class Game {
    constructor() {
        this.grid = Array(4).fill().map(() => Array(4).fill(0));
        this.score = 0;
        this.init();
    }

    init() {
        this.addNewTile();
        this.addNewTile();
        this.updateDisplay();
    }

    addNewTile() {
        const emptyCells = [];
        for (let i = 0; i < 4; i++) {
            for (let j = 0; j < 4; j++) {
                if (this.grid[i][j] === 0) {
                    emptyCells.push({x: i, y: j});
                }
            }
        }
        if (emptyCells.length > 0) {
            const randomCell = emptyCells[Math.floor(Math.random() * emptyCells.length)];
            this.grid[randomCell.x][randomCell.y] = Math.random() < 0.9 ? 2 : 4;
        }
    }

    updateDisplay() {
        for (let i = 0; i < 4; i++) {
            for (let j = 0; j < 4; j++) {
                const cell = document.getElementById(`cell-${i}-${j}`);
                const value = this.grid[i][j];
                cell.className = 'grid-cell' + (value ? ` tile tile-${value}` : '');
                cell.textContent = value || '';
            }
        }
        document.getElementById('score').textContent = this.score;
    }

    move(direction) {
        let moved = false;
        const oldGrid = JSON.stringify(this.grid);

        switch(direction) {
            case 'left':
                moved = this.moveLeft();
                break;
            case 'right':
                moved = this.moveRight();
                break;
            case 'up':
                moved = this.moveUp();
                break;
            case 'down':
                moved = this.moveDown();
                break;
        }

        if (moved) {
            this.addNewTile();
            this.updateDisplay();
            if (this.isGameOver()) {
                alert('Game Over!');
            }
        }
    }

    moveLeft() {
        let moved = false;
        for (let i = 0; i < 4; i++) {
            let row = this.grid[i].filter(cell => cell !== 0);
            for (let j = 0; j < row.length - 1; j++) {
                if (row[j] === row[j + 1]) {
                    row[j] *= 2;
                    this.score += row[j];
                    row.splice(j + 1, 1);
                    moved = true;
                }
            }
            while (row.length < 4) {
                row.push(0);
            }
            if (row.join(',') !== this.grid[i].join(',')) {
                moved = true;
            }
            this.grid[i] = row;
        }
        return moved;
    }

    moveRight() {
        let moved = false;
        for (let i = 0; i < 4; i++) {
            let row = this.grid[i].filter(cell => cell !== 0);
            for (let j = row.length - 1; j > 0; j--) {
                if (row[j] === row[j - 1]) {
                    row[j] *= 2;
                    this.score += row[j];
                    row.splice(j - 1, 1);
                    row.unshift(0);
                    moved = true;
                }
            }
            while (row.length < 4) {
                row.unshift(0);
            }
            if (row.join(',') !== this.grid[i].join(',')) {
                moved = true;
            }
            this.grid[i] = row;
        }
        return moved;
    }

    moveUp() {
        let moved = false;
        for (let j = 0; j < 4; j++) {
            let column = [];
            for (let i = 0; i < 4; i++) {
                column.push(this.grid[i][j]);
            }
            column = column.filter(cell => cell !== 0);
            for (let i = 0; i < column.length - 1; i++) {
                if (column[i] === column[i + 1]) {
                    column[i] *= 2;
                    this.score += column[i];
                    column.splice(i + 1, 1);
                    moved = true;
                }
            }
            while (column.length < 4) {
                column.push(0);
            }
            for (let i = 0; i < 4; i++) {
                if (this.grid[i][j] !== column[i]) {
                    moved = true;
                }
                this.grid[i][j] = column[i];
            }
        }
        return moved;
    }

    moveDown() {
        let moved = false;
        for (let j = 0; j < 4; j++) {
            let column = [];
            for (let i = 0; i < 4; i++) {
                column.push(this.grid[i][j]);
            }
            column = column.filter(cell => cell !== 0);
            for (let i = column.length - 1; i > 0; i--) {
                if (column[i] === column[i - 1]) {
                    column[i] *= 2;
                    this.score += column[i];
                    column.splice(i - 1, 1);
                    column.unshift(0);
                    moved = true;
                }
            }
            while (column.length < 4) {
                column.unshift(0);
            }
            for (let i = 0; i < 4; i++) {
                if (this.grid[i][j] !== column[i]) {
                    moved = true;
                }
                this.grid[i][j] = column[i];
            }
        }
        return moved;
    }

    isGameOver() {
        // Check for empty cells
        for (let i = 0; i < 4; i++) {
            for (let j = 0; j < 4; j++) {
                if (this.grid[i][j] === 0) {
                    return false;
                }
            }
        }
        
        // Check for possible merges
        for (let i = 0; i < 4; i++) {
            for (let j = 0; j < 4; j++) {
                if (j < 3 && this.grid[i][j] === this.grid[i][j + 1]) return false;
                if (i < 3 && this.grid[i][j] === this.grid[i + 1][j]) return false;
            }
        }
        return true;
    }
}

// Initialize game
let game = new Game();

// Event listeners
document.addEventListener('keydown', (event) => {
    switch(event.key) {
        case 'ArrowLeft':
            game.move('left');
            break;
        case 'ArrowRight':
            game.move('right');
            break;
        case 'ArrowUp':
            game.move('up');
            break;
        case 'ArrowDown':
            game.move('down');
            break;
    }
});

document.getElementById('new-game').addEventListener('click', () => {
    game = new Game();
});