score`: `number`.
    *   `gameOver`: `boolean`.
    *   `won`: `boolean`.

    *   `move(direction)`:
        *   Calculate target positions for each tile.
        *   Identify merges.
        *   Update tiles array.
        *   Update score.
        *   If changed, spawn new tile.

    *   Instead of re-creating all DOM elements, I should have a pool or just manage tiles carefully.
    *   Actually, for a simple 2048, it's easier to:
        1.  Update the underlying 2D grid (values and "merged" status).
        2.  Translate the visual representation to match the grid.
        3.  For smooth movement, it's better to track the actual DOM elements.

    Wait, let's refine the "Smooth Movement" part.
    To animate smoothly:
    1.  Each tile has a unique ID.
    2.  When a move occurs:
        a.  Determine where each existing tile ends up.
        b.  Some tiles might merge (one tile stays, the other disappears or changes value).
        c.  Some tiles might be newly spawned.
    3.  Actually, the easiest way to handle smooth animations in a simple way is:
        -   Have a fixed number of DOM elements (16 for the grid, plus some for tiles).
        -   Wait, that's not quite right for 2048.
        -   Better: Keep a set of DOM elements for tiles.
        -   `render()` function:
            1.  Look at current grid.
            2.  Identify tiles that need to move, merge, or appear/disappear.
            3.  For tiles that exist in old grid and new grid: `transform: translate(...)`.
            4.  For tiles that exist in old grid but not new: `opacity: 0`.
            5.  For tiles that exist in new grid but not old: `opacity: 1`.

    *   Use `requestAnimationFrame` or just rely on CSS transitions.
    *   Since I need to implement it in one file, I'll keep it simple.

    *   Board: 4x4 grid.
    *   Tiles: 16 possible.
    *   States:
        -   `board`: `[ [null, null,...],... ]` where each entry is a tile object or `null`.
        -   `score`.
        -   `win`: `boolean`.
        -   `over`: `boolean`.

    *   `addTile()`:
        -   Find empty cells.
        -   If empty cells, pick one.
        -   Value = 2 (90%) or 4 (10%).

    *   `move(direction)`:
        -   Get all tiles.
        -   Sort them by position along the direction of movement.
        -   For each row/col (depending on direction):
            -   `compact(line)`:
                -   `new_line = line.filter(t => t!= null)`
                -   `merged_line = []`
                -   `for i = 0 to new_line.length - 1`:
                    -   `if new_line[i].value == new_line[i+1].value and!new_line[i].merged`:
                        -   `new_line[i].value *= 2`
                        -   `score += new_line[i].value`
                        -   `new_line[i].merged = true`
                        -   `push new_line[i]`
                        -   `i++`
                    -   `else: push new_line[i]`
                -   `return merged_line`

    Actually, the "merge once per tile per move" rule is important. The standard way is to process the line by collapsing and then merging.

    Wait, a simpler `move` logic:
    1.  Get all tiles in the current row/column.
    2.  Remove empty cells.
    3.  Merge adjacent identical tiles.
    4.  Re-fill with empty cells.
    5.  Check if anything changed.

    Example: `[2, 2, 2, 2]` -> `[4, 4]` -> `[4, 4, 0, 0]`.
    Example: `[2, 2, 4, 4]` -> `[4, 8, 0, 0]` NO.
    Wait, `[2, 2, 4, 4]` -> `[4, 8]` is WRONG for 2048.
    It should be `[2, 2, 4, 4]` -> `[4, 8]` is NOT what happens.
    In 2048: `[2, 2, 4, 4]` -> `[4, 8]`.
    Wait, let's re-read: "Adjacent equal tiles merge once per tile per move".
    `[2, 2, 4, 4]` -> the first `2, 2` merge to `4`. The `4, 4` merge to `8`.
    Actually, standard 2048: `[2, 2, 4, 4]` left -> `[4, 8, 0, 0]`.
    Wait, `[2, 2, 2, 2]` left -> `[4, 4, 0, 0]`.
    Let's check standard 2048:
    Row: `[2, 2, 2, 2]` -> `[4, 4]` -> `[4, 4, 0, 0]`. Correct.
    Row: `[2, 2, 4, 4]` -> `[4, 8]` -> `[4, 8, 0, 0]`. Correct.
    Row: `[4, 2, 2, 0]` -> `[4, 4, 0, 0]`. Correct.

    Wait, the prompt says:
    - `[2, 2, 2, 2]` left produces `[4, 4, 0, 0]`. (This is standard)
    - `[2, 2, 4, 0]` left produces `[4, 4, 0, 0]`. (Wait, `2+2=4`, then `4, 4`? No, `[2, 2, 4, 0]` -> `[4, 4, 0, 0]`. Yes.)

    Implementation detail:
    For a given row/column:
    1.  Extract non-zero values: `vals = [2, 2, 4]`.
    2.  Create new row:
        `new_vals = []`
        `i = 0`
        `while i < vals.length`:
           `if i < vals.length - 1 and vals[i] == vals[i+1]`:
               `new_vals.push(vals[i] * 2)`
               `score += vals[i] * 2`
               `i += 2`
           `else`:
               `new_vals.push(vals[i])`
               `i += 1`
    This logic handles `[2, 2, 2, 2]` -> `[4, 4]`.

    *   HTML structure: `div#game-container` -> `div#score-container`, `div#board`, `div#message-container`.
    *   CSS:
        -   `.tile`: `position: absolute`, `width: 100px`, `height: 100px`, `transition: transform 0.1s`.
        -   Colors for 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048.
    *   JS:
        -   `class Tile`: `id`, `value`, `row`, `col`.
        -   `class Game`: `grid` (4x4), `score`, `tiles` (Map: id -> Tile), `nextId`, `gameOver`.
        -   `updateDOM()`:
            -   Clear tile elements? No, that's not efficient.
            -   Better:
                -   `tilesMap`: `Map<id, DOMElement>`.
                -   For each tile in current state:
                    -   If it's new, create DOM element and append to board.
                    -   If it's existing, update `transform` and `textContent`.
                    -   If it's gone (merged or moved out), remove from DOM.

    Actually, for simplicity and robustness in a single file:
    -   Maintain a `currentTiles` set of objects `{id, value, row, col}`.
    -   Maintain a `previousTiles` set.
    -   On each move:
        1.  `previousTiles = copy(currentTiles)`.
        2.  Perform move logic.
        3.  Update `currentTiles`.
        4.  Render:
            -   For each tile in `currentTiles`:
                -   If `id` is not in `previousTiles`, create element.
                -   Update `transform` using `row` and `col`.
                -   Update `textContent` and `className`.
            -   For each tile in `previousTiles` but not in `currentTiles`, remove element.

    *   Prevent default for arrow keys: `event.preventDefault()`.
    *   `isGameOver`: Check if any non-empty cells are adjacent and equal OR if any cells are empty.

    *   `#game-board`: `position: relative`, width/height via JS or fixed.
    *   `.tile`: `position: absolute`, width/height `25%` of board.
    *   Grid: 4x4.
    *   Transition: `transition: transform 0.1s ease-in-out`.

    Wait, "Animate movement and merging without allowing rapid input to corrupt board state."
    This means we should probably use a `isMoving` flag or wait for transitions to end. But `0.1s` is very fast. Let's use a small debounce or just ensure the logic is synchronous and the DOM updates correctly.

    *   A tile `id` must be unique.
    *   When two tiles merge, one tile survives with a new `id` and a new `value`.
    *   Actually, it's better to say:
        -   Tile A and Tile B merge.
        -   Tile A is removed.
        -   A new Tile C is created (or Tile B is transformed) with the new value.
        -   To make it look like a merge, Tile A and Tile B should both move to the same position, then Tile A disappears and Tile B becomes the new value.

    Wait, let's simplify:
    1.  Tiles have `(value, row, col, id)`.
    2.  Move logic produces a new set of tiles with new `id`s (to signify new values).
    3.  Rendering:
        -   Compare `oldTiles` and `newTiles`.
        -   `toRemove`: `id` in `old` but not in `new`.
        -   `toUpdate`: `id` in both `old` and `new`.
        -   `toAdd`: `id` in `new` but not in `old`.

    Wait, if a tile merges, it's better if the two tiles move to the same cell and then one "explodes" into the new value.
    Actually, a simpler way to handle merges visually:
    1.  Identify which tiles will merge.
    2.  For those tiles:
        -   Set their `target_row`, `target_col` to the same cell.
        -   After transition, remove them and add the new tile.

    This is getting complicated. Let's stick to a simpler version first:
    1.  Calculate new state.
    2.  For every tile in the new state:
        -   If it's a new tile (not in old state), `opacity: 0` -> `opacity: 1`.
        -   If it's an existing tile, update `transform`.
        -   If it's a merged tile (a tile that was part of a merge), its `id` will be different.
    Actually, the easiest way:
    -   Every tile has an `id`.
    -   When merging:
        -   Tile 1 and Tile 2 merge into Tile 3.
        -   Tile 1 and Tile 2 are removed.
        -   Tile 3 is added at the merge position.

    Let's refine the move logic:
    ```javascript
    function move(direction) {
      // 1. Get rows/cols based on direction
      // 2. For each line:
      //    a. Collect non-zero tiles
      //    b. Check for merges:
      //       For i = 0 to len - 2:
      //         If tiles[i].value == tiles[i+1].value:
      //           Create new tile with value * 2
      //           Mark tiles[i], tiles[i+1] as merged
      //           Break to next i? No, check how many merges can happen.
      //           In 2048, [2, 2, 2, 2] -> [4, 4].
      //           So we skip the next index.
      //    c. Update score.
      // 3. If anything changed:
      //    a. Spawn new tile.
      //    b. Check for win/loss.
    }
    ```

    Wait, if `[2, 2, 2, 2]` becomes `[4, 4, 0, 0]`, we have two tiles.
    The score increase is `4 + 4 = 8`. Wait, the prompt says: "the score increases by the value of each new merged tile."
    Example: `[2, 2, 2, 2]` -> `[4, 4, 0, 0]`.
    Tiles were `2, 2, 2, 2`.
    New tiles are `4, 4`.
    Each merge produces a `4`. Total score increase = `4 + 4 = 8`.
    Wait, if we had `[2, 2, 4, 0]` -> `[4, 4, 0, 0]`.
    One merge: `2+2=4`. Score increases by `4`. Correct.

    Wait, let's double check the prompt:
    "the score increases by the value of each new merged tile."
    `[2, 2, 2, 2]` -> `[4, 4, 0, 0]`.
    The merges are: (2,2) -> 4 and (2,2) -> 4.
    So score increases by 4 + 4 = 8. Correct.

    *   Board size: 400x400.
    *   Cell size: 100x100.
    *   Tiles: `position: absolute`, `width: 100px`, `height: 100px`, `transition: transform 0.1s, background-color 0.1s`.

    *   `tiles`: Map of `id -> { value, row, col }`.
    *   `score`: `number`.
    *   `isGameOver`: `boolean`.
    *   `isWon`: `boolean`.

    *   `render()` function:
        *   `oldTiles` (before move)
        *   `newTiles` (after move)
        *   Logic to handle transitions.

    Actually, to make it simpler, we can just rebuild the DOM for tiles every frame, but that's not efficient and doesn't allow transitions.
    Correct way:
    -   Maintain `activeTiles`: `Map<id, DOMElement>`.
    -   When `render(oldTiles, newTiles)`:
        -   `newIds = new Set(newTiles.map(t => t.id))`
        -   `oldIds = new Set(oldTiles.map(t => t.id))`
        -   For each `id` in `oldIds`:
            -   If `id` not in `newIds`: `element.remove()`
            -   Else (it's a tile that survived):
                -   `t = newTiles.find(nt => nt.id == id)`
                -   `element.style.transform = translate(t.row, t.col)`
                -   `element.textContent = t.value`
                -   `element.className = 'tile value-' + t.value`
        -   For each `id` in `newIds`:
            -   If `id` not in `oldIds`:
                -   `element = createTileElement(t)`
                -   `element.style.opacity = 0`
                -   `element.style.transform = translate(t.row, t.col)`
                -   `appendChild(element)`
                -   `requestAnimationFrame(() => element.style.opacity = 1)`

    Wait, a tile that merges:
    -   Tile 1 (val 2, row 0, col 0, id: 1)
    -   Tile 2 (val 2, row 0, col 1, id: 2)
    -   They merge into Tile 3 (val 4, row 0, col 0, id: 3)
    -   `oldTiles` = `{1, 2}`
    -   `newTiles` = `{3}`
    -   `toRemove`: 1, 2.
    -   `toAdd`: 3.
    -   Visual: 1 and 2 disappear, 3 appears.
    -   This is okay, but it doesn't show the "merge" animation perfectly.
    -   To show merge animation:
        -   The surviving tile should be one of the merging tiles.
        -   But the value changes.
    -   Let's try this:
        -   In `move()`, if tiles merge:
            -   Identify which tile will "survive" (e.g., the one with the lower index).
            -   The other tile is removed.
            -   The surviving tile's `value` is updated.
            -   This way, the DOM element for the surviving tile stays, and its `textContent` and `className` change.

    Let's trace `[2, 2, 2, 2]` -> `[4, 4, 0, 0]`:
    -   Initial: `id:1 (v2, r0,c0), id:2 (v2, r0,c1), id:3 (v2, r0,c2), id:4 (v2, r0,c3)`.
    -   Move Left:
        -   Merge 1 & 2 -> New Tile 1 (v4, r0,c0).
        -   Merge 3 & 4 -> New Tile 3 (v4, r0,c1).
        -   `oldTiles`: `[1, 2, 3, 4]`
        -   `newTiles`: `[1, 3]` (with new values)
        -   `id 1`: `transform` (no change), `value` (2 -> 4).
        -   `id 2`: removed.
        -   `id 3`: `transform` (r0,c2 -> r0,c1), `value` (2 -> 4).
        -   `id 4`: removed.
    -   This looks much better!

    *   Use `grid` for the board layout.
    *   `position: relative` on board.
    *   `position: absolute` on tiles.
    *   Use CSS variables or `calc()` for tile positions.
    *   Tile positions:
        -   `left: col * 25%`
        -   `top: row * 25%`

    *   `index.html`
    *   `style` tag
    *   `script` tag

    ```javascript
    class Game {
      constructor() {
        this.board = Array(4).fill().map(() => Array(4).fill(null));
        this.score = 0;
        this.gameOver = false;
        this.win = false;
        this.nextId = 0;
        this.tiles = new Map(); // id -> {value, row, col}
      }

      init() {
        this.addTile(0, 0, 2);
        this.addTile(1, 1, 2); // random placement is better
      }
    }
    ```
    Wait, the requirement says "Start with two tiles." and "After a move that changes the board, spawn exactly one tile in an empty cell: 2 with 90% probability, otherwise 4."

    Let's refine `addTile(row, col, value)`:
    -   Find empty cells.
    -   Pick one.
    -   Assign `id`, `value`, `row`, `col`.

    Wait, `addTile` should just pick a random empty cell.

    `isGameOver()`:
    -   Are there any empty cells?
    -   Are there any adjacent tiles with the same value?

    *   `keydown` event on `window`.
    *   `event.preventDefault()` for ArrowUp, ArrowDown, ArrowLeft, ArrowRight.
    *   Check if the move changed anything.
    *   Update score.
    *   Spawn new tile.
    *   Check win/loss.

    *   Win: Reaching 2048.
    *   Game Over: No moves possible.
    *   New Game button: Resets everything.

    *   `2`: `#eee4da` (light beige) / text `#776e65`
    *   `4`: `#ede0c8` (light beige) / text `#776e65`
    *   `8`: `#f2b179` (orange) / text `#fff`
    *   `16`: `#f59564` (darker orange) / text `#fff`
    *   `32`: `#f67c5f` (pinkish orange) / text `#fff`
    *   `64`: `#f65e3b` (reddish orange) / text `#fff`
    *   `128`: `#edcf7a` (yellow) / text `#fff`
    *   `256`: `#edc22e` (yellow) / text `#fff`
    *   `512`: `#edb120` (yellow) / text `#fff`
    *   `1024`: `#ed9121` (yellow) / text `#fff`
    *   `2048`: `#ed6d27` (orange) / text `#fff`

    *   `tile-container`: `position: relative`, `width: 400px`, `height: 400px`, `background: #bbada0`, `border-radius: 6px`.
    *   `tile`: `position: absolute`, `width: 90px`, `height: 90px`, `border-radius: 3px`, `transition: transform 0.1s, background-color 0.1s`.
    *   Use `top` and `left` for initial position and `transform` for animation.
    *   Actually, it's better to use `top` and `left` for the initial position and then `transform: translate(x, y)` for movement if we want smooth transitions. But if we use `top`/`left` for everything, the transition will work if we change `top` and `left`. Let's use `top` and `left` and `transition: top 0.1s, left 0.1s`.

    Wait, `top`/`left` is easier for 4x4 grid.
    `top = row * 100px`
    `left = col * 100px`

    ```javascript
    move(direction) {
        let changed = false;
        // 1. Get all current tiles
        // 2. Group tiles by row/col
        // 3. For each group (line):
        //    a. compact (slide)
        //    b. merge (if same value, combine)
        //    c. update score and state
        // 4. If anything changed, spawn new tile
    }
    ```

    Wait, I need to be careful with the merge logic to match "Adjacent equal tiles merge once per tile per move".
    Example: `[2, 2, 2, 2]` left -> `[4, 4, 0, 0]`.
    `[2, 2, 4, 4]` left -> `[4, 8, 0, 0]` - wait, is that right?
    Let's re-check `[2, 2, 4, 4]` in standard 2048.
    - Left: `2, 2` merge to `4`. `4, 4` merge to `8`.
    - Result: `4, 8`.
    Yes.

    How about `[2, 2, 2, 0]` left?
    - `2, 2` merge to `4`. Remaining: `2`.
    - Result: `4, 2`.
    Yes.

    Implementation of `processLine(line)`:
    ```javascript
    function processLine(line) {
      // line is an array of tile objects {value, id,...}
      // 1. Filter out nulls
      let nonNulls = line.filter(t => t!== null);
      let newLine = [];
      let mergedIds = new Set();

      for (let i = 0; i < nonNulls.length; i++) {
        if (i < nonNulls.length - 1 && nonNulls[i].value === nonNulls[i+1].value) {
          // Merge!
          const mergedValue = nonNulls[i].value * 2;
          // Create a new tile object for the merged result
          // BUT we'll keep the ID of the first tile to simplify rendering
          // Actually, let's just return the new line of objects
          new_line.push({
            id: nonNulls[i].id,
            value: mergedValue,
            merged: true
          });
          score += mergedValue;
          i++; // skip next
        } else {
          new_line.push({
            id: nonNulls[i].id,
            value: nonNulls[i].value,
            merged: false
          });
        }
      }
    }
    ```
    Wait, if I use the same ID, I have to update its value.
    If I use a new ID, I have to remove the old ones and add a new one.
    Let's go with "keep the first tile's ID and update its value".

    *   `id`: unique identifier for a tile.
    *   `value`: 2, 4, 8...
    *   `row`, `col`: position.

    Move(direction):
    1.  `oldTiles = Map(tiles)`
    2.  `changed = false`
    3.  For each row/col (depending on direction):
        -   `line = []`
        -   `for r, c in line: if (tiles[r][c]) line.push(tiles[r][c])`
        -   `processedLine = processLine(line)`
        -   `if (processedLine.length!= line.length or values changed) changed = true`
        -   `update tiles[r][c]`
    4.  If `changed`:
        -   `spawnNewTile()`
    5.  `render(oldTiles, newTiles)`

    *Wait*, `processLine` needs to be very careful.
    Let's try:
    ```javascript
    function processLine(lineTiles) {
        let newTiles = [];
        for (let i = 0; i < lineTiles.length; i++) {
            let current = lineTiles[i];
            let next = lineTiles[i+1];
            if (next && current.value === next.value) {
                // Merge
                current.value *= 2;
                score += current.value;
                // Mark for removal of 'next'
                newTiles.push(current);
                i++; 
            } else {
                newTiles.push(current);
            }
        }
        return newTiles;
    }
    ```
    And then we need to clear the old positions in the grid.

    ```javascript
    function updateGrid(direction) {
        // 1. For each row/column, collect non-empty cells
        // 2. For each line:
        //    a. Slide (remove nulls)
        //    b. Merge
        //    c. Update the tiles and the grid
    }
    ```

    Wait, the `processLine` must return the *new* configuration of that line.
    ```javascript
    function getLine(row, col, direction) {
        // returns array of {value, id}
    }
    ```

    Let's simplify. The grid is always 4x4.
    ```javascript
    let grid = [
      [id1, id2, null, id3],
      [null, id4, id5, null],
      ...
    ]
    ```
    This is harder. Let's use an array of 16 elements.
    `grid[index]` = `id` or `null`.
    `tilesMap[id]` = `{value, row, col}`.

    When moving left:
    1.  For each row `r`:
        -   `line = []`
        -   `for c in 0..3`:
            -   `id = grid[r * 4 + c]`
            -   `if (id) line.push({id, value: tilesMap.get(id).value})`
        -   `new_line_data = processLine(line)`
        -   `for c in 0..3`:
            -   `grid[r * 4 + c] = (new_line_data[c]? new_line_data[c].id : null)`
        -   `updateTiles(new_line_data)`

    Let's trace `[2, 2, 2, 2]` left.
    - `line = [{id:1,v:2}, {id:2,v:2}, {id:3,v:2}, {id:4,v:2}]`
    - `processLine(line)`:
        - `i=0`: `line[0].v == line[1].v` -> `new_line.push({id:1, v:4})`, `i=2`
        - `i=2`: `line[2].v == line[3].v` -> `new_line.push({id:3, v:4})`, `i=4`
        - returns `[{id:1, v:4}, {id:3, v:4}]`
    - `grid[r*4+0] = 1`
    - `grid[r*4+1] = 3`
    - `grid[r*4+2] = null`
    - `grid[r*4+3] = null`
    - `tilesMap.set(1, {v:4, r, c:0})`
    - `tilesMap.set(3, {v:4, r, c:1})`
    - Done.

    -   `tilesMap` stores the state.
    -   `grid` stores the IDs.
    -   `oldTilesMap` stores state before move.
    -   `newTilesMap` stores state after move.

    *Wait*, what if a tile is removed because it was merged?
    -   In `processLine`, we return the new IDs.
    -   If an ID is not in the `new_line_data`, it's not in the `grid` and not in `newTilesMap`.

    `processLine(line)`:
    ```javascript
    function processLine(line) {
      const new_line = [];
      for (let i = 0; i < line.length; i++) {
        const current = line[i];
        const next = line[i + 1];
        if (next && current.value === next.value) {
          // Merge
          const mergedValue = current.value * 2;
          new_line.push({ id: current.id, value: mergedValue, merged: true });
          score += mergedValue;
          i++; // Skip next
        } else {
          new_line.push({ id: current.id, value: current.value, merged: false });
        }
      }
      return new_line;
    }
    ```

    Wait, the `score` update:
    If `[2, 2, 2, 2]` -> `[4, 4]`.
    `i=0`: `current.v=2, next.v=2`. `mergedValue=4`. `score += 4`. `new_line.push({id:1, v:4})`. `i=1`.
    `i=2`: `current.v=2, next.v=2`. `mergedValue=4`. `score += 4`. `new_line.push({id:3, v:4})`. `i=3`.
    Total score increment = 8. Correct.

    - `grid`: `Array(16).fill(null)`
    - `tilesMap`: `Map<id, {value, row, col}>`
    - `nextId`: `counter`
    - `score`: `0`

    - `init()`:
        - `addTile(rand_r, rand_c, 2)`
        - `addTile(rand_r, rand_c, 2)`

    - `addTile(r, c, v)`:
        - `id = nextId++`
        - `grid[r*4 + c] = id`
        - `tilesMap.set(id, {value: v, row: r, col: c})`

    - `move(dir)`:
        - `oldTilesMap = new Map(tilesMap)`
        - `changed = false`
        - For each row/column:
            - `line = []`
            - `for i in 0..3`:
                - `idx =...`
                - `id = grid[idx]`
                - `if (id) line.push({id, value: tilesMap.get(id).value})`
            - `new_line_data = processLine(line)`
            - `// if new_line_data.length!= line.length, or any values changed, changed = true`
            - `// Update grid and tilesMap`
        - `if (changed)`:
            - `spawnTile()`
            - `checkWinGameOver()`
        - `render(oldTilesMap, tilesMap)`

    Wait, `changed` needs to be more precise. A move is a "change" if the grid or the tile values in the grid change.

    ```javascript
    function checkGameOver() {
        // No empty cells and no adjacent same-value tiles
        // 1. Check for empty cells
        for (let i = 0; i < 16; i++) {
            if (grid[i] === null) return false;
        }
        // 2. Check for adjacent same-value tiles
        for (let r = 0; r < 4; r++) {
            for (let c = 0; c < 4; c++) {
                const id = grid[r * 4 + c];
                const val = tilesMap.get(id).value;
                // Check right
                if (c < 3) {
                    const idR = grid[r * 4 + c + 1];
                    if (idR && tilesMap.get(idR).value === val) return false;
                }
                // Check down
                if (r < 3) {
                    const idD = grid[(r + 1) * 4 + c];
                    if (idD && tilesMap.get(idD).value === val) return false;
                }
            }
        }
        return true;
    }
    ```

    ```javascript
    function spawnTile() {
        const emptyCells = [];
        for (let i = 0; i < 16; i++) {
            if (grid[i] === null) emptyCells.push(i);
        }
        if (emptyCells.length > 0) {
            const cellIdx = emptyCells[Math.floor(Math.random() * emptyCells.length)];
            const r = Math.floor(cellIdx / 4);
            const c = cellIdx % 4;
            const val = Math.random() < 0.9? 2 : 4;
            addTile(r, c, val);
        }
    }
    ```

    - Two tiles: `2, 2`.
    - Score: `0`.
    - Win: `2048`.
    - Game Over: No moves.
    - New Game: reset.

    Wait, one more thing: "Animate movement and merging without allowing rapid input to corrupt board state."
    I'll add a `isProcessing` flag that is set to `true` during the move and set back to `false` after the DOM updates are done.

    ```javascript
    let isProcessing = false;

    // In the keydown event:
    if (isProcessing) return;
    isProcessing = true;
    move(direction).then(() => {
      isProcessing = false;
    });
    ```
    Actually, since the `move` function is synchronous, `isProcessing` might not be needed unless I make `move` asynchronous.
    But the `render` function needs to wait for transitions?
    The requirement is: "Animate movement and merging without allowing rapid input to corrupt board state."
    If the logic is synchronous, the board state is always consistent. The "animation" is just a visual delay. The user might trigger multiple moves during the 100ms animation.
    If the `move` function is synchronous, then:
    1.  `move()` updates `grid` and `tilesMap` immediately.
    2.  `render()` updates the DOM.
    3.  The DOM takes 100ms to finish the transition.
    The user can trigger another `move()` during that 100ms.
    Since the `grid` and `tilesMap` are already updated, the next `move()` will start from the *new* state. This is actually fine and correct! The animation is just "catching up".

    Wait, the only issue is if `move()` takes time. But it's fast.
    The user might be able to press keys so fast that the animations look weird. But the logic will stay consistent.
    To be safe, I'll use a `isProcessing` flag.

    *   4x4 board.
    *   Single file `index.html`.
    *   Score, Win, Game Over.
    *   New Game.
    *   Arrow keys (no scroll).
    *   Correct merge logic (`[2,2,2,2]` -> `[4,4]`).
    *   Spawn 2 or 4.
    *   Score updates by merged value.

    -   Container: 400x400.
    -   Cells: 100x100.
    -   Tile: `position: absolute`, `width: 100px`, `height: 100px`.
    -   Gap between tiles: I'll use `padding` or `box-sizing`.
    -   Actually, let's use a grid with gaps.
    -   Wait, `position: absolute` is better for animating `top` and `left`.

    *   `index.html`
    *   CSS:
        - `.tile` needs `transition: top 100ms, left 100ms, background-color 100ms, opacity 100ms`.
        - `.tile-new` (for spawning): `opacity: 0` -> `opacity: 1`.
    *   JS:
        - `move(dir)`: returns a boolean (true if changed).
        - `render()`:
            - `oldTilesMap`, `newTilesMap`?
            - I'll just use a simple `render` that:
                1.  Identifies which tiles are new (not in `oldTilesMap`).
                2.  Identifies which tiles are removed (in `oldTilesMap` but not in `newTilesMap`).
                3.  Identifies which tiles are moved/updated (in both).

    Let's refine `render(oldTilesMap)`:
    ```javascript
    function render(oldTilesMap) {
        // 1. Find tiles to remove (in old but not in current)
        for (const [id, tile] of tilesMap) {
            //...
        }
        // 2. Find tiles to add (in current but not in old)
        // 3. Find tiles to update (in both)
    }
    ```

    Wait, the problem is that when a tile is merged, it's not just "one tile removed", the *values* of the tiles in the grid change.
    Let's use the `id` to track tiles.
    When merging tile A (id: 1) and B (id: 2) into tile C (id: 3, value: 4):
    - `oldTilesMap`: `{1: {v:2}, 2: {v:2}}`
    - `newTilesMap`: `{3: {v:4}}`
    - `toRemove`: 1, 2.
    - `toAdd`: 3.

    Wait, if I do this, there's no smooth transition for the "merging" tile.
    To make it look smooth, let's use the "surviving ID" approach:
    - When merging tile A and B into tile C:
        - We'll set `C.id = A.id`.
        - We'll set `C.value = A.value * 2`.
        - We'll remove B from the grid.
        - The DOM for A will stay, its `value` will change, and it will have a new `class` (color).
        - The DOM for B will be removed.
    - This is much better!

    Let's trace `[2, 2, 2, 2]` left:
    - `id:1 (v2), id:2 (v2), id:3 (v2), id:4 (v2)`
    - Merge 1 and 2:
        - `tile 1: value = 4`
        - `tile 2: removed`
    - Merge 3 and 4:
        - `tile 3: value = 4`
        - `tile 4: removed`
    - Result: `id:1 (v4, col:0), id:3 (v4, col:1)`.

    This is clean and handles everything.

    ```javascript
    function processLine(line) {
      const new_line = [];
      for (let i = 0; i < line.length; i++) {
        const current = line[i];
        const next = line[i + 1];
        if (next && current.value === next.value) {
          current.value *= 2;
          score += current.value;
          new_line.push(current);
          i++; // skip next
        } else {
          new_line.push(current);
        }
      }
      return new_line;
    }
    ```

    Wait, one detail: `changed` must be true if:
    -   Number of tiles in line changed.
    -   Any value in the line changed.
    -   Any tile moved to a new position.

    - `(2, 2, 2, 2)` -> `(4, 4, 0, 0)`.
    - `(2, 2, 4, 0)` -> `(4, 4, 0, 0)`.
    - `(4, 2, 2, 0)` -> `(4, 4, 0, 0)`.
    - `(2, 2, 4, 4)` -> `(4, 8, 0, 0)`.
    All match standard 2048.

    - Win: reaches 2048.
    - Loss: no more moves (checked when move is done).
    - New game: resets `grid`, `tilesMap`, `score`, `nextId`, `gameOver`, `win`.

    *   Create the `index.html` file with all components.

    *   `grid` is `Array(16)`
    *   `tilesMap` is `Map<id, {value, row, col}>`
    *   `render(oldTilesMap)`:
        1.  `const currentIds = new Set(tilesMap.keys())`
        2.  `const oldIds = new Set(oldTilesMap.keys())`
        3.  `for (id of oldIds)`:
            - `if (!currentIds.has(id)) { // tile was removed`
            - `  element.remove()`
            - `} else { // tile survived`
            - `  const tile = tilesMap.get(id)`
            - `  element.style.left = tile.col * 100 + 'px'`
            - `  element.style.top = tile.row * 100 + 'px'`
            - `  element.textContent = tile.value`
            - `  element.className = 'tile ''+ 'val-' + tile.value`
            - `}`
        4.  `for (id of currentIds)`:
            - `if (!oldIds.has(id)) { // tile is new`
            - `  create and append element`
            - `  element.style.opacity = 0`
            - `  element.style.left = tile.col * 100 + 'px'`
            - `  element.style.top = tile.row * 100 + 'px'`
            - `  requestAnimationFrame(() => element.style.opacity = 1)`
            - `}`

    Wait, one edge case: what if a tile is both updated (value changed) and moved (row/col changed)?
    The "survived" logic handles it: `if (currentIds.has(id))` it updates the position and value.

    *   `id: 1 (v2, r0,c0)`
    *   `id: 2 (v2, r0,c1)`
    *   `id: 3 (v2, r0,c2)`
    *   `id: 4 (v2, r0,c3)`
    *   `move(LEFT)`:
        -   `line = [{id:1,v:2}, {id:2,v:2}, {id:3,v:2}, {id:4,v:2}]`
        -   `processLine(line)`:
            -   `i=0`: `1.v=4`, `score+=4`, `new_line=[{id:1,v:4}]`, `i=1`
            -   `i=2`: `3.v=4`, `score+=4`, `new_line=[{id:1,v:4}, {id:3,v:4}]`, `i=3`
        -   `grid`: `[1, 3, null, null]`
        -   `tilesMap`: `{1: {v:4,r:0,c:0}, 3: {v:4,r:0,c:1}}`
        -   `oldTilesMap`: `{1: {v:2,r:0,c:0}, 2: {v:2,r:0,c:1}, 3: {v:2,r:0,c:2}, 4: {v:2,r:0,c:3}}`
        -   `render(oldTilesMap)`:
            -   `id 1`: `currentIds.has(1)`? Yes. `element.value=4`, `element.top=0`, `element.left=0`.
            -   `id 2`: `currentIds.has(2)`? No. `element.remove()`.
            -   `id 3`: `currentIds.has(3)`? Yes. `element.value=4`, `element.top=0`, `element.left=100`.
            -   `id 4`: `currentIds.has(4)`? No. `element.remove()`.
            -   `id (new)`: None.
        -   Visual result: Tiles 1 and 3 are updated, 2 and 4 are gone. Perfect.

    *Wait*, what if `new_line_data` returned an ID that *was* in `oldTilesMap` but its value changed?
    My `processLine` does that. It modifies the object `current` in place.
    So `current.value *= 2`. This is fine because `current` is a reference to the tile object in the `line` array, which itself comes from the `tilesMap`.
    Wait, I should be careful not to modify the `tilesMap` until I'm sure I want to.
    Let's copy the tile objects first.

    ```javascript
    function processLine(line) {
      const new_line = [];
      for (let i = 0; i < line.length; i++) {
        const current = {...line[i] }; // Copy!
        const next = line[i+1];
        if (next && current.value === next.value) {
          current.value *= 2;
          new_line.push(current);
          i++;
        } else {
          new_line.push(current);
        }
      }
      return new_line;
    }
    ```
    This is safer.

    Final logic for `move(dir)`:
    1.  `oldTilesMap = new Map(tilesMap)`.
    2.  `oldGrid = [...grid]`.
    3.  `changed = false`.
    4.  For each row/column:
        -   `line = getLine(row, col, dir)`
        -   `new_line_data = processLine(line)`
        -   `if (new_line_data.length!= line.length or any value/id mismatch) changed = true`
        -   `updateGridAndTiles(new_line_data, dir)`
    5.  If `changed`:
        -   `spawnTile()`
        -   `checkWinGameOver()`
    6.  `render(oldTilesMap)`

    Let's refine `getLine` and `updateGridAndTiles`:

    ```javascript
    function move(dir) {
      const oldTilesMap = new Map();
      for (let [id, tile] of tilesMap) {
        oldTilesMap.set(id, {...tile });
      }
      const oldGrid = [...grid];

      let changed = false;

      // Get lines based on direction
      const lines = [];
      if (dir === 'up') {
        for (let c = 0; c < 4; c++) {
          let line = [];
          for (let r = 0; r < 4; r++) {
            const id = grid[r * 4 + c];
            if (id) line.push({ id, value: tilesMap.get(id).value });
          }
          lines.push({ type: 'column', index: c, line });
        }
      } else if (dir === 'down') {
        for (let c = 0; c < 4; c++) {
          let line = [];
          for (let r = 3; r >= 0; r--) {
            const id = grid[r * 4 + c];
            if (id) line.push({ id, value: tilesMap.get(id).value });
          }
          lines.push({ type: 'column', index: c, line });
        }
      } else if (dir === 'left') {
        for (let r = 0; r < 4; r++) {
          let line = [];
          for (let c = 0; c < 4; c++) {
            const id = grid[r * 4 + c];
            if (id) line.push({ id, value: tilesMap.get(id).value });
          }
          lines.push({ type: 'row', index: r, line });
        }
      } else if (dir === 'right') {
        for (let r = 0; r < 4; r++) {
          let line = [];
          for (let c = 3; c >= 0; c--) {
            const id = grid[r * 4 + c];
            if (id) line.push({ id, value: tilesMap.get(id).value });
          }
          lines.push({ type: 'row', index: r, line });
        }
      }

      for (const { type, index, line } of lines) {
        const new_line_data = processLine(line);
        // If anything in new_line_data is different from original line, changed = true
        // Or if length of new_line_data!= line.length
        if (new_line_data.length!== line.length) {
          changed = true;
        } else {
          for (let i = 0; i < line.length; i++) {
            if (line[i].value!== new_line_data[i].value) {
              changed = true;
              break;
            }
          }
        }
        // wait, that's not enough, we also need to check if values changed but length didn't
        // actually, the above is fine.

        // Update grid and tilesMap
        // 1. Clear the row/column in the grid
        for (let i = 0; i < 4; i++) {
          if (type === 'row') grid[index * 4 + i] = null;
          else grid[i * 4 + index] = null;
        }
        // 2. Fill with new_line_data
        for (let i = 0; i < new_line_data.length; i++) {
          const { id, value } = new_line_data[i];
          if (type === 'row') {
            // We need to know the column
            // This is tricky. Let's make processLine return the column index too.
          }
        }
      }
    }
    ```

    Let's rethink. The `processLine` should be simpler.
    ```javascript
    function processLine(line) {
      const new_line = [];
      for (let i = 0; i < line.length; i++) {
        const current = {...line[i] };
        const next = line[i+1];
        if (next && current.value === next.value) {
          current.value *= 2;
          score += current.value;
          new_line.push(current);
          i++;
        } else {
          new_line.push(current);
        }
      }
      return new_line;
    }
    ```
    Then for each line, we find where the `new_line` elements go.
    If it's a row, `new_line[i]` goes to `(row, i)`.
    Wait, that's only if the `new_line` elements are pushed to the *start* of the row.
    Example `[2, 2, 2, 2]` -> `[4, 4]`. The `4, 4` should be at `col 0` and `col 1`.
    So yes, `new_line[i]` goes to `(row, i)` if it's a row, or `(i, col)` if it's a column.

    Wait, `[2, 2, 2, 2]` left:
    - `line = [id:1, v:2, c:0; id:2, v:2, c:1; id:3, v:2, c:2; id:4, v:2, c:3]`
    - `new_line = [{id:1, v:4}, {id:3, v:4}]`
    - `grid[r*4+0] = 1`
    - `grid[r*4+1] = 3`
    - `grid[r*4+2] = null`
    - `grid[r*4+3] = null`
    - `tilesMap.set(1, {v:4, r, c:0})`
    - `tilesMap.set(3, {v:4, r, c:1})`

    Yes! This works for any direction.
    - `up`: `new_line[i]` goes to `(i, col)`
    - `down`: `new_line[i]` goes to `(3 - (len - 1 - i), col)`? No.
    Let's make it simple:
    - Always process from the "movement" end.
    - If `dir` is `left`, process `c = 0..3`.
    - If `dir` is `right`, process `c = 3..0`.
    - If `dir` is `up`, process `r = 0..3`.
    - If `dir` is `down`, process `r = 3..0`.

    Then `new_line[i]` goes to `(row, i)` or `(i, col)`?
    If we always process from `0` to `3`, then `new_line[i]` goes to `(i, col)` for `up` and `(row, i)` for `left`.
    Wait, if `dir` is `right`, the `new_line` elements should be at the *end* of the row?
    No, because we are "sliding" everything to the right.
    So `[2, 2, 0, 0]` right becomes `[0, 0, 2, 2]`. No, it stays `[0, 0, 2, 2]`? No, `[2, 2]` right -> `[0, 0, 2, 2]`? No, it stays `[0, 0, 2, 2]`.
    Actually, standard 2048:
    `[2, 2, 0, 0]` left -> `[4, 0, 0, 0]`
    `[2, 2, 0, 0]` right -> `[0, 0, 0, 4]` - wait, the `4` is at the end.

    Let's use this:
    1.  For each row/column:
        -   `line = []`
        -   `for i in 0..3`:
            -   `id = grid[row, col]`
            -   `if (id) line.push({id, value: tilesMap.get(id).value})`
        -   `new_line = processLine(line)`
        -   `clear row/column`
        -   `if (dir is left or right)`:
            -   `start_idx = (dir is left? 0 : 4 - new_line.length)`
            -   `for i in 0..new_line.length-1`:
                -   `grid[row, start_idx + i] = new_line[i].id`
        -   `else (up or down)`:
            -   `start_idx = (dir is up? 0 : 4 - new_line.length)`
            -   `for i in 0..new_line.length-1`:
                -   `grid[start_idx + i, col] = new_line[i].id`

    Actually, it's simpler:
    -   `[2, 2, 0, 0]` left -> `[4, 0, 0, 0]`
    -   `[2, 2, 0, 0]` right -> `[0, 0, 0, 4]`
    -   `[2, 2, 0, 0]` up -> `[4, 0, 0, 0]` (but in column)
    -   `[2, 2, 0, 0]` down -> `[0, 0, 0, 4]` (but in column)

    Okay, let's use this logic:
    ```javascript
    // 1. Collect line
    // 2. Process line
    // 3. Clear row/column
    // 4. If dir == left:
    //      for i in 0..new_line.length-1: grid[r, i] = new_line[i].id
    //    if dir == right:
    //      for i in 0..new_line.length-1: grid[r, 4 - new_line.length + i] = new_line[i].id
    //    if dir == up:
    //      for i in 0..new_line.length-1: grid[i, c] = new_line[i].id
    //    if dir == down:
    //      for i in 0..new_line.length-1: grid[4 - new_line.length + i, c] = new_line[i].id
    ```

    Test `[2, 2, 0, 0]` right:
    - `line = [id1, id2]`
    - `new_line = [{id1, v4}]`
    - `grid[r, 4-1+0] = id1` -> `grid[r, 3] = id1`. Correct.

    Test `[2, 2, 0, 0]` down:
    - `line = [id1, id2]`
    - `new_line = [{id1, v4}]`
    - `grid[4-1+0, c] = id1` -> `grid[3, c] = id1`. Correct.

    This logic is solid!

    - `2`: `#eee4da`, text: `#776e65`
    - `4`: `#ede0c8`, text: `#776e65`
    - `8`: `#f2b179`, text: `#fff`
    - `16`: `#f59564`, text: `#fff`
    - `32`: `#f67c5f`, text: `#fff`
    - `64`: `#f65e3b`, text: `#fff`
    - `128`: `#edcf7a`, text: `#fff`
    - `256`: `#edc22e`, text: `#fff`
    - `512`: `#edb120`, text: `#fff`
    - `1024`: `#ed9121`, text: `#fff`
    - `2048`: `#ed6d27`, text: `#fff`

    -   `index.html`
    -   `style`
    -   `script`
        -   `Game` class
        -   `render` function
        -   `move` function
        -   `event listeners`

    One small thing: "After a move that changes the board, spawn exactly one tile in an empty cell: 2 with 90% probability, otherwise 4."
    The `move` function should return `true` if the `grid` or `tilesMap` changed.

    `checkGameOver()`:
    - `grid` has no `null`.
    - no `grid[i]` has `tilesMap.get(grid[i]).value === tilesMap.get(grid[neighbor]).value`.

    `win` condition: `tilesMap` has a tile with `value === 2048`.

    ```javascript
    function checkWin() {
        for (const tile of tilesMap.values()) {
            if (tile.value === 2048) return true;
        }
        return false;
    }
    ```

    Wait, if the user wins, should they be able to continue?
    "allow continuing after a win."
    Yes, the win message should be displayed, but the game doesn't stop.
    The `isGameOver` and `isWon` check should be part of the `move` logic.

    - `keydown`: `arrowup`, `arrowdown`, `arrowleft`, `arrowright`.
    - `preventDefault`.
    - `move(direction)`.

    Let's make sure `move` is robust.

Okay, the `move` function in the previous `write` was incomplete. I need to implement the `move` logic correctly. I'll rewrite the `Game` class in `index.html` with a complete and working `move` method.

I'll also make sure the animation (even if simple) and the scoring are correct.

Plan:
1. Rewrite `index.html` with a complete `Game` class.
2. Implement `move` logic:
    - Extract lines (rows or columns) based on direction.
    - For each line:
        - Filter out empty cells.
        - Merge adjacent identical tiles.
        - Update score for each merge.
        - Re-populate the line with the new values and empty cells.
    - Check if any tile's position or value changed.
    - If changed:
        - Update `this.grid` and `this.tilesMap`.
        - Spawn a new tile.
        - Check for win/game over.
    - Render the new state.
3. Ensure the grid and tile positioning logic is correct.
4. Verify with `browser` tool.

Let's refine the `move` logic:
- For a given direction, we can think of the board as a set of lines.
- `left`: rows, index 0-3, cells 0-3
- `right`: rows, index 0-3, cells 3-0
- `up`: cols, index 0-3, cells 0-3
- `down`: cols, index 0-3, cells 3-0

A more unified way:
1. Create a function `getLine(index, direction)` that returns the indices of a line.
2. Create a function `setTile(index, id, value, row, col)` to update the tile.

Wait, I'll just implement it directly in `move`.

```javascript
            move(direction) {
                this.isProcessing = true;
                const oldTilesMap = new Map();
                for (const [id, tile] of this.tilesMap) {
                    oldTilesMap.set(id, { ...tile });
                }

                let changed = false;
                const newGrid = [...this.grid];
                const newTilesMap = new Map();
                let newScore = this.score;
                let nextId = this.nextId;

                // Helper to get line indices
                const getLineIndices = (dir) => {
                    const indices = [];
                    if (dir === 'left') {
                        for (let r = 0; r < 4; r++) for (let c = 0; c < 4; c++) indices.push(r * 4 + c);
                    } else if (dir === 'right') {
                        for (let r = 0; r < 4; r++) for (let c = 3; c >= 0; c--) indices.push(r * 4 + c);
                    } else if (dir === 'up') {
                        for (let c = 0; c < 4; c++) for (let r = 0; r < 4; r++) indices.push(r * 4 + c);
                    } else if (dir === 'down') {
                        for (let c = 0; c < 4; c++) for (let r = 3; r >= 0; r--) indices.push(r * 4 + c);
                    }
                    return indices;
                };

                // This is still a bit messy because 'left' and 'right' share rows, 
                // but the order of processing matters.
                // Actually, for 2048, we can process each row/column independently.

                const lines = [];
                if (direction === 'left' || direction === 'right') {
                    for (let r = 0; r < 4; r++) {
                        const lineIndices = [];
                        if (direction === 'left') {
                            for (let c = 0; c < 4; c++) lineIndices.push(r * 4 + c);
                        } else {
                            for (let c = 3; c >= 0; c--) lineIndices.push(r * 4 + c);
                        }
                        lines.push(lineIndices);
                    }
                } else {
                    for (let c = 0; c < 4; c++) {
                        const lineIndices = [];
                        if (direction === 'up') {
                            for (let r = 0; r < 4; r++) lineIndices.push(r * 4 + c);
                        } else {
                            for (let r = 3; r >= 0; r--) lineIndices.push(r * 4 + c);
                        }
                        lines.push(lineIndices);
                    }
                }

                // We need to track which tiles are merged to avoid double merging in one move.
                // But since we process lines independently, we only need to worry about 
                // tiles within a single line.
                
                // Let's use a more robust approach.
                // For each line:
                // 1. Extract all tiles in that line.
                // 2. Merge them.
                // 3. Place them back in the line.

                const processedLines = []; // Will store {indices, tiles: [{id, value}, ...]}

                for (const lineIndices of lines) {
                    const lineTiles = [];
                    for (const idx of lineIndices) {
                        const id = this.grid[idx];
                        if (id !== null) {
                            lineTiles.push({ id, value: this.tilesMap.get(id).value });
                        }
                    }

                    const mergedLineTiles = [];
                    for (let i = 0; i < lineTiles.length; i++) {
                        if (i + 1 < lineTiles.length && lineTiles[i].value === lineTiles[i + 1].value) {
                            const mergedValue = lineTiles[i].value * 2;
                            newScore += mergedValue;
                            // We need a way to handle the ID of the merged tile.
                            // In 2048, the merged tile usually takes the ID of one of the two, 
                            // or a new one. Let's say it's a new ID to keep it simple, 
                            // but we must remove the old ones.
                            
                            // Actually, let's keep it simpler: 
                            // The merged tile will have a new ID, and the two old tiles are gone.
                            // But we need to know which ID to use. 
                            // Let's use the ID of the first tile and just update its value.
                            const id1 = lineTiles[i].id;
                            const id2 = lineTiles[i+1].id;
                            
                            // To avoid complexity with IDs, let's just create a new tile entry
                            // for the merged result and remove the old ones.
                            
                            // Wait, if I use a new ID, I need to manage it.
                            // Let's just update the first tile's value and mark the second as "consumed".
                            
                            // Let's try this:
                            // mergedLineTiles.push({ id: lineTiles[i].id, value: mergedValue, merged: true });
                            // i++;
                            // But wait, if I change the ID, I need to update the map.
                        }
                    }
                    // ... (this is getting complicated)
                }
```

Actually, a better way to implement 2048's move is:
1. For each line (row or col):
   a. Extract all non-empty tile values.
   b. Merge adjacent equal values (once).
   c. The new line is the result of these merges, padded with zeros.
2. Check if the new grid is different from the old grid.
3. If different, update the grid and spawn a new tile.

Wait, I need to maintain tile IDs for animation.

Let's try this:
For each line (row or col) in the direction of movement:
1. Get all tiles in the line (id and value).
2. Create a new list of tiles by merging.
   Example: `[{id:1, val:2}, {id:2, val:2}, {id:3, val:4}]` moving left -> `[{id:1, val:4}, {id:3, val:4}]`.
   *Wait*, the example `[2, 2, 2, 2]` -> `[4, 4, 0, 0]` is correct.
3. The new list of tiles will have:
   - If merged: a new tile with a new ID and the merged value.
   - If not merged: the original tile.
4. Update the `grid` and `tilesMap`.

Let's refine the `processLine` and `move` logic in the actual code.

```javascript
            move(direction) {
                this.isProcessing = true;
                
                const oldGrid = [...this.grid];
                const oldTilesMap = new Map();
                for (const [id, tile] of this.tilesMap) {
                    oldTilesMap.set(id, { ...tile });
                }

                let moved = false;
                let scoreGained = 0;

                // Get all lines (rows or columns)
                const lines = [];
                if (direction === 'left' || direction === 'right') {
                    for (let r = 0; r < 4; r++) {
                        const line = [];
                        for (let c = 0; c < 4; c++) {
                            const col = (direction === 'left') ? c : (3 - c);
                            line.push(r * 4 + col);
                        }
                        lines.push(line);
                    }
                } else {
                    for (let c = 0; c < 4; c++) {
                        const line = [];
                        for (let r = 0; r < 4; r++) {
                            const row = (direction === 'up') ? r : (3 - r);
                            line.push(row * 4 + c);
                        }
                        lines.push(line);
                    }
                }

                // To keep track of which tiles are removed and which are merged
                const tilesToRemove = new Set();
                const tilesToUpdate = []; // {id, value, newIdx}
                const tilesToCreate = []; // {value, newIdx} (for merges)

                for (const lineIndices of lines) {
                    // 1. Extract non-empty tiles from this line
                    const lineTiles = [];
                    for (const idx of lineIndices) {
                        const id = this.grid[idx];
                        if (id !== null) {
                            lineTiles.push({ id, value: this.tilesMap.get(id).value, originalIdx: idx });
                        }
                    }

                    // 2. Merge tiles
                    const mergedLine = [];
                    for (let i = 0; i < lineTiles.length; i++) {
                        if (i + 1 < lineTiles.length && lineTiles[i].value === lineTiles[i + 1].value) {
                            const newValue = lineTiles[i].value * 2;
                            scoreGained += newValue;
                            
                            // Create a merged tile
                            mergedLine.push({ 
                                type: 'merge', 
                                value: newValue, 
                                originalIds: [lineTiles[i].id, lineTiles[i+1].id] 
                            });
                            
                            tilesToRemove.add(lineTiles[i].id);
                            tilesToRemove.add(lineTiles[i+1].id);
                            i++; // Skip next
                        } else {
                            mergedLine.push({ 
                                type: 'move', 
                                id: lineTiles[i].id, 
                                value: lineTiles[i].value 
                            });
                        }
                    }

                    // 3. Assign new positions
                    // The mergedLine contains the tiles that will be placed at the beginning of the line
                    // (because we've already ordered the lineIndices in the direction of movement)
                    for (let i = 0; i < mergedLine.length; i++) {
                        const tileInfo = mergedLine[i];
                        const targetIdx = lineIndices[i];
                        if (tileInfo.type === 'move') {
                            // If the tile's position changed, it's a move
                            const oldIdx = lineTiles.find(t => t.id === tileInfo.id).originalIdx;
                            if (oldIdx !== targetIdx) {
                                moved = true;
                            }
                            tilesToUpdate.push({ id: tileInfo.id, value: tileInfo.value, newIdx: targetIdx });
                        } else {
                            // It's a merge. A merge always counts as a move/change.
                            moved = true;
                            // We'll create a new tile for the merge result
                            tilesToCreate.push({ value: tileInfo.value, newIdx: targetIdx });
                        }
                    }
                    
                    // Check if any existing tiles in the line were NOT in the mergedLine 
                    // (they've been shifted/merged)
                    // Actually, if the number of tiles in the line changed, or their positions changed, moved = true.
                    if (mergedLine.length !== lineTiles.length) {
                        moved = true;
                    } else {
                        // Even if count is same, positions might have changed if there were empty cells
                        // but we only added non-empty tiles to lineTiles.
                        // So if lineTiles.length is the same as mergedLine.length, 
                        // we still need to check if their originalIdx matches their targetIdx.
                        // (Already done in the 'move' type check)
                    }
                    
                    // If there were empty cells in the line, the tiles must have shifted.
                    // But we only know about non-empty tiles.
                    // Let's re-evaluate 'moved' more simply.
                    // A move is 'moved' if the set of occupied indices in the line changes, 
                    // OR if any tile's ID/value changes.
                    
                    // Let's simplify: A move is changed if the grid changes.
                    // We will build the new grid first, then compare.
                }
            }
```

Actually, I will just use a much simpler approach for the `move` function.

1. For each direction:
   - Create a `newGrid` (all null).
   - Create a `newTilesMap`.
   - For each line (row/column):
     - Get all current non-null tiles in that line.
     - Merge them into a new list of `(value, maybe_new_id)` pairs.
     - Place these pairs into the `newGrid` at the start of the line.
   - Compare `newGrid` with `oldGrid`.
   - If they are different, update `this.grid`, `this.tilesMap`, `this.score`, and spawn a new tile.

This is much cleaner and less error-prone.

```javascript
            move(direction) {
                this.isProcessing = true;
                const oldGrid = [...this.grid];
                const oldTilesMap = new Map();
                for (const [id, tile] of this.tilesMap) {
                    oldTilesMap.set(id, { ...tile });
                }

                const newGrid = new Array(16).fill(null);
                const newTilesMap = new Map();
                let nextId = this.nextId;
                let scoreGained = 0;

                const lines = this.getLines(direction);

                for (const lineIndices of lines) {
                    // 1. Get tiles in this line
                    const lineTiles = [];
                    for (const idx of lineIndices) {
                        const id = this.grid[idx];
                        if (id !== null) {
                            lineTiles.push({ id, value: this.tilesMap.get(id).value });
                        }
                    }

                    // 2. Merge
                    const mergedValues = [];
                    for (let i = 0; i < lineTiles.length; i++) {
                        if (i + 1 < lineTiles.length && lineTiles[i].value === lineTiles[i+1].value) {
                            const val = lineTiles[i].value * 2;
                            mergedValues.push({ value: val, isMerge: true });
                            scoreGained += val;
                            i++;
                        } else {
                            mergedValues.push({ value: lineTiles[i].value, isMerge: false, id: lineTiles[i].id });
                        }
                    }

                    // 3. Place back into newGrid
                    for (let i = 0; i < mergedValues.length; i++) {
                        const item = mergedValues[i];
                        const targetIdx = lineIndices[i];
                        if (item.isMerge) {
                            const id = nextId++;
                            newGrid[targetIdx] = id;
                            newTilesMap.set(id, { value: item.value, row: Math.floor(targetIdx / 4), col: targetIdx % 4 });
                        } else {
                            newGrid[targetIdx] = item.id;
                            newTilesMap.set(item.id, { value: item.value, row: Math.floor(targetIdx / 4), col: targetIdx % 4 });
                        }
                    }
                }

                // Check if anything changed
                let changed = false;
                for (let i = 0; i < 16; i++) {
                    if (this.grid[i] !== newGrid[i]) {
                        changed = true;
                        break;
                    }
                }
                // Also check if any tile's value changed (in case of merge)
                if (!changed) {
                    for (const [id, tile] of newTilesMap) {
                        const oldTile = oldTilesMap.get(id);
                        if (!oldTile || oldTile.value !== tile.value) {
                            changed = true;
                            break;
                        }
                    }
                }

                if (changed) {
                    this.grid = newGrid;
                    this.tilesMap = newTilesMap;
                    this.score += scoreGained;
                    this.nextId = nextId;
                    this.spawnNewTileAfterMove();
                    this.updateScore();
                    if (this.checkWin()) {
                        this.win = true;
                        this.showMessage("You Win!");
                    } else if (this.checkGameOver()) {
                        this.gameOver = true;
                        this.showMessage("Game Over!");
                    }
                }

                this.render(oldTilesMap);
                this.isProcessing = false;
            }
```

Wait, I also need `getLines(direction)`.

```javascript
            getLines(direction) {
                const lines = [];
                if (direction === 'left' || direction === 'right') {
                    for (let r = 0; r < 4; r++) {
                        const line = [];
                        if (direction === 'left') {
                            for (let c = 0; c < 4; c++) line.push(r * 4 + c);
                        } else {
                            for (let c = 3; c >= 0; c--) line.push(r * 4 + c);
                        }
                        lines.push(line);
                    }
                } else {
                    for (let c = 0; c < 4; c++) {
                        const line = [];
                        if (direction === 'up') {
                            for (let r = 0; r < 4; r++) line.push(r * 4 + c);
                        } else {
                            for (let r = 3; r >= 0; r--) line.push(r * 4 + c);
                        }
                        lines.push(line);
                    }
                }
                return lines;
            }
```

And the `render` function needs to handle removing old tiles correctly.

I'll now write the complete, correct `index.html`.

Okay, I have written the complete `index.html`. Now I will verify it using the `browser` tool. I'll check if the tiles are rendered, if I can move them, if scores update, and if New Game works.

Ah, I see. The `browser` tool doesn't have a `navigate` action. It seems it automatically navigates to `index.html` when I call it, or I should just use `action: 'screenshot'` or something to see the initial state. Actually, the documentation says: "Run index.html, inspect DOM/console, interact, and see screenshots or motion frames."

Let me try `action: 'screenshot'` to see if it loaded.

The user wants me to build a 2048 game.
I have already written a version of the code to `index.html`.
When I tried to run it in the browser, I got a console error: `error: Cannot read properties of undefined (reading 'keys')`.
This error happened in the `render` method: `const oldIds = new Set(oldTilesMap.keys());`.
Looking at the `move` method, `oldTilesMap` is defined as `const oldTilesMap = new Map();`.
Wait, in the `move` method, `oldTilesMap` is a `Map`.
In the `render(oldTilesMap)` call, if `move` is called, `oldTilesMap` should be a Map.
However, in the `constructor`, `this.reset()` is called.
`this.reset()` calls `this.render()`.
`this.render()` is called with no arguments, so `oldTilesMap` is `undefined`.
`const oldIds = new Set(oldTilesMap.keys());` will throw because `oldTilesMap` is undefined.

I need to fix the `render` method to handle the case where `oldTilesMap` is not provided.

Also, I noticed that the `render` function in `move` is called like this: `this.render(oldTilesMap);`.
And in `reset`, it's `this.render();`.

Let's fix the `render` method and also check the tile sizing/positioning logic.
The current `tile` CSS has `width: 87.5px` and `height: 87.5px`.
The `tiles-container` has `width: 380px` and `height: 380px`.
The grid is `400x400` with `10px` padding, so `380x380`.
`380 / 4 = 95`. But there are gaps.
If there are 4 cells and 3 gaps of 10px, then `4*C + 3*10 = 380` => `4*C = 350` => `C = 87.5`.
So `87.5` is correct for the cell size.
The position for cell `(r, c)` should be:
`left = c * (87.5 + 10) = c * 97.5`
`top = r * (87.5 + 10) = r * 97.5`
Wait, if `c=0`, `left=0`. If `c=1`, `left=97.5`. If `c=2`, `left=195`. If `c=3`, `left=292.5`.
`292.5 + 87.5 = 380`. This is correct.

The error is definitely the `oldTilesMap.keys()` when `oldTilesMap` is undefined.

Let's fix the `render` method in `index.html`.

I also want to make sure the `move` logic is correct.
In `move`, I have:
```javascript
                const oldTilesMap = new Map();
                for (const [id, tile] of this.tilesMap) {
                    oldTilesMap.set(id, { ...tile });
                }
```
This is fine.

And in `render`:
```javascript
            render(oldTilesMap) {
                const currentIds = new Set(this.tilesMap.keys());
                const oldIds = oldTilesMap ? new Set(oldTilesMap.keys()) : new Set();
```

Let's also check the `move` implementation details.
In `move`, I am trying to calculate `newGrid` and `newTilesMap`.
The `getLines` function returns arrays of indices.
For 'left', it's `[0, 1, 2, 3]` for row 0.
The `processLine` part:
```javascript
                    const new_line_data = this.processLine(line); // Wait, I changed this in my mind but didn't update the code in the last write? 
```
Wait, my last `write` used a different `move` implementation than my first one.
Let's look at the last `write`'s `move` implementation.

```javascript
            move(direction) {
                this.isProcessing = true;
                const oldTilesMap = new Map();
                for (const [id, tile] of this.tilesMap) {
                    oldTilesMap.set(id, { ...tile });
                }

                let changed = false;
                const oldGrid = [...this.grid];
                // ... (lines calculation)
                const newGrid = new Array(16).fill(null);
                const newTilesMap = new Map();
                let nextId = this.nextId;
                let scoreGained = 0;

                for (const lineIndices of lines) {
                    const lineTiles = [];
                    for (const idx of lineIndices) {
                        const id = this.grid[idx];
                        if (id !== null) {
                            lineTiles.push({ id, value: this.tilesMap.get(id).value });
                        }
                    }

                    const mergedValues = [];
                    for (let i = 0; i < lineTiles.length; i++) {
                        if (i + 1 < lineTiles.length && lineTiles[i].value === lineTiles[i + 1].value) {
                            const val = lineTiles[i].value * 2;
                            mergedValues.push({ value: val, isMerge: true });
                            scoreGained += val;
                            i++; // Skip the next element as it's merged
                        } else {
                            mergedValues.push({ value: lineTiles[i].value, isMerge: false, id: lineTiles[i].id });
                        }
                    }

                    for (let i = 0; i < mergedValues.length; i++) {
                        const item = mergedValues[i];
                        const targetIdx = lineIndices[i];
                        if (item.isMerge) {
                            const id = nextId++;
                            newGrid[targetIdx] = id;
                            newTilesMap.set(id, { value: item.value, row: Math.floor(targetIdx / 4), col: targetIdx % 4 });
                        } else {
                            newGrid[targetIdx] = item.id;
                            newTilesMap.set(item.id, { value: item.value, row: Math.floor(targetIdx / 4), col: targetIdx % 4 });
                        }
                    }
                }
                // ...
```

Wait, if I move 'right', the `lineIndices` for row 0 would be `[3, 2, 1, 0]`.
`mergedValues` would be filled from the end of the row.
`targetIdx = lineIndices[i]`.
If `i=0`, `targetIdx = 3`.
If `i=1`, `targetIdx = 2`.
This seems correct for 'right' and 'down' too.

The `changed` detection:
```javascript
                let changed = false;
                for (let i = 0; i < 16; i++) {
                    if (this.grid[i] !== newGrid[i]) {
                        changed = true;
                        break;
                    }
                }
                if (!changed) {
                    for (const [id, tile] of newTilesMap) {
                        const oldTile = oldTilesMap.get(id);
                        if (!oldTile || oldTile.value !== tile.value) {
                            changed = true;
                            break;
                        }
                    }
                }
```
Actually, if `changed` is true, I update `this.grid = newGrid`.
But `this.grid` is an array of IDs. `newGrid` is also an array of IDs.
So `this.grid[i] !== newGrid[i]` is a correct way to check if the board state changed.

One more thing: `this.isProcessing = false;` is at the end of `move`.
If `changed` is true, I spawn a tile.

I'll fix the `render` method and the `move` method (I'll make it cleaner).
And I'll make sure `oldTilesMap` is always passed or handled.

Wait, I see another potential issue:
In `render`, I am doing:
```javascript
                    el.className = `tile val-${tile.value}`;
```
But I have classes like `.val-2`, `.val-4` etc.
If `tile.value` is `2048`, `.val-2048` should exist. It does.
But for values that are not in the CSS, it might not have a background.
2048 is the max I defined.

Let's apply the fix for the `render` method first.